81. The vasa recta 
A. supplies blood to the glomerulus.
B. alters the cortical solute gradient.
C. collects excess water and solutes from the medullary interstitial fluid.
D. collects water and solutes reabsorbed from the proximal convoluted tubule.
E. surrounds the collecting duct.

82. The countercurrent multiplier system 
A. is found in the collecting duct.
B. is assisted by hormones.
C. has fluid flowing in parallel tubes in opposite directions.
D. maintains the solute concentration of the medullary interstitial fluid.
E. has fluid flowing in parallel tubes in opposite directions and maintains the solute concentration of the medullary interstitial fluid.

83. A countercurrent mechanism is in 
A. the afferent and efferent arterioles.
B. the glomerulus and Bowman’s capsule.
C. the proximal and distal convoluted tubules.
D. the loop of Henle only.
E. both the loop of Henle and the vasa recta.

84. In which of the following locations is filtrate osmolality highest? 
A. end of proximal convoluted tubule
B. tip of the loop of Henle
C. end of distal convoluted tubule
D. beginning of distal convoluted tubule
E. descending loop of Henle

85.  Which of the following helps maintain a high solute concentration in the kidney medulla?  
A.  high urea concentration in medulla
B.  active transport of solutes from ascending limb
C.  the vasa recta
D.  functions of the loop of Henle
E.  All of these choices are correct

86. Which of the following would help maintain the medullary concentration gradient? 
A. movement of urea from the distal convoluted tubule to the proximal convoluted tubule
B. urea movement from the collecting duct to medullary interstitial fluid
C. movement of water from the ascending limb into the medullary interstitial fluid
D. active transport of sodium and chloride from the medullary interstitial fluid into the collecting duct
E. urea movement into the collecting duct

87. Starting from the collecting duct, indicate the order in which urea molecules move to
maintain the medullary concentration gradient.
(1) ascending limb
(2) distal convoluted tubule
(3) interstitial fluid
(4) collecting duct
(5) descending limb 
A. 2, 5, 1, 4, 3
B. 1, 3, 5, 2, 4
C. 3, 5, 1, 2, 4
D. 4, 2, 1, 3, 5
E. 4, 3, 5, 1, 2

88.  Which of the following statements is true?  
A.  Only sodium diffuses from the descending limb.
B.  Solutes are actively transported from the ascending limb.
C.  Na+ and Cl- ions are actively transported into the vasa recta.
D.  The osmolality of the descending limb decreases as filtrate moves through.
E.  Water moves by osmosis out of the ascending loop.

89. Which structure removes excess water and solutes from the kidney’s medullary interstitial fluid? 
A. distal convoluted tubule
B. collecting duct
C. loop of Henle
D. vasa recta
E. proximal convoluted tubule

90.  The kidney dialysis machine is an example of a mechanical  
A.  active transport system.
B.  countercurrent system.
C.  cotransport system.
D.  sorting system.
E.  toy.