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b) Reabsorption

The primary urine (glomerular filtrate) enters the proximal convoluted tubule where its volume is reduced by about 80%. This is accomplished by active reabsorption of many useful substances like sodium chloride, glucose, amino acids, etc. from the tubular fluid. The water diffuses out of tubular fluid into surrounding tissue (cortical interstitium). This is called obligatory reabsorption of water. Thus, in proximal tubule there is a drastic reduction in fluid-volume without any change in its osmolarity (maintains osmotic balance).

As the tubular fluid passes down the descending limb of Henle’s loop, water leaves and sodium enters the tubular fluid, the fluid becomes progressively more concentrated and hyperosmotic to blood. The tubular fluid passes through the hair-pin bend and enters the ascending limb of Henle’s loop. The walls of the ascending limb do not permit diffusion of water and salts. In the ascending limb the chloride ions are actively transported out of the nephron and into the surrounding tissue and sodium ions follow passively.

c) Secretion

During ultrafiltration, certain harmful substances like ammonia, urea, potassium, hydrogen ions etc. may escape filtration and pass in the surrounding capillary network. Such substances are secreted back into the distal convoluted tubule and then into the common collecting duct. This results in the formation of secondary urine, which is then excreted. Once urine is in the collecting duct, no more absorbtion of water or salts takes place.

About 1600 liters of blood passes through the human kidney daily. The amount of plasma in the human body is only about three liters. Thus every drop of plasma in the blood passes through the kidneys, where its contents are monitored, checked and altered, about 560 times in a day. About 180 liters of filtrate passes through the nephron in a day, at the rate of about 125 ml per minute. Most of it is reabsorbed, so that only about one liter of urine is produced and excreted from the body daily.

iii) Regulation of kidney function (Hormonal control)

The amount of salts, water and production of urine depends upon kidney functions like filtration, reabsorption, and secretion.  These are regulated by certain hormones from the pituitary and adrenal gland.

a) Antidiuretic Hormone (ADH): Under conditions of water diuresis (i.e., when there is no need for water conservation), the hyposomotic tubular fluid (urine) passes into the collecting tubule unchanged in its osmolarity. When water needs to be conserved by the body, ADH (also called vasopressin) is secreted from the brain, and , the distal convoluted tubule and collecting tubules reabsorb more water, so that urine becomes more concentrated. The water level in blood is restored in this manner. In the absence of ADH, reabsorption of water is reduced, leading to excretion of increased amounts of more dilute urine.

b) Aldosterone: Reabsorption of sodium ions is controlled by the hormone aldosterone, produced in the cortex of the adrenal gland. Increased aldosterone production results in increased reabsorption of sodium ions, whereas lowered levels of aldosterone causes increased excretion of sodium.

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Table of Contents

19.0 - Introduction
19.1 - Ammonotelism, Ureotelism and Uricotelism
19.2 - Excretory System of Man
19.3 - Skin and Lungs as Accessory Excretory Organs

Chapter 20





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