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PART 4 Regulation and Maintenance

Blood Pressure and the Effect of Gravity

In a standing person, hydrostatic pressure caused by gravity increasesblood pressure below the heart and decreases pressure above the heart.

21.9 Regulation of Mean Arterial Pressure(p. 762)

Mean arterial pressure (MAP) is proportional to cardiac output timesperipheral resistance.

21.8 Control of Blood Flow in Tissues(p. 759)

Blood flow through tissues is highly controlled and matched closely tothe metabolic needs of tissues.

Short-Term Regulation of Blood Pressure

1. Baroreceptors are sensory receptors sensitive to stretch. ■ Baroreceptors are located in the carotid sinuses and the aortic arch. ■ The baroreceptor reflex changes peripheral resistance, heart rate,and stroke volume in response to changes in blood pressure.2. Chemoreceptors are sensory receptors sensitive to O 2 , CO 2 , and pHlevels in the blood.3. Epinephrine and norepinephrine are released from the adrenal medullaas a result of sympathetic stimulation. They increase heart rate, strokevolume, and vasoconstriction.4. The CNS ischemic response, which results from high CO 2 or low pHlevels in the medulla, increases peripheral resistance.

Local Control of Blood Flow in Tissues

1. Blood flow through a tissue is usually proportional to the tissue’smetabolic needs. Exceptions are tissues that perform functions thatrequire additional blood.2. Control of blood flow by the metarterioles and precapillary sphincterscan be regulated by vasodilator substances or by lack of O 2 and nutrients.3. Only large changes in blood pressure have an effect on blood flowthrough tissues.4. If the metabolic activity of a tissue increases, the number and thediameter of capillaries in the tissue increase over time.

Long-Term Regulation of Blood Pressure

1. In the renin-angiotensin-aldosterone mechanism, renin is released bythe kidneys in response to low blood pressure. Renin promotes theproduction of angiotensin II, which causes vasoconstriction and anincrease in aldosterone secretion.2. The antidiuretic hormone (vasopressin) mechanism causes ADHrelease from the posterior pituitary in response to a substantialdecrease in blood pressure. ADH acts directly on blood vessels tocause vasoconstriction.3. The atrial natriuretic mechanism causes atrial natriuretic hormonerelease from the cardiac muscle cells when atrial blood pressureincreases. It stimulates an increase in urine production, causing adecrease in blood volume and blood pressure.4. The fluid shift mechanism causes fluid shift, which is the movementof fluid between the interstitial spaces and capillaries in response tochanges in blood pressure to maintain blood volume.5. The stress-relaxation response is an adjustment of the smooth musclesof blood vessels in response to a change in blood volume.

Autoregulation of Blood Flow

1. Autoregulation refers to changes in blood flow in response tochanges in O 2 , nutrients, and metabolic by-products, which altervasoconstriction and contraction of precapillary sphinters to adjustblood flow through tissues.2. Long-term regulation of blood flow results in alteration in capillarydiameter and number of capillaries in a tissue.

Nervous and Hormonal Control of BloodFlow in Tissues

1. The sympathetic nervous system (vasomotor center in the medulla)controls blood vessel diameter. Other brain areas can excite or inhibitthe vasomotor center.2. Vasomotor tone is a state of partial contraction of blood vessels.3. The nervous system is responsible for routing the flow of blood andmaintaining blood pressure.4. Sympathetic action potentials stimulate epinephrine and norepinephrinerelease from the adrenal medulla, and these hormones cause vaso-constriction in most blood vessels.

1. Given these blood vessels:(1) arteriole (3) elastic artery(2) capillary (4) muscular artery

(5) vein(6) venule

3. In which of these blood vessels are elastic fibers present in the largestamounts?a. large arteries c. arterioles e. large veinsb. medium arteries d. venules

4. Comparing and contrasting arteries and veins, veins havea. thicker walls.b. a greater amount of smooth muscle than arteries.c. a tunica media, but arteries do not.d. valves, but arteries do not.e. All of these are correct.

5. The structures that supply the walls of blood vessels with blood area. venous shunts. d. vasa vasorum.b. tunic channels. e. coronary arteries.c. arteriovenous anastomoses.

Choose the arrangement that lists the blood vessels in the order a redblood cell passes through them as it leaves the heart, travels to a tis-sue, and returns to the heart.a. 3,4,2,1,5,6 c. 4,3,1,2,5,6 e. 4,2,3,5,1,6b. 3,4,1,2,6,5 d. 4,3,2,1,6,5

2. Given these structures:(1) metarteriole (3) thoroughfare channel(2) precapillary sphincterChoose the arrangement that lists the structures in the order a redblood cell encounters them as it passes through a tissue.a. 1,3,2 b. 2,1,3 c. 2,3,1 d. 3,1,2 e. 3,2,1