44. Moment-to-moment changes in total peripheral resistance are normally due to changes in
(A) the viscosity of blood.
(B) the radius of certain blood vessels in the systemic circuit.
(C) the lengths of blood vessels in the systemic circuit.
(D) the cardiac output.

40. Which of these statements about gamma interferon is false? (A) It stimulates the immune system to attack infected cells and tumors. (B) It is produced by almost all cells of the body. (C) It is a polypeptide regulator. (D) It can be produced in response to viral infections.

41. Which of these statements about intrinsic factor is false? (A) It helps prevent pernicious anemia. (B) It is secreted by the pancreas. (C) It promotes absorption of vitamin B12 in the intestine. (D) It is a polypeptide.

42. Which of the following would tend to reduce the concentration of lactic acid that accumulates in a muscle cell as a result of contractile activity? (A) increasing the diameter of the cell (B) increasing the number of mitochondria in the cell (C) decreasing the oxygen supply to the cell (D) increasing the concentration of glycolytic enzymes. 

43. Which of these statements about fat digestion and absorption is false? (A) Triglycerides are resynthesized from monoglycerides and fatty acids in the intestinal epithelial cells. (B) Triglycerides are hydrolyzed by the action of pancreatic lipase. (C) Triglycerides, as particles called chylomicrons, are absorbed into blood capillaries within the villi. (D) Emulsification by bile salts increases the rate of fat digestion.

45. The motor end plate is (A) the specialized region of an effector organ innervated by an autonomic postganglionic neuron. (B) the specialized synaptic terminal of autonomic postganglionic neurons. (C) the specialized synaptic terminal of the motor neuron. (D) the specialized region of skeletal muscle innervated by a motor neuron.

46. Which of the following is most likely to cause a decrease in the stroke volume of the left ventricle? (A) an increase in end-diastolic volume (B) an increase in the activity of sympathetic nerves to the heart (C) an increase in mean arterial pressure (D) an increase in end-diastolic pressure

47. Lymphatic capillaries differ from blood capillaries in that (A) lymphatic capillaries are not connected to any other vessels. (B) lymphatic capillaries have a lower permeability to water. (C) lymphatic capillaries have a lower permeability to small solutes. (D) lymphatic capillaries are blind ended.

48. If arterial pressure is elevated, baroreceptor signals trigger which of the following responses? (A) an increase in epinephrine secretion (B) a rise in vasopressin secretion (C) a fall in plasma angiotensin II levels (D) increased activity in sympathetic vasoconstrictor nerves

49. In respiring tissues, a rise in blood Pco2 causes all of the following except (A) a rise in the concentration of carbaminohemoglobin. (B) an increase in the affinity of hemoglobin for oxygen. (C) an increase in the hydrogen ion concentration. (D) a rise in bicarbonate concentration.

50. Which of the following is not true about helper T cells? (A) They secrete antibodies. (B) When activated, they secrete IL-2 and other cytokines. (C) They are subject to infection by HIV. (D) They function in both cell-mediated and humoral immune responses.