Define Excretion. Write a note on the physiology of urine formation.

Hint: The process of removal of metabolic waste from the body is called excretion. It is mainly carried out by lungs, kidneys and skin. The urine is the excretory product of the kidney. Urine contains water and other excess salt which is not needed by our body.

Complete answer:
To solve this question, we must know about the physiology of urine formation.
Excretion is a biological process of eliminating waste and toxins from the body of an organism. Waste is excreted from the human body in the form of urine. Urine is composed of 95% water and rest is the mineral waste. The kidney is the chief organ in which the formation of urine takes place.

The urine formation takes place in three steps-
1.Filtration- It takes place in the glomerulus of the nephron by the process of ultrafiltration of blood. All the constituents of the blood plasma reach Bowman ’s capsule except protein. Wastes like urea, creatinine are excreted from the blood.
2.Reabsorption- The filtrate obtained from the glomerulus is reabsorbed in the renal tubules. Most of the water is reabsorbed by the Henle’s loop.
3.Secretion- By this process the ionic, acid-base balance of our body is maintained. The tubular cells secrete ions like hydrogen, sodium potassium into the filtrate.

Note: The urine formed is collected in the collecting duct. The urine is stored in the urinary bladder. It is excreted outside through urethra. The main function of the kidney is osmoregulation i.e. to maintain the osmotic pressure and homeostasis of the body.