Urine concentrating mechanismsDR. EMAN EL ETER

The kidney can excrete urine as dilute as 5 mOsm/L and as concentrate as high as 100-1400 mOsm/L.

The kidney can excrete large volume or small volume of urine without affecting the rate of solute excretion

What is the relation of ADH to urine osmolarity, i.e. diluted or concentrated urine?

Changes in the osmolarity of tubular fluid throughout the nephrone

Changes in the osmolarity of tubular fluid throughout the nephrone

Counter current multiplier mechanism

Responsible for the high osmolality of the renal medulla

1200-1400 mOsm. The renal medulla must accumulates solutes in great excess of water.

It is maintained by the balanced inflow and outflow of solutes and water in medulla.

A countercurrent mechanisms ; the inflow is parallel to, close to but opposite to outflow.

Mechanisms responsible for creation of hyperosmolar medulla:

1- Active transport of Na+ and co-transport of K+, Cl- and other ions out of loop of Henle into medullary interstitium.

2- Active transport of ions from collecting ducts into medullary interstituim. 3- Diffusion of large amounts of urea from the inner medullary collecting ducts into the medullary interstituim.

4- Diffusion of only small amounts of water from the medullary collecting tubules into

the interstitium, less than the reabsorption of solutes into medullary interstitium.

5- No water diffusion into the medulla from the ascending loop of Henle.

Counter current multiplier

ISF= interstitial fluid

Counter current exchanger in the vasa recta:

Function: Maintains the hyperosmolarity of the renal medulla. Why?

Vasa Recta has two main features: 1- Blood flow is slow (1-2% of total renal blood flow).

2- The vasa recta act as a counter current exchanger to minimize washout of solutes from the medullary interstitium. This is due to the U shape of vasa recta capillaries.

Disorders of urininary concentrating ability

Diabetes insipidus: Cause: inability to produce or release ADH Urine: low fixed specific gravity (diluted urine) Polyuria Polydypsia. Nephrogenic diabetes insipidus: Cause: inability of kidney to respond to ADH Diabetes mellitus: High specific gravity urine