Download Physiology Ch 15 p167-175 [4-25

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