Question

Process of formation of urine

Answer 1

Urine is the liquid waste product of the human body. It contains urea, uric acid, salts, water and other waste products that are the result of various metabolic processes occurring in the body. It is formed in the primary excretory organs- the kidneys. The structural and functional unit of the kidneys is called the nephrons. Millions of nephrons are involved in the process of urine formation.

The formation process occurs in 3 steps or phases:

Glomerular Filtration

Tubular Reabsorption

Tubular Secretion

Process of Urine Formation:

Glomerular Filtration

This process occurs in the glomerular capillaries. The process of filtration leads to the formation of an ultrafiltrate. The blood gushes into these capillaries with high pressure and gets filtered across the thin capillary walls. Everything except the blood cells and proteins are pushed into the capsular space of the Bowman’s capsule to form the ultrafiltrate. The glomerular filtration rate (GFR) is 125ml/min or 180 Litres/day.

Tubular Reabsorption

During glomerular filtration, all substances except blood cells and proteins are pushed through the capillaries at high pressure. At the level of proximal convoluted tubule(PCT), some of the substances from the filtrate are reabsorbed. These include sodium chloride, potassium, glucose, amino acids, bicarbonate, and 75% of water.

Absorption of some substances is passive, some substances are actively transported while others are co-transported. The absorption depends upon the permeability of different parts of the nephron. The distal convoluted tubule shows selective absorption. The substances and water which is reabsorbed are taken up by the peritubular capillaries to be returned to the blood.

Tubular Secretion

The peritubular capillaries that help in transporting the reabsorbed substances into the bloodstream, also help in actively secreting substances like H+ ions, K+ ions. Whenever excess K+ is secreted into the filtrate, Na+ ions are actively reabsorbed to maintain the Na-K balance. Some drugs are not filtered in the glomerulus and so are actively secreted into the filtrate during the tubular secretion phase.

Answer 2

The urine is formed by the filtration of blood that takes place inside the kidney. The nephrons which are the basic filtration units of kidney, when blood passes through it through many tubes present in it, absorbs the nutrients and other substances which are useful and the waste materials such as urea, uric acid etc., in the blood will be left at the last. This is called urine then it comes out of the kidney and goes to the urinary bladder through ureter and gets stored in it until the person gets the urge to urinate.

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