BIO 3520 Notes, 10/30/06
CARDIOVASCULAR SYSTEM II
I. Blood Pressure. [Widmaier, pp. 414-418]
A. Introduction.
1. As blood flows through a tube, molecules collide with walls, producing
resistance to flow.
2. When ventricles contract, they force blood to flow against this resistance
and pressure develops within vessels.
3. Flow = D pressure
resistance
4. First direct blood pressure measurement -- Stephen Hales (1733).
5. Blood pressure is measured in mmHg.
6. BP is highest in arteries, lower in capillaries, lowest in veins
(fig. 12-29).
7. Arterial BP is highest during systole, lowest at the end of diastole.
8. Venous BP is fairly constant throughout the cardiac cycle.
B. Definitions.
1. Systolic BP = Maximum arterial BP reached during ventricular ejection
(mid-systole).
a. Normal systolic BP = 120 mmHg.
2. Diastolic BP = Minimum arterial BP reached during ventricular
relaxation (end-diastole).
a. Normal diastolic BP = 80 mmHg.
3. Pulse pressure = Pressure change produced by ventricular
contraction.
a. Pulse pressure = systolic BP - diastolic BP
4. Mean arterial pressure = Average arterial pressure over the whole
cardiac cycle.
a. MAP = Diastolic BP + 1/3 pulse pressure
C. Indirect measurement of blood pressure (figs. 12-32, figure).
1. Korotkoff sounds represent turbulent flow of blood through the
brachial artery.
II. Control of Arterial BP. [pp. 390-392, 421, 437-442]
A. Major controlled variable is mean arterial pressure (MAP).
B. Rearrange flow equation:
D Pressure = flow x resistance
MAP = Cardiac output x total peripheral resistance
C. Regulation of cardiac output was discussed earlier.
1. Systolic BP and pulse pressure are especially sensitive to changes
in stroke volume.
D. Control of resistance.
1. Diameter of the blood vessel is the most important factor affecting
resistance.
a. Resistance is inversely proportional to radius to the 4th power (r4)
(fig. 12-5).
2. Major resistance vessels are arterioles.
a. Small in diameter.
b. Muscular.
3. Primary mechanism of BP regulation is control of diameter of
arterioles.
4. Diastolic BP is especially sensitive to changes in resistance.
5. Arterial diameter is controlled by sympathetic n.s. and circulating
epinephrine.
a. NE or epi cause contraction of arteriolar smooth muscle ---->
decreased diameter (vasoconstriction) ----> increased resistance
----> increased BP.
b. Exception -- blood flow to skeletal muscle.
1. Circulating epinephrine causes relaxation of skeletal muscle
arterioles ----> increased diameter (vasodilation) ---->
increased blood flow to muscle.
6. No significant parasympathetic effect.
E. Baroreceptor reflex.
1. Elements (figure).
a. Sensory receptor -- arterial baroreceptors in carotid sinus and
aortic arch (fig. 12-53).
1. Firing is proportional to MAP (fig. 12-54).
b. Afferent pathway -- sensory neurons (ex. carotid sinus nerve).
c. Control center -- medullary cardiovascular center located in
brainstem.
d. Efferent pathway -- autonomic motor neurons (ex. vagus nerve).
e. Effectors -- heart and blood vessels.
2. Baroreceptor reflex at work: Postural adjustment (figure).
a. Stand up ----> gravity causes shift of blood to lower limbs ---->
veins expand to accommodate extra blood ----> decreased venous
return ----> decreased CO ----> decreased MAP.
b. Decreased MAP ----> decreased baroreceptor discharge.
c. Activates medullary CV center.
1. Increased symp. activity, decreased parasymp. activity.
d. Effects of sympathetic activation.
1. Increased HR.
2. Increased myocardial contractility ----> increased SV.
3. Vasoconstriction ----> increased resistance.
e. Effect of decreased parasympathetic activity ----> increased HR.
F. Hemorrhage and shock (fig. 12-56).
1. Loss of blood (hemorrhage) will lower MAP (fig. 12-52).
a. Decreased blood volume ----> decreased venous return ---->
decreased CO ----> decreased MAP
2. Baroreceptor reflex will restore MAP towards normal (ex. after donating
a pint of blood).
3. Circulatory shock.
a. Caused by loss of more than 20% of blood volume.
b. Circulatory shock = Reduced CO to the extent that tissues are
damaged from inadequate blood flow.
c. If unable to restore MAP, then reflexes will maintain blood flow to heart
and brain at the expense of other organs (ex. skin is cold and dry).
d. If myocardium does not receive adequate blood flow, it deteriorates
----> decreased CO ----> death.
G. Hypotension and hypertension.
1. Normal arterial BP = 120/80 mmHg.
a. Varies with age and gender (see table in ECG lab handout).
2. Hypotension = Arterial BP less than 100/60 mmHg.
a. Postural hypotension -- failure to adjust blood pressure.
b. Inadequate blood flow to brain ----> fainting.
3. Hypertension = Arterial BP greater than 140/90 mmHg.
a. About 20% of adults in U.S. have hypertension.
b. Consequences of hypertension.
1. Increased work of the heart ----> heart failure.
2. Atherosclerosis ----> heart attack.
3. Increased cerebral blood pressure ----> stroke.
III. Cardiovascular Adjustments During Exercise. [pp. 448-451]
(figs. 12-61, 12-62, 12-63, table 12-7)
A. Increased O2 consumption of skeletal muscle.
B. Increased sympathetic activity.
C. Increased HR and SV.
1. Exercising muscle returns more blood to heart -- skeletal muscle pump
(fig. 12-45).
2. CO may increase up to 35 L/min.
3. Increased systolic BP and pulse pressure.
D. Increased coronary blood flow.
E. Dilation of skeletal muscle arterioles.
1. Distributes extra cardiac output to skeletal muscle.
2. Diastolic BP may decrease, due to decreased resistance.
IV. Capillaries. [pp. 423-431]
A. Function -- Exchange of gases, nutrients, and waste products.
B. Principal means of exchange -- simple diffusion.
C. Properties contributing to efficient exchange of molecules.
1. Accessibility.
a. Capillary network is so diffuse that every cell lies within 0.1 mm of
a capillary.
2. Thin walls.
a. One layer of flattened epithelial cells (fig. 12-37).
b. No smooth muscle.
c. Pores between cells.
1. Pores are large in glomerular capillaries of kidney.
2. Pores are small in cerebral circulation (blood-brain barrier).
3. Slow blood velocity.
a. Blood velocity is slowest in capillaries (fig. 12-39).
b. Allows time for exchange of nutrients.
D. Balance between volume of interstitial fluid and blood plasma (fig. 12-41).
1. Extracellular fluid = interstitial fluid (80%) + plasma (20%).
2. Distribution of water between these two compartments depends on
balance between osmotic pressure and hydrostatic pressure in the
capillaries.
3. Osmotic pressure.
a. Most solutes distribute evenly between plasma and interstitial fluid.
b. Major exception -- protein stays in plasma.
c. Resulting osmotic pressure favors movement of water into capillaries.
4. Hydrostatic pressure = capillary blood pressure.
a. At arterial end of capillary, hydrostatic pressure is greater than
osmotic pressure ----> water leaves capillary (filtration)
(fig. 12-42).
b. At venous end of capillary, hydrostatic pressure is less than osmotic
pressure ----> water returns to capillary (absorption).
c. Same volume of fluid is usually absorbed as was filtered.
5. If capillary pressure is increased (ex. by increased blood volume),
net movement of fluid is out of capillaries.
a. Accumulation of fluid in tissues -- edema.
b. Example: Water retention in pregnancy.
V. Congestive Heart Failure. [pp. 454-455]
A. Inability of the heart to pump enough blood to meet the body's metabolic
needs.B. Affects 2.5 million Americans.
C. Reduced myocardial contractility.
D. Causes.
1. 90% of all CHF patients have hypertension, coronary artery disease,
or both.
a. Coronary artery disease limits blood flow to myocardium.
b. Hypertension leads to myocardial fatigue.2. Cardiomyopathy (rare) -- diseased cardiac muscle.
E. Sequence of events.
1. Decreased contractility ----> decreased SV ----> decreased CO ---->
decreased arterial BP.2. Baroreceptor reflex attempts to raise BP.
3. Reflex conservation of water by kidneys ----> increased plasma vol. ---->
increased venous return to heart ----> increased end-diastolic volume
----> increased SV.F. New steady state is reached with increased ventricular volume ---->
enlarged heart.G. Extra load on heart causes progressive failure.
H. Formation of edema due to increased hydrostatic pressure.
1. Swelling of legs and feet.
2. Since left heart is usually sicker than right heart, CO is lower on left than
on right.
a. Increased volume and pressure in pulmonary circulation.
b. Drives water into lung tissues ----> pulmonary edema.
c. Congestion interferes with gas exchange ----> shortness of breath
(dyspnea).I. With impaired gas exchange, tissues receive less oxygen ---->
more myocardial damage ----> further decline in CO.J. Treatment.
1. Digitalis (from foxglove plant).
a. Increases myocardial contractility.2. Diuretics.
a. Increases renal excretion of salt and water ---->
decreases extracellular fluid volume.3. If severe, oxygen may be required.
4. If extremely severe, heart transplant or artificial heart.
VI. Heart Transplants and Artificial Heart.
A. First human heart transplant -- 1967.
1. Over 2,000 heart transplants per year in U.S.
2. 40,000 on waiting list.B. Artificial heart.
1. Description.
a. Mechanical pumps placed in chest to replace ventricles.
b. Until recently, driven by an external electrical motor.2. First total artificial heart was implanted in a dog in 1957.
3. First human implantation of total artificial heart -- 1969.
a. Intended as a temporary measure until a human heart could be
transplanted.4. The first permanent total artificial heart was implanted in 1982.
a. First patient died of a GI infection after 112 days.
b. In the five years following, three other patients received permanent
artificial hearts and 40 others received temporary ones while awaiting
transplants.c. One patient was able to leave the hospital for short time periods and
survived almost two years.a. Implanted July 9, 2001.
b. Size and shape of a large heart.
c. Two chambers (no atria).
d. Blood is pumped by an expandable pair of membranes located
between the right and left ventricles.
e. Powered by external battery pack and energy-transfer coils that
transmit power without wires.
