Chapter 19 Physiology of Circulation.doc

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Chapter 19: Physiology of Circulation
Blood Flow
-the volume of blood flowing through a vessel, organ, or the entire circulation in a given
period in terms of (mL / min)
-blood flow is equivalent to cardiac output (CO)
-relatively constant under resting conditions
-blood flow through individual body organs vary widely and is related to their immediate
Blood Pressure
-the force per unit area exerted on a vessel wall by the contained blood
-expressed in millimetres of mercury (mm Hg)
-blood pressure refers to the systemic arterial blood pressure in the largest arteries near the
-the pressure gradient (the differences in blood pressure within the vascular system)
provides the driving force that keeps blood moving from an area of higher pressure to
lower pressure in the body
-the opposition to flow and is a measure of the amount of friction that blood encounters as
it passes through the vessels
-most friction is encountered in the peripheral (systemic) circulation, away from the heart,
so it is called peripheral resistance
-there are 3 sources of resistance:
1) Blood Viscosity
-the internal resistance to flow that exists in all fluids is known as viscosity
-it is related to the thickness of a fluid
-the greater the viscosity, the less easily molecules slide past one another and the more
difficult it is to get and keep fluid moving
-blood is more viscous than water
-it contains formed elements and plasma proteins
-blood flows more slowly under the same conditions
-blood viscosity is fairly constant
-polycythemia (excessive RBCs) can increase blood viscosity and peripheral resistance will
-anemia (low RBC count) can decrease blood viscosity and peripheral resistance will
2) Total Blood Vessel Length
-the longer the vessel, the greater the resistance
-an infant's blood vessels lengthen as they grow to adulthood and peripheral resistance and
blood pressure increase
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3) Blood Vessel Diameter
-changes in blood vessel diameter are frequent
-fluid close to the wall of a tube is slowed by friction as it passes along the wall and fluid in
the centre of the tube flows more freely and faster
-the relative speed and position of fluid in the different regions of the tube's cross section
remain constant - this is known as laminar flow or streamlining
-the smaller the tube, the greater the friction since more of the fluid contacts the tube wall
-resistance varies inversely with the 4th power of the vessel radius
-if the radius of a vessel is doubled, the resistance drops to 1/16 of its original
-large arteries close to the heart do not change dramatically in diameter and contribute little
to peripheral resistance
-small diameter arterioles that can enlarge or constrict that can enlarge or constrict in
response to neural or chemical controls are the major determinants of peripheral resistance
-when blood encounters an abrupt change in tube size or rough protruding areas of the tube
wall (i.e.: fatty plaques), the smooth laminar blood flow is replaced by turbulent flow
which is an irregular fluid motion where blood from different laminae mixes
-turbulence increases resistance
Relationship Between Flow, Pressure, and Resistance
-blood flow (F) is directly proportional to the difference in blood pressure (∆P) between 2
points in circulation (blood pressure or hydrostatic pressure gradient)
-when ∆P increases, blood flow speeds up
-when ∆P decreases, blood flow declines
-blood flow (F) is inversely proportional to the peripheral resistance (R) in the systemic
-if R increases, F decreases
F = P/R
-of the 2 factors, R is more important than P in influencing blood flow because R can
easily be changed by altering blood vessel diameter
-when the arterioles serving a particular tissue dilate (i.e.: decreasing the
resistance), blood flow to that tissue increases
Systemic Blood Pressure
-blood flows through the blood vessels along a pressure gradient; from a higher to lower
pressure area
-the pumping action of the heart generates blood flow and pressure results when flow is
opposed by resistance
-systemic blood pressure is highest in the aorta and declines throughout the pathway to
finally reach 0 mm Hg in the right atrium
-the steepest drop in blood pressure occurs in the arterioles which have the greatest
resistance to blood flow
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-as long as a pressure gradient exists, blood continues to flow until it completes
the circuit back to the heart
Arterial Blood Pressure
-reflects 2 factors:
1) Compliance and Distensibility
-how much the elastic arteries close to the heart can be stretched
2) the volume of blood forced into them at any time
-if the amounts of blood entering and leaving the elastic arteries in a given period
were equal, arterial pressure would be constant
-blood pressure actually rises and falls in a regular fashion in the elastic arteries
near the heart
-blood pressure near the heart is pulsatile
-as the left ventricle contracts and expels blood into the aorta, it gives kinetic energy to the
blood, which stretches the elastic aorta as aortic pressure peaks
-this pressure peak is known as systolic pressure
-it averages 120 mmHg
-blood moves forward in the arterial bed because the pressure in the aorta is higher
than the pressure in the more distal vessels
-during diastole, the aortic valve closes and prevents blood from flowing back into the
-the walls of the aorta and other elastic arteries recoil to maintain sufficient
pressure to keep the blood flowing forward into smaller vessels
-aortic pressure drops to its lowest level (70-80 mmHg) and this is known as
diastolic pressure
-elastic arteries act as pressure reservoirs to keep blood circulating throughout
diastole (when the heart is relaxed)
-the volume and energy of blood stored in the elastic arteries during
systole are given back during diastole
-pulse pressure is the difference between systolic and diastolic pressure
-it is felt as a throbbing pulsation in an artery (i.e.: a pulse) during systole as the
elastic arteries are expanded by blood being forced into them by ventricular
-increased stroke volume and faster blood ejection from the heart (due to
increased contractility) cause a temporary increase in pulse pressure
-pulse pressure increases with arteriosclerosis as elastic arteries become stiff
-mean arterial pressure (MAP) is the pressure that propels the blood to tissues
-it is the important pressure figure to consider
-it is equal to the diastolic pressure plus 1/3 of the pulse pressure (since diastole
lasts longer than systole)
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