Formation of Urine

Kidney contains a large number of minute tubular structures called nephrons that are located partly in the renal cortex and partly in the renal medulla. They form urine and drain it ultimately into the pelvis of the kidney from where the ureters transport the urine to the urinary bladder.

Nephrons carry out excretory and osmoregulatory functions in the following steps:

  1. Ultra-filtration
  2. Selective Reabsorption
  3. Tubular Secretion

1. Ultra-filtration

Each glomerular capillary receives blood flowing under high pressure through a branch of renal artery. There is continuous process of ultra filtration (filtration under pressure). All small molecules like water, glucose, minerals, amino acids, urea and uric acid are filtered out of the blood plasma into the Bowman’s capsule through the capillary walls. Proteins remain in the glomerular blood. Thus a protein free filtrate is collected in the lumen of the Bowman’s capsule. The hydrostatic pressure of the circulating blood provides the pressure for filtration.

2. Selective Reabsorption or Tubular Reabsorption

As the glomerular filtrate flows through the tubules several substances useful to the body such as glucose and amino acids and mineral ions needed to maintain the water and salt balance are reabsorbed through the walls of the renal tubule. The blood capillary passing over the nephrons absorb these substances.

  1. About 65%- 85% of filtrate is reabsorbed in Proximal Convoluted tubule (PCT). It includes water, glucose, amino acids, and salts.
  2. About 5% of water is reabsorbed in the descending limb.
  3. Ascending limb is impermeable to water; hence only salts are reabsorbed here.
  4. In Distal convoluted tubule (DCT) and collecting duct Na+ is reabsorbed under the influence of the hormone aldosterone (secreted by adrenal cortex). Water is absorbed under the influence of ADH (Anti diuretic Hormone) secreted by posterior pituitary.

3. Tubular Secretion

Cells of the renal tubule also directly serecte certain unwanted substances from the blood into the filtrate. These include uric acid, K+ ions and ammonia. The filtrate is now known as urine.

Storage of Urine

The urine passes into urinary bladder via ureters and is stored there. The bladder can hold 400-500 cm3 of urine. When about 200 cm3 or more urine gets collected in urinary bladder, stretch receptors are stimulated leading to the desire to discharge urine.