Question

Trace how urine is filtered in the human excretory system.

Answer 1

The urine formation includes three processes:-

A. Glomerular filtration

B. Tubular reabsorption

C.Tubular secretion.

Among these three processes filtration is the function of the glomerulus. Reabsorption and secretion are the functions of tubular portion of the nephron.

*GLOMERULAR FILTRATION = Glomerular filtration is the process by which the blood is filtered while passing through the glomerular capillaries by filtration membrane. It is the first process of urine formation.

Filtration membrane is formed by three layers: 1.Glomerular capillary membrane 2.Basement membrane 3.Visceral layer of Bowman capsule.

Ultrafiltration :- Glomerular filtration is called ultrafiltration because even the minute particles are filtered. But, the plasma proteins are not filtered due to their large molecular size. The protein molecules are larger than the slit pores present in the endothelium of capillaries. Thus, the glomerular filtrate contains all the substances present in plasma except the plasma proteins.

*TUBULAR REABSORPTION= Tubular reabsorption is the process by which water and other substances are transported from renal tubules back to the blood. When the glomerular filtrate flows through the tubular portion of nephron, both quantitative and qualitative changes occur. Large quantity of water (more than 99%), electrolytes and other substances are reabsorbed by the tubular epithelial cells. The reabsorbed substances move into the interstitial fluid of renal medulla. And, from here, the substances move into the blood in peritubular capillaries. Since the substances are taken back into the blood from the glomerular filtrate, the entire process is called tubular reabsorption.

*TUBULAR SECRETION = Tubular secretion is the process by which the substances are transported from blood into renal tubules. It is also called tubular excretion.

1.Potassium is secreted actively by sodium potassium pump in proximal and distal convoluted tubules and collecting ducts

2.Ammonia is secreted in the proximal convoluted tubule

3.Hydrogen ions are secreted in the proximal and distal convoluted tubules. Maximum hydrogen ion secretion occurs in proximal tubule

4.Urea is secreted in loop of Henle.

Thus, urine is formed in nephron by the processes of glomerular filtration, selective reabsorption and tubular secretion.