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

8. Temporal summation of presynaptic signals summarizes potentials from (A) two different excitatory presynaptic neurons (B) two overlapping depolarization signals of the same synapse (C) two different inhibitory presynaptic neurons (D) two overlapping hyperpolarization signals of the same synapse.

9. Which of following respiratory status has the lowest intrapleural pressure (Pip)? (A) beginning of inspiration (B) middle of inspiration (C) end of inspiration and beginning of expiration (D) middle of expiration.

10. The percentage of oxygen-saturated hemoglobin in the systemic venous under resting state is about (A) 100 (B) 75 (C) 40 (D) 20.

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

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.

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.

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