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 kidneysC.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).
