Excretion: During cellular respiration various metabolic reactions occur in the body, leading to the formation of various waste products, such as carbon dioxide, urea. These waste products are harmful. Therefore, the removal of these waste products is a must. Thus, excretion is the process of removal of metabolic wastes from the body.
Formation of Urine: This is a physical reaction. Nephrons of the kidney are separate and independent units for the formation of urine.
(i) Three process occur in nephrons:
(ii) Glomerulus ultra-filtration
(iii) Selective re-absorption and
(iv) Tubular excretion.
The blood enters the kidney by the renal artery. After entering the kidney, the renal artery is divided and sub-divided to form a network of blood capillaries. These capillaries form a dense network in the Bowman’s capsule, which is called glomerulus. The blood capillaries entering glomerulus are called afferent arterioles. The diameter of afferent arterioles entering the glomerulus is greater than efferent arterioles, resulting in the increase of blood pressure in the glomerulus. Due to this pressure, the excess of water, urea, glucose uric acid and some salts pass from blood to the Bowman’s capsule. This is glomerulus ultrafiltration. When this filterate passes through the glandular part, glucose, useful salts and some part of water are re-absorbed. This is re-absorption. The liquid left after reabsorption is now called urine. This urine then passes through the ureter into the urinary bladder.
Urinary bladder has involuntary muscles. As soon as the urinary bladder fills up, the involuntary muscles contract and urine are expelled out of the body through the urethra.
