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Physiology Ch 15 p167-175 Vascular Distensibility and Functions of the Arterial and Venous Systems -all blood vessels are distensible, which allows them to accommodate pulsatile output of the heart and to average out the pressure pulsations to provide smooth, continuous flow of blood through small vessels of the tissues -most distensible vessels are the veins; slight increases in venous pressure causes veins to store 0.5-1L of extra blood, and so veins provide a reservoir function for storing large quantities of blood that can be used elsewhere whenever needed Units of Vascular Distensibility – expressed as fractional increase in volume for each mmHg rise: uses the following formula: Vascular Distensibility = increase in V / (increase in P * original V) -if 1mmHg causes vessel originally containing 10mm of blood to increase V by 1mm, the Distensibility would be 0.1 per mmHg, or 10% per mmHg Difference in Distensibility of Arteries and Veins – walls of arteries are stronger than veins, and thus, veins are 8x more distensible than arteries; an increase in pressure causes 8x as much increase in blood in a vein than in an artery -in pulmonary circulation, pulmonary vein distensibility is similar to that of systemic circulation -pulmonary arteries operate at a lower pressure than systemic arteries, and have 6x the distensibility of systemic arteries Vascular Compliance (Vascular Capacitance) – total quantity of blood that can be stored in a given portion of circulation for each mmHg rise in pressure; Compliance = Increase in V/Increase in P -a highly distensible vessel that has slight volume may have far less compliance than a much less distensible vessel that has a large volume because compliance = distensibility * volume -compliance of systemic vein is 24 times that of its artery because it is 8* as distensible and has a volume 3x as great Volume-Pressure Curves of Arterial and Venous Circulations – volume-pressure curves express the relation of pressure to volume in a vessel or in any portion of circulation -when arterial system is filled with 700mL of blood, the arterial pressure is 100mmHg; when it is filled with 400mmHg, the pressure falls to 0 -the entire venous system volume normally ranges from 20003500mL, and a change of several 100mL in volume is required to change venous pressure only 3-4mmHg Effect of Sympathetic Stimulation or Sympathetic Inhibition on Volume-Pressure Relations of Arterial and Venous Systems – increase in vascular smooth muscle tone caused by sympathetic stimulation increases the pressure at each volume of arteries and veins; sympathetic inhibition decreases pressure at each volume -an increase in vascular tone throughout systemic circulation causes large volumes of blood to shift into the heart, which is one of the methods that body uses to increase heart pumping. -sympathetic control of vascular capacitance is also highly important during hemorrhage; enhancement of sympathetic tone, especially to veins, reduces the vessel sizes enough that the circulation continues to operate normally even if 25% of the total blood volume has been lost Delayed Compliance (Stress-Relaxation) of Vessels – a vessel exposed to increased volume at first exhibits a large increase in pressure, but progressive delayed stretching allows pressure to return back toward normal over a period of minutes to hours -in this figure to the right, pressure is recorded in small segment of vein that is occluded at both ends; pressure begins to decrease immediately and approaches 9mmHg after several minutes, after which it decreases dramatically -immediate elastic distention of vein is followed by smooth muscle fibers creeping to longer lengths to decrease tensions; called Stress-relaxation Arterial Pressure Pulsations – each beat of the heart fills the arteries, and the distensibility of the arterial system reduces pressure pulsations caused during cardiac systole so that no pulsations occur near the capillaries; therefore, tissue blood flow is mainly continuous with little pulsation -top of each pulse in the diagram of pressure pulse in the aorta to the right is the systolic pressure (120mmHg) -the lowest point is the diastolic pressure (80mmHg) -difference between the 2 pressures is called pulse pressure -two factors affect pulse pressure: 1. stroke volume output of heart 2. compliance (total distensibility) of arterial tree -greater the stroke volume output, the greater the amount of blood that must be accommodated in the arterial tree with each heartbeat, and the greater the pressure rise and fall during systole/diastole, causing a greater pulse pressure -the less compliance of arterial system, the greater the rie in pressure for a given stroke volume of blood pumped into arteries -arteriosclerosis causes double the pulse pressure because arteries are hardened and noncompliant -pulse pressure is determined by ratio of stroke volume output to compliance of arterial tree: -Pulse Pressure = Stroke Volume / Arterial Compliance Abnormal Pressure Pulse Contours – 1. Aortic Stenosis – diameter of aortic valve is reduced, and aortic pressure pulse is decreased because of diminished flow outword through stenotic valve 2. Patent Ductus Arteriosus – 50% of blood is pumped by left ventricle back into pulmonary artery, allowing diastolic pressure to fall very low before the next heartbeat 3. Aortic Regurgitation – aortic valve is absent of doesn’t close; after each heartbeat, blood in aorta flows back into left ventricle, to cause aortic pressure to fall all the way to 0 between heartbeats a. There is no incisura because there are no valves to close Transmission of Pressure Pulses to Peripheral Arteries – when heart ejects blood into aorta during systole, at first only proximal part of aorta becomes distended because inertia of blood prevents sudden blood movement all the way to the periphery -rising pressure in proximal aorta quickly overcomes this inertia, and the wave front of distention spreads farther along the aorta, called transmission of pressure pulse in arteries -velocity of pressure pulse transmission in normal aorta is 3-5 m/s; 7-10m/s in small arteries, and 1535m/s in small arteries -the greater the compliance of each vascular segment, the slower the velocity, explaining the slower transmission in the aorta and faster transmission in the less compliant distal arteries -in aorta, velocity of transmission of pressure pulse is 15x the velocity of blood flow because pressure pulse is simply a moving wave of pressure that involves little forward movement of blood volume Damping of Pressure Pulses in Smaller Arteries, Arterioles, and Capillaries – intensity of pulsation becomes less and less in smaller arteries, arterioles, and especially in capillaries -damping is due to (1) resistance of blood movement in vessels and (2) compliance of vessels -the resistance damps pulsations because a small amount of blood must flow forward at the pulse wave front to distend the next segment of vessel (greater resistance = more difficult for this to occur) -compliance damps pulsations because the more compliant a vessel, the greater quantity of blood required at pulse wave to cause an increase in pressure Clinical Methods for Measuring Systolic and Diastolic Pressures – uses mainly the auscultatory method Auscultatory Method – stethoscope is placed over antecubital arteryand a BP cuff is inflated around the upper arm; as long as cuff continues to compress the arm with too little pressure to close the brachial artery, no sounds are heard from the antecubital artery with stethoscope -when cuff pressure is great enough to close artery during part of arterial pressure cycle, a sound is heard with each pulsation, called Korotkoff sounds, caused by blood jetting through partly occluded vessel and by vibrations of the vessel wall -jet causes turbulence in vessel beyond the cuff, and this sets up vibrations heard with stethoscope -to determine BP, pressure in cuff is elevated well above arterial systolic pressure; when cuff pressure > arterial systolic pressure, the brachial artery is collapsed so that no blood jets into lower artery and no Korotkoff sounds are heard in the lower artery -when cuff pressure is reduced below systolic pressure, blood slips through artery and you hear tapping sounds from antecubital artery in synchrony with heartbeat -further reduction causes the Korotkoff sounds to change in quality, having less of tapping and more of a rhythmical and harsher quality -when cuff falls near diastolic pressure, sounds change to muffled quality -as cuff pressure falls further, artery is no longer closed during diastole; the sound is no longer present Normal Arterial Pressures as Measured by Auscultatory Method – there is a progressive increase in BP with age, resulting from aging effects on BP control mechanisms (kidneys responsible for long-term control of BP) -a slight increase in systolic pressure occurs after age 60 resulting from decreasing distensibility or “hardening” of arteries, often a result of atherosclerosis Mean Arterial Pressure – average of arterial pressures measured millisecond by millisecond over a period of time; NOT equal to average of systolic and diastolic pressures because at normal heart rates, a greater fraction of cardiac cycle is spent in diastole -mean arterial pressure is 60% diastolic and 40% systolic; mean is closer to diastolic pressure Veins and Their Functions – veins are capable of constricting and enlarging to store large quantities of blood and making it available when it is required by the rest of circulation; peripheral veins can also propel blood forward by means of a venous pump, and they help regulate cardiac output Venous Pressures – Right Atrial Pressure (Central Venous Pressure) and Peripheral Venous Pressures – blood from all systemic veins flows into the R atrium of the heart, and so the pressure in R atrium is called the central venous pressure -R atrial pressure is regulated by balance between (1) ability of heart to pump blood out of R atrium and ventricle into lungs, and (2) the tendency for blood to flow from peripheral veins into the R atrium -if R heart is pumping strongly, the R atrial pressure decreases, and weakness of heart elevates R atrial pressure -increase in venous return also increase atrial pressure; several factors are: 1. increased blood volume 2. increased large vessel tone throughout body with resultant increased in peripheral pressures 3. dilation of the arterioles, which decreases peripheral resistance and allows rapid flow of blood from arteries into the veins -same factors that regulate R atrial pressure also contribute to regulating cardiac output becaue amount of blood pumped by heart depends on both ability of heart to pump and tendency of blood to flow into the heart from peripheral vessels -the NORMAL R ATRIAL PRESSURE is 0mmHg, which is equal to atmospheric pressure around the body -this can increase to 20-30mmHg under serious heart failure or massive transfusion of blood to increase total blood volume -the LOWER limit to R atrial pressure is -3 to -5mmHg below atmospheric pressure, which is also the pressure in the chest cavity that surrounds heart -R atrial pressure becomes low when heart pumps with exceptional vigor or when blood flow to the heart from periphery is depressed, such as after severe hemorrhage Venous Resistance and Peripheral Venous Pressure – large veins have such little resistance to flow when they are distended that the resistance is almost 0 and is almost not important -large veins entering thorax are compressed at many points by surrounding tissues so that blood flow is impeded, such as veins entering arms impeded by first rib -pressure in neck veins falls so low that the atmospheric pressure on outside of the neck causes these veins to collapse -veins in abdomen are often compressed by different organs and intraabdominal pressure, so they are always partially collapsed -this causes large veins to offer some resistance to blood flow, and this causes pressure in more peripheral small veins in person lying down to be +4 to +6 mmHg greater than R atrial pressure Effect of High Right Atrial Pressure on Peripheral Venous Pressure – when R atrial pressure is above 0mmHg, blood begins to back up in the large veins: this enlarges the veins and even collapse points open up when pressure rises to +4 to +6 mmHg; additional increases cause peripheral venous pressure to rise in limbs and elsewhere Effect of Intra-abdominal Pressure on Venous Pressures of the Leg – pressure in the abdominal cavity of a recumbent person averages +6mmHg, but can rise to +15 to +30 mmHg as a result of pregnancy, large tumors, abdominal obesity, or excessive fluid (ascites) in the abdominal cavity -when intraabdominal pressure rises, pressure in the veins must rise ABOVE abdominal pressure before abdominal veins will open and allow blood to flow from legs to the heart Effect of Gravitational Pressure on Venous Pressure – for a body of water exposed to air, pressure at surface = atmospheric pressure, but rises 1mmHg for each 13.6mm below the surface; pressure results from weight of water and is called gravitational pressure or hydrostatic pressure -when person is standing, pressure in R atrium remains about 0mmHg because heart pumps into arteries any excess blood that attempts to accumulate at that point -in an adult standing STILL, the pressure of veins in the feet is +90mmHg because of the gravitational weight of blood in the veins and heart and feet -venous pressures at other levels of the body are between 0 and 90 mmHg -in arm veins, pressure at level of top rib is +6mmHg because of compression of subclavian vein as it passes over this rib -gravitational pressure down the arm is determined by distance below the rib added to the 6mmHg caused by compression of the vein as it crosses the rib -the neck veins of a person standing still collapse almost completely all the way to the skull because of atmospheric pressure outside of the neck -this collapse causes pressure in these veins to remain at 0 along the entire extent -veins INSIDE the skull are in a noncollapsible chamber (the skull) and so they cannot collapse -negative pressure can exist in the dural sinuses of the head -in standing position, venous pressure in sagittal sinus at top of the brain is about -10mmHg because of the hydrostatic suction between top of the skull and base of the skull -surgery opening sagittal sinus can cause air to be sucked immediately into the venous system, to cause air embolism in the heart and death Effect of Gravitational Factor on Arterial and Other Pressures – gravitational factor also affects pressures in peripheral arteries and capillaries in addition to its effects in veins -standing person has mean arterial pressure of 100mmHg at level of the heart would have a mean arterial pressure in the feet of 190mmHg; BP stated is that of the heart Venous Valves and the “Venous Pump”: Effects on Venous Pressure – without valves, venous pressure in feet would always be 90mmHg in a standing adult -whenever we move the legs, we tighten the muscles and compress the veins adjacent to muscles to squeeze the blood out of the veins -valves are arranged in such a manner that the direction of venous flow can only be toward the heart -every time person moves the legs or tenses leg muscles, a certain amount of venous blood is propelled toward the heart; this is called the venous pump and is efficient enough such that venous pressure in the feet remains less than +20mmHg -person standing perfectly still causes venous pump not to work and pressures in lower legs increase to about 90mmHg in 30seconds, and pressures in capillaries also increase greatly, causing fluid to leak from circulatory system into the tissue spaces; causing leg swelling and blood volume reduction Venous Valve Incompetence Causes “Varicose” Veins – valves of venous system can become incompetent or destroyed, true when veins have been overstretched by excess venous pressure lasting weeks or months -stretching of veins increases cross-sectional areas, but leaflets of the valves no longer close completely -when this develops, the pressure in the veins of the legs increases greatly because of failure of venous pump, which further increases sizes of veins and destroys the valves entirely; person develops varicose veins; large, bulbous protrusions of veins beneath the skin of the entire lower leg -when people with varicose veins stand for more than a few minutes, venous and capillary pressures become very high and leakage of fluid from capillaries causes constant edema in the legs; edema prevents adequate diffusion of nutritional materials from capillaries to muscle and skin cells, so muscles become painful and weak and skin becomes gangrenous and ulcerates -best treatment is to elevate the legs as high as the heart Clinical Estimation of Venous Pressure – can often be estimated by degree of distention of peripheral veins, especially of neck veins -in the sitting position, neck veins are never distended in the normal person -when R atrial pressure increases to as much as +10mmHg, the lower veins of the neck begin to protrude, and at +15mmHg atrial pressure the veins in entire neck become distended Direct Measurement of Venous Pressure and Right Atrial Pressure – Venous pressure can be measured by inserting needle into a vein and connecting it to a pressure recorder -R atrial pressure can only be measured by inserting a catheter through peripheral veins and into R atrium; pressures measured through central venous catheters are used routinely Pressure Reference Level for Measuring Venous and Other Circulatory Pressures – there is one point in the circulatory system at which gravitational pressure factors caused by changes in body position of a healthy person usually do not affect pressure measurement by more than 1-2 mmHg -this is near the tricuspid valve; therefore all circulatory pressure measurements are referred to this level, called the reference level for pressure measurement -heart automatically prevents significant gravitational changes in pressure at triscupid valve this way: -if pressure at tricuspid valve rises slightly above normal, the R ventricle fills to a greater extent than usual, causing the heart to pump blood more rapidly and decrease the pressure at tricuspid valve back to normal value -if pressure at tricuspid valve falls, R ventricle fails to fill adequetly, its pumping decreases, and blood dams up in venous system until pressure at tricuspid level again rises to normal value -heart acts as a FEEDBACK REGULATOR OF PRESSURE at tricuspid valve -a person lying on their back has a tricuspid valve located at 60% of chest thickness in front and back, this is the zero pressure reference level for a person lying down Blood Reservoir Function of the Veins – more than 60% of all blood is in the veins; because of this and because veins are compliant, venous system serves as a blood reservoir for circulation -when blood is lost from body and arterial pressure falls, nerve signals are elicited from carotid sinuses to elicit nerve signals from brain and spinal cord through sympathetic nerves to the veins, causing them to CONSTRICT which takes up much of the slack in circulatory system caused by lost blood -can lose as much as 20% of blood and still function normally because of the reservoir function Specific Blood Reservoirs – specific blood reservoirs include the spleen, which can release 100mL of blood, the liver, can release hundreds of mL, the large abdominal veins (300mL), the venous plexus beneath the skin, the heart and lungs (heart shrinks during sympathetic stimulation) Spleen as a Reservoir for Storing Red Blood Cells – spleen can store blood in the venous sinuses and the pulp -in the pulp, capillaries are so permeable that whole blood, including RBC, oozes through the capillary walls into a trabecular mesh, forming the red pulp; RBC are trapped by trabeculae, while plasma flows into the venous sinuses and into general circulation -as a consequence, red pulp of spleen is a special reservoir containing large quantities of concentrated RBCs which can be expelled into circulation upon sympathetic nervous system excitation causing spleen to contract to release 50mL of concentrated RBC, raising hematocrit by 1-2% -white pulp of the spleen houses white blood cells Blood-Cleansing Function of Spleen-Removal of Old Cells – RBC passing through splenic pulp before entering sinuses undergo squeezing, and fragile cells can not withstand the trauma -many RBC destroyed in spleen, releasing hemoglobin and cell stroma, which are digested by reticuloendothelial cells of the spleen, and the digestion products are reused by the body as nutrients, forming new RBC Reticuloendothelial Cells of the Spleen – pulp of the spleen contains many large phagocytic reticuloendothelial cells, and venous sinuses are lined with similar cells; these cells function as part of cleansing system for the blood, acting in concert with similar system of reticuloendothelial cells of liver -when blood is invaded by infectious agents, these cells of spleen rapidly remove debris, bacteria, parasites, etc. -in many chronic infectious processes, spleen enlarges in the same manner that lymph nodes enlarge and then performs its cleansing function even more avidly