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Lecture: Circulatory Physiology
I. Factors Involved in Blood Circulation
A. Blood Flow - the actual VOLUME of blood moving through a particular site (vessel or organ) over
a certain TIME period (liter/hour, ml/min)
B. Blood Pressure - the FORCE exerted on the wall of a blood vessel by the blood contained within
(millimeters of Mercury; mm Hg)
blood pressure = the systemic arterial pressure of large vessels of the body (mm Hg)
C. Resistance to Flow (Peripheral Resistance) - the FORCE resisting the flow of blood through a
vessel (usually from friction)
1. viscosity - a measure of the "thickness" or "stickiness" of a fluid flowing through a pipe
a. V water < V blood < V toothpaste
b. water flows easier than blood
2. tube length - the longer the vessel, the greater the drop in pressure due to friction
3. tube diameter - smaller diameter = greater friction
D. Relation Between Blood Flow, Pressure, Resistance
difference in blood pressure ( P)
Blood Flow (F) = -------------------------------------------peripheral resistance (R)
a. increased P -> increased flow
b. decreased P -> decreased flow
b. increased R (vasoconstriction) -> DECREASED flow
c. decreased R (vasodilation) -> INCREASED flow
II. Systemic Blood Pressure
A. Blood Pressure Near the Heart
1. HEART produces blood pressure by pumping the blood
2. Blood pressure decreases with distance from Heart
3. systolic arterial blood pressure - pressure in aorta (& major arteries) in middle of ventricular
contraction (120 mm Hg in healthy adult)
4. diastolic arterial blood pressure - pressure in aorta (& major arteries) during ventricular
diastole, when semilunar valves are closed (80 mm Hg in healthy adult)
5. mean arterial pressure (MAP) - the "average" blood pressure produced by the heart (93 mm
Hg in healthy adult)
mean arterial pressure = diastolic pressure
+
1/3 pulse pressure
** pulse pressure = systolic pressure - diastolic pressure
6. 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
7. venous return - venous blood pressure is so low, other factors contribute to venous blood flow
a. respiratory pump - breathing action of thorax "squeezes" blood back toward the heart
b. muscular pump - contraction/relaxation of skeletal muscles "milk" blood up veins to heart
III. Factors Affecting Blood Pressure
A. Cardiac Output ( = stroke volume X heart rate)
CO = SV (ml/beat) x HR (beats/min)
= 70 ml/beat
x
60 beats/min = 4200 ml/min
1. increased cardiac output -> increased blood pressure
2. increased stroke volume -> increased blood pressure
3. increased heart rate -> increased blood pressure
B. Peripheral Resistance
1. arteriole constriction ---> increased blood pressure
2. resistance inversely proportional to the "fourth power" of the radius change
C. Blood Volume
1. hemorrhage - decrease in blood pressure
2. salt/fluid - increase in blood pressure
3. polycythemia - increase in blood viscosity
4. RBC anemia - decrease in blood viscosity
IV. Regulation of Blood Pressure
A. Nervous System Control
1. control of arteriole diameter
2. directs blood flow to proper organs and tissues that need it
3. REFLEX PATHWAY:
baroreceptors/chemoreceptors/brain -->
afferent nerve fibers -->
medulla (vasomotor center) -->
vasomotor (efferent) nerve fibers -->
smooth muscle of arterioles
B. Vasomotor Fibers to Smooth Muscle of Arterioles
1. sympathetic fibers that release norepinephrine (NE); cause vasoconstriction of arterioles
C. 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
D. Baroreceptors
1. blood pressure receptors large arteries (carotid sinuses, aortic arch, neck/thorax arteries)
2. send blood pressure information to vasomotor center of medulla
increased pressure -->
inhibits vasomotor center ->
vasodilation
decreased pressure -->
stimulates vasomotor center ->
vasoconstriction
E. Chemoreceptors
1. located in aortic arch and carotid arteries
a. 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
F. Higher Brain Centers Control on BP
1. hypothalamus & cortex also effect vasomotor area
G. Chemical Controls of Blood Pressure
1. hormones of adrenal medulla - "fight-or-flight" response to fear; release of norepinephrine and
epinephrine from adrenal medulla; causes vasoconstriction and increased BP
2. atrial natriuretic factor (ANF) - secreted by the atria of the heart, promotes general decline
in blood pressure kidney releasing more Na+ and water, reducing fluid volume
3. antidiuretic hormone (ADH) - released by the hypothalamus, causes increase in blood pressure
by getting the kidneys to conserve water in the body; eg. during hypotensive situations
4. endothelium derived factors
a. endothelin - strong vasoconstrictor
b. endothelium derived relaxing factor - vasodilation
5. alcohol - causes vasodilation
H. Renal (Kidney) Regulation
1. direct regulation - fluid loss through urine
a. low pressure/volume --> conserve water
b. high pressure/volume --> release more water
2. renin-angiotensin mechanism
low blood pressure -->
release of renin -->
formation of angiotensin II --> vasoconstriction
release of aldosterone --> Na+/water reabsorption (by kidney)
V. Variations in Blood Pressure
A. Measuring Blood Pressure
1. vital signs - blood pressure, pulse, respiratory rate, and body temperature
2. auscultory method of blood pressure measurement
a. "sphygmomanometer" wrapped around upper arm
b. inflate above systolic pressure of brachial a.
c. pressure released, first sounds - systolic pr.
d. disappearance of sounds - diastolic pr.
B. Hypotension (below normal blood pressure, < 100/60)
1. factors - age, physical conditioning, illness
2. orthostatic hypotension - generally in elderly, drop in blood pressure during postural changes
3. chronic hypotension - ongoing low blood pressure
a. low blood protein levels (nutrition)
b. Addison's disease (adrenal cortex malfunction)
c. hypothyroidism
d. also sign of various types of cancer
C. Hypertension (above normal blood pressure at rest, > 140/90)
1. factors - weight, exercise, emotions, stress
2. chronic hypertension - ongoing high blood pressure
a. prevalent in obese and elderly
b. leads to heart disease, renal failure, stroke
c. also leads to more arteriosclerosis
d. primary hypertension - unidentified source
i. high Na+, cholesterol, fat levels
ii. clear genetic component (in families)
iii. diuretics - promote water removal
iv. NE blockers - slow vasoconstriction
e. secondary hypertension - identifiable disorder
i. kidney disorders
ii. endocrine (hormone) disorders
iii. arteriosclerosis
VI. Blood Flow in the Body
A. 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
5. normal blood flow at rest
abdominal organs
skeletal muscle
kidneys
brain
heart
other
24%
20%
20%
13%
4%
15%
B. Velocity of Blood Flow
1. velocity directly related to the TOTAL cross-sectional area of the vessel(s)
FASTEST
SLOWEST
aorta
arteries
arterioles
capillaries
40-50 cm/s
20-40 cm/s
1-20 cm/s
0.1-1 cm/s
C. Local Regulation of Blood Flow
1. autoregulation - regulation of blood flow by altering arteriole diameter
a. oxygen and carbon dioxide levels
b. prostaglandins, histamines, kinins
c. needy areas --> more blood flow
2. myogenic response - change in flow through arteriole in response to stretch of smooth muscle
3. reactive hyperemia - increase in blood flow to area where an occlusion has occurred
4. increased vasculature - results from prolonged lack of oxygen/nutrients to an area (eg. heart)
D. Blood Flow to Skeletal Muscles
1. active (exercise) hyperemia - increased blood flow to muscles during heavy activity
a. decreased oxygen and increased lactic acid
b. visceral organ blood flow is decreased
E. 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
F. Blood Flow to The Skin
1. 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)
G. Blood Flow to the Lungs
1. short pathway from heart, less pressure required
2. low oxygen level --> vasoconstriction
H. Blood Flow to the Heart
1. blood to coronary arteries during diastole
2. vasodilation from ADP and carbon dioxide
VII. Blood Flow in the Capillaries
A. Exchange of Gases and Nutrients
1. diffusion - all molecules move DOWN the concentration gradient (from HIGH to LOW) into
or out of the blood
2.
oxygen/nutrients
carbon dioxide/ wastes
(blood ------> body cells)
(body cells ------> blood)
B. Fluid Movements
1. hydrostatic pressure - force of the capillary wall on the blood itself
a. filtration pressure - the pressure forcing fluid and solutes through capillary clefts
2. osmotic pressure - force driving fluid in the direction of HIGHER solute concentration
3.
movement out: Hydrostatic pressure > Osmotic difference
movement in : Hydrostatic pressure < Osmotic difference
4. normal fluid movement - 1.5 ml/min in the entire body
C. Circulatory Shock
1. circulatory shock - blood pressure gets so low that blood will not flow adequately
2. hypovolemic shock - circulatory shock resulting from loss of fluid (bleeding, diarrhea, burn)
a. heart rate increases rapidly
b. general vasoconstriction of vessels
3. vascular shock - extreme vasodilation causes sudden drop in blood pressure
a. snake and spider bites with NE blockers
b. septicemia - bacterial infection
4. cardiogenic shock - heart is unable to provide sufficient blood pressure