Renal physiology | PGI Based MCQs and Explanations

Which of the following is true about Renal physiology?

A. Plasma flow in kidney is 600 ml/min 
B. About 25% of cardiac output is received by kidneys
C.Fluid reabsorption across the proximal tubule is isosmotic and accounts for reabsorption of approximately two-thirds of the filtered Na+ and H2O.
D. The urine in DCT is hyperosmotic 
E. None of the above

Answer. A. Plasma flow in kidney is 600 ml/min ; (B)About 25% of cardiac output is received by kidneys ; (C) Fluid reabsorption across the proximal tubule is isosmotic and accounts for reabsorption of approximately two-thirds of the filtered Na+ and H2O.

• Cardiac output                                5000 ml /min

• Kidney perfusion                             1000 ml/min (20 % of CO)

• Plasma flow in kidneys                   600 ml/min

• GFR                                      120 ml/min (20%-filtr. fraction)

• Final urine                            2000 ml/24 hours, i.e. 1,4 ml/min

• Fluid reabsorption across the proximal tubule is isosmotic and accounts for reabsorption of approximately two-thirds of the filtered Na+ and H2O.

• The major portions of the filtered HCO3-, amino acids, glucose, and phosphate are reabsorbed in the early proximal convoluted tubule. Reabsorption of glucose and amino acids is coupled to Na+ transport and thereby generates a negative potential difference within the tubule lumen. At the same time, HCO3_ is reabsorbed by a non electrogenic mechanism, via H+ secretion. The active transport of these solutes results in trans epithelial concentration and effective osmotic pressure gradients promoting H2O flow across the proximal tubule, into the peritubular capillaries.

• The rise in tubule fluid Cl- concentration is a necessary reciprocal consequence of the decreased luminal HCO3 concentration. The resultant high concentration of Cl_ becomes an important force for the outward passive transport of Cl_ down its concentration gradient, resulting in a lumen-positive potential difference in the late proximal convoluted tubule.

The pars recta of the proximal tubule is capable of active electrogenic transport of Na_ independent of organic solute transport. Under normal conditions, approximately one-third of the glomerular filtrate enters the descending limb of Henle’s loop. This segment is incapable of active outward NaCl transport and is characterized by low permeability to Na_ but high

• H2O permeability, H2O is abstracted passively as the fluid approaches the bend of Henle’s loop.

• Hypertonic fluid with a greater NaCl concentration but lower urea concentration than the surrounding medullary interstitium thus enters the thin ascending limb of Henle, which is largely impermeable to H2O and urea but highly permeable to NaCl. This permits passive outward diffusion of NaCl. Active Na:K:2Cl transport across the water-impermeable thick ascending limb of Henle enables tubule fluid to become dilute and the medullary interstitium hypertonic.

• Irrespective of the final osmolality of the urine, the fluid that enters the distal convoluted tubule (DCT) is always hypoosmotic. This segment exhibits active Na_ reabsorption. All but the terminal portion of the DCT is water-impermeable, even in the presence of arginine vasopressin (AVP).