CHAPTER 4 BIOOD ACIRCULATION 主讲：黄文英

1. CHAPTER 4 BIOOD ACIRCULATION主讲：黄文英

2. Section 1 Blood Components Section 2 Heart Physiology Section 3 Vessel Physiology Section 4 Circulation and Sports BIOOD ACIRCULATION

3. This chapter describes summary The muscle cell is unique in regard to its ability to increase its metabolic rate. The maintenance of a constant milieu interne in the cell during the transition from rest to vigorous exercise necessarily represents, at times, a tremendous challenge to the circulation. The function of individual cells within the body depends on the constancy of internal and surrounding environment. Claude Bernard recognized that an evolution of higher forms of organisms could not have taken place without the establishment of a stable milieu interne its composition being guarded by regulatory mechanisms.

4. Section 1 Blood Components （2）Blood Cells The solid elements of the blood that are visible under the ordinary microscope are red cells, or erythrocytes; white cells, or leukocytes; and platelets, or thrombocytes. In adult men, the average Hb content is 15.8g per 100 ml of blood; in women it is 13.9g per 100 ml. From a statistical point of view, a normal range (within which 95% of all people fall)is from 14.0 to 18.0g for men and 11.5 to16.0g for women per 100 ml of blood. Each gram of Hb can maximally combine with 1.34 ml of oxygen. （1）Plasma The maintenance of a constant milieu interne in the cell during the transition from rest to vigorous exercise necessarily represents, at times, a tremendous challenge to the circulation. The total amount of electrolytes in the plasma is 9g. L. The main ions are sodium ion and chloride ion. The plasma normally contains about 5 mmol/L of glucose. It also contains free fatty acids, amino acids, hormones, various enzymes, and about 25 different electrolytes in varying amounts.

5. 2 Hemodynamics （1）Plasma The pressure in a vessel is created by continuous bombardment by the molecules of the fluid against the inner surface of the vessel. The pressure is equal at all points lying in the same horizontal plane in a static liquid. The hydrostatic pressure in a fluid at rest, under the influence of gravity, increases uniformly with depth under the free surface

6. 2 Hemodynamics （2）Tension The pressure within the heart and vessels will more or less distend the walls the resistance to this force, which produces tension, depends on the thickness of the wall and its content of elastic and collagen fibers and active smooth muscle fibers. The elastic tissue can balance the pressure without any energy output, but the contraction of the muscles requires a continuous expenditure of energy. Heart muscle follows the same law as skeletal muscle: Its ability to produce tension increases with length. By applying Laplace’s law, one finds that the heart has to pay more to maintain a given pressure if a reduction of muscular strength is compensated for by a dilation.

7. 2 Hemodynamics （3）Flow and Resistance The resistance to flow of blood results from the inner friction or viscosity of the blood. A cohesive force between the blood and the wall of the vessel “retards” the flow of the layers close to the wall. The nearer the center of the vessel, the higher the speed of each lamina of fluid, and this “friction” phenomenon results in maximal speed in the very center of the vessel. The hydraulic energy provided to the blood by the contracting heart muscle is gradually spant and transformed to heart. The factors of importance for flow can be described in more detail by presenting the Poiseuille Hagen foemula , although the equation is only applicable for nonpulsatile flow of homogeneous fluids. Actually , floe in the blood vessels is normally laminar except in and close to the heart .

8. 2 Hemodynamics （3）Flow and Resistance The resistance to flow of blood results from the inner friction or viscosity of the blood. A cohesive force between the blood and the wall of the vessel “retards” the flow of the layers close to the wall. The nearer the center of the vessel, the higher the speed of each lamina of fluid, and this “friction” phenomenon results in maximal speed in the very center of the vessel. The hydraulic energy provided to the blood by the contracting heart muscle is gradually spant and transformed to heart. The factors of importance for flow can be described in more detail by presenting the Poiseuille Hagen foemula , although the equation is only applicable for nonpulsatile flow of homogeneous fluids. Actually , floe in the blood vessels is normally laminar except in and close to the heart .

9. 3 Blood Function （1）Buffer Action, Blood pH At rest, the pH of arterial blood is 7.4 and of mixed venous blood approximately 7.37. In the catabolism of the cells, CO is formed, and in the case of anaerobic metabolism, there is a net production of hydrogen ion (H), with the hydrolysis of adenosine ATP as the dominant source. The oxidation of phosphorous and sulfur in protein leads to the formation of phosphoric and sulfuric acids.

10. 3 Blood Function Potentially, this reaction gives place for more CO in the solution without a change in the pH. Although more HCO will be formed, this is only an intermediate step in the formation of H and HCO. In this sense, the supply of a hydrogen accepter actually determines the final equilibrium. Protein anions can serve as such a hydrogen accepter. The CO is produced in the tissue and diffuses into the red cell. At the same time, oxygen diffuses out to the tissue, where the oxygen concentration is lower than in the capillaries (HbO). Reduced Hb is a weaker acid than HbO, and when reduced, it binds some of the dissociated H ions (CO+ HO + Hb + H-Hb).

11. 3 Blood Function （2）Carbon Dioxide Transport ①The most important reactions in the oxygen and CO exchange between blood and tissue are(a) the formation of a weak acid, reduced Hb, from the stronger one, HbO, and (b) the formation of H+ HCO from CO+ HO ,with carbonic anhydrates serving as an enzyme in the intermediate formation of HCO.The interplay between reactions (a) and (b) can be illustrated quantitatively by the following example: For each millimole of HbOreduced, about 0.7 mmol of Hcan be taken up, and consequently 0.7 mmol of CO can enter the blood without changing pH. ②Some of the remaining COcan combine directly with Hb, forming carbaminohemoglobin(CO+), and the simultaneous reduction of HbO greatly favors this formation.

12. 3 Blood Function （2）Carbon Dioxide Transport ①The most important reactions in the oxygen and CO exchange between blood and tissue are(a) the formation of a weak acid, reduced Hb, from the stronger one, HbO, and (b) the formation of H+ HCO from CO+ HO ,with carbonic anhydrates serving as an enzyme in the intermediate formation of HCO.The interplay between reactions (a) and (b) can be illustrated quantitatively by the following example: For each millimole of HbOreduced, about 0.7 mmol of Hcan be taken up, and consequently 0.7 mmol of CO can enter the blood without changing pH. ②Some of the remaining COcan combine directly with Hb, forming carbaminohemoglobin(CO+), and the simultaneous reduction of HbO greatly favors this formation.

13. 3 Blood Function ③There is an increase in the volume of dissolved CO, so that the venous blood at rest contains about 0.003L of CO per liter more than the arterial blood. Together, these three factors explain how the COproduced at rest can be transported with the very small change in pH of 0.03 to the acid side. The difference in acidity between Hb and HbO2 minimizes the variation in the pH of the blood as it passes the tissue, takes up CO2 , and subsequently releases it though the lungs. In the kidneys, excess H+ produced in the metabolism of “nonvolatile” acids can be excreted as free ions or ions bound to NH+. and as NaH2PO4. In other words, there are three strategies or lines of defense against increasing acidity: buffers, respiration, and renal excretion of protons.

14. 3 Blood Function （3）Filtration and Osmosis The figure shows the driving forces in a process outlined by Starling When blood flows continuously through a capillary, the hydrostatic pressure is normally well above the pressure in the interstitial fluid (ISF).

15. Section 2 Heart 1 The characteristics of Cardiac Muscle Physiology

16. 2 Pumping Function of Heart The systole starts with an isometric contraction, because the mitral values close rapidly as the pressure in the ventricle exceeds that of the atrium. Within about 0.5s, the ventricular pressure is increased above the level in the aorta, and the aortic values open. The isotonic contraction increases the pressure further. The peripheral resistance does not permit the same volume of blood to escape from the aorta as is ejected into it. Part of this volume is "stored" in the distended aorta and its large branches. Then, as the pressure decreases in the ventricle during the relaxation of the muscle, the aortic values close, and the elastic property of the aortic wall can propel the stored blood out into the arterial tree. The intermittent energy outbursts of the heart would give an intermittent flow if the vessels were rigid tubes. However, part of the potential energy is taken up by the elastic arterial wall and then released during diastole of the heart, keeping the hydraulic energy level close to the heart continuously high.

17. 心室收缩与射血 心房初级泵血功能 等容收缩期 快速射血期 减慢射血期 等容舒张期 快速充盈期 减慢充盈期 心室舒张与血液充盈

18. 3 The Regulating of Pumping Heart （1）Efficiency of the Heat From studies of cardiac output the energy output and work efficiency of the heart the following factors seem of special interest: Factors1: A given stroke volume can be ejected with a minimum of myocardial shortening if the contraction starts at a larger volume. Factors2: energy losses in the from of frictionand tension developed within the heart wall are also at a minimum in a dilated heart. Factors3: the stretched muscle fiber can within limits, provide a higher tension than the unstretched one. Factors4: loss of energy is larger when the contraction occurs rapidly, that is, with a high heart rate as compared with a slower contraction rate. Factors5: on the other hand, the greater the volume of the heart, the higher the tension of the myocardial fibers necessary to sustain a particular intraventricular pressure (as a consequence of Laplace’s law, as discussed earlier in this chapter)

19. （2）Heart rate In many types of exercise, the increase in heart rate is linear with the increase in rate of exercise. There are exceptions, and those exceptions are perhaps more frequent among untrained. When the subject performs very heavy exercise, the (a-v) O2 difference may increase so that the oxygen uptake increases relatively more than the cardiac output。 Prolonged exercise in a hot environment causes a higher heart rate than exercise at a low ambient temperature. The regulation of the circulation in exercise is probably guided primarly by factors sensitive to an adequate cardiac output to secure the oxygen supply to the exercising skeletal muscles. There is a remarkable constancy in the relationship between oxygen uptake and cardiac output.

20. （3）Stroke volume Two factors affecting the stroke volume are the venous return to the heart and the distensibility of the ventricles. The heart adjusts itself to changing conditions by an inherent self-regulatory mechanism. Starling(1896), using his famous lung-heart preparations, found that the normal heart tended to empty itself almost completely. It was distended to a greater diastolic volume in response to either a greater venous return or an increase of arterial pressure. It has been emphasized repeatedly that the increased in length of a muscle (within limits) will improve its force-generating potential. In addition, catecholamines will elicit a similar effect plus an increase in the heart rate. It has long been believed that the stroke volume reaches a plateau when oxygen uptake exceeds 40%to 50% of maximal aerobic power.

21. （4）Heart volume A high correlation has been established between heart volume and various parameters, such as blood volume, total amount of Hb, and stroke volume in healthy younger individuals. Data from longitudinal studies of former endurance athletes who later become relatively inactive confirm the results from the study of the girl swimmer. Many of these previous athletes still had a large heart. There are exceptions, however. Rost and Hollmann(1983) reported a heart volume of 1700ml in a professional bicyclist; his maximal aerobic power was above 6 L/min. four years after cessation of training, his heart volume was reduced to 980ml. In former endurance-trained athletes, the correlation between maximal oxygen uptake and heart volume is low. This may be explained, at least in part, by their different levels of habitual physical activity.

22. 4 Regulation of the Heart （1）Innervation of the Heart The heart receives a rich supply of sympathetic and parasympathetic nerve fibers. Parasympathetic fibers, terminating in the region of the pacemaker(sinoatrial node), release primarily acetylcholine (Ach), which shows the heart rate. Sympathetic postganglionic fibers releasing primarily noradrenaline are distributed not only to the conducting system but to the entire myocardium. On the basis of purely pharmacological criteria, we can distinguish two main groups of receptors, α- and ß-adrenergic receptors. The α receptors are abundant in the cell membrane of all vascular smooth muscle cells, and throughout the body ą-adrenergic activity causes vasoconstriction. In the heart, however, the ą receptors are relatively sparse, making noradrenaline and adrenaline to activate mainly ß receptors. An increased sympathetic drive will, as just mentioned, elevate the heart rate, and the heart beat becomes more forceful. This will increase the myocardial oxygen uptake and coronary blood flow.

23. （2）The Heart and the Effect of Nerve Impulses The heart has its own pacemaker, the sinoatrial node, initiating 110 impulses/min if left alone. Normally, however, it is under the influence of nervous activity, and at rest inhibitory impulses will dominate. Both sympathetic and parasympathetic activity from a cardioinhibitory center via the vagus nerve (and acetylcholine) slows the heart rate as well as blood borne adrenaline can increase heart rate. The sympathetic nerves can increase the contractile force of the heart muscle fibers, but sympathetic nervous control of the vasomotor tone in the heart vessels is probably insignificant. As mentioned, the blood vessels of the heart dilate willingly when affected by, for example, metabolites, endothelium-derived NO, and hypoxia.

24. Section 3 Vessel Physiology （1）Arteries Blood （2）Veins Vessels （3）Capillaries

25. ① Arterioles The arteries are elastic, and forward flow is continuous because of the recoil of the vessels walls that have been stretched during the systole. For this reason the arteries are sometimes referred to as Windkessel vessels. The expansion of the arterial wall during the ejection of blood causes a pressure wave that travels along the peripheral blood vessels at a speed of 5 to 9 m.s-1.In comparison, the velocity of the blood in the aorta at rest is 5m.s-1.The more elastic the arterial wall, the lower the speed of the pulse wave. In practical application, the frequency of this wave is counted as the pulse rate, because it easily can be felt over the radial or carotid arteries. （1）Arteries

26. ② Arterial Blood Pressure and Vasomotor Tone The blood pressure in the aorta is maintained by an integration of the following factors: (1) cardiac output, (2)peripheral resistance, (3) elasticity of the main arteries, (4) viscosity of the blood, and (5) blood volume. Evidently, a regulation of the arterial blood pressure can use factors 1 and 2, because factors 3, 4 and 5 are normally not at its disposal for rapid modifications. The local blood flow is determined mainly by the pressure load and the diameter of the actual vessels. The smooth muscles of the arterioles and veins in many regions continuously receive nerve impulses that keep the lumen of the vessels more or less constricted. The vasomotor tone is important keeping arterial blood pressure and cardiac output at an economicallevel. The splanchnic area could contain the whole blood volume after maximal dilation of the vessels. Also, the vascular bed of skin and muscles has a similarly large capacity.

27. ② Arterial Blood Pressure and Vasomotor Tone A decrease in the vasomotor tone, which actually relaxes the smooth muscles in the vessel wall and therefore causes vasodilation, can be obtained principally in two ways. The peripheral to blood flow is determined by the vasomotor tone. The degree of contraction of the smooth muscles in the arterioles is of special importance for the local blood flow as well as the total resistance. In some tissues, this tone is probably partly spontaneous, the smooth muscles contractions being trigging by mechanical stretch induced by the intravascular pressure, but a sympathetic vasomotor tone is superimposed. In other regions, this sympathetic vasomotor tone is dominating. A vasodilation can occur when the smooth muscles are affected by varies dilating agents liberated locally or delivered from the blood. Vasodilation also can be caused by a decreased discharge in the sympathetic vasomotor nerves. Regular physical activity results in vascular adaptation that enhances perfusion and flow capacity.

28. ③ Baroreceptors in Systemic Arteries The systemic arterial receptors are located in the tissue of the carotid sinus, aortic arch, right subclavian artery, and common carotid artery. Mechanical deformation, or stretch, of the walls of the vessels is the normal stimulus of the receptors. They respond to the rate of the blood pressure increase as well as to the amplitude of pulse pressure with a discharge transmitted to the CNS. The baroreceptors can report a decrease as well as an increase in blood pressure to the cardiovascular centers, primarily the medullary vasomotor areas. At rest, the baroreceptors exert a retraining influence on the cardiovascular system, causing a reflex bradycardia and reflex inhibition of the medullary vasomotor center.

29. （2）Veins The veins within a skeletal muscle will be compressed mechanically during the muscular contraction. Blood is squeezed from the veins, and because of the higher resistance on the capillary side and the design of the valves, the blood return to the heart is facilitated by this skeletal muscle pump. During muscular relation, the venous pressure decreases, and therefore blood can flow from the superficial veins, anatomizing the deep veins. Normally, the venous systems contain some 60% to 70% of the total blood volume. The veins therefore are often referred to as capacitance vessels. The pulmonary veins account for about 10% to 15% of the total blood volume. At rest, the blood in the capillaries is estimated to be 5% of the total

30. ① Venous Blood Return The increase in venous return to the heart as exercise starts will enable an immediate increase in cardiac output. An increased filling pressure in the heart may stimulate the stretch receptors in the myocardium and in a reflex manner accelerate the heart and improve its stroke. The increase in venous return to the heart as exercise starts will enable an immediate increase in cardiac output. The venous return to the heart is determined by the balance between the filling pressure and the distensibility of the heart, that is, the intraventricular pressure minus the intrathoracic pressure. The filling is enhanced by(1)the variation in intrathoracic and intra-abdominal pressures during the respiratory cycle, (2)the effect of the muscle pump during muscular movements, and (3) a vasoconstriction in the postcapillary vessels. Changes in body position will, at least temporarily, affect the volume of blood in central veins.

31. ② Oxygen Content of Arterial and Mixed Venous Blood During exercise, there is a hemoconcentration of the blood, which is explained partly by the mentioned withdrawal of fluid to the active muscle cells and by the interstitial fluid. The increased capillary pressure and surface area also increase outward filtration. This hemoconicentration makes the blood more viscous, but it also increases the transport capacity per liter of blood for both oxygen and carbon dioxide The arterial PH, however, may be below7.2 and the blood temperature may be markedly elevated, and therefore the shift in the oxygen dissociation curve to the right gives a noticeable effect on the oxygen saturation even at a high oxygen tension in the blood. An increased oxygen tension in the inspired air will increase maximal oxygen uptake and improve performance. In the studies by Ekblom et al. eight subjects breathing 50% oxygen in nitrogen at sea level showed an average 12% increase in maximal aerobic power.

32. The purpose of this brief summary is to illustrate that the maximal oxygen uptake (maximal aerobic power) in exercise engaging large muscle groups is apparently not limited by the capacity of the muscle mitochondria to consume oxygen. A period of physical conditioning will increase the volume of mitochondria in trained muscles, increasing their aerobic energy potential, the crucial question is whether there are enzymes in the skeletal muscles that serve as a bottleneck for maximal aerobic energy yield. So far, the central circulation and the capillary bed available for perfusion have been considered to be the limitations for an individual’s maximal oxygen uptake. Saltin and Gollnick(1983) estimated that the potential of enzyme systems of the skeletal muscles to consume oxygen by far exceeds the maximum actually attained.

33. （3）Capillaries The arterioles branch off into terminal or metarterioles. There the smooth muscle fibers are arranged in circular or spiral alignment and constitute a single layer. The metarteriole ends in a capillary-like channel or thoroughfare channel (preferential channel), from which the capillaries branch. In skeletal muscles, the true capillaries are 8 to 10 times as numerous as the preferential channels. In other tissues, there are relatively fewer capillaries. The capillaries and the preferential channels drain into venules, which in turn drain into the larger veins. The circulation of blood from arteries to veins can take various routes via thoroughfare channels, true capillaries, and arteriovenous or arteriolovenous shunts, The pressure gradient and the activity of the smooth muscle cells in the walls or the precapillary sphincters decide which route the blood flow will take.

34. （3）Capillaries The arterioles branch off into terminal or metarterioles. There the smooth muscle fibers are arranged in circular or spiral alignment and constitute a single layer. The metarteriole ends in a capillary-like channel or thoroughfare channel (preferential channel), from which the capillaries branch. In skeletal muscles, the true capillaries are 8 to 10 times as numerous as the preferential channels. In other tissues, there are relatively fewer capillaries. The capillaries and the preferential channels drain into venules, which in turn drain into the larger veins. The circulation of blood from arteries to veins can take various routes via thoroughfare channels, true capillaries, and arteriovenous or arteriolovenous shunts, The pressure gradient and the activity of the smooth muscle cells in the walls or the precapillary sphincters decide which route the blood flow will take.

35. Capillary Structure and Transport Mechanisms The very thin walls of the capillaries (and venules) have a simple endothelium cell layer resting on a basal iamina. Lipid-soluble substances, which include gasses, can diffuse freely through the endothelial cells. For the filtration and diffusion of water-soluble particles and large molecules, the pathways are thought to be pores or other types of interruptions of the continuous lining of endothelial cells. In morphological studies, pinocytotic vesicles have been shown to constitute a discontinuous route of passage through endothelial cells for large molecules.

36. Simultaneous measurement of intra-arterial blood pressure in a peripheral artery and in the aorta during exercise gives a significantly higher systolic end pressure in the peripheral artery, but the mean and diastolic pressures are about the same as in the aorta. The systolic pressure in a peripheral artery is higher in a resting than in an exercising limb. The progressive increase in systolic pressure along an artery is attributable, at least in part, to a distortion in the transmission because of summation of the centrifugal wave and the reflected waves from the periphery. The importance of the wave reflection increases when the peripheral resistance is high, 2 Blood pressure

37. In the control of arterial blood pressure, tbaroreceptors in some artery walls play, as mentioned, a key role in that they perform some sort of a “buffer” function. If, at rest, there is a pressure decrease, the reduced activity will activate the sympathetic system with reciprocal inhibition of the parasympathetic antagonist. The reaction to an increase in arterial blood pressure is the reverse. How do these receptors react to the “hypertension” induced by exercise? The answer is that they do respond and have a reflex buffer function even during exercise that will prevent marked deviations in arterial pressure form normal values. The maximal firing rate is reached around 180 mmHg, and with a further increase in blood pressure, the activity of the receptors in the carotid sinus does not change. 2 Blood pressure

38. The contraction of the heart muscle interferes mechanically with the coronary blood flow. In a dog at rest, the left ventricle completely stops during the isometric contraction and a backflow actually is established, whereas a peak flow is reached when the systolic pressure is at its maximum, during the early ejection phase. A second peak flow is reached during early diastole, after which it decreases gradually to about 70% of maximum just at the end of diastole (see figure 4.12b). In the right ventricle, where pressure is lower during systole, the fluctuations are not so pronounced as in the left ventricle. In healthy individuals, coronary blood flow and cardiac metabolic demands are well matched. The heart rate is well correlated with coronary blood flow. Adenosine and other factors CO2，K, and endothelium-derived NO act as local metabolic vasodilating transmitters between the cardiac cells and the coronary vascular smooth muscle cells. There is evidence that resting NO release is increased by physical training. 3 Blood Flow

39. Section 4 Circulation and Sports 1 Vascularization of Skeletal Muscles 2 Regulation of Circulation at Rest 3 Regulating of Circulation During Exercise 4 Control and Effects by the Central Nervous System

40. 1 Vascularization of Skeletal Muscles Therefore, two different fiber types often share common capillaries. On the average, the capillary density around the slow- switch fibers is higher (there to four capillaries per fiber) than around the fast-twitch fiber In the untrained muscle, the average tissue area that a capillary supplies is 20% to 30% larger for type ⅡⅩ and 10% to 20% more for type ⅡA fibers compared with the type Ⅰ fibers . As mentioned, training increases the capillary density; disuse of a muscle will reduce the density The opening or closing is supposed to be operated by local chemical factors of a hypoxic or metabolic nature and only to a smaller degree by nerve activity (see subsequent discussion). At a certain level of metabolic activity in the tissue, the number of working capillaries is fairly constant, but the individual capillary can be intermittently open or closed

41. 1 Vascularization of Skeletal Muscles Nerves and vessels enter the muscle at a neuromuscular hilus often located at half-length of the muscle. The artery enters the muscle belly and branches freely in its course along the perimysium By anastomoses; a primary arterial network is established. Finer arteries arise and create a secondary network infiltrating the muscle tissue. The smallest arteries and terminal arterioles branch off, usually transversely to the long axis of the muscle fibers and at fairly regular intervals of 1 mm. The arterioles then supply the capillary network oriented parallel to the individual muscle fibers but also from frequent transverse links over or under the intervening fibers, thereby forming a delicate network is especially well developed around the motor endplate. The veins have valves directing the blood flow toward the heart, and they follow the course of the arterioles and arteries.

42. 2 Regulation of Circulation at Rest Any change in cellular activity should be met by a corresponding variation in local blood flow through the capillary bed. If an individual cell, in one way or another, could control its environment by varying the blood supply in balance with the actual nutritional demand, that cell would benefit. But other cells or tissue might suffer if some cells take more than their share. Hence, coordinative mechanisms are essential if the distribute of blood is to be balance properly. An active regulatory mechanism ensures that more active and less active cells, as well as more susceptible and less susceptible organs, are supplied according to their need and to the capacity of the whole circulation.

43. 3 Regulating of Circulation During Exercise At rest, the kidneys receive about 25% of the cardiac output. There are no tonic impulses from the CNS to renal blood vessels, but electrical stimulation of the renal nerves causes intense renal vessel constriction with associated changes in blood flow (down to 250 ml/min) and in excretion of water and electrolytes. Exercise, postural changes, and circulatory stress in general can profoundly alter renal function, mediated through the hemodynamic effects of renal nerves. This summarizes the main tools available for regulation of the circulatory system during exercise. The approximate blood distribution to the various organs At rest the skeletal muscles receive only some 15% of the minute blood flow, and their arterioles are constricted by a continuous vasoconstrictor activity and spontaneous vascular tone. Sympathetic adrenergic vasoconstrictor nerves act on the vessels of abdominal organs and skin so that a decreasing share of the cardiac output flow though those tissues. The veins become constrictor fibers. In the activated muscles, the increased metabolism causes changes in the environment that locally dilate arterioles and open capillaries.

44. （1）Type of Exercise The cardiac output at a biven oxygen uptake is consistently 1 to 2L less per minute in the erect position than when the subject is recumbent; the heart rate is about the same. The heart rate is about the same. The compensation for the lower cardiac output must, by definition, be an increased (a-V)O2 difference when the person is erect. Peak oxygen uptake during cycling in the supine position is lower than during exercise in the sitting position on a cycle ergometer (figure4.15) Similarly, cardiac output is somewhat lower during maximal exercise with the legs in the supine position. This difference in response may be explained in the following way: Rhythmic muscular contractions squeeze out blood from the veins, lowering the average venous pressure considerably and hence raising the effective perfusion pressure of flow.

45. （2）Age The heart rate reached during maximal exercise decreases with age. The value typical for a 10-year-old girl or boy is 210, for a 25-year-old 195, and for a 50-year-old 175 beats/min. Unfortunately, our knowledge of the cardiovascular response to exercise in the aged is rather scanty. One of the reasons for this is the problem of selecting a random sample. It Is relatively simple to define a “normal healthy subject” when dealing with a young population. But, who can be called a “normal healthy subject ”when approaching the age of 70? Such a group of older individuals is already a very selected group of older individuals is already a very selected group of subjects compared with younger counterparts.

46. （3）Posture A change in body position inevitably will affect the circulation as long as the individual stays under the influence of gravity. A head-up position primarily will increase the blood volume in the legs and decrease the central blood volume and cardiac output. Secondary variations in arterial blood pressure are reported from the baroreceptors in some arteries to the cardiovascular centers in the brain. The activity in sympathetic and parasympathetic nerves varies by reciprocal innervation in such a way that the arterial blood pressure and cardiac output return to a level fairly ciose to the one typical for the individual in a supine position.

47. （4）Cardiac Output and Oxygen Uptake At rest in the supine position, the cardiac output is 4 to 6L/min with an extraction of 40 to 50ml of oxygen per liter of blood and a total oxygen uptake of 0.2 to 0.3L/min. when a subject strapped to a tilting table is tilted from the horizontal to the feet down position, the cardiac output can decrease from 5 to 4L/min. This decrease is attributable to the previously discussed venous pooling. The stroke volume is reduced and the heart rate is usually increased. Activation of the skeletal muscle pump propels the blood toward the heart, and the heart rate may even decrease as the stroke volume increase. In the passive feet-down position, the oxygen uptake unchanged, and hence the (a-v)O2 difference is increased. With CO present, carboxyhemoglobin is formed. Such a conversion, affects the HbO2 dissociation curve with a shift. The effect of CO on oxygen transport is therefore twofold: it reduces the amount of Hb available for oxygen transport, and it interferes with the unloading of oxygen in the tissues.

48. （4）Cardiac Output and Oxygen Uptake

49. （5）Training and Cardiac Output The stroke volume to a large extent determines maximal cardiac output. The most pronounced difference between the sexes is the smaller stroke volume and higher heart rate during exercise of a given severity for women compared with men. During maximal exercise, however, the cardiac output, oxygen uptake. And heart rate are remarkably fixed to values typical for the individual even if the performance is made under adverse conditions. In this situation, apparently all circulatory functions of decisive importance for a maximal oxygen supply to the active muscle are actually devoted to this task. Irrespective of environment, external and internal maximal vasoconstricition occurs in the blood vessels of the viscera and skin, so that practically the entire cardiac output is diverted to the vigorously contracting muscles. Maximal exercise involving large muscle groups creates an emergency reaction in the circulatory adjustment that favors the exercising muscles, including the heart, at expense of all other tissues with exception of the central nervous system.

50. 4 Control and Effects by the Central Nervous System Important and essential are located in the brain stem, particularly in the medulla oblongata. Traditionally, textbooks have discussed a medulla vasomotor area, divided into a vasoconstrictor center and a vasodepressor area, the latter operating through inhibition of the sympathetic vasoconstrictor outflow. Keele, Neil, and Joels (1982) suggested the term medullary cardiovascular centers. Anatomically, they are found in the reticular formation. Such terms may be convenient but they give a false impression of well-defined collections of neurons with precise effects. This is not so. The neurogentic vasomotor tone of the blood vessels originates essentially in the brain stem, and a continuous, somewhat rhythmic discharge synchronous with the pulse can be recorded in some areas. This discharge is probably mediated via baroreceptors. The nuclei eliciting vasodilation are essentially relay stations without spontaneous activity, but they can be activated by incoming impulses from peripheral receptors and other areas in the CNS.