Describe the process of urine formation in the kidney

The regulatory activities of kidneys form urine as a by-product. Urine formation involves three main steps—the glomerular filtration, the tubular reabsorption, and the tubular secretion.

a. Glomerular Filtration (Ultrafiltration):

Glomerulus filters out substances of low molecular weight from the blood with the retention of substances of high molecular weight, especially the proteins. Therefore, proteins are retained in the glomeruli and are not normally found in urine. If protein is detected in the urine, it indicates the kidney damage or other disease which ef­fect the glomerular membrane.

In normal adult, two million nephrons filter one li­tre of blood each minute to give about 1200 ml of glomerular filtrate (primary urine) at Bowman’s Capsule. Therefore, the Glomerular Filtration Rate (GFR) in adult is about 120 ml per minute. The hydrostatic pressure of the blood in the glomerular capillaires (Pg) is the main force for driving the fluid (Water and sol­ute) out of the glomerulus.

b. Tubular Reabsorption:

The rate of forma­tion of the primary urine is 120 ml/minute, while the rate of urine passing to the blad­der under the same condition is 1-2 ml/ minute. Therefore, it indicates that about 99 per cent of the glomerular filtrate is reabsorbed during its passage through the different segments of the renal tubule.

The reabsorption of different solids takes place at different sites in the renal tubules. Amino acids, glucose, and small amounts of protein that pass through the glomeru­lus are reabsorbed in the first part of the proximal tubule.

Sodium, chloride, and bi­carbonate are reabsorbed uniformly along the entire length of the proximal tubule and also in the distal tubule. Potassium is reabsorbed in the proximal and secreted in the distal tubule.

c. Tubular Secretion:

Although, most of the substances are reabsorbed by the tubular cells, some substances are actively trans­ported or actively excreted into the tubu­lar lumen. The secreted substance by the tubular epithelium in man are creatinine and potassium. The tubular epithelium also removes a number of foreign sub­stances that are introduced into the body for therapeutic and diagnostic purposes.

The hydrogen ions and ammo­nia formed in the distal tubular cells are also actively excreted into tubular lumen and thus pass to urine.