(a) The Excretory system (urinary system) in human beings consists of a pair of kidneys, a pair of ureters, a urinary bladder and a urethra.
(b) In the kidney, the wastes are converted to urine by three processes:
(i) Ultrafiltration: A large amount of water, along with certain harmful substances like urea, uric acid, K, ammonium salts, creatinine, etc., and certain useful substances like glucose, amino acids, Na, etc., pass through glomerular capillaries and glomerular membrane into the cavity of Bowman’s capsule of nephrons under pressure.
The filtrate so formed is called nephric filtrate, which is moved towards the ureter.
(ii) Selective reabsorption: A large amount of water and sodium, a whole of glucose and amino acids, and a small amount of urea are passed back from the nephric filtrate into the blood capillaries.
It occurs either by back diffusion (i.e., water and urea) or active transport (i.e., Na, glucose, and amino acids). It generally occurs in the PCT (Proximal convoluted tubule) of nephrons.
(iii) Tubular secretion: Certain harmful chemicals like uric acid, creatinine, K, etc., are passed from blood capillaries surrounding the nephron into nephric filtrate by active transport.
It generally occurs in the DCT (Distal convoluted tubule) of nephrons.
Now, the fluid is termed as urine and is excreted out of the excretory organs.
