Urine conc

Question 1

Sodium Reabsorption Mechanisms

Answer 1

Proximal tubule (50-55%):
Cotransport with glucose, amino acids, phosphate
Countertransport with H+ (Na+/H+ exchange)

Thick ascending limb (35-40%): Na+,K+,2Cl- cotransport

Early distal convoluted tubule (5-8%): Na+,Cl- cotransport

Late distal convoluted tubule, collecting duct (2-3%): luminal Na+ membrane channels

Question 2

Water reabsorption:

Answer 2

Always passive; can be transcellular or paracellular

Follows osmotic gradients established by reabsorption of sodium, other solutes

Question 3

Chloride reabsorption:

Answer 3

Always linked, either directly or indirectly, to Na+ reabsorption (Cl- can balance the + charges)

Specific mechanisms differ in different segments

Question 4

Loop of Henle Descending limb:

Answer 4

Freely permeable to water

Impermeable to Na+, Cl-

Question 5

Loop of Henle Ascending Limb:

Answer 5

Always impermeable to water

Thin segment: NaCl reabsorption mechanism is controversial

Thick segment: active Na+,K+,2Cl- cotransport

Question 6

Late DCT and Collecting Duct-

Answer 6

The major site of physiological control of salt and water balance

Aldosterone stimulates Na+ reabsorption, K+ secretion, H+ secretion in this segment-

Antidiuretic hormone [aka arginine vasopressin (AVP)] stimulates water reabsorption-

Atrial natriuretic peptide inhibits Na+ reabsorption (medullary collecting duct)-

Question 7

Free water clearance (CH2O)

Answer 7

Definition: excretion of water in excess of amount needed to excrete isosmotic urine, i.e. excretion of solute-free water by the kidneys

CH2O = V – Cosm
= 2.0 ml/min – 4.0 ml/min = -2 ml/min
If Uosm < Posm, CH2O is positive; pure water is cleared from the body
If Uosm > Posm, CH2O is negative; pure water is retained
Does AVP affect CH2O?