NCERT Class 11-Biology: Chapter –19 Excretory Products and Their Elimination Part 6

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Question 9:

Comment upon the hormonal regulation of selective reabsorption.


Selective reabsorption is the process in which some molecules like glucose, ions and certain amino acids get reabsorbed from the filtrate as they pass through the neuron. Sodium reabsorption in the late distal tubule and collecting duct is regulated by hormones, which stimulate or inhibit sodium reabsorption as necessary.

Kidney function is monitored by hormones like: ADH i.e. Antidiuretic hormone and ANF i.e. Atrial Natriuretic Factor.

ADH facilitates reabsorption of water from the DCT and collecting duct by increasing the water and salt permeability as well as by accelerating ion and water transfer by osmotic gradient creation.

The adrenal cortex is activated by Angiotensin II as a result of which aldosterone is released.

Aldosterone in turn induces the DCT to absorb more ions and water.

ANF regulates blood flow by causing vasodilation and increasing excretion of sodium.

Long Answer Type Questions

Question 1:

Explain the mechanism of formation of concentrated urine in mammals.


Mammals have the ability to produce a concentrated urine. The Henle’s loop and vasa recta play an important role in the same.

The transport of sodium chloride in the ascending limb of Henle’s loop results in an osmolality gradient. This is passive in the thin region of the loop while active in the thick region. The creation of a hypertonic medulla is facilitated by the renal tubules and vasa recta in the following manner:There is an active absorption of urea that occurs from the collecting tubule.

Passive absorption of water without takes place in the descending limb of the loop of Henle, which helps to concentrate as it enters the ascending limb.

In the vasa recta, absorption of water into the blood takes place and return of the absorbed solute to the interstitium follows. This is called the counter current exchange mechanism.

Human kidneys can produce urine nearly four times concentrated than the initial filtrate which is initially formed.

Question 2:

Draw a labelled diagram showing reabsorption and secretion of major substances at different parts of the nephron.


When we compare volume of the filtrate formed per day with that of the urine released per day, we find that nearly 99 per cent of the filtrate has been reabsorbed by the renal tubules. This is called reabsorption. During urine formation, the tubular cells also secrete certain into the filtrate. Tubular secretion is crucial as it helps in the maintenance of ionic balance in the body fluids.

Proximal Convoluted Tubule has simple cuboidal epithelium that increases the surface area for reabsorption. Nearly all of the essential nutrients, and 70-80 per cent of electrolytes and water are reabsorbed by this segment. It also maintains the pH and ionic balance of the body fluids by selective secretion.

Henle’s Loop: The process of reabsorption is very little in its ascending limb. The descending limb is permeable to water but almost impermeable to electrolytes. This concentrates the filtrate or urine.

Distal Convoluted Tubule is where conditional reabsorption of sodium ions and water takes place. It is also responsible for reabsorption of bicarbonate ions and selectively secreting hydrogen and potassium ion to maintain the ionic balance and .

Collecting Duct: From this long duct, large amounts of water could be reabsorbed to produce concentrated urine. It plays a role in the maintenance of pH and ionic balance of blood by the selective secretion of ions as well as maintain osmolality.

Question 3:

Explain briefly, micturition and disorders of the excretory system.


Urine is formed by the nephrons which are the functional unit of the kidneys which is ultimately carried to the urinary bladder. It is stored there till a voluntary signal is given by the central nervous system (CNS) for the release of the urine. This signal stretched the urinary bladder as it gets filled with urine. In response, the stretch receptors on the bladder walls would now send signals to the CNS that passes on motor messages that would lead to contraction of the muscles of the bladder leading to relaxation of the sphincter. This releases the urine and this process is called micturition.

Disorders of the excretory system:

Glomerulonephritis: It is the inflammation of the glomeruli of kidney. If this inflammation pursues, the kidneys may stop working completely, resulting in kidney failure. There are two types of glomerulonephritis known—acute and chronic.

Uremia: Malfunctioning of kidneys would lead to accumulation of toxic urea in blood. This condition is called Uremia, which is highly harmful and can lead to failure of the kidney.

Renal calculi: Stone or insoluble mass of crystallised salts like those of calcium oxalates, etc. are formed within the kidney resulting in kidney stones.