15. The frequency of action potentials generated by the SA node is:
(A) increased by parasympathetic stimulation.
(B) decreased by sympathetic stimulation.
(C) increased by acetylcholine
(D) increased by epinephrine.

11. Firings of central chemoreceptors are sensitive directly to increase concentration of (A) H+ (B) O2 (C) CO2 (D) lactic acid.

12. Which of the following changes would most increase the resistance to blood flow in a blood vessel? (A) halving the diameter of the vessel (B) doubling the diameter of the vessel (C) halving the length of the vessel (D) doubling the length of the vessel

13. The QRS complex in an electrocardiogram best represents: (A) depolarization of the atria (B) repolarization of the atria (C) depolarization of the ventricles (D) repolarization of the ventricles

14. Which of the following statements about the cardiac cycle is TRUE? (A) The duration of systole is greater than that of diastole in a subject at rest. (B) During isovolumetric ventricular relaxation, blood flows from the atria into the ventricles. (C) Closure of the atrioventricular valves occurs at the onset of ventricular systole. (D) The QRS complex occurs at approximately the same time as the closure of the semilunar valves.

16. Which of the following statements about blood vessel structure and function is FALSE? (A) Veins are more compliant than arteries. (B) The aorta and other large arteries serve as a pressure reservoir for the circulatory system. (C) Arterioles are small vessels whose resistance to blood flow can be varied. (D) Arterioles, capillaries, and veins all have valves that allow only one-way flow of blood.

17. In the systemic circuit, which of these best describes active hyperemia? (A) increased metabolic activity that results in vasoconstriction (B) increased metabolic activity that results in vasodilation (C) decreased blood flow to tissues that results in vasoconstriction (D) decreased blood flow to tissues that results in vasodilation

18. Normally, the hydrostatic pressure difference between capillary fluid and interstitial fluid favors movement of fluid _____ a tissue capillary. The protein osmotic pressure difference between capillary fluid and interstitial fluid normally favors movement of fluid _____ a tissue capillary. (A) into; into (B) into; out of (C) out of; out of (D) out of; into

19. Which is TRUE about the resting membrane potential? (A) It has the same value in all cells. (B) It is oriented so that the cell's interior is positive with respect to the extracellular fluid. (C) Only nerve and muscle cells have a potential difference across the membrane at rest. (D) It can be altered by changing concentration gradients of permeating ions.

20. At the peak of the action potential in normal physiological conditions, the membrane potential is usually: (A) exactly at the equilibrium potential of . (B) close to but more positive than the equilibrium potential of . (C) close to but less positive than the equilibrium potential of . (D) exactly at 0 mV.

21. At very low concentrations, epinephrine causes an artery to dilate. At higher concentrations epinephrine causes the same artery to constrict. How can these different effects be explained? (A) There is one type of epinephrine receptor that uses two second-messenger systems. (B) There are two types of epinephrine receptors with different affinities for epinephrine that use two different second-messenger systems. (C) There are two types of receptors for epinephrine that use the same second-messenger system. (D) At higher concentrations epinephrine can pass through the plasma membrane and directly stimulate contraction within the cell.