Question

In tubular secretion from the peritubular capillary,

-what substances are secreted?

-if penicillin is found in the filtrate, why is it not filtered at the renal corpuscle?

-what is creatinine? is it secreted by the nephrons?

-where in the nephron does most secretion occur?

Answer 1

Tubular secretion is the process by which the substance s are transported into renal tubules from blood via tubular epithelial cells thus also called tubular excreation.Dye phenol red was the first substance found to be secreted in renal tubules in experimental conditions.later many substances found-para aminohippuric acid(PAHA),diodrast,5 hydroxy indole acetic acid,amino derivatives , penicillin etc.

Substances secreted in different tubules of nephron-1.potassium secreted actively by sodium potassium pump in proximal and distal convoluted tubules.2.Ammonia is secreted in proximal tubules3. Hydrogen secretion in proximal >distal convoluted tubules.4.Urea secreted in loop of Henle.

Penicilin is a beta lactum antibiotic flows is plasma in protein bound state which makes it difficult for filteration in glomerular corpuscles (proteins are not filtered negatively charged and also larger size).Active transporters eg- organic acid transporter present mostly in PCT(proximal convoluted tubules) are responsible for active transport of penicillin across yhe tubules which reduces concentration of its free form in the peitububular capillaries and promotes dissociation of protein bound drugs.

Creatinin is a product of metabolism -produced from creatine phosphate which is a product of protein Metabolism in muscles also produced in liver. It is important as its level signifies functional clearance of kidney so used in renal function test. It is a substnce which is neither reabsorbed nor secreted in the tubules of nephron just filtrated so signifies renal clearance and measure of GFR.

Tubular secretion occurs in all parts of tubules but most secretions occour in proximal convoluted tubules.