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The direction and magnitude of water movement across the capillary wall can be estimated as the algebraic sum of the hydrostatic and osmotic pressure that exists across the wall medicine everyday therapy buy discount citalopram 20 mg line. An increase in intracapillary hydrostatic pressure favors movement of fluid from the vessel interior to the interstitial space, whereas an increase in the concentration of osmotically active particles within vessels favors movement of fluid into the vessels from the interstitial space. Instead, it depends on arterial and venous pressure and on precapillary resistance (in the arterioles) and postcapillary resistance (in the venules and small veins). An increase in arterial or venous pressure elevates capillary hydrostatic pressure, whereas a reduction in arterial or venous pressure has the opposite effect. An increase in arteriolar resistance or closure of arteries reduces capillary pressure, whereas a greater resistance to flow in venules and veins increases capillary pressure. A given change in Pv produces a greater effect on capillary hydrostatic pressure than does the same change in Pa. Average values, obtained from direct measurements in human skin, are approximately 32 mm Hg at the arterial end of capillaries and approximately 15 mm Hg at the venous end of capillaries at the level of the heart. As discussed previously, when a person stands, hydrostatic pressure increases in the legs and decreases in the head. Tissue pressure, or, more specifically, interstitial fluid pressure (Pi) outside the capillaries, opposes capillary filtration. Normally, Pi is close to zero, and so Pc essentially represents the hydrostatic driving force. The key factor that restrains fluid loss from capillaries is the osmotic pressure of plasma proteins (such as albumin). The total osmotic pressure of plasma is approximately 6000 mm Hg (reflecting the presence of electrolytes and other small molecules, as well as plasma proteins), whereas oncotic pressure is only approximately 25 mm Hg. This low level of oncotic pressure is an important factor in fluid exchange across the capillary because plasma proteins are essentially confined to the intravascular space, whereas electrolytes are virtually equal in concentration on both sides of the capillary endothelium. The relative permeability of solute by water influences the actual magnitude of osmotic pressure. The reflection coefficient is the relative impediment to the passage of a substance through the capillary membrane. The reflection coefficient of water is 0, and that of albumin (to which the endothelium is essentially impermeable) is 1. In addition, different tissues have different reflection coefficients for the same molecule. Hence, movement of a given solute across the endothelial wall varies with the tissue. The actual oncotic pressure of the plasma (p) is defined by the following equation (see also Chapter 1): Equation 17. In this section, the capillary wallisformed bya single endothelialcell (Nu,endothelial nucleus). Lymphatic vessels H2O 32 mm Hg Absorption 15 mm Hg 25 mm Hg Oncotic pressure static pressu re Filtration Solutes protein (albumin) Hydro Balance of Hydrostatic and Osmotic Forces. The relationship between hydrostatic pressure and oncotic pressure and the role of these forces in regulating fluid passage across the capillary endothelium were expounded by Frank Starling in 1896. Albumin exerts an osmotic force greater than can be accounted for solely on the basis of its concentration in plasma. Therefore, it cannot be replaced on a mole-bymole basis by inert substances of appropriate molecular size, such as dextran. This additional osmotic force becomes disproportionately great at high concentrations of albumin (as in plasma), and this force is weak to absent in dilute solutions of albumin (as in interstitial fluid). The reason for this activity of albumin is its negative charge at normal blood pH and the attraction and retention of cations (principally Na+) in the vascular compartment (Gibbs-Donnan effect). Traditionally, filtration was thought to occur at the arterial end of the capillary, and absorption was thought to occur at its venous end because of the gradient of hydrostatic pressure along the capillary. However, in well-perfused capillaries, arteriolar vasoconstriction can reduce Pc in such a way that absorption at the arteriolar end can occur transiently. With continued vasoconstriction, absorption diminishes with time because Pi increases. Hence, in the normal state, filtration and absorption across the capillary wall are well balanced. However, a change in precapillary resistance influences fluid movement across the capillary wall. The rate of fluid movement (Qf) across the capillary membrane depends not only on the algebraic sum of the hydrostatic and osmotic forces across the endothelium (P) but also on the area (Am) of the capillary wall available for filtration, the distance (x) across the capillary wall, the viscosity of the filtrate, and the filtration constant (k) of the membrane. Because the thickness of the capillary wall and the viscosity of the filtrate are relatively constant, they can be included in the filtration constant k. If the area of the capillary membrane is not known, the rate of filtration can be expressed per unit weight of tissue. In any given tissue, the filtration coefficient per unit area of capillary surface, and hence capillary permeability, is not changed by various physiological conditions, such as arteriolar dilation and capillary distention, or by such adverse conditions as hypoxia, hypercapnia, or reduced pH. When capillaries are injured (as by toxins or severe burns), significant amounts of fluid and protein leak out of the capillaries into the interstitial space. This increase in capillary permeability is reflected by an increase in the filtration coefficient. Because capillary permeability is constant under normal conditions, the filtration coefficient can be used to determine the relative number of open capillaries. For example, the increased metabolic activity of contracting skeletal muscle relaxes the precapillary resistance vessels and hence opens more capillaries. The change in pressure may be countered by adjustments in precapillary resistance vessels (autoregulation; see Chapter 18) so that hydrostatic pressure remains constant in the open capillaries.

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In humans symptoms 9 days after embryo transfer cheap citalopram 40 mg buy on line, the auditory canal has a resonant frequency of about 3500 Hz, and this resonance contributes to the low perceptual threshold for sounds in that range. Three ossicles are present and serve to link the tympanic membrane to the oval window of the inner ear. A, Location of the right human cochlea in relation to the vestibular apparatus, the middle ear, and the external ear. B,Relationshipsbetweentheouter, middle, and inner ear spaces; the cochlea is depicted unrolled for clarity. Adjacent to the oval window is the round window, another membrane-covered opening between the middle ear and inner ear. Behind the oval window is a fluid-filled component of the inner ear, the vestibule. Inward movement of the tympanic membrane by a sound pressure wave causes the chain of ossicles to push the footplate of the stapes into the oval window. This movement of the stapes footplate in turn displaces the fluid within the scala vestibuli. The pressure wave that ensues within the fluid is transmitted through the basilar membrane of the cochlea to the scala tympani (described later), and it causes the round window to bulge into the middle ear. The tympanic membrane and the chain of ossicles serve as an impedance-matching device. The ear must detect sound waves traveling in air, but the neural transduction mechanism depends on movement in the fluid-filled cochlea, where acoustic impedance is much higher than that of air. Therefore, without a special device for impedance matching, most sound reaching the ear would simply be reflected, as are voices from shore when a person is swimming under water. Impedance matching in the ear depends on (1) the ratio of the surface area of the large tympanic membrane to that of the smaller oval window and (2) the mechanical advantage of the lever system formed by the ossicles. This impedance matching is sufficient to increase the efficiency of energy transfer by nearly 30 dB in the range of hearing from 300 to 3500 Hz. The bony labyrinth is a complex but continuous series of spaces in the temporal bone of the skull, whereas the membranous labyrinth consists of a series of soft tissue spaces and channels lying inside the bony labyrinth. In humans, the spiral consists of 2 3 4 turns from a broad base to a narrow apex, although its internal lumen is small at the base and wide at the top. Continuous with the vestibule is the scala vestibuli, the spiral-shaped chamber that extends to the apex of the cochlea, where it meets and merges with the scala tympani at the helicotrema. The scala tympani is another spiral-shaped space that winds back down the cochlea and ends at the round window. Separating the two, except at the helicotrema, is the scala media enclosed in the membranous labyrinth. The fluid in the bony labyrinth, including the scala vestibuli and scala tympani, is perilymph, which closely resembles cerebrospinal fluid. The fluid in the membranous labyrinth, including the scala media, is endolymph, which is very different from perilymph. Endolymph, generated by the stria vascularis, contains high [K+] (about 145 mM) and low [Na+] (about 2 mm) and has a high positive potential (about +80 mV) with regard to the perilymph. As a result, a very large potential gradient (about 140 mV) exists across the membranes of the hair cell cilia that extend into the endolymph. It lies on the basilar membrane and consists of several components, including three rows of outer hair cells, a single row of inner hair cells, a gelatinous tectorial membrane, and a number of types of supporting cells. Located on the apical surface of the hair cells are stereocilia, which can be described as nonmotile cilia that contact the tectorial membrane. The 32,000 auditory afferent fibers in humans originate in sensory ganglion cells in the spiral ganglion. These nerve fibers penetrate the organ of Corti and terminate at the bases of the hair cells. Approximately 90% of the fibers end on inner hair cells, and the remainder end on outer hair cells. Thus approximately 10 afferent fibers supply each inner hair cell, whereas other afferent fibers diverge to supply about five outer hair cells each. In addition to afferent fibers, the organ of Corti is supplied by efferent fibers, most of which terminate on the outer hair cells. These cochlear efferent fibers originate in the superior olivary nucleus of the brainstem and are often called olivocochlear fibers. The length of the outer hair cells varies; this characteristic suggests that changes in outer hair cell length may affect the sensitivity, or "tuning," of the inner hair cells. Such a mechanism could conceivably influence the sensitivity of the cochlea and the way that the brain recognizes sound. Other efferent fibers that end on cochlear afferent fibers may be inhibitory, and they may help improve frequency discrimination. Sound waves that reach the ear cause the tympanic membrane to oscillate, and these oscillations are transmitted to the scala vestibuli by the ossicles. This creates a pressure difference between the scala vestibuli and the scala tympani. Because of the shear forces set up by the relative displacement of the basilar and tectorial membranes, the stereocilia of the hair cells bend. Upward displacement bends the stereocilia toward the tallest cilium, which depolarizes the hair cells; downward deflection bends the stereocilia in the opposite direction, which hyperpolarizes the hair cells. Sound Transduction In view of the wide range of frequencies and amplitudes of sound stimuli, it is no surprise that hair cell transduction must provide for a fast response.

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Structure of the Pulmonary Circulation the arteries of the pulmonary circulation have thin walls treatment trichomoniasis buy citalopram, with minimal smooth muscle. They are seven times more compliant than systemic vessels, and they are easily distensible. This highly compliant state of the pulmonary arterial vessels requires lower pressure for blood flow through the pulmonary circulation than do the more muscular, noncompliant arterial walls of the systemic circulation. The vessels in the pulmonary circulation, under normal circumstances, are in a dilated state and have larger diameters than do similar arteries in the systemic system. All of these factors contribute to a very compliant, low-resistance circulatory system, which aids in the flow of blood through the pulmonary circulation via the relatively weak pumping action of the right ventricle. This low-resistance, low-work system also explains why the right ventricle is less muscular than the left ventricle. The pressure gradient differential for the pulmonary circulation from the pulmonary artery to the left atrium is only 6 mm Hg (14 mm Hg in the pulmonary artery minus 8 mm Hg in the left atrium). This pressure gradient differential is less than 7% of the pressure gradient differential of 87 mm Hg present in the systemic circulation (90 mm Hg in the aorta minus 3 mm Hg in the right atrium). Structures of the Extra-Alveolar and Alveolar Vessels and the Pulmonary Microcirculation Although not well defined anatomically, vessels in the pulmonary circulation can be divided into three categories (extra-alveolar, alveolar, and microcirculation) on the basis of differences in their physiological properties. They are not influenced by alveolar pressure changes, but they are affected by intrapleural and interstitial pressure changes. Thus the caliber of extra-alveolar vessels is affected by lung volume and by lung elastin. At high lung volumes, the decrease in pleural pressure increases the caliber of extra-alveolar vessels, whereas at low lung volumes, an increase in pleural pressure decreases vessel caliber. In contrast, alveolar capillaries reside within the interalveolar septa, and they are very sensitive to changes in alveolar pressure but not to changes in pleural or interstitial pressure. Positive-pressure ventilation increases alveolar pressure and compresses these capillaries and thus blocks blood flow. The pulmonary microcirculation comprises the small vessels that participate in liquid and solute exchange in maintenance of fluid balance in the lung. Structure of the Alveolar-Capillary Network the sequential branching of the pulmonary arteries culminates in a dense mesh-like network of capillaries that surround alveoli. This alveolar-capillary network is composed of thin epithelial cells of the alveolus and endothelial cells of the vessels and their supportive matrix, and it has an alveolar surface area of approximately 85 m2 (the approximate size of a tennis court). The structural matrix and the tissue components of this alveolar-capillary network provide the only barrier between gas in the airway and blood in the capillary. Surrounded mostly by air, this alveolar-capillary network is an ideal environment for gas exchange. In addition to gas exchange, the alveolar-capillary network regulates the amount of fluid within the lung. At the pulmonary capillary level, the balance between hydrostatic and oncotic pressure across the wall of the capillary results in a small net movement of fluid out of the vessels into the interstitial space. The fluid is then removed from the lung interstitium by the lymphatic system and enters the circulation via the vena cava in the area of the lung hilus. In normal adults, an average of 30 mL of fluid per hour is returned to the circulation via this route. Venous blood from the capillaries of the bronchial circulation flows to the heart through either true bronchial veins or bronchopulmonary veins. True bronchial veins are present in the region of the lung hilus, and blood flows into the azygos, hemiazygos, or intercostal veins before entering the right atrium. The bronchopulmonary veins are formed through a network of tributaries from the bronchial and pulmonary circulatory vessels that anastomose and form vessels with an admixture of blood from both circulatory systems. Blood from these anastomosed vessels returns to the left atrium through pulmonary veins. Approximately two thirds of the total bronchial circulation is returned to the heart via the pulmonary veins and this anastomosis route. The bronchial circulation receives only approximately 1% of total cardiac output; in comparison, the pulmonary circulation receives almost 100%. In the presence of diseases such as cystic fibrosis, the bronchial arteries, which normally receive only 1% to 2% of cardiac output, increase in size (hypertrophy) and receive as much as 10% to 20% of the cardiac output. The erosion of inflamed tissue into these vessels as a result of bacterial infection is responsible for the hemoptysis (coughing up blood) that can occur in this disease. The autonomic nervous system has four distinct components: parasympathetic, sympathetic, nonadrenergic noncholinergic inhibitory, and nonadrenergic noncholinergic stimulatory. Stimulation of the parasympathetic system leads to airway smooth muscle constriction, blood vessel dilation, and increased glandular cell secretion, whereas stimulation of the sympathetic system causes relaxation of the airway smooth muscle, constriction of blood vessels, and inhibition of glandular secretion (see Chapter 26. The parasympathetic innervation of the lung originates from the medulla in the brainstem (cranial nerve X, the vagus nerve). Preganglionic fibers from the vagal nuclei descend in the vagus nerve to ganglia adjacent to airways and blood vessels in the lung. Postganglionic fibers from the ganglia then complete the network by innervating smooth muscle cells, blood vessels, and bronchial epithelial cells (including goblet cells and submucosal glands). In the lungs, both preganglionic and postganglionic fibers Bronchial Circulation the bronchial circulation is a distinct system, separate from the pulmonary circulation in the lung, that provides systemic arterial blood to the trachea, upper airways, surface secretory cells, glands, nerves, visceral pleural surfaces, lymph nodes, pulmonary arteries, and pulmonary veins. Acetylcholine and substance P are neurotransmitters of excitatory motor neurons; dynorphin and vasoactive intestinal peptide are neurotransmitters of inhibitory motor neurons. Parasympathetic stimulation through the vagus nerve is responsible for the slightly constricted smooth muscle tone in a normal resting lung. Parasympathetic fibers also innervate the bronchial glands, and these fibers, when stimulated, increase the synthesis of mucus glycoprotein, which increases the viscosity of mucus. Parasympathetic innervation is greatest in the larger airways and most limited in the smaller conducting airways in the periphery.

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Active transport is further divided into primary active and secondary active transport medicine rash buy on line citalopram. Secondary active transport occurs with coupled solute carriers, for which passive movement of one or more molecules drives the active transport of other molecules. What is steady-state balance, and, with water balance as an example, what are the elements needed to achieve steady-state balance What are the volumes of the body fluid compartments, and how do they change under various conditions How do cells regulate their volume in isotonic, hypotonic, and hypertonic solutions What are the structural features of epithelial cells, how do they carry out vectorial transport, and what are the general mechanisms by which transport is regulated N ormal cellular function requires that the intracellular composition-with regard to ions, small molecules, water, pH, and a host of other substances-be maintained within a narrow range. This is accomplished by the transport of many substances and water into and out of the cell via membrane transport proteins, as described in Chapter 1. In addition, each day, food and water are ingested, and waste products are excreted from the body. In a healthy individual, these processes occur without significant changes in either the volume of the body fluid compartments or their composition. The maintenance of constant volume and composition of the body fluid compartments (and their temperature in warm-blooded animals and humans) is termed homeostasis. The human body has multiple systems designed to achieve homeostasis, the details of which are explained in the various chapters of this book. In this chapter, the basic principles that underlie the maintenance of homeostasis are outlined. In addition, the volume and composition of the various body fluid compartments are defined. Concept of Steady-State Balance the human body is an "open system," which means that substances are added to the body each day and, similarly, substances are lost from the body each day. The amounts added to or lost from the body can vary widely, depending on the environment, access to food and water, disease processes, and even cultural norms. In such an open system, homeostasis occurs through the process of steady-state balance. To illustrate the concept of steady-state balance, consider a river on which a dam is built to create a synthetic lake. At the same time, water is lost through the spillways of the dam and by the process of evaporation. Because the addition of water is not easily controlled and the loss by evaporation cannot be controlled, the only way to maintain a constant level of the lake is to regulate the amount that is lost through the spillways. To understand steady-state balance as it applies to the human body, the following key concepts are important. There must be a "set point" so that deviations from this baseline can be monitored. The sensor or sensors that monitor deviations from the set point must generate "effector signals" that can lead to changes in either input or output, or both, to maintain the desired set point. Although transient periods of imbalance can be tolerated, prolonged states of positive or negative balance are generally incompatible with life. For example, the amount of water lost through the lungs depends on the humidity of the air and the rate of respiration. Similarly, the amount of water lost as sweat varies according to ambient temperature and physical activity. Finally, water loss via the gastrointestinal tract can increase from a normal level of 100 to 200 mL/day to many liters with acute diarrhea. Of these inputs and outputs, the only two that can be regulated are increased ingestion of water in response to thirst and alterations in urine output by the kidneys (see Chapter 35). Cells within the hypothalamus of the brain monitor body fluid osmolality for deviations from the set point (normal range: 280-295 mOsm/kg H2O). The other is hormonal (antidiuretic hormone, also called arginine vasopressin), which regulates the amount of water excreted by the kidneys. With appropriate responses to these two signals, water input, water output, or both are adjusted to maintain balance and thereby keep body fluid osmolality at the set point. Volumes and Composition of Body Fluid Compartments Unicellular organisms maintain their volume and composition through exchanges with the environment they inhabit. The billions of cells that constitute the human body must maintain their volume and composition as well, but their task is much more difficult. This challenge, as well as its solution, was first articulated by the French physiologist Claude Bernard (1813-1878). He recognized that although cells within the body cannot maintain their volume and composition through exchanges with the environment, they can do so through exchanges with the fluid environment that surrounds them. He also recognized that the organ systems of the body are designed and function to maintain a constant milieu interieur or a "constant internal environment. The cardiovascular system delivers nutrients to and removes waste products from the cells and tissues and keeps the extracellular fluid well mixed. Finally, the nervous and endocrine systems provide regulation and integration of these important functions. To provide background for the study of all organ systems, this chapter presents an overview of the normal volume and composition of the body fluid compartments and describes how cells maintain their intracellular composition and volume. Included is a presentation on how cells generate and maintain a membrane potential, which is fundamental for understanding the function of excitable cells. Finally, because epithelial cells are so central to the process of regulating the volume and composition of the body fluids, the principles of solute and water transport by epithelial cells are also reviewed. Because the water content of adipose tissue is lower than that of other tissue, increased amounts of adipose tissue reduce the fraction of water in the total body weight. In some pathological conditions, additional fluid may accumulate in what is referred to as a third space. The pathways and driving forces for this water movement are different across cell membranes, in comparison to the capillary walls.

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Thermal Transduction the receptor that binds capsaicin (the molecule in chili peppers responsible for their spiciness) has been identified medicine in the 1800s cheap citalopram 20 mg without prescription, and either it or one of a family of related proteins has been found to be expressed in populations of dorsal root ganglion cells. Nevertheless, touch and pain sensitivity is altered in such knockdown mutants, so they may still play a modulatory role in the transduction process. Currently, Piezo2 is thought to be channel protein underlying the transduction for cutaneous mechanical rapidly adapting responses, because it forms a nonselective cation pore that opens in response to mechanical stimuli. Note how the response ranges of the afferents largely match up with those of individual heat-sensitive channels. These substances and others released from the damaged cells cause neurogenic inflammation (edema and redness of the surrounding skin). In addition to causing a local reaction, these substances may serve to activate the insensitive or silent nociceptors mentioned earlier, such that they can henceforth respond to any subsequent damaging stimuli. Sensitization of silent nociceptors has been suggested to underlie allodynia (elicitation of painful sensations by stimuli that were innocuous before an injury) and hyperalgesia (increase in the level of pain felt to already painful stimuli). The Piezo2 current is increased by substances known to cause mechanical hyperalgesia and allodynia, suggesting changes in this current underlie these phenomena. Centrifugal Control of Somatosensation Sensory experience is not just the passive detection of environmental events. Thus sensory information is often received as a result of activity in the motor system. Furthermore, transmission in pathways to the sensory centers of the brain is regulated by descending control systems. These systems allow the brain to control its input by filtering the incoming sensory messages. Important information can be attended to and unimportant information can be ignored. The tactile and proprioceptive somatosensory pathways are regulated by descending pathways that originate in the S-I and motor regions of the cerebral cortex. Of particular interest is the descending control system that regulates transmission of nociceptive information. For example, it is well known that soldiers on the battlefield, accident victims, and athletes in competition often feel little or no pain at the time a wound occurs or a bone is broken. Although the descending regulatory system that controls pain is part of a more general centrifugal control system that modulates all forms of sensation, the pain control system is so important medically that it is distinguished as a special system called the endogenous analgesia system. Several centers in the brainstem and pathways descending from these centers contribute to the endogenous analgesia system. Each of the proteins listed is expressed in at least some dorsal root ganglion cells, but they are also expressed inothercelltypes. In this model, gating of transmission of pain information would be due to a balance of the excitatory and inhibitory activity in the descending pathways. Other inhibitory pathways originate in the sensorimotor cortex, hypothalamus, and reticular formation. The endogenous analgesia system can be subdivided into two components: one component uses endogenous opioid peptides as neurotransmitters and the other does not. Endogenous opioids are neuropeptides that activate one of several types of opiate receptors. Opiate analgesia can generally be prevented or reversed by the narcotic antagonist naloxone. Therefore naloxone is frequently used to determine whether analgesia is mediated by an opioid mechanism. The opioid-mediated endogenous analgesia system can be activated by exogenous administration of morphine or other opiate drugs. Thus one of the oldest medical treatments of pain depends on the triggering of a sensory control system. One hypothesis is that the descending analgesia system is under tonic inhibitory control by inhibitory interneurons in both the midbrain and medulla. The action of opiates would inhibit the inhibitory interneurons and thereby disinhibit the descending analgesia pathways. Some endogenous analgesia pathways operate by neurotransmitters other than opioids and thus are unaffected by naloxone. One way of engaging a nonopioid analgesia pathway is through certain forms of stress. Serotonin can inhibit nociceptive neurons and presumably plays an important role in the endogenous analgesia system. Other brainstem neurons release catecholamines, such as norepinephrine and epinephrine, in the spinal cord. These catecholamines also inhibit nociceptive neurons; therefore catecholaminergic neurons may contribute to the endogenous analgesia system. In addition, there is evidence for the existence of endogenous opiate antagonists that can prevent opiate analgesia. Sensory neurons have cell bodies in sensory nerve ganglia: (1) dorsal root ganglia for neurons innervating the body and (2) cranial nerve ganglia for neurons innervating the face, oral and nasal cavities, and dura, except for proprioceptive neurons, which are in the trigeminal mesencephalic nucleus. They connect peripherally to a sensory receptor and centrally to second-order neurons in the spinal cord or brainstem. A and C nociceptors detect noxious mechanical, thermal, and chemical stimuli and may be sensitized by release of chemical substances from damaged cells. Peripheral release of substances, such as peptides, from nociceptors themselves may contribute to inflammation. Large primary afferent fibers enter the dorsal funiculus through the medial part of the dorsal root; collaterals synapse in the deep dorsal horn, intermediate zone, and ventral horn.

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This is accomplished primarily through action potentials medications during labor order citalopram, which propagate down the axon to the presynaptic terminals and cause neurotransmitter release, signaling the postsynaptic cells. As already explained, the regenerative nature of action potentials allows them to carry signals regardless of the length of the axon, whereas local signals, such as receptor or synaptic potentials (see Chapter 6), decay with distance and are therefore not suitable for this purpose. The tradeoff, however, is that the all-or-none nature of action potentials means that their shape and size do not generally convey information in the way gradations of local potentials do. Instead, the variations in the rate or timing of action potentials appear to be used primarily as the "codes" for transmission of information between neurons. Rate coding refers to information being coded in the firing rate of a neuron, where firing rate is defined as the number of spikes fired per unit time, usually expressed as spikes/second, also called hertz (Hz). For example, the force of a mechanical stimulus to the skin can be encoded in the firing rate of the primary afferent neuron that innervates the skin; the greater the force applied to the skin, the larger the resulting receptor potential in the primary afferent neuron will be and, as a consequence, the faster the rate of action potentials triggered by the receptor potential will be. Research has shown many neurons employ rate coding in the sense that the firing rate of a neuron shows a consistent relationship to particular parameters of sensory stimuli, upcoming movements, or other aspects of behavior. The upper limit of this range is set by the maximal frequency that a neuron can fire action potentials, which is determined by the duration of the absolute and relative refractory periods. The lower limit of the firing range is, of course, 0 Hz, as neurons cannot fire at negative rates. Timing, or temporal coding, refers to spike codes in which the specific timing of spikes rather than the overall firing rate encodes information. One often-studied version of temporal coding is the synchronization of spikes across neurons. Synchronization of neuronal spiking has been shown to occur in a number of brain regions and has been related to function in a number of instances. An advantage of temporal coding is that it can convey information more quickly than can rate coding, inasmuch as it does not require averaging, which takes time. Moreover, rate coding and temporal coding are not mutually exclusive, inasmuch as overall firing rates can be varied while synchronous events are superimposed. Such multiplexing of codes may increase the information transmission capacity of neuronal pathways. The encoded information is an abstraction based on (1) which sensory receptors are activated, (2) the responses of sensory receptors to the stimulus, and (3) information processing in the sensory pathway. Some stimulus parameters that can be encoded include sensory modality, location, intensity, frequency, and duration. Other aspects of stimuli that are encoded are described in relation to particular sensory systems in later chapters. For example, sustained mechanical stimuli applied to the skin result in sensations of touch or pressure, and transient mechanical stimuli may evoke sensations of flutter or vibration. Vision, audition, taste, and smell are examples of noncutaneous sensory modalities. The specific sensory receptors define the normal energy associated with the modality of a sensory pathway. For example, the visual pathway includes photoreceptors, neurons in the retina, the lateral geniculate nucleus of the thalamus, and the visual areas of the cerebral cortex (see Chapter 8). Thus neurons of the visual system can be regarded as a labeled line, which, when activated by whatever means, results in a visual sensation. The location of a stimulus is signaled by activation of the particular population of sensory neurons whose receptive fields are affected by the stimulus. For example, a somatotopic map is formed by arrays of neurons in the somatosensory cortex that receive information from corresponding locations on the body surface (see Chapter 7). In the visual system, points on the retina are represented by neuronal arrays that form retinotopic maps (see Chapter 8). Because action potentials have a uniform magnitude, some sensory neurons encode intensity by their frequency of discharge (rate coding). The relationship between stimulus intensity and response can be plotted as a stimulus-response function. For many sensory neurons, the stimulus-response function approximates an exponential curve with an exponent that can be less than, equal to , or greater than 1. Stimulusresponse functions with fractional exponents characterize many mechanoreceptors. Thermoreceptors, which detect changes in temperature, have linear stimulus-response curves (exponent of 1). Nociceptors, which detect painful stimuli, may have linear or positively accelerating stimulus-response functions. The positively accelerating stimulus-response functions of nociceptors help explain the urgency that is experienced as the pain sensation increases. Another way in which stimulus intensity is encoded is according to the number of sensory receptors that are activated. A stimulus at the threshold for perception may activate only one or only a few primary afferent neurons of an appropriate class, whereas a strong stimulus of the same type may recruit many similar receptors. Central sensory neurons that receive input from sensory receptors of this particular class would be more powerfully affected as more primary afferent neurons discharge. Greater activity in central sensory neurons may be perceived as a stronger stimulus. Stimuli of different intensities may also activate different sets of sensory receptors.

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In addition symptoms just before giving birth purchase citalopram pills in toronto, via its intimate connections with limbic and, by extension, hypothalamic structures, it provides input to subconscious mechanisms related to emotions, memory, and sexual behavior. Light enters the eye through the cornea and lens and is focused on the retina, which lines the back of the eye. The cornea is the most powerful refractive surface, but the lens has a variable power that allows images of near objects to be focused on the retina. The iris regulates depth of field and the amount of illumination that enters the eye. Photoreceptors synapse on retinal bipolar cells, which in turn synapse on other interneurons and on ganglion cells. The optic disc, where the optic nerve leaves the retina, contains no photoreceptors and is therefore a blind spot. The portion of the retina with the highest degree of spatial resolution is the fovea and the surrounding macula. Rod photoreceptors have high sensitivity, do not discriminate among colors, and function best under low light levels. Color vision relies on the three types of cones that have different spectral sensitivities. Bipolar cells and many ganglion cells have concentric receptive fields with an on-center/off-surround or off-center/on-surround organization. Photoreceptor, bipolar, and horizontal cells respond to stimulation by modulating their membrane potential and their release of neurotransmitters, but ganglion cells respond by generating action potentials. The axons of ganglion cells in the temporal retina project to the brain ipsilaterally; those in the nasal retina cross in the optic chiasm. Because the lens inverts the image that falls on the retina, each side of the visual field is projected to the contralateral side of the brain for both eyes. Within the map, information from each eye maps to alternating adjacent points to create ocular dominance columns that extend vertically in the cortex. Striate cortical neurons outside of layer 4 respond best to bar or edge stimuli oriented in a particular way. Cells that "prefer" a particular stimulus orientation are grouped in orientation columns. Some in the inferotemporal cortex are influenced chiefly by P cells, and they function in form detection, color vision, and face discrimination. M cells influence regions of the middle temporal and parietal cortex, which function in motion detection and the control of eye movements. The pinna and auditory canal convey airborne sound waves to the tympanic membrane. The three small bones (ossicles) of the middle ear transmit the vibrations of the tympanic membrane to the oval window of the fluid-filled inner ear. Hearing is most sensitive at about 3000 Hz because of the dimensions of the auditory canal and the mechanics of the ossicles. The cochlea of the inner ear has three main compartments: the scala vestibuli, the scala tympani, and the intervening scala media (cochlear duct). The cochlear duct is bounded on one side by the basilar membrane, on which lies the organ of Corti, the sound transduction mechanism. When the basilar membrane oscillates in response to pressure waves introduced into the scala vestibuli at the oval window, the stereocilia of the hair cells of the organ of Corti are subjected to shear forces, which open mechanoreceptor K+ channels. This results in a membrane conductance change that modulates the release of neurotransmitters on to cochlear nerve fibers. High-frequency sounds best activate the hair cells near the base of the cochlea, and low-frequency sounds activate cells near the apex. Such a tonotopic organization is also present in central auditory structures, including the cochlear nuclei, superior olivary complex, inferior colliculus, medial geniculate nucleus, and primary auditory cortex. Auditory processing at many sites in the central auditory pathway contributes to sound localization, frequency and intensity analysis, and speech recognition. It includes three semicircular canals (horizontal, anterior, and posterior) and two otolith organs (utricle and saccule) on each side. The three semicircular canals are mutually orthogonal, so they can resolve angular acceleration of the head about any axis of rotation. In each semicircular canal, there are sensory hair cells whose cilia extend into a cupula, which blocks the 16. Angular head acceleration displaces the endolymph and the cupula, bending the cilia. If the stereocilia bend toward the kinocilium, the hair cell is depolarized, which causes an increase in the firing rate in the afferent fiber. Acceleration of the head, as with linear movement, or change in position in relation to gravity displaces the otolithic membrane (because of the mass of the otoliths) and changes the firing patterns of the hair cells, depending on their orientation. Central vestibular pathways include afferent connections to the vestibular nuclei and the cerebellum. Activation of the vestibular afferent fibers is detected by the brain as head acceleration or position change and is relayed via the vestibular nuclei to pathways that mediate compensatory eye movements, neck movements, and adjustments to posture. Taste buds are located on several kinds of papillae on the tongue and in the pharynx and larynx. Five types of taste-receptor cells detect the five elementary qualities of taste: salty, sweet, sour, bitter, and umami. Complex flavors are signaled by the patterned activity of multiple classes of taste receptor and by central correlation with accompanying olfactory input.

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The umbilical arterial line tip may be at L4 to L5 (bifurcation of the aorta) or at T8 above the diaphragm symptoms 2 dpo generic 40 mg citalopram with amex. The course of the umbilical venous line is as follows: umbilical vein umbilical recess left portal vein ductus venosus inferior vena cava right atrium. The umbilical arterial line dips inferiorly in the umbilical artery to join the internal iliac artery (creating a deep loop in the pelvis) and then rises posteriorly in the abdominal aorta. The lateral view (not shown) confirms which catheter is arterial (equivalent to posterior) and which is venous (equivalent to anterior). Supine and upright radiographs are obtained, although left lateral decubitus views are used in newborns and in ill or uncooperative patients. Single supine examinations may be obtained when the clinical suspicion is constipation or foreign body ingestion or if the examination is being performed for tube or catheter localization. On a cross-table lateral view, it may be difficult to differentiate intraluminal air from extraluminal air, and decubitus or upright views are more useful in that context. The presence of air in normal-caliber large and small bowel is a common and normal finding in newborns and infants. In adynamic (without peristalsis) ileus (distention), the small bowel is less distended than the colon, and there is gas in the rectum. In infants and older children, adynamic ileus occurs (1) after surgery, (2) with sepsis, (3) with gastroenteritis, (4) subsequent to electrolyte disturbances such as dehydration and hypokalemia, and (5) after administration of drugs such as opiates and anticholinergics. AbdominalRadiograph:ChildVersusAdult the liver takes up a relatively larger space in the peritoneal cavity of a child. The angle between the umbilical arteries and the internal iliac arteries may make catheterization via the umbilical artery more difficult. C, Supine radiograph of the abdomen of a 3-day-old newborn shows the umbilical venous line (blue arrow) with the tip in the right atrium, and the umbilical arterial line (red arrow) with the tip in the aorta at T8. The prone film is helpful in directing gas to the rectum for assessment of its caliber when bowel obstruction is a concern; a prone horizontal-beam (cross-table lateral) film of the rectum is helpful in these cases. The most common causes of mechanical obstruction in the neonate are as follows: (1) duodenal atresia or stenosis, (2) malrotation with midgut volvulus, and (3) obstructing peritoneal bands (Ladd bands). In older children, valvulae conniventes and haustral markings may distinguish dilated small bowel and large bowel, respectively, and the distended colon is more peripheral in location than the more centrally located distended small bowel. These findings may not be applicable in the neonate or the child with malrotation and midgut volvulus. Anatomy Bone develops by intramembranous (flat bones) and endochondral (long bones) ossification. The long bone of a child is made up of four parts: the physis, or growth plate, and the epiphysis, metaphysis, and diaphysis. Each long bone (humerus, tibia) has a physis, and thus metaphyses and epiphyses, on either end. For example, hematogenous osteomyelitis is visible most often in the metaphysis; similarly, metastases travel hematogenously to the metaphysis. In children the periosteum is less firmly attached to the diaphysis (or shaft) of the long bone and is more likely to tear and thus be elevated by trauma or hematoma formation. In contrast, the periosteum at the ends of the long bones is not loosely attached but rather firmly adherent to the metaphyseal regions. This type of "bucket-handle" injury or avulsion "corner fracture" is commonly found in cases of child abuse (see Chapter 6). Pneumoperitoneum Free air in the peritoneal cavity most commonly results from perforation of a hollow viscus. Large amounts of free air are readily identifiable on supine abdominal radiographs, which may show the presence of the Rigler sign (where both sides of the bowel wall can be visualized), the football sign (where the liver is blacker than the adjacent soft tissues), and visualization of the falciform ligament. A cross-table lateral supine view may show a long colonic collection of air mimicking pneumoperitoneum. Perforation of a hollow viscus typically leads to intraperitoneal air/fluid levels as intraluminal fluid, as well as air leaks into the peritoneal cavity. Other causes of pneumoperitoneum include (1) postoperative air and (2) tracking of air from pneumomediastinum, usually in children undergoing pressure ventilation and in asthmatic patients. The latter condition results when mediastinal air extends into the retroperitoneum and then along the course of the mesenteric vessels, resulting in subserosal air, which can rupture into the peritoneal cavity. It can be differentiated from visceral perforation by the lack of intraperitoneal air/fluid levels on horizontal-beam radiographs. When imaging a long bone, both the proximal and distal joints must be included so that the entirety of the bone is imaged. Contralateral comparison views are not routinely obtained but frequently aid in differentiating normal developmental variation from pathology. Comparison views are most helpful in areas of complex anatomy, such as the elbow with its six ossification centers. The greenstick fracture is characterized by a bowed long bone with a break on the convex surface but apparent cortical continuity on the concave surface. If there is suspicion that a fracture has occurred but none is definitely identified, repeat films in 7 to 10 days, or a bone scan may help. Periosteal reaction around a fracture indicates healing and means that the injury is certainly more than several days old. Periosteal reaction, with the exception of so-called physiologic appositional new bone found symmetrically in infants 2 to 6 months old, should be regarded as abnormal.

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The numerous minor calyces expand into two or three open-ended pouches symptoms with twins discount citalopram 20 mg without a prescription, the major calyces. The pelvis represents the upper expanded region of the ureter, which carries urine from the pelvis to the urinary bladder. The walls of the calyces, pelvis, and ureters contain smooth muscle that contracts to propel the urine toward the urinary bladder. The glomerular capillaries come together to form the efferent arteriole, which leads into a second capillary network, the peritubular capillaries, which supply blood to the nephron. The vessels of the venous system run parallel to the arterial vessels and progressively form the interlobular vein, arcuate vein, interlobar vein, and renal vein, which courses beside the ureter. The nephron consists of a renal corpuscle, proximal tubule, loop of Henle, distal tubule, and collecting duct systema. The proximal tubule exits this structure and initially forms several coils, followed by a straight piece that descends toward the medulla. The next segment is the loop of Henle, which is composed of a the organization of the nephron is actually more complicated than presented here. However, for simplicity and clarity of presentation in subsequent chapters, the nephron is divided into five segments. However, again for simplicity, we consider the collecting duct system part of the nephron. Near the end of the thick ascending limb, the nephron passes between the afferent and efferent arterioles of the same nephron. This short segment of the thick ascending limb abutting the glomerulus is called the macula densa. The distal tubule begins a short distance beyond the macula densa and extends to the point in the cortex where two or more nephrons join to form a cortical collecting duct. The cortical collecting duct enters the medulla and becomes the outer medullary collecting duct and then the inner medullary collecting duct. Each nephron segment is made up of cells that are uniquely suited to perform specific transport functions. Proximal tubule cells have an extensively amplified apical membrane (the ultrafiltrate or urine side of the cell) called the brush border, which is present only in the proximal tubule. The basolateral membrane (the interstitial or blood side of the cell) is highly invaginated. In contrast, the descending and ascending thin limbs of the loop of Henle have poorly developed apical and basolateral surfaces and few mitochondria. The cells of the thick ascending limb and the distal tubule have abundant mitochondria and extensive infoldings of the basolateral membrane. The collecting duct is composed of two cell types: principal cells and intercalated cells. Principal cells have a moderately invaginated basolateral membrane and contain few mitochondria. Principal cells play an important role in reabsorption of NaCl (see Chapters 34 and 35) and secretion of K+ (see Chapter 36). Intercalated cells, which play an important role in regulating acid-base balance, have a high density of mitochondria. The final segment of the nephron, the inner medullary collecting duct, is composed of inner medullary collecting duct cells, which have poorly developed apical and basolateral surfaces and few mitochondria. All cells in the nephron except intercalated cells have in their apical plasma membrane a single nonmotile primary cilium that protrudes into the tubule fluid. As described in more detail in Chapter 36, increased flow of tubule fluid in the collecting duct is a strong stimulus for secretion of K+. The increase in [Ca++] activates K+ channels in the apical plasma membrane, which enhances secretion of K+ from the cell into the tubule fluid. The corresponding loops of Henle are short, and associated efferent arterioles branch into peritubular capillaries that surround its associated nephron segments as well as adjacent nephrons. This capillary network conveys oxygen and important nutrients to the nephron segments in the cortex, delivers substances to individual nephron segments for secretion. The glomerulus of each juxtamedullary nephron is located in the region of the cortex adjacent to the medulla. When compared with superficial nephrons, juxtamedullary nephrons differ anatomically in two important ways: the loop of Henle is longer and extends deeper into the medulla, and the efferent arteriole forms not only a network of peritubular capillaries but also a series of accompanying vascular loops called the vasa recta. To appreciate this process, one must understand the anatomy of the glomerulus, which consists of a network of capillaries supplied by the afferent arteriole and drained by the efferent arteriole. The endothelial cells of glomerular capillaries are covered by a basement membrane surrounded by podocytes. The filtration barrier is composed of three layers: the endothelium, basement membrane, and foot processes of the podocytes. Note the filtration slit diaphragm bridging the floor of the filtration slits (arrows). B, Scanning electron micrograph of the inner surface (blood side) of a glomerular capillary. In addition to their role as a barrier to filtration, the endothelial cells synthesize a number of vasoactive substances. Because both the basement membrane and filtration slits contain negatively charged glycoproteins, some proteins are held back. Another important component of the renal corpuscle is the mesangium, which consists of mesangial cells and the mesangial matrix. Mesangial cells, which possess many properties of smooth muscle cells, provide structural support for the glomerular capillaries, secrete extracellular matrix, exhibit phagocytic activity by removing macromolecules from the mesangium, and secrete prostaglandins and proinflammatory cytokines.

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Because of this very low resistance administering medications 7th edition order citalopram now, all the veins can be viewed as having a common venous compliance in the model of the circulatory system illustrated in Chapter 19. Because of the intermittent venous compression exerted by the contracting leg muscles, and because of the operation of the venous valves, blood is forced from the veins toward the heart. Hence, muscular contraction lowers the mean Pv in the legs and serves as an auxiliary pump. Furthermore, muscular contraction prevents venous pooling and lowers capillary hydrostatic pressure. In this way, muscular contraction reduces the tendency for edematous fluid to collect in the feet during standing. The capillaries, whose walls consist of a single layer of endothelial cells, allow rapid exchange of gases, water, and solutes with interstitial fluid. The muscular arterioles, which are the major resistance vessels, regulate regional blood flow to the capillary beds. The lymphatic system is composed of lymphatic vessels, nodes, and lymphoid tissue. This system collects the fluid and proteins that have escaped from blood and transports them back into the veins for recirculation in blood. In this section, the network of the smallest blood vessels of the body, as well as the lymphatic vessels, are examined in detail. The circular structures on the arteriole and venule represent smooth muscle fibers, and the branching solid lines represent sympatheticnervefibers. Microcirculation the microcirculation is defined as the circulation of blood through the smallest vessels of the body: arterioles, capillaries, and venules. Metarterioles can bypass the capillary bed and connect to venules, or they can connect directly to the capillary bed. Arterioles that give rise directly to capillaries regulate flow through these capillaries by constriction or dilation. The capillaries form an interconnecting network of tubes with an average length of 0. Functional Properties of Capillaries In metabolically active organs, such as the heart, skeletal muscle, and glands, capillary density is high. In less active tissues, such as subcutaneous tissue or cartilage, capillary density is low. Passage through these tiny vessels requires the erythrocytes to become temporarily deformed. The average velocity of blood flow in capillaries is approximately 1 mm/second; however, it can vary from zero to several millimeters per second in the same vessel within a brief period. The rhythmic oscillatory behavior of capillaries is caused by contraction and relaxation (vasomotion) of the precapillary vessels. Vasomotion is an intrinsic contractile behavior of vascular smooth muscle and is independent of external input. Changes in transmural pressure (intravascular pressure minus extravascular pressure) also influence the contractile state of precapillary vessels. An increase in transmural pressure, caused either by an increase in Pv or by dilation of arterioles, results in contraction of the terminal arterioles. For example, when increased transmural pressure causes the precapillary vessels to contract, the contractile response can be overridden and vasomotion abolished. This effect is accomplished by metabolic (humoral) factors when the O2 supply becomes too low for the requirements of parenchymal tissue, as occurs in skeletal muscle during exercise. Although a reduction in transmural pressure relaxes the terminal arterioles, blood flow through the capillaries cannot increase if the reduction in intravascular pressure is caused by severe constriction of the upstream microvessels. However, their contraction usually does not completely occlude the lumen of the vessel and arrest blood flow, whereas contraction of the terminal arterioles may arrest blood flow. Thus the flow rate in capillaries may be altered by contraction and relaxation of small arteries, arterioles, and metarterioles. Conversely, blood flow that bypasses the capillaries as it passes from the arterial to the venous side of the circulation via metarterioles has been termed nonnutritional, or shunt, flow. In tissues with metarterioles, nonnutritional flow may be continuous from arteriole to venule during low metabolic activity, when many precapillary vessels are closed. When metabolic activity increases in these tissues, more precapillary vessels open to allow capillary perfusion. True capillaries lack smooth muscle and are therefore incapable of active constriction. Nevertheless, the endothelial cells that form the capillary wall contain actin and myosin, and they can alter their shape in response to certain chemical stimuli. This property can be explained in terms of the law of Pierre-Simon Laplace: Equation 17. Wall tension opposes the distending force (Pr) that tends to pull apart a theoretical longitudinal slit in the vessel. Transmural pressure in a blood vessel in vivo is essentially equal to intraluminal pressure because extravascular pressure is generally negligible.

Asam, 64 years: Normal growth is associated with cellular hypertrophy, caused by the addition of more myofibrils and more sarcomeres at the ends of the cell to match skeletal growth. Monosaccharide derivatives are important metabolic products, although excesses or deficiencies of some contribute to pathogenic conditions. Activation of the superficial receptors results in apnea, cough, and expiratory movements that protect the lower respiratory tract from aspirating foreign material.

Oelk, 52 years: The primary transporter responsible for such uptake is called peptide transporter 1 (PepT1) and is a symporter that transports peptides in conjunction with protons. Muscular contractions also form functional "valves" in the rectum that retard movement of feces and are important in delaying the loss of feces until it is convenient, at least in adults. The main differences between the responses of dorsal column neurons and primary afferent neurons are as follows: (1) dorsal column neurons have larger receptive fields because multiple primary afferent fibers synapse on a given dorsal column neuron, (2) dorsal column neurons sometimes respond to more than one class of sensory receptor because of the convergence of several different types of primary afferent fibers on the second-order neurons, and (3) dorsal column neurons often have inhibitory receptive fields that are mediated through local interneurons.

Tukash, 53 years: These humoral substances reinforce the effects of the sympathetic nervous activity listed previously. Conversely, a decrease in cardiac output, anemia, hyperthermia, and exercise increase O2 extraction. Variations in venous return are achieved by adjustments in venomotor tone, respiratory activity (see Chapter 19), and orthostatic stress or gravity.

Rufus, 43 years: During growth and pregnancy, intestinal absorption exceeds urinary excretion, and these ions accumulate in newly formed fetal tissue and bone. Thermal Transduction the receptor that binds capsaicin (the molecule in chili peppers responsible for their spiciness) has been identified, and either it or one of a family of related proteins has been found to be expressed in populations of dorsal root ganglion cells. However, if a -adrenergic receptor antagonist is given to dogs with denervated hearts, exercise performance is impaired.

Rendell, 38 years: The hippocampal formation is important for storing declarative and spatial memory. The slow wave will initiate a contraction in smooth muscle when it reaches a threshold amplitude. Positron-emitting radionuclides include carbon-11 (11C), oxygen-15 (15O), and nitrogen-13 (13N), and these can be combined with various biologic tracers to image physiologic or metabolic processes.

Vibald, 37 years: Normally, membrane depolarization to threshold or beyond triggers an action potential; however, the explosive depolarization of the action potential can occur only if a critical number of Na+ channels are recruited. They consist of type I (glomus) cells that are rich in mitochondria and endoplasmic reticulum. The response of the kidneys to abrupt changes in NaCl intake typically takes several hours to several days, depending on the magnitude of the change.

Hamil, 41 years: True bronchial veins are present in the region of the lung hilus, and blood flows into the azygos, hemiazygos, or intercostal veins before entering the right atrium. In this example the fractional excretion of Na+ is 140 mEq/day � 25,200 mEq/day = 0. For example, it is well known that soldiers on the battlefield, accident victims, and athletes in competition often feel little or no pain at the time a wound occurs or a bone is broken.

Roland, 29 years: If the neck is bent (without tilting of the head), the neck muscle spindles evoke tonic neck reflexes without interference from the vestibular system. Because of their nonpolar nature, steroid hormones are not readily soluble in blood. Cardiac muscle must contract repetitively for a lifetime, and hence it requires a continuous supply of O2.

Shakyor, 24 years: If the coronary vasculature of an excised heart is artificially perfused with blood or an oxygenated electrolyte solution, rhythmic cardiac contractions may persist for many hours. It is involved in preventing acid reflux from the stomach back into the esophagus. As described subsequently, Diuresis is the term used for excretion of a large volume of urine.

Giores, 39 years: This in turn can reflect stimulus parameters, local ionic conditions, and the properties of the synapse. Reduction in fructose 2,6-bisphosphate concentration relieves inhibition of fructose 1,6-bisphosphatase (rate-limiting enzyme) and reduces activation of phosphofructokinase 1. The polarity and magnitude of the transepithelial voltage is determined by the specific membrane transporters in the apical and basolateral membranes, as well as by the permeability characteristics of the tight junction.

Aidan, 27 years: The olfactory mucosa is exposed to odorant molecules by ventilatory air currents or from the oral cavity during feeding. The alternating bursts of discharges in turn intermittently excite neurons in the cerebral cortex. The contractile apparatus of adjacent cells is mechanically coupled by the links between membrane-dense areas.

Spike, 63 years: For example, serum albumin, an anionic protein that has an effective molecular radius of 35. In addition to its central role in homeostasis, the autonomic nervous system also participates in appropriate and coordinated responses to external stimuli that are required for the optimal functioning of the somatic nervous system in performing voluntary behaviors. This secretion of anions drives the entry of Na+ and thus water into the acinar lumen across the relatively leaky tight junctions.

Campa, 35 years: However, their contraction usually does not completely occlude the lumen of the vessel and arrest blood flow, whereas contraction of the terminal arterioles may arrest blood flow. As a result, a very large potential gradient (about 140 mV) exists across the membranes of the hair cell cilia that extend into the endolymph. Alveolar pressure is the sum of the pleural pres sure and elastic recoil pressure (Pel) of the lung: Equation 21.

Akascha, 21 years: Phosphorylated cross-bridges cycle until they are dephosphorylated by myosin phosphatase. Under certain conditions, however, this increase in Pa could significantly alter the function of the cardiovascular system. Subtraction Technique In the subtraction technique 99mTc-sestamibi/123I or 99mTc-pertechnetate subtraction images are used.

Pyran, 47 years: Regulation of Plasma [K+] Several hormones, including epinephrine, insulin, and aldosterone, increase uptake of K+ into skeletal muscle, liver, bone, and red blood cells (Box 36. In Alport syndrome, the glomerular basement membrane becomes irregular in thickness and fails to serve as an effective filtration barrier to blood cells and protein. Images are acquired in the right posterior oblique position, which allows fluid in the gastric fundus to flow into the antropyloric area, distending this region.

Goran, 26 years: The chemical responsible was found to be acetylcholine, which we now know is also a neurotransmitter at the neuromuscular junction and at other synapses in the peripheral and central nervous systems. A normal ventilation/perfusion ratio does not mean that ventilation and perfusion of that lung unit are normal; it simply means that the relationship between ventilation and perfusion is normal. The pancreas also provides additional important secretory products that are vital for normal digestive function.

Kliff, 36 years: This figure illustrates the proteins that make up the slit diaphragm between two adjacent foot processes. Spread of inflammation occurs in 20% to 40% of postpubertal males with acute epididymitis, producing epididymo-orchitis. With more prolonged and severe hemorrhage, however, renal vasoconstriction becomes intense.

Leon, 59 years: By the time Na+ channels are de-inactivated (have returned to their closed state and would be able to open) the depolarization of membrane at that site has ended (because the action potential lasts only for 1 msec). The liver synthesizes all the nonessential amino acids (see Chapter 30) that do not need to be supplied in the diet, in addition to participating in interconverting and deaminating amino acids so the products can enter biosynthetic pathways for carbohydrate synthesis. The thebesian vessels of the left ventricular myocardium drain directly into the left ventricle (rather than into the coronary sinus in the right atrium), and some bronchial and mediastinal veins drain into the pulmonary veins.