Question

What are the different stages of urine formation? Explain the role of glomerulus of nephron-in the formation of urine.

Answer 1

Urine formation involves three main processes namely, glomerular filtration, reabsorption and secretion, that takes place in different parts of the nephron. The first step in urine formation is the filtration of blood, which is carried out by the glomerulus and is called glomerular filtration. Nearly 99 per cent of the filtrate has to be reabsorbed by the renal tubules which is known as reabsorption. The tubular epithelial cells in different segments of nephron perform this either by active or passive mechanisms. Also, during urine formation, the tubular cells secrete substances like H+,K+ and ammonia into the filtrate (secretion).

Filtration of blood, which is carried out by the glomerulus and is called glomerular filtration. On an average, 1100-1200 ml of blood is filtered by the kidneys per minute which constitute roughly 1/5th of the blood pumped out by each ventricle of the heart in a minute. The glomerular capillary blood pressure causes filtration of blood through 3 layers, i.e., the endothelium of glomerular blood vessels, the epithelium of Bowman’s capsule and a basement membrane between these two layers. The epithelial cells of Bowman’s capsule called podocytes are arranged in an intricate manner so as to leave some minute spaces called filtration slits or slit pores. Blood is filtered so finely through these membranes, that almost all the constituents of the plasma except the proteins pass onto the lumen of the Bowman’s capsule.