Urinary System
Answers
254.The answer is c.(Kierszenbaum, pp 124, 389, 413, 456. Junqueira,
pp 294, 296, 353.)Carbonic anhydrase is a critical enzyme that plays an
essential role in a number of cells by catalyzing the hydration of carbon
dioxide and the dehydration of bicarbonate. In the pancreas, blocking car-
bonic anhydrase results in a reduction in secretion of bicarbonate into the
pancreatic juice by the pancreatic duct cells. Blockage of carbonic anhy-
drase results in metabolic acidosis not alkalosis (answer a)because of the
decrease in renal tubular excretion of hydrogen ions from the kidney
(answer b).In osteoclasts, blockage of carbonic anhydrase would result in
decreased bone resorption (answer d).In parietal cells blocking carbonic
anhydrase activity results in the reduction in protons moving toward the
lumen and bicarbonate toward the blood (answer e).
255.The answer is a. (Kierszenbaum, pp 375, 377. Junqueira, pp
373–379.)The cell labeled in the transmission electron micrograph of the
renal corpuscle is a mesangial cell that synthesizes extracellular matrix
including the basement membrane of the glomerulus for the support of the
capillary wall. Mesangial cells are morphologically similar to pericytes found
in association with other systemic blood vessels. The mesangial cells sur-
round glomerular capillaries as illustrated in this electron micrograph. Other
proposed functions for the mesangial cells include phagocytosis and regula-
tion of glomerular blood flow. Mesangial cells synthesize cyclooxygenase-2
[Cox-2 (a critical enzyme in prostaglandin synthesis)] and nitric oxide and
are therefore involved in vasoactive regulation. Like endothelial cells they
synthesize endothelin. Endothelial cells are anti-thrombogenic (answer b).
An endothelial cell within a glomerular capillary is also shown below the
mesangial cell. A podocyte with its processes in close association with the
glomerular capillary is observed below the mesangial cell. The podocytes
form the visceral layer of the Bowman’s capsule (answer c)and possess
processes that interdigitate to form the pedicels (answer e).The outer
layer of the Bowman’s capsule is formed by parietal cells, one of which is
located in the lower left corner of the micrograph. The urinary space and
the blood are separated by the glomerular basement membrane (answer d)
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