Excretory Products and their Elimination

The glomerular filtration rate (GFR) is the amount of filtrate formed by the kidneys per minute. In a healthy individual, it is approximately 125 mL per minute.

The kidneys regulate GFR by an autoregulatory mechanism involving the juxta glomerular apparatus (JGA). If GFR falls, the JG cells in the JGA release renin. Renin increases glomerular blood flow and raises the GFR back to normal. Thus, the JGA helps maintain a fairly constant filtration rate.

Micturition is the release of urine, and it is carried out by a reflex called the micturition reflex.

ADH helps in water reabsorption, not elimination. It prevents diuresis and helps produce concentrated urine, not hypotonic urine.

During glomerular filtration, almost all plasma constituents except proteins pass into the Bowman’s capsule. So the statement is true.

Henle’s loop plays an important role in concentrating the urine by maintaining the osmotic gradient in the medulla.

In the proximal convoluted tubule (PCT), glucose is actively reabsorbed along with amino acids, Na$^+$ and other useful substances.

The counter current mechanism operates between the two limbs of Henle’s loop and the two limbs of vasa recta. The filtrate in Henle’s loop and blood in vasa recta flow in opposite directions. This arrangement helps maintain an increasing osmolarity from the cortex to the inner medulla, mainly due to NaCl and urea. Because of this medullary osmotic gradient, water can move out easily from the collecting duct, producing concentrated urine.

Liver, lungs and skin also help in excretion. The lungs remove **CO$_2$ and some water vapour. The liver secretes bile-containing excretory substances such as bilirubin, biliverdin, cholesterol, degraded steroid hormones, vitamins and drugs, which leave the body with digestive wastes. The skin helps through sweat and sebum: sweat contains NaCl, small amounts of urea, lactic acid, etc., and sebaceous glands remove substances like sterols, hydrocarbons and waxes**.

Micturition is the process of release of urine. When the urinary bladder fills, its walls stretch and stretch receptors send signals to the CNS. The CNS sends motor impulses that cause contraction of bladder smooth muscles and relaxation of the urethral sphincter, allowing urine to be expelled. The neural mechanism involved is called the micturition reflex.

Matching each term:
- Ammonotelism → bony fish (iii)
- Bowman’s capsule → part of the renal tubule (v)
- Micturition → urinary bladder (iv)
- Uricotelism → birds (i)
- ADH → water reabsorption (ii)

So the correct matching is a-iii, b-v, c-iv, d-i, e-ii.