- Excretion and its Importance
to the Body
The process by which non-gaseous nitrogenous wastes are eliminated from
the body is called excretion. During excretion metabolic wastes from the
cell, the tissue or the body of an organism are eliminated. The process
of excretion regulates the pH and osmotic pressure of the cells of the
body. Excretion also maintains the acid-base balance of the body.
Excretion regulates the water content of the body, which is essential for
the survival of an organism.
- Functions of Renal Tubule
The renal tubule plays an important role in selective reabsorption of glucose, water, salts, etc. The
cells of the renal tubule absorb substances like ammonia, potassium,
hydrogen ions, creatinine, etc, from the blood
capillaries surrounding the tubule and discharge them into the filtrate
of the tubule. This is called tubular secretion.
- Functions of Uriniferous Tubule:
The uriniferous tubule plays an important role in ultrapurification of blood containing excretory
substances. The cells of uriniferous tubule
reabsorb essential substances like glucose, water, salts, etc; when the
filtrate passes through it. The urea absorbed by the cells of the uriniferous tubule is secreted by them into the lumen
of the tubule. This process is called renal or tubular secretion.
- Selective Reabsorption
The glomerular filtrate contains plenty of
water, glucose and salts. When this filtrate passes through the uriniferous tubule, its cells reabsorb the essential
substances like glucose, water and certain salts. This process is called
- Chemical Composition of
The chemical analysis of normal urine has shown the presence of number
of organic and inorganic constituents. These constituents and their
percentage in normal human urine are as follows:
Urea - 2.5%
Uric acid - 0.05%
Creatinine - 1%
Glucose - nil
Water - 95%
Sodium chloride - 1%
Calcium and magnesium - 0.5%
Ammonia - traces
The salt content of urine varies according to the season. It is higher
in winter and less in summer. This is because during summer, greater
amount of salt is lost through sweating.
- Differences Between Tubular Reabsorption and Tubular Secretion
Tubular reabsorption is concerned with the
selective absorption of useful substances from the glomerular
filtrate. Tubular secretion is concerned with the removal of unwanted
substances from the glomerular filtrate.
Tubular reabsorption involves the
participation of microvilli. Tubular secretion
does not involve the participation of microvilli.
In tubular reabsorption, useful substances
like glucose, amino acids and some salts are reabsorbed and supplied
back to the blood. In tubular secretion, unwanted substances like
ammonia, sodium and potassium salts, hydrogen ions and creatinine are removed from the blood and discharged
along with the urine. Tubular reabsorption
occurs in proximal convoluted tubule, Henle's
loop, distal convoluted tubule and collecting duct. Tubular secretion
occurs in proximal and distal convoluted tubules only.
- Role of Liver and Alimentary
Canal as Excretory Organs
Liver is an excretory organ because a variety of metabolic by-products
are secreted in the liver. For example, bile pigments like bilirubin and biliverdin
are formed in liver due to decomposition of dead and injured red blood
corpuscles. It also forms toxins due to detoxification in the liver.
Liver plays an important role in eliminating urea formed due to deamination of excess amino acids. The bile pigments
and toxins are excreted into the alimentary canal from where they are
eliminated along with the faecal matter. Owing
to these functions, liver and alimentary canal are considered as
Some Diseases Related to Excretion
Uremia is excess of urea in the blood.
Anuria is the failure of kidney to form
Painful urination is termed dysuria
Polyuria is a condition when large amount
of urine is excreted.
Kidney stones are formed by the precipitation of uric acid and
accumulation of oxalate crystals.
- Paget's disease is a type of carcinoma of
breast that involves the nipple or areola and large ducts.
- Bright disease is inflamation
of the kidney that involves glomeruli and is
caused by infection of streptococci bacteria.