Question

explain the terms ultrafiltration, tubular reabsorption and tubular secretion where do these functions take place in nephron?

Answer 1

ultrafiltration-

the blood flows through the glomerulus under great pressure which is much greater than in the cappilaries elsewhere.The reason for this greater pressure is that the efferent arteriole is narrower than afferent arteriole.This high pressure causes the liquid part of blood to filter out from glomerulus into renal tubule.This filtration under extraordinary force is called ultrafiltration.

tubular reabsorption-

The glomerular filtrate entering the renal tubule is not urine.It is an extremely dilute soln.containing lot of useful materials including glucose and some sodium salts. As the filtrate passes down the tubule much water is reabsorbed together with the useful substances. But their reabsorption is only to the extent that the normal concentration of the blood is not disturbed. This is called selective absorption.

Tubular secretion-

certain substances like potassium in the normal course and a large number of foreign Chemicals including drugs like penicillin are passed into the forming urine in the distal convoluted tubule. This passage involves the activity of cells of tubular wall and hence it is called tubular secretion.

it takes place in malphigian capsule(bowman's capsule+glomerulus)

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Answer 2

i. Glomerular Filtration/Ultrafiltration:

 Filtration of blood occurs through three layers, i.e. endothelium of glomerular blood vessels, epithelium of Bowman's capsule and basement substance between these two layers.

 Epithelial cells of Bowman's capsule called podocytes are arranged in a 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 plasma except proteins pass into the lumen of Bowman's capsule. This process is called ultrafiltration.

 The filtrate that enters the tubules of nephrons is called glomerular filtrate or ultra-filtrate.

Note: Glomerular filtrate contains essentially all constituents of blood except blood cells and plasma proteins.

 The amount of filtrate formed by kidneys per minute is called glomerular filtration rate (GFR).

 GFR in a healthy individual is about 125 ml/minute, i.e. 180 litres per day.

ii. Tubular Reabsorption:

 A comparison of volume of filtrate formed per day (180 litres) with that of the urine released (1.5 litres), suggests that nearly 99% of the filtrate is reabsorbed by the renal tubules. This process is called tubular reabsorption.

 Selective transport of substances across tubular epithelium from ultra-filtrate to blood is called reabsorption.

iii. Tubular Secretion:

 Transport of substances across tubular epithelium from blood to filtrate is called secretion.

 Tubular secretion helps in the maintenance of ionic and acid-base balance of body fluids.

 PCT maintains pH and ionic balance of body fluid by selective secretion of H+, K+ and NH3 into filtrate.

 DCT and collecting duct also maintain pH and ionic balance by secreting H+ and K+.

Note: PCT and DCT are the main sites of tubular secretion.

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