Excretory Products and Their EliminationNEET Zoology · Class 11 · NCERT Chapter 16

You can expect 1 to 2 questions from Excretory Products and Their Elimination in NEET 2027. The most reliable scoring areas are: the three modes of nitrogenous excretion with their typical animals, parts of the nephron and what each part does, the value of GFR (about 125 mL per minute), the counter-current mechanism in the Loop of Henle, and the effects of ADH and aldosterone.

Ammonotelic animals excrete ammonia. Ammonia is very toxic but very soluble, so animals that excrete it need lots of water. Examples: bony fish, aquatic amphibians (tadpoles). Ureotelic animals excrete urea. Urea is less toxic than ammonia and needs less water. Examples: mammals (including humans), adult amphibians, sharks. Uricotelic animals excrete uric acid as a near-solid paste. Uric acid is the least toxic and needs almost no water. Examples: reptiles (except a few), birds, insects, land snails.

A nephron has six main parts in order. (1) Glomerulus: a tuft of capillaries where blood is filtered. (2) Bowman capsule: cup that catches the filtrate. (3) Proximal convoluted tubule (PCT): reabsorbs about 70 to 80 percent of water and most useful solutes (glucose, amino acids, Na+). (4) Loop of Henle: descends into the medulla then ascends, sets up the salt gradient via counter-current mechanism. (5) Distal convoluted tubule (DCT): fine reabsorption and tubular secretion of H+, K+ and ammonia. (6) Collecting duct: ADH-controlled final water reabsorption; concentrates the urine.

The counter-current mechanism is how the kidney concentrates urine. The descending limb of the Loop of Henle is permeable to water but not salts; water moves out, the filtrate becomes more concentrated. The ascending limb is permeable to salts but not water; salts move out, the filtrate becomes more dilute. This sets up a high osmolarity gradient in the medulla (about 300 mOsm in cortex to 1200 mOsm at the medullary tip). The collecting duct passes through this gradient, and water can move out into the medulla if ADH is present, producing concentrated urine.

GFR (Glomerular Filtration Rate) is the volume of blood plasma filtered by the glomeruli of both kidneys per minute. In a healthy adult it is about 125 mL per minute, which is roughly 180 litres per day. Almost all of this is reabsorbed; only about 1 to 1.5 litres becomes urine. GFR is kept constant by the juxtaglomerular apparatus (JGA), which adjusts blood flow to the glomerulus and releases renin when GFR drops.

ADH (anti-diuretic hormone, from posterior pituitary) makes the collecting duct more permeable to water. Result: more water is reabsorbed, urine is more concentrated, urine volume drops. ADH responds to low blood volume or high blood osmolarity. Aldosterone (from adrenal cortex) makes the DCT and collecting duct reabsorb more Na+ (and water follows passively) and excrete more K+. Both raise blood pressure but through different mechanisms.

Hemodialysis sends the patient blood through a machine where it passes over a semi-permeable membrane against a clean dialysing solution. Wastes (urea, creatinine, excess salts) diffuse out across the membrane. Typically done in a hospital, 3 sessions per week, several hours each. Peritoneal dialysis uses the patient own peritoneum (the membrane lining the abdomen) as the filter. Dialysing fluid is infused into the abdominal cavity, left there for a few hours, then drained. Can be done at home. Both are used when the kidneys can no longer remove waste on their own (renal failure). Kidney transplantation is the long-term cure.