Vascular Distensibility and Functions PDF

VASCULAR DISTENSIBILITY & FUNCTIONS OF THE ARTERIAL & VENOUS SYSTEMS Belén Merck MD PhD  VASCULAR DISTENSIBILITY A valuable characteristic of the vascular system is that all blood vessels are distensible. The distensible nature...

VASCULAR DISTENSIBILITY & FUNCTIONS OF THE ARTERIAL & VENOUS SYSTEMS Belén Merck MD PhD  VASCULAR DISTENSIBILITY A valuable characteristic of the vascular system is that all blood vessels are distensible. The distensible nature of the arteries allows them to accommodate the pulsatile output of the heart and to average out the pressure pulsations. This provides smooth, continuous flow of blood through the very small blood vessels of the tissues. The most distensible by far of all the vessels are the veins. Even slight increases in venous pressure cause the veins to store 0.5 to 1.0 liter of extra blood. Therefore, the veins provide a reservoir function for storing large quantities of extra blood that can be called into use whenever required elsewhere in the circulation.  VASCULAR DISTENSIBILITY Vascular distensibility normally is expressed as the fractional increase in volume for each mm Hg rise in pressure, in accordance with the following formula: That is, if 1 mm Hg causes a vessel that originally contained 10 millimeters of blood to increase its volume by 1 milliliter, the distensibility would be 0.1 per mm Hg, or 10% per mm Hg.  VASCULAR DISTENSIBILITY Anatomically, the walls of the arteries are far stronger than those of the veins. Consequently, the veins, on average, are about eight times more distensible than the arteries. That is, a given increase in pressure causes about eight times as much increase in blood in a vein as in an artery of comparable size. In the pulmonary circulation, the pulmonary vein distensibilities are similar to those of the systemic circulation. But the pulmonary arteries normally operate under pressures about one sixth of those in the systemic arterial system, and their distensibilities are correspondingly greater, about six times the distensibility of systemic arteries.  COMPLIANCE OR CAPACITANCE In hemodynamic studies, it is much more important to know the total quantity of blood that can be stored in a given portion of the circulation for each mm Hg pressure rise than to know the distensibilities of the individual vessels. This value is called the compliance or capacitance of the respective vascular bed; that is,  COMPLIANCE OR CAPACITANCE Compliance and distensibility are quite different. A highly distensible vessel that has a slight volume may have far less compliance than a much less distensible vessel that has a large volume because compliance is equal to distensibility times volume. Vascular distensibility= ʌ volume/ ʌ pressure x Original volume The compliance of a systemic vein is about 24 times that of its corresponding artery because it is about 8 times as distensible and it has a volume about 3 times as great 8 × 3 = 24).  VOLUME-PRESSURE CURVES A convenient method for expressing the relationship of pressure to volume in a vessel or in any portion of the circulation is to use a volume-pressure curve. When the arterial system of the average adult person (including all the large arteries, small arteries, and arterioles) is filled with about 700 ml of blood, the mean arterial pressure is 100 mm Hg but, when it is filled with only 400 ml of blood, the pressure falls to zero.  VOLUME-PRESSURE CURVES In the entire systemic venous system, the volume normally ranges from 2000 to 3500 milliliters, and a change of several hundred millimeters in this volume is required to change the venous pressure only 3 to 5 mm Hg. This mainly explains why as much as one half liter of blood can be transfused into a healthy person in only a few minutes without greatly altering function of the circulation.  VOLUME-PRESSURE CURVES Increase in vascular smooth muscle tone caused by sympathetic stimulation increases the pressure at each volume of the arteries or veins, whereas sympathetic inhibition decreases the pressure at each volume. Control of the vessels in this manner by the sympathetics is a valuable means for diminishing the dimensions of one segment of the circulation, thus transferring blood to other segments.  VOLUME-PRESSURE CURVES For instance, an increase in vascular tone throughout the systemic circulation often causes large volumes of blood to shift into the heart, which is one of the principal methods that the body uses to increase heart pumping. Sympathetic control of vascular capacitance is also highly important during hemorrhage. Enhancement of sympathetic tone, especially to the veins, reduces the vessel sizes enough that the circulation continues to operate almost normally even when as much as 25% of the total blood volume has been lost.  DELAYED COMPLIANCE The term delayed compliance means that a vessel exposed to increased volume at first exhibits a large increase in pressure, but progressive delayed stretching of smooth muscle in the vessel wall allows the pressure to return back toward normal over a period of minutes to hours.  DELAYED COMPLIANCE The pressure is recorded in a small segment of a vein that is occluded at both ends. An extra volume of blood is suddenly injected until the pressure rises from 5 to 12 mm Hg. Even though none of the blood is removed after it is injected, the pressure begins to decrease immediately and approaches about 9 mm Hg after several minutes.  DELAYED COMPLIANCE The volume of blood injected causes immediate elastic distention of the vein, but then the smooth muscle fibers of the vein begin to “creep” to longer lengths, and their tensions correspondingly decrease. This effect is a characteristic of all smooth muscle tissue and is called stress-relaxation.  DELAYED COMPLIANCE Delayed compliance is a valuable mechanism by which the circulation can accommodate extra blood when necessary, such as after too large a transfusion. Delayed compliance in the reverse direction is one of the ways in which the circulation automatically adjusts itself over a period of minutes or hours to diminished blood volume after serious hemorrhage.  ARTERIAL PRESSURE PULSATIONS With each beat of the heart a new surge of blood fills the arteries. Were it not for distensibility of the arterial system, all of this new blood would have to flow through the peripheral blood vessels almost instantaneously, only during cardiac systole, and no flow would occur during diastole. However, the compliance of the arterial tree normally reduces the pressure pulsations to almost no pulsations by the time the blood reaches the capillaries; therefore, tissue blood flow is mainly continuous with very little pulsation.  ARTERIAL PRESSURE PULSATIONS In the healthy young adult, the pressure at the top of each pulse, called the systolic pressure, is about 120 mm Hg. At the lowest point of each pulse, called the diastolic pressure, it is about 80 mm Hg. The difference between these two pressures, about 40 mm Hg, is called the pulse pressure.  ARTERIAL PRESSURE PULSATIONS Two major factors affect the pulse pressure: (1) the stroke volume output of the heart and (2) the compliance (total distensibility) of the arterial tree. A third, less important factor, is the character of ejection from the heart during systole. In general, the greater the stroke volume output, the greater the amount of blood that must be accommodated in the arterial tree with each heartbeat, and, therefore, the greater the pressure rise and fall during systole and diastole, thus causing a greater pulse pressure. Conversely, the less the compliance of the arterial system, the greater the rise in pressure for a given stroke volume of blood pumped into the arteries.  ARTERIAL PRESSURE PULSATIONS The pulse pressure in old age to as much as twice normal, because the arteries have become hardened and therefore are noncompliant. Pulse pressure is determined by the ratio of stroke volume output to compliance of the arterial tree. Any condition of the circulation that affects either of these two factors also affects the pulse pressure: Pulse pressure ∝ stroke volume/arterial compliance  TRANSMISSION OF PRESSURE PULSES When the heart ejects blood into the aorta during systole, at first only the proximal portion of the aorta becomes distended because the inertia of the blood prevents sudden blood movement all the way to the periphery. However, the rising pressure in the proximal aorta rapidly overcomes this inertia, and the wave front of distention spreads farther and farther along the aorta. This is called transmission of the pressure pulse in the arteries.  TRANSMISSION OF PRESSURE PULSES The velocity of pressure pulse transmission in the o normal aorta is 3 to 5 m/sec; o in the large arterial branches, 7 to 10 m/sec; and o in the small arteries, 15 to 35 m/sec. In general, the greater the compliance of each vascular segment, the slower the velocity, which explains the slow transmission in the aorta and the much faster transmission in the much less compliant small distal arteries.  TRANSMISSION OF PRESSURE PULSES The progressive diminution of the pulsations is called damping of the pressure pulses. The cause of this is twofold: 1. resistance to blood movement in the vessels and 2. compliance of the vessels. The resistance damps the pulsations because a small amount of blood must flow forward at the pulse wave front to distend the next segment of the vessel; the greater the resistance, the more difficult it is for this to occur.  TRANSMISSION OF PRESSURE PULSES The compliance damps the pulsations because the more compliant a vessel, the greater the quantity of blood required at the pulse wave front to cause an increase in pressure. Therefore, the degree of damping is almost directly proportional to the product of resistance times compliance.  VEINS AND THEIR FUNCTIONS Veins are capable of constricting and enlarging and thereby storing either small or large quantities of blood and making this blood available when it is required by the remainder of the circulation. The peripheral veins can also propel blood forward by means of a so-called venous pump, and they even help to regulate cardiac output.  VEINS AND THEIR FUNCTIONS Blood from all the systemic veins flows into the right atrium of the heart; therefore, the pressure in the right atrium is called the central venous pressure. Right atrial pressure is regulated by a balance between (1) the ability of the heart to pump blood out of the right atrium and ventricle into the lungs and (2) the tendency for blood to flow from the peripheral veins into the right atrium.  VEINS AND THEIR FUNCTIONS If the right heart is pumping strongly, the right atrial pressure decreases. Conversely, weakness of the heart elevates the right atrial pressure. Also, any effect that causes rapid inflow of blood into the right atrium from the peripheral veins elevates the right atrial pressure.  VEINS AND THEIR FUNCTIONS Some of the factors that can increase this venous return (and thereby increase the right atrial pressure) are (1) increased blood volume, (2) increased large vessel tone throughout the body with resultant increased peripheral venous pressures, and (3) dilatation of the arterioles, which decreases the peripheral resistance and allows rapid flow of blood from the arteries into the veins.  VEINS AND THEIR FUNCTIONS The same factors that regulate right atrial pressure also contribute to regulation of cardiac output because the amount of blood pumped by the heart depends on both the ability of the heart to pump and the tendency for blood to flow into the heart from the peripheral vessels. The normal right atrial pressure is about 0 mm Hg, which is equal to the atmospheric pressure around the body.  VEINS AND THEIR FUNCTIONS It can increase to 20 to 30 mm Hg under very abnormal conditions, such as 1. serious heart failure or 2. after massive transfusion of blood, which greatly increases the total blood volume and causes excessive quantities of blood to attempt to flow into the heart from the peripheral vessels. The lower limit to the right atrial pressure is usually about −3 to −5 mm Hg below atmospheric pressure. This is also the pressure in the chest cavity that surrounds the heart.  VEINS AND THEIR FUNCTIONS 1 The right atrial pressure approaches these low values when the heart pumps with exceptional vigor or 2 when blood flow into the heart from the peripheral vessels is greatly depressed, such as after severe 3 hemorrhage. 4  VEINS AND THEIR FUNCTIONS Finally, veins coursing through the abdomen are often compressed by different organs and by the intraabdominal pressure, so they usually are at least partially collapsed to an ovoid or slit like state. For these reasons, the large veins do usually offer some resistance to blood flow, and because of this, the pressure in the more peripheral small veins in a person lying down is usually +4 to +6 mm Hg greater than the right atrial pressure.  VEINS AND THEIR FUNCTIONS When the right atrial pressure rises above its normal value of 0 mm Hg, blood begins to back up in the large veins. This enlarges the veins, and even the collapse points in the veins open up when the right atrial pressure rises above +4 to +6 mm Hg. Then, as the right atrial pressure rises still further, the additional increase causes a corresponding rise in peripheral venous pressure in the limbs and elsewhere.  VEINS AND THEIR FUNCTIONS The pressure in the abdominal cavity of a recumbent person normally averages about +6 mm Hg, but it can rise to +15 to +30 mm Hg as a result of pregnancy, large tumors, abdominal obesity, or excessive fluid (called “ascites”) in the abdominal cavity. When the intraabdominal pressure does rise, the pressure in the veins of the legs must rise above the abdominal pressure before the abdominal veins will open and allow the blood to flow from the legs to the heart. Thus, if the intra-abdominal pressure is +20 mm Hg, the lowest possible pressure in the femoral veins is also about +20 mm Hg.  VEINS AND THEIR FUNCTIONS In any body of water that is exposed to air, the pressure at the surface of the water is equal to atmospheric pressure, but the pressure rises 1 mm Hg for each 13.6 millimeters of distance below the surface. This pressure results from the weight of the water and therefore is called gravitational pressure or hydrostatic pressure. Gravitational pressure also occurs in the vascular system of the human being because of weight of the blood in the vessels.  VEINS AND THEIR FUNCTIONS When a person is standing, the pressure in the right atrium remains about 0 mm Hg because the heart pumps into the arteries any excess blood that attempts to accumulate at this point. However, in an adult who is standing absolutely still, the pressure in the veins of the feet is about +90 mm Hg simply because of the gravitational weight of the blood in the veins between the heart and the feet. The venous pressures at other levels of the body are proportionately between 0 and 90 mm Hg.  VEINS AND THEIR FUNCTIONS The neck veins of a person standing upright collapse almost completely all the way to the skull because of atmospheric pressure on the outside of the neck. This collapse causes the pressure in these veins to remain at zero along their entire extent. The reason for this is that any tendency for the pressure to rise above this level opens the veins and allows the pressure to fall back to zero because of flow of the blood. Conversely, any tendency for the neck vein pressure to fall below zero collapses the veins still more, which further increases their resistance and again returns the pressure back to zero. The veins inside the skull, are in a no collapsible chamber so that they cannot collapse. https://vimeo.com/219987015  VENOUS VALVES AND THE “VENOUS PUMP” Were it not for valves in the veins, the gravitational pressure effect would cause the venous pressure in the feet always to be about +90 mm Hg in a standing adult. However, every time one moves the legs, one tightens the muscles and compresses the veins in or adjacent to the muscles, and this squeezes the blood out of the veins. But the valves in the veins are arranged so that the direction of venous blood flow can be only toward the heart.  VENOUS VALVES AND THE “VENOUS PUMP” Consequently, every time a person moves the legs or even tenses the leg muscles, a certain amount of venous blood is propelled toward the heart. This pumping system is known as the “venous pump” or “muscle pump,” and it is efficient enough that under ordinary circumstances, the venous pressure in the feet of a walking adult remains less than +20 mm Hg. If a person stands perfectly still, the venous pump does not work, and the venous pressures in the lower legs increase to the full gravitational value of 90 mm Hg in about 30 seconds.  VENOUS VALVES AND THE “VENOUS PUMP” The pressures in the capillaries also increase greatly, causing fluid to leak into the tissue spaces. As a result, the legs swell and the blood volume diminishes. Indeed, 10 to 20%of the blood volume can be lost from the circulatory system within the 15 to 30 minutes of standing absolutely still, as often occurs when a soldier is made to stand at rigid attention.  SPECIFIC BLOOD RESERVOIRS Certain portions of the circulatory system are so extensive and/or so compliant that they are called “specific blood reservoirs.” These include (1) the spleen, which sometimes can decrease in size sufficiently to release as much as 100 ml of blood into other areas of the circulation; (2) the liver, the sinuses of which can release several hundred milliliters of blood into the remainder of the circulation; (3) the large abdominal veins, which can contribute as much as 300 ml; and (4) the venous plexus beneath the skin, which also can contribute several hundred milliliters.  SPECIFIC BLOOD RESERVOIRS The heart and the lungs, although not parts of the systemic venous reservoir system, must also be considered blood reservoirs. The heart, for instance, shrinks during sympathetic stimulation and in this way can contribute some 50 to 100 milliliters of blood/beat; the lungs can contribute another 100 to 200 milliliters when the pulmonary pressures decrease to low values.