Question

What is excretion?

Answer 1

Animals accumulate ammonia, urea, uric acid, carbon dioxide, water and ions like ${ Na }^{ + }$, ${ K }^{ + }$, ${ Cl }^{ - }$, phosphate, sulphate, etc, either by metabolic activities or by means like excess ingestion. These substances have to be removed totally or partially as these are waste products. These are eliminated by the process known as excretion.

Complete answer:
Excretion is the process of removal of metabolic wastes from the body. Ammonia, Urea, and uric acids are the main forms of nitrogenous wastes that are excreted by the animals. A pair of kidneys, one pair of ureters, a urinary bladder, and a urethra constitutes the human excretory system. Urine formation involves three main processes namely, - Glomerular filtration, reabsorption, and secretion take place in the different parts of the nephron.
- Glomerular filtration: It is the first step which includes the filtration of the blood which is carried out by the glomerulus and thus known as glomerular filtration. Blood is filtered so finely through the podocytes, that almost all the constituents of plasma except the proteins pass into the lumen of the Bowman’s capsule. Therefore, it is considered as ultra- filtration.
- Tubular reabsorption: A comparison of the volume of the filtrate formed per day (180L per day) with that of urine released (1.5L), suggest that nearly 99% of the filtrate is reabsorbed by the renal tubules. This process is called reabsorption.
- Tubular secretion: The phenomenon of secretion of metabolic wastes by tubular cells into the filtrate is known as tubular secretion. It includes secretion of ${ K }^{ + }$, ${ H }^{ + }$, Ammonia and other wastes like creatinine, drugs, pigments, toxins, etc.

Additional Information: - The bean- shaped structures situated between the levels of the last thoracic and third lumbar vertebra, close to the dorsal inner wall of the abdominal cavity are called Kidneys. Towards the centre of the inner surface of the kidney, there is a notch called hilum. Through which blood vessels and nerves pass.
- Ureters are the pair of fine distensible muscular tubes of 25- 30 cm length, about 3mm in diameter. They start from the hilum of the kidneys, descend along the abdominal wall, bend obliquely inwards and upwards to open into the urinary bladder.
- Urinary bladder is a pyriform sac which varies in the shape, size, and position. The fully distended bladder becomes ovoid in outline.
- Urethra starts from the neck of the urinary bladder and opens outside the body.

Note: - Urethra is present only in mammals. - The structural and functional unit of the kidney is known as uriniferous tubules, also known as nephrons. - On average, an adult human secretes 1- 1.5L urine per day which is light yellow coloured, watery fluid, slightly acidic (pH- 6.0) and has a characteristic odour. - Other than kidney organs such as lungs, liver, and skin also help in the excretion of wastes.