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However spasms upper left abdomen discount nimodipine 30 mg on line, as we discuss later in the chapter, if the returning blood does become more than the heart can pump, then the heart becomes the limiting factor that determines cardiac output. Cardiac output is equal to venous return and is the sum of tissue and organ blood flows. Except when the heart is severely weakened and unable to pump the venous return adequately, cardiac output (total tissue blood flow) is determined mainly by the metabolic needs of the tissues and organs of the body. Chronic effect of different levels of total peripheral resistance on cardiac output, showing a reciprocal relationship between total peripheral resistance and cardiac output. Cardiac output curves for the normal heart and for hypoeffective and hypereffective hearts. Arterial pressure Cardiac output = Total peripheral resistance Thus, any time the long-term level of total peripheral resistance changes (but no other functions of the circulation change), the cardiac output changes quantitatively in exactly the opposite direction. Limits for the Cardiac Output There are definite limits to the amount of blood that the heart can pump, which can be expressed quantitatively in the form of cardiac output curves. Note that the plateau level of this normal cardiac output curve is about 13 L/min, 2. This means that the normal human heart, functioning without any special stimulation, can pump a venous return up to about 2. The uppermost curves are for hypereffective hearts that are pumping better than normal. The lowermost curves are for hypoeffective hearts that are pumping at levels below normal. In Chapter 9, we saw that a combination of sympathetic stimulation and parasympathetic inhibition does two things to increase the pumping effectiveness of the heart: (1) it greatly increases the heart rate-sometimes, in young people, from the normal level of 72 beats/min up to 180 to 200 beats/min-and (2) it increases the strength of heart contraction (called increased contractility) to twice its normal strength. Factors That Cause a Hypereffective Heart Two general types of factors that can make the heart a stronger pump than normal are nervous stimulation and hypertrophy of the heart muscle. A long-term increased workload, but not so much excess load that it damages the heart, causes the heart muscle to increase in mass and contractile strength in the same way that heavy exercise causes skeletal muscles to hypertrophy. For example, the hearts of marathon runners may be increased in mass by 50% to 75%. This factor increases the plateau level of the cardiac output curve, sometimes 60% to 100%, and therefore allows the heart to pump much greater than the usual amounts of cardiac output. When one combines nervous excitation of the heart and hypertrophy, as occurs in marathon runners, the total effect can allow the heart to pump as much 30 to 40 L/min, about 2. The solid curves demonstrate the effect in the normal dog of intense dilation of the peripheral blood vessels caused by administering the drug dinitrophenol, which increased the metabolism of virtually all tissues of the body about fourfold. With nervous control mechanisms intact, dilating all the peripheral blood vessels caused almost no change in arterial pressure but increased the cardiac output almost fourfold.
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The limited ability of the human kidney to concentrate the urine to only about 1200 mOsm/L explains why severe dehydration occurs if one attempts to drink seawater quercetin muscle relaxant buy generic nimodipine 30 mg line. Drinking 1 liter of seawater with a concentration of 1200 mOsm/L would provide a total sodium chloride intake of 1200 milliosmoles. If the maximal urine concentrating ability is 1200 mOsm/L, the amount of urine volume needed to excrete 1200 milliosmoles would be 1. The answer is that the kidney must also excrete other solutes, especially urea, which contribute about 600 mOsm/L when the urine is maximally concentrated. Urine specific gravity, however, is a measure of the weight of solutes in a given volume of urine and is therefore determined by the number and size of the solute molecules. In contrast, osmolarity is determined only by the number of solute molecules in a given volume. Urine specific gravity is generally expressed in grams per milliliter (g/ml) and, in humans, normally ranges from 1. This relationship between specific gravity and osmolarity is altered when there are significant amounts of large molecules in the urine, such as glucose, radiocontrast media used for diagnostic purposes, or some antibiotics. In these cases, urine specific gravity measurements may falsely suggest a highly concentrated urine, despite a normal urine osmolality. Dipsticks are available that measure approximate urine specific gravity, but most laboratories measure specific gravity with a refractometer. The countercurrent multiplier mechanism depends on the special anatomical arrangement of the loops of Henle and vasa recta, the specialized peritubular capillaries of the renal medulla. In humans, about 25% of the nephrons are juxtamedullary nephrons, with loops of Henle and vasa recta that go deeply into the medulla before returning to the cortex. Some of the loops of Henle dip all the way to the tips of the renal papillae that project from the medulla into the renal pelvis. Paralleling the long loops of Henle are the vasa recta, which also loop down into the medulla before returning to the renal cortex. And finally, the collecting ducts, which carry urine through the hyperosmotic renal medulla before it is excreted, also play a critical role in the countercurrent mechanism. The major factors that contribute to the buildup of solute concentration into the renal medulla are as follows: 1. Active transport of sodium ions and co-transport of potassium, chloride, and other ions out of the thick portion of the ascending limb of the loop of Henle into the medullary interstitium 2. Active transport of ions from the collecting ducts into the medullary interstitium 3. Facilitated diffusion of urea from the inner medullary collecting ducts into the medullary interstitium 4. A major reason for the high medullary osmolarity is active transport of sodium and co-transport of potassium, chloride, and other ions from the thick ascending loop of Henle into the interstitium. This pump is capable of establishing about a 200-mOsm/L concentration gradient between the tubular lumen and interstitial fluid.
Cutaneous and subcutaneous flow regulates heat loss from the body by metering the flow of heat from the core to the surface of the body spasms when falling asleep discount nimodipine 30 mg buy, where heat is lost to the environment. Skin blood flow is controlled largely by the central nervous system through the sympathetic nerves, as discussed in Chapter 74. Although skin blood flow is only about 3 ml/min/100 g of tissue in cool weather, large changes from that value can occur as needed. When humans are exposed to body heating, skin blood flow may increase greatly, to as high as 7 to 8 L/min for the entire body. When body temperature is reduced, skin blood flow decreases, falling to barely above zero at very low temperatures. Even with severe vasoconstriction, skin blood flow is usually great enough to meet the basic metabolic demands of the skin. Control of Tissue Blood Flow: Endothelium-Derived Relaxing or Constricting Factors the endothelial cells lining the blood vessels synthesize several substances that when released, can affect the degree of relaxation or contraction of the vascular wall. For many of these endothelium-derived relaxing or constrictor factors, the physiological roles are just beginning to be understood. The flow of blood through the arteries and arterioles causes shear stress on the endothelial cells because of viscous drag of the blood against the vascular walls. Without such a response, the effectiveness of local blood flow control would be decreased because a significant part of the resistance to blood flow is in the upstream small arteries. If untreated, this may eventually cause vascular injury and damage to vulnerable tissues such as the heart, kidneys, and brain. The most important of these is endothelin, a large, 27amino acid peptide that requires only minute amounts (nanograms) to cause powerful vasoconstriction. This substance is present in the endothelial cells of all or most blood vessels but greatly increases when the vessels are injured. The usual stimulus for release is damage to the endothelium, such as that caused by crushing the tissues or injecting a traumatizing chemical into the blood vessel. After severe blood vessel damage, local release of endothelin and subsequent vasoconstriction helps prevent extensive bleeding from arteries as large as 5 millimeters in diameter that might have been torn open by crushing injury. Increased endothelin release is also believed to contribute to vasoconstriction when the endothelium is Chapter 17 Local and Humoral Control of Tissue Blood Flow damaged by hypertension. Drugs that block endothelin receptors have been used to treat pulmonary hypertension but generally have not been used for lowering blood pressure in patients with systemic arterial hypertension. Yet, even after full activation of these acute mechanisms, the blood flow usually is adjusted only about three quarters of the way to the exact additional requirements of the tissues. For example, when the arterial pressure suddenly increases from 100 to 150 mm Hg, the blood flow increases almost instantaneously, by about 100%. Then, within 30 seconds to 2 minutes, the flow decreases back to about 10% to 15% above the original control value. This example illustrates the rapidity of the acute mechanisms for local blood flow regulation, but also demonstrates that the regulation is still incomplete because a 10% to 15% excess blood flow remains in some tissues. However, over a period of hours, days, and weeks, a long-term type of local blood flow regulation develops in addition to the acute control.
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Riordian, 44 years: Other parts of the heart can also exhibit intrinsic rhythmical excitation in the same way as the sinus nodal fibers; this is particularly true of the A-V nodal and Purkinje fibers. Even so, certain modalities of sensation are transmitted only in this system and not at all in the dorsal columnmedial lemniscal system. However, under two separate conditions, the damping factors can be overridden, and Cheyne-Stokes breathing does occur: 1. The normal right atrial pressure is about 0 mm Hg, which is equal to the atmospheric pressure around the body.
Dargoth, 64 years: In the case of the mechanoreceptors, the receptor that has been studied in greatest detail is the Pacinian corpuscle. The lymphoid progenitor cells that are eventually destined to form activated T lymphocytes first migrate to and are preprocessed in the thymus gland; thus, they are called T lymphocytes to designate the role of the thymus. Table 28-2 summarizes the factors that can influence the peritubular capillary reabsorption rate. Note that a few large myelinated fibers can transmit impulses at velocities as great as 120 m/sec, covering a distance that is longer than a football field in 1 second.
Larson, 23 years: An increasing pressure gradient then develops across the calcified valve, reaching 75 to 100 mm Hg in severe cases of aortic valve stenosis. These curves are contrasted with the much higher aortic pressure curve shown in the upper portion of the figure. This form of metabolic acidosis can be serious and can cause death, especially in young children. Thus, if a person remains under water at a deep level for only a few minutes, not much nitrogen dissolves in the body fluids and tissues, whereas if the person remains at a deep level for several hours, both the body water and body fat become saturated with nitrogen.
Milten, 25 years: Collaterals also terminate in the inferior olivary nuclei, and from there, secondary olivocerebellar fibers transmit signals to multiple areas of the cerebellum. Transport, storage, and most important feature of the hemoglobin molecule is its ability to combine loosely and reversibly with oxygen. Only a small amount of additional O2 dissolves in the fluid of the blood, as will be discussed subsequently. For example, a red area is often contrasted against a green area, a blue area against a red area, or a green area against a yellow area.
Gonzales, 40 years: The bronchiolar obstruction increases airway resistance and results in greatly increased work of breathing. Therefore, whenever plasmin is formed, it can cause lysis of a clot by destroying many of the clotting factors, thereby sometimes even causing hypocoagulability of the blood. Finally, these vesicles release their transmitter at the neuronal terminals in response to action potentials in the same manner as for small-molecule transmitters. The procedures for categorizing the types of alkalosis involve the same basic steps.
Saturas, 48 years: For example, surgical reduction of kidney mass or injury to the kidney due to hypertension, diabetes, or various kidney diseases all cause blood pressure to be more sensitive to changes in salt intake. Excess workload on the heart leads to early heart failure and coronary heart disease, often causing death as a result of a heart attack. Indeed, even when the lumbar portion of the spinal cord is separated from the remainder of the cord and a longitudinal section is made down the center of the cord to block neuronal connections between the two sides of the cord and between the two limbs, each hindlimb can still perform individual stepping functions. For potassium ions, the concentration gradient is 120 mEq/L inside the neuron and 4.
Pakwan, 46 years: Thus, chronic acidosis leads to a loss of potassium, whereas acute acidosis leads to decreased potassium excretion. Therefore, the potential of the electrocardiogram at this instant is at zero voltage. Also, the pressure in the neck veins often falls so low that the atmospheric pressure on the outside of the neck causes these veins to collapse. These functions of the auditory association areas and their relation to the overall intellectual functions of the brain are discussed in Chapter 58.
