Chapter 42

Keeping the mammalian cardiac cycle balanced is/are: a. an \(\mathrm{AV}\) node between the right atria and right ventricle, which signals the Purkinje fibers. b. pacemaker cells, which compose the AV node and signal the SA node. c. an insulating layer that isolates the SA node from the right atrium. d. ion channels in pacemaker cells, which close to depolarize their plasma membranes. e. neurogenic stimuli from the nervous system.

Characteristics of veins and venules are: a. thick walls. b. large muscle mass in walls. c. a large quantity of elastin in the walls. d. low blood volume compared with arteries. e. one-way valves to prevent backflow of blood.

When capillaries exchange substances: a. red blood cells move through the capillary lumens in double file. b. blood flow resistance is lower than it is in arteries and veins. c. water, ions, glucose, and erythrocytes pass freely between blood and tissues. d. diffusion along a concentration gradient and bulk flow are operating. e. diffusion is greatest closest to the arterioles.

To increase cardiac output: a. the adrenal medulla and sympathetic nervous system secrete epinephrine and norepinephrine. b. baroreceptors in the brain signal the sympathetic nerves. c. the brain stem signals the baroreceptors, causing the heart to beat faster. d. chemoreceptors and decreases the force of the heartbeat. c. chemoreceptors, stimulated by excessive blood oxygen, increase the rate of the heartbeat.