Process of Excretion in Nephron

The Process of Excretion in nephron: The process of excretion may be divided into three stages—filtration, selective reabsorption and tubular secretion.

Filtration: Filtration of blood occurs under high pressure in the nephrons of the kidney. Blood enters the glomerulus through the afferent arteriole (with a wider lumen) and leaves through the efferent arteriole (with a narrow lumen). Therefore, blood passes through the glomerulus under pressure. This results in filtration of blood. Water and small molecules are forced out of the walls of the capillaries of the glomerulus and Bowman’s capsule and enter the tubule of the nephron. Large molecules remain in the blood of the glomerulus. The filtrate contains water, glucose, salts, urea, vitamins, etc. It is called the glomerular filtrate.

Selective reabsorption: Some molecules of the glomerular filtrate are selectively reabsorbed into the blood. The glomerular filtrate flows through the proximal convoluted tubule, the U-shaped Henle’s loop and the distal convoluted tubule. It contains many useful substances such as glucose, amino acids and salts. These are reabsorbed by a process, which requires energy. Without reabsorption, these nutrients could have been lost with the urine.

The filtrate now contains urea, some salts and water. Reabsorption of solutes into the blood increases the water concentration of the filtrate. Then water is reabsorbed into the blood by the process of osmosis, and the osmotic balance is restored.

The amount of water reabsorbed depends on the amount of excess water in the body and that of the dissolved waste to be excreted. This reabsorption of water from the filtrate to maintain the water balance of the body fluid is known as osmoregulation.

In this way the kidneys serve as water-conserving organs. After reabsorption from 180 L of filtrate in the kidney, only 1—2 L of urine is produced.

Tubular secretion: Some nitrogenous waste products like creatinin and some other substances like potassium ions are removed from the blood by the distal convoluted tubule, and are then added to the urine. This is called tubular secretion.

Control of Excretion: The urine that is formed continually collects in the urinary bladder. As the bladder expands, its pressure creates an urge to pass urine through the urethra. As the bladder is muscular, the urge to urinate is under voluntary nervous control.

Sample Questions

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Question : 1

Which of the following are correct ? (a) The filtrate contains water, glucose, salts, urea, vitamins, etc. It is called the glomerular filtrate. (b) The glomerular filtrate flows through the proximal convoluted tubule, the U-shaped Henle’s loop and the distal convoluted tubule. (c) As the bladder expands, its pressure creates an urge to pass urine through the urethra.
A. Only (a) and (b) is correct .	B. Only (b) and (c) is correct .	C. Only (a) and (c) is correct .	D. All of these .
Right Option : D
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Question : 2

Which of the following are correct ? (a) Blood enters the glomerulus through the afferent arteriole (with a wider lumen) and leaves through the efferent arteriole (with a narrow lumen). (b) The filtrate contains urea, some salts and water. Re-absorption of solutes into the blood increases the water concentration of the filtrate . (c) Some nitrogenous waste products are removed from the blood by the distal convoluted tubule, and are then added to the urine. This is called control of excretion .
A. Only (a) and (b) is correct .	B. Only (b) and (c) is correct .	C. Only (a) and (c) is correct .	D. All of these .
Right Option : A
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Question : 3

Filtration of blood occurs under high pressure in the nephrons of the _________________ .
A. Kidney	B. Skin	C. Testis	D. None of these .
Right Option : A
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