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Circulatory Physiology


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the FORCE resisting the flow of blood through a vessel (usually from friction)
Resistance to Flow (Peripheral Resistance) -
a measure of the "thickness" or "stickiness" of a fluid flowing through a pipe
viscosity -
viscosity -
water < V blood < V toothpaste
water flows easier than blood
the longer the vessel, the greater the drop in pressure due to friction
tube length -
smaller diameter = greater friction
tube diameter -
Relation Between Blood Flow, Pressure, Resistance
increased P -> increased flow
decreased P -> decreased flow
increased R (vasoconstriction) -> DECREASED flow
decreased R (vasodilation) -> INCREASED flow
Blood Pressure Near the Heart
Systemic Blood Pressure
HEART produces blood pressure by
pumping the blood
Blood pressure ? with distance from Heart
pressure in aorta (& major arteries) in middle of ventricular contraction (120 mm Hg in healthy adult)
systolic arterial blood pressure -
pressure in aorta (& major arteries) during ventricular diastole, when semilunar valves are closed (80 mm Hg in healthy adult)
diastolic arterial blood pressure -
the "average" blood pressure produced by the heart (93 mm Hg in healthy adult)
mean arterial pressure (MAP) -
mean arterial pressure =
diastolic pressure + 1/3 pulse pressure
pulse pressure =
systolic pressure - diastolic pressure
blood pressure decreases throughout system
L ventricle -->120 mm Hg
arteries -->120 - 60 mm Hg
arterioles -->60 - 40 mm Hg
capillaries -->40 - 20 mm Hg
venous -->20 - 10 mm Hg
R atrium -->10 - 0 mm Hg
venous blood pressure is so low, other factors contribute to venous blood flow
venous return -
venous return

breathing action of thorax "squeezes" blood back toward the heart
respiratory pump -
venous return

contraction/relaxation of skeletal muscles "milk" blood up veins to heart
muscular pump -
endothelium derived factors

strong vasoconstrictor
endothelin -
Cardiac Output =
stroke volume X heart rate

CO = SV (ml/beat) x HR (beats/min)

CO = 70 ml/beat x 60 beats/min = 4200 ml/min
Cardiac Output

increased cardiac output ->
increased blood pressure
Cardiac Output

increased stroke volume ->
increased blood pressure
Cardiac Output

increased heart rate ->
increased blood pressure
Peripheral Resistance

arteriole constriction --->
increased blood pressure
Peripheral Resistance

resistance inversely proportional to the "fourth power" of the radius change
for every radius change, resistance is increased by a factor of 4
Regulation of Blood Pressure

Nervous System Control
1. control of arteriole diameter
2. directs blood flow to proper organs and tissues that need it
baroreceptors/chemoreceptors/brain -->
afferent nerve fibers -->
medulla (vasomotor center) -->
vasomotor (efferent) nerve fibers -->
smooth muscle of arterioles
Regulation of Blood Pressure

Vasomotor Fibers to Smooth Muscle of Arterioles
sympathetic fibers that release norepinephrine (NE); cause vasoconstriction of arterioles
Regulation of Blood Pressure

Vasomotor Center of the Medulla
1. sympathetic neuron cell bodies in the medulla
2. receive input from baroreceptors, chemoreceptors, and brain
3. vasomotor tone - general constricted state of arterioles set by vasomotor center
Regulation of Blood Pressure

1. blood pressure receptors large arteries (carotid sinuses, aortic arch, neck/thorax arteries)
2. send blood pressure information to vasomotor center of medulla
Regulation of Blood Pressure


blood pressure receptors large arteries:
carotid sinuses, aortic arch, neck/thorax arteries
Regulation of Blood Pressure

send blood pressure information to vasomotor center of medulla
(increased pressure)
increased pressure -->
inhibits vasomotor center -->
Regulation of Blood Pressure

send blood pressure information to vasomotor center of medulla
(decreased pressure)
decreased pressure -->
stimulates vasomotor center ->
Regulation of Blood Pressure

1. located in aortic arch and carotid arteries
carotid and aortic bodies

2. monitor OXYGEN and pH levels of the blood
low OXYGEN or low pH -------> increase blood pressure, return blood to lungs quickly
Higher Brain Centers Control on BP
hypothalamus & cortex also effect vasomotor area
Chemical Controls of Blood Pressure
1. hormones of adrenal medulla -
2. atrial natriuretic factor (ANF) -
3. antidiuretic hormone (ADH) -
4. endothelium derived factors
5. alcohol -
"fight-or-flight" response to fear; release of norepinephrine and epinephrine from adrenal medulla; causes vasoconstriction and increased BP
hormones of adrenal medulla -
secreted by the atria of the heart, promotes general decline in blood pressure kidney releasing more Na+ and water, reducing fluid volume
atrial natriuretic factor (ANF) -
endothelium derived factors

endothelium derived relaxing factor
vasodilation -
Chemical Controls of Blood Pressure

causes vasodilation
alcohol -
released by the hypothalamus, causes increase in blood pressure by getting the kidneys to conserve water in the body; e.g. during hypotensive situations
antidiuretic hormone (ADH) -
Renal (Kidney) Regulation of Blood Pressure

fluid loss through urine
direct regulation -

a. low pressure/volume --> conserve water
b. high pressure/volume --> release more water
Renal (Kidney) Regulation of Blood Pressure

renin-angiotensin mechanism
low blood pressure -->
release of renin -->
formation of angiotensin II--> vasoconstriction
release of aldosterone --> Na+/water reabsorption (by kidney)
blood pressure, pulse, respiratory rate, and body temperature
vital signs -
the actual VOLUME of blood moving through a particular site (vessel or organ) over a certain TIME period (liter/hour, ml/min)
Blood Flow -
the FORCE exerted on the wall of a blood vessel by the blood contained within (millimeters of Mercury; mm Hg)
Blood Pressure -
blood pressure =
the systemic arterial pressure of large vessels of the body (mm Hg)
auscultory method of blood pressure measurement
1. “sphygmomanometer” wrapped around upper arm
2. inflate above systolic pressure of brachial a.
3. pressure released, first sounds - systolic pr.
4. disappearance of sounds - diastolic pr.
below normal blood pressure, < 100/60

factors -
age, physical conditioning, illness
generally in elderly, drop in blood pressure during postural changes
orthostatic hypotension -
ongoing low blood pressure
chronic hypotension -
chronic hypotension -
1. low blood protein levels (nutrition)
2. AddisonÂ’s disease (adrenal cortex malfunction)
3. hypothyroidism
4. also sign of various types of cancer
above normal blood pressure at rest, > 140/90

factors -
weight, exercise, emotions, stress
ongoing high blood pressure
chronic hypertension -
chronic hypertension -
1. prevalent in obese and elderly
2. leads to heart disease, renal failure, stroke
3. also leads to more arteriosclerosis
4. primary hypertension - unidentified source
primary hypertension - unidentified source
1. high Na+, cholesterol, fat levels
2. clear genetic component (in families)
3. diuretics - promote water removal
4. NE blockers - slow vasoconstriction
secondary hypertension - identifiable disorder
i. kidney disorders
ii. endocrine (hormone) disorders
iii. arteriosclerosis
Blood Flow in the Body

General Features
1. delivery of oxygen and removal of carbon dioxide
2. gas exchange in the lungs
3. absorption and delivery of nutrients from GI tract
4. processing/waste removal in the kidneys
normal blood flow at rest
abdominal organs 24%
skeletal muscle 20%
kidneys 20%
heart 4%
other 15%
Velocity of Blood Flow

velocity directly related to:
TOTAL cross-sectional area of the vessel(s)
Velocity of Blood Flow
aorta 40-50 cm/s (FASTEST)
arteries 20-40 cm/s
arterioles 1-20 cm/s
capillaries 0.1-1 cm/s (SLOWEST)
Local Regulation of Blood Flow

regulation of blood flow by altering arteriole diameter
autoregulation -
autoregulation - regulation of blood flow by altering arteriole diameter
1. oxygen and carbon dioxide levels
2. prostaglandins, histamines, kinins
3. needy areas --> more blood flow
change in flow through arteriole in response to stretch of smooth muscle
myogenic response -
increase in blood flow to area where an occlusion has occurred
reactive hyperemia -
results from prolonged lack of oxygen/nutrients to an area (eg. heart)
increased vasculature -
Blood Flow to Skeletal Muscles

increased blood flow to muscles during heavy activity
active (exercise) hyperemia -
active (exercise) hyperemia - increased blood flow to muscles during heavy activity
1. decreased oxygen and increased lactic acid
2. visceral organ blood flow is decreased
Blood Flow to The Brain
1. MUST maintain constant blood flow (750 ml/min)
2. sensitive to low pH and high carbon dioxide
3. blood pressure tightly regulated in the brain
a. fainting -> below 60 mm Hg
b. edema (brain swelling) -> above 180 mm Hg
Blood Flow to The Skin

intimately involved in temperature regulation
increased body temperature ->
hypothalamic inhibition of vasomotor area ->
vasodilation of vessels in skin ->
increased blood flow ->
sweating ->
(bradykinin -> more vasodilation)

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