Respiration and Elimation of Nitrogenous Wastes: Structure of Kidney and Formation of Urine

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Structure of Kidney

Image showing Microscopic structure of human kidney.

Image Showing Microscopic Structure of Human Kidney

  • Each kidney contains about a million tiny units called nephrons. Nephrons are the main functional units that filter blood and remove the wastes. Nephrons are located partly in the renal cortex and partly in the renal medulla. Inside each kidney there are about one million of nephrons structures which are responsible for filtering the blood and removing waste products. They form urine and drain it ultimately into the pelvis of the kidney from where the ureters transport the urine to the urinary bladder.

  • Renal corpuscle (composed of cup-shaped Bowman’s capsule and a tuft of capillaries called glomerulus). Glomerulus receives the blood from a branch of renal artery.

  • Proximal convoluted tubule (PCT), Descending limb of loop of henle, ascending limb of loop of henle, distal convoluted tubule (DCT), collecting duct, collecting ducts of all nephrons join and ultimately form the pelvis from where the ureters arise. Peritubular blood capillaries passing over the tubules and they join and form the renal vein.

Formation of Urine

Urine formation requires three distinct processes: (i) pressure filtration at the glomerular capsule, (ii) reabsorption, and (iii) tubular secretion at the distal convoluted tubule.

Glomerular Capsule or Ultra-Filtration

When blood enters the glomerulus, blood pressure is sufficient to cause small molecules such as water, salts, nutrients and wastes to move from the glomerular to the inside of glomerular capsule. The molecules the leave the blood and enter the glomerular capsule are called the glomerular filtrate. Blood proteins and blood cells are large to be part of this filtrate, so they remain in the blood as it flows into the efferent arteriole.

Selective Reabsorption

Reabsorption from the nephron to the blood takes place through the walls of the proximal convoluted tubule. Nutrients, water and even some waste molecules diffuse possibly back into the peritubular capillary network. Osmotic pressure further influences the movement of water. (a) About 65%- 85% of filtrate is reabsorbed in Proximal Convoluted tubule (PCT). It includes water, glucose, amino acids, salts etc. (b) About 5% of water is reabsorbed in the descending limb. (c) Ascending limb is impermeable to water; hence only salts are reabsorbed here. (d) 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.

Tubular Secretion

Tubular secretion refers to the transport of substances into the tubular lumen by means other than glomerular filtration. Tubular secretion may be particularly associated with the distal convoluted tubule. Substances such as foreign uric acid, hydrogen ions, ammonia and penicillin are eliminated by tubular secretion.

Image showing steps in urine formation.

Image Showing Steps in Urine Formation

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 collects in bladder, stretch receptors are stimulated leading to the desire to discharge urine.

Composition of urine: -

Table Showing Composition of Urine
Table showing Composition of urine.

Normal components

Abnormal components








Diabetes mellitus


25-30 gms


Kidney disease

Uric acid

0.7 gms


Diabetes mellitus, starvation


1.2 gms


Infection in urinary system


0.6 gms


Large numbers indicate infection in urinary system


10-15 gms

Uric acid crystals



2.5 gms


0.2 gms


1.7 gms


2.0 gms

Minute amounts of fatty acids, amino acids, pigments, mucin, enzymes, hormones, vitamins.

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