1. Formation of Urine:

The formation of urine takes place by the following three processes:

        I.            Ultra Filtration or Glomerular Filtration

      II.            Selective Reabsorption

    III.            Tubular secretion

Ultra Filtration:

Blood enters the glomerulus at a hydrostatic pressure of 60mmHg to filter out some fine components of blood. The glomerular filtration pressure is about 10mmHg and is responsible for filtering a large amount of water, glucose, amino acids, vitamins, Na⁺, K⁺ and number of harmful substances like urea, uric acid, creatinine, ammonium salts and pigments. The proteins, fats and carbohydrates are not filtered out. The glomerular filtrate is same as blood minus cells and proteins. It resembles the protein free plasma in composition and Osmotic pressure.

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About 650ml of blood passes through the glomerular capillaries in all nephrons of both kidneys, but in one minute about 125ml of filtrate is filtered out. So about 150-180 liters of nephric filtrate are formed per day but only 1.5-1.8 liter of urine is excreted out per day and that is called urine output.

Selective Reabsorption:

As the nephric filtrate moves in the nephron towards the collecting tubule, about 99% of water, whole of glucose and amino acids, most of the Na⁺, and Cl^– , some urea and uric acid are reabsorbed back into back into the blood. This process by which only useful substances are reabsorbed from the nephric filtrate is called selective reabsorption. The main function of reabsorption is maintaining homeostasis so that volume, electrolytic balance composition, pH and osmotic pressure of blood are kept constant.

Tubular Secretion:

The third process by which the kidneys clean blood is called tubular secretion and involve in substances being added to the tubular fluid. This removes excessive quantities of certain dissolved substances from the body, and also maintains the blood at a normal healthy pH. The substances that are secreted into the tubular fluid for removal from the body include:

·         Potassium Ions (k⁺)

·         Hydrogen Ions (H⁺)

·         Ammonium Ions (NH⁴⁺)

·         Creatinine

·         Urea

·         Some hormones

·         Some drugs e.g. Penicillin

As a result of these three processes, the wastes are changed into urine.

Role of kidney in excretion:

·         The kidneys are excretory organs that perform three main functions to produce urine, filtration, reabsorption and secretion. They are situated at the back of the peritoneal cavity.

·         The kidney monitors and maintains the body’s balance of water, ensuring that our tissues receive enough water to function properly and be healthy.

·         The kidneys also regulate blood pressure and the level of vital salts in the blood.  By regulating salt levels through production of an enzyme called rennin (as well as other substances).

·         The kidneys also secrete the hormone erythropoietin, which stimulates and controls red blood cell production.

·         The kidneys help regulate the acid-base balance (or the pH) of the blood and body fluids, which is necessary for the body to function normally.