Mechanism of urine formation

Urine is continually formed by each nephron and the processes
involved in the formation of urine are

Glomerular ultra filtration, Tubular Reabsorption, Tubular Secretion

Glomerular ultra filtration (Mechanism of urine formation)

Ultrafiltration of blood takes place in the malpighian body which acts as a biological filter. A malpighian body comprises Bowmann’s capsule and glomeruli.

Dynamics of filtration (Mechanism of urine formation)

The kidneys normally receive an abundant blood supply of about
1200ml/min or about 20 to 25 percent of the cardiac output. It flows through
the capillaries of glomerulus where the blood pressure is comparatively high.
The high blood pressure brings about effective filtration. The hydrostatic
pressure (forward pressure 75mm/Hg.) of the blood in the afferent
glomerular capillaries and the cumulative effect of the opposition pressures
and renal intratubular pressure (10mm/Hg.) play an important role in
producing the glomerular filtrate. The hydrostatic pressure of the blood is
always greater than the opposing pressures existing in the plasma protein
and renal capillaries. Thus the available net filtering force (75-50mm/Hg. =
25mm/Hg.) is chiefly responsible for glomerular filtration. The fluid in the
capsule which is obtained by the process is termed glomerular filtrate. The
volume of the glomerular filtrate produced each minute is called glomerular
filtration rate (GFR). In man it is about 125ml/min. In 24 hours the total
volume of glomerular filtrate is 170 to 180 liters.

Tubular Reabsorption (Mechanism of urine formation)

This is the second step in the urine formation. The glomerular filtrate
contains many useful substances such as glucose, amino acids, mineral salts
and vitamins dissolved in large amount of water. Reabsorption takes place in
the uriniferous tubules. Reabsorption of useful substances is a differential or
selective process. Substances such as glucose, sodium and calcium, are called
“high threshold substance”. They are actively reabsorbed in
considerable quantities. Substances like urea and uric acid etc which are
called “low threshold substances” are reabsorbed in small quantities by a
simple diffusion process or passive reabsorption. Substances like creatinine
are not reabsorbed. They are completely eliminated.

Reabsorption in Proximal Convoluted Tubule (Mechanism of urine formation)

Proximal convoluted tubule is responsible for the reabsorption of
water, glucose, sodium phosphate and bicarbonates. The urine is found to

be isotonic in the proximal convoluted tubule. Isotonic condition of a
solution indicates no passage of water across the membrane separating two
such solutes.

Reabsorption in Henle’s loop

Urine becomes more and more hypertonic as it passes through the
descending limb of the loop of Henle’s. This is due to the fact that the thin
descending portions of the Henles loop are freely permeable to sodium. As
the urine slowly passes through the thick ascending limb of the loop of Henle,
it becomes less hypertonic since the sodium is actively transported from the
ascending limb to the descending limb through the interstitial tissue space.

Reabsorption in distal convoluted tubule

On entering the distal convoluted tubule, the urine becomes nearly
isotonic to the surrounding tissue fluid due to the active transport of sodium
and passive transport of water. Summary of Renal Filtration and
Reabsorption in 24 hours

Reabsorption in collection tubule

As urine (isotonic) passes into the collecting tubule it becomes once
more hypertonic by the osmotic reabsorption of water under the
influence of the hormone ADH. The release of ADH is controlled by the
osmoreceptors in the hypothalamus in response to changes in the osmotic
pressure of the plasma circulating through the collecting tubule. Thus the urine
formed contains 96% of water, 2% urea and 2% of the metabolic products.

Tubular Secretion

This is the final step in the formation of urine during which the walls
of tubule actively remove additional waste substances which are harmful to
the body from the blood that have escaped filtration.