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Capillaries are specialized for exchange of
materials
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Pressure-driven movement of fluid and solutes from blood capillaries into interstitial fluid is called
filtration. Oxygen, glucose, and other nutrients are delivered to the cells by filtration.
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Pressure-driven movement from interstitial fluid into blood capillaries is called
reabsorption. Carbon dioxide, acid, urea, and other wastes are returned to the capillaries through reabsorption.
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The balance between hydrostatic pressure and interstitial fluid osmotic pressure forces is called the
starling forces
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The equation which relates hydrostatic pressure and interstitial fluid osmotic pressure is called
starling's law of the capillary
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Interstitial fluid osmotic pressure about the same throughout capillary, but hydrostatic pressure drops, this means the capillary delivers
nutrients on the arteriole side, and picks up wastes on the venule side
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At the arterial end of the capillary, forces favor filtration:
- Blood hydrostatic pressure (pushing), generated by the pumping action of the heart.
Blood hydrostatic pressure decreases from 35 to 16 mmHg from the arterial to the venous end of the capillary.
- Interstitial fluid osmotic pressure (pulling), which is constant at about 1 mmHg.
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At the venous end, forces favor reabsorption:
- Blood colloid osmotic pressure (pulling), is due to the presence of plasma proteins too large to cross the capillary wall. Averages 36 mmHg
- Interstitial fluid hydrostatic pressure, (pushing), which is normally close to zero mmHg, becomes a significant factor only in states of edema.
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Autoregulation in Capillaries:
Precapillary sphincters are collars of
smooth muscle.
when relaxed, blood flows through capillary bed
when contracted, blood bypasses capillary bed and takes thoroughfare channel
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The ability of capillaries to regulate blood flow is called
autoregulation
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Autoregulation is due to
low oxygen in tissues
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Autoregulation increases the capillary blood flow
- muscles undergoing metabolic demand
- brain in areas of greater neural activity
- skin autoregulates oxygen and nutrients; neural mechanisms control body temperature
- lungs operate in an opposite
- low O2 ------> vasoconstriction
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- high O2 ------->
vasodilation
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These Vasoconstrictors raise blood pressure
norepinephrine
epinephrine
antidiuretic hormone (ADH)
angiotensin II
endothelium-derived factors (released in low blood flow)
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These Vasodilators lower blood pressure
arterial natriuretic peptide
nitric oxide
- inflammatory mediators
- histamine, prostacyclin, kinins
- ethanol
- inhibits ADH & vasomotor center
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Blood Flow:
PA - Pv
flow = ------------
R
- R is resistance
- V is the pressure difference between arteries and veins
- Arterial pressure is PA
- Venous pressure is Pv
- Substitute these for V and we get I (flow)
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Blood Flow:
R = nL/r4
- n is blood viscosity
- L is length of all blood vessels in the body
- both of these don't change much
- - we can and do change r (the radius of a vessel)
- -r4 decreases resistance
- -remember conductance = 1/R so conductance is related to the radius of the vessel to the fourth power: flow "a" r4
- - the symbol "a" means - is proportional to-
- - that is, flow = a constant times r4
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Blood flow:
flow "a" r4
- when arteries lose their elasticity, patients lose the ability to regulate vessel diameter
- Vessel diameter depends on the action of smooth muscle in arteries, arterioles and capillaries
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Decreased elasticity ------>
hypertension (high blood pressure)
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Laminar flow describes the most efficient way
liquids (such as blood) can flow
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Friction with walls of vessel ______ velocity at edges
reduces
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Flow rate is similar in concentric shells (laminae) so flow is called
laminar
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Turbulent flow occurs when
laminar flow is disrupted
Turbulent flow----> platelet shearing----> blood clotting
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Blood pressure (BP) is usually measured in the
larger conducting arteries where the high and low pulsations of the heart can be detected.
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Systolic BP is the
higher pressure measured during left ventricular systole, when the aortic valve is open
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Diastolic BP is the
lower pressure measured during left ventricular diastole when the aortic valve is closed.
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A normal BP is less than ______ mmHg systolic and less than _____ mmHg diastolic. In young adult females, the pressures are often _____ mmHg less. People who are in good physical condition may have even ______ BPs.
120; 80; 8-10; lower
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When blood pressure is measured, the blood pressure cuff, which is
sphygmomanometer is inflated to a pressure greater than the systolic pressure of the blood. Blood flow is stopped and no sounds are heard.
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Slowly releasing air in the cuff, drops the pressure and blood begins to flow. The first sound heard is the .... Air continues to be released for the cuff. the ________ ______ is measured when no blood sounds are heard.
systolic pressure; diastolic pressure
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Difference between systolic and diastolic pressure is greatest at
aorta
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As arteries decrease in caliber,
pressure and pressure difference both drop
By the time the blood reaches the venae cavae, pressure nearly zero.
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Muscles act as pumps to help
venous blood return to heart
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