Question

Write short notes on ultrafiltration and selective reabsorption.

Answer 1

Ultrafiltration begins in the nephron in the kidney. Blood travels through a coiled structure of capillaries called the glomerulus surrounded by the Bowman's capsule. The hydrostatic pressure of the flow of blood allows small molecules within the blood to pass through the capillary membrane and through the gaps in the podocytes surrounding the Bowman's capsule. These molecules include small ions such as sodium and potassium, glucose, water, urea and amino acids. Large molecules such as erythrocytes (red blood cells) and plasma proteins cannot cross the plasma membrane due to their size. 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.

Nearly 99 per cent of the filtrate has to be reabsorbed by the renal tubules. Selective reabsorption involves the reuptake of useful substances from the filtrate and occurs in the convoluted tubules (proximal and distal). The tubular epithelial cells in different segments of nephron perform this either by active or passive mechanisms. The majority of selective reabsorption occurs in the proximal convoluted tubule, which extends from the Bowman’s capsule.