Excretory Products and their Elimination

Metabolism, especially of proteins, produces nitrogenous wastes that must be removed; their excretion keeps the internal environment stable. The three main nitrogenous wastes differ in toxicity and the water needed to remove them, and matching each to its animals is a guaranteed NEET fact. Ammonia is the most toxic and needs the most water to flush out; animals that excrete it are ammonotelic (most bony fishes and aquatic amphibians), and they simply let it diffuse into the surrounding water.

Urea is far less toxic and needs less water; animals excreting it are ureotelic — including mammals (humans), terrestrial amphibians and marine fishes. In humans, ammonia produced in the tissues is converted into urea in the liver and carried by the blood to the kidneys. Uric acid is the least toxic and needs the least water (it is almost insoluble and is removed as a paste or pellet); animals excreting it are uricotelic — reptiles, birds, insects and land snails — an adaptation that conserves water on land.

Different animal groups use different excretory organs: protonephridia (flame cells) in flatworms, nephridia in annelids (earthworm), Malpighian tubules in insects (cockroach), and kidneys in vertebrates. Humans have a pair of kidneys as the main excretory organs.

The human excretory system consists of a pair of kidneys, two ureters, a urinary bladder and a urethra. Each kidney has an outer cortex and an inner medulla (with cone-shaped renal pyramids), and the urine drains through the calyces into the renal pelvis and out via the ureter. Inside each kidney are about one million nephrons, the microscopic tubular units that actually make urine — the subject of the next topic. For NEET, fix the three waste types with their toxicity/water order and examples, and the basic plan of the excretory system.

The nephron is the microscopic tubular unit that makes urine, and its parts must be known precisely. It begins with the Malpighian (renal) corpuscle — a tuft of capillaries called the glomerulus cupped inside the Bowman's capsule. The glomerulus is fed by an afferent arteriole and drained by a narrower efferent arteriole, which raises the pressure for filtration. The filtrate then passes along the proximal convoluted tubule (PCT), the hairpin Loop of Henle (descending and ascending limbs), the distal convoluted tubule (DCT), and finally into a collecting duct. Nephrons whose loops dip deep into the medulla (juxtamedullary nephrons) are specialised for concentrating urine.

Urine is made in three steps, a core NEET sequence. The first is glomerular filtration (ultrafiltration): blood pressure forces water and small solutes out of the glomerulus into the Bowman's capsule, producing a filtrate that is essentially plasma minus the large proteins and blood cells. The rate of this filtration, the glomerular filtration rate (GFR), is about 125 mL per minute — roughly 180 litres a day.

Clearly we do not pass 180 litres of urine daily, because of the second step, tubular reabsorption: about 99% of the filtrate is reabsorbed back into the blood. Most of this happens in the PCT, which reclaims nearly all the glucose and amino acids and much of the sodium and water. The third step is tubular secretion, in which the tubule cells actively add substances such as H⁺, K⁺ and ammonia from the blood into the filtrate; this helps regulate the body's ionic and acid–base balance.

Putting it together, the filtrate that leaves the glomerulus is progressively modified along the tubule — losing most of its water and useful solutes by reabsorption and gaining wastes by secretion — until it becomes urine. The numbers and locations are heavily examined: ultrafiltration at the glomerulus (GFR ~125 mL/min, ~180 L/day), bulk reabsorption (~99%) mainly in the PCT, and secretion of H⁺/K⁺/NH₃ along the tubule. Recognising the nephron parts in order completes this topic.

A defining feature of the mammalian kidney is its ability to produce concentrated (hypertonic) urine, so that water is conserved. This is achieved by the counter-current mechanism, which depends on two structures running side by side in opposite directions: the hairpin Loop of Henle and the parallel capillary, the vasa recta. Together they set up and maintain a steep osmotic (concentration) gradient in the medulla, with the fluid becoming more and more concentrated from the cortex toward the inner medulla.

The two limbs of the loop have different permeabilities, which is the key to the gradient. The descending limb is permeable to water but not to salts, so water leaves and the filtrate becomes concentrated as it descends. The ascending limb is impermeable to water but actively pumps out sodium (and salts), which concentrates the surrounding medullary tissue while the filtrate inside becomes dilute. The vasa recta carries away the reabsorbed water and salts without destroying the gradient. Because of this mechanism, human urine can be about four times more concentrated than the blood plasma.

The kidneys are the chief excretory organs, but several other organs share the load. The lungs remove large amounts of carbon dioxide (and some water vapour) each day. The liver makes urea and also disposes of bile pigments, cholesterol and breakdown products of drugs.

The skin contributes through its glands: sweat glands secrete sweat (mainly water with some salts, urea and lactic acid), and sebaceous glands remove some sterols and waxes through sebum. Although sweating is primarily for cooling, it does eliminate small amounts of waste. So excretion is a shared task, with the kidneys doing the precise, regulated work and the lungs, liver and skin assisting. For NEET, the must-knows are the counter-current roles (descending limb = water out, ascending limb = salt out, vasa recta maintains the gradient) and the accessory roles of lungs, liver and skin.

Kidney function is finely regulated by hormones so that blood volume, pressure and ionic balance stay constant. The first system is the renin–angiotensin–aldosterone system (RAAS). When blood pressure or the glomerular filtration rate falls, the juxtaglomerular apparatus (JGA) of the nephron releases the enzyme renin; this triggers the formation of angiotensin II, a powerful vasoconstrictor that raises blood pressure and stimulates the adrenal cortex to release aldosterone, which increases the reabsorption of sodium and water — restoring blood volume and pressure.

The second key hormone is ADH (antidiuretic hormone, vasopressin), released from the posterior pituitary. When the body is short of water, ADH is secreted and promotes the reabsorption of water in the distal tubule and collecting duct, so less water is lost and concentrated urine is formed. A lack of ADH causes diabetes insipidus, with the loss of large volumes of dilute urine. Counterbalancing these is the atrial natriuretic factor (ANF), released by the walls of the heart's atria when blood pressure rises; ANF causes vasodilation and lowers blood pressure, acting as a check on the RAAS.

The release of urine itself, micturition, is a reflex. As the urinary bladder fills, stretch receptors in its wall send signals to the central nervous system; the resulting micturition reflex relaxes the sphincters and contracts the bladder so that urine is expelled. In adults this reflex is under voluntary control.

Several disorders complete the chapter. Uraemia is the dangerous accumulation of urea in the blood when the kidneys fail; it is treated by haemodialysis using an artificial kidney, which removes the excess urea from the patient's blood. Other disorders include kidney stones (renal calculi) — hard crystals (often of oxalate) formed in the kidney — and glomerulonephritis, an inflammation of the glomeruli. For NEET, the high-yield points are the actions of RAAS, ADH and ANF, the micturition reflex, and the link between uraemia/kidney failure and dialysis.