Key Points

Animals excrete three major forms of nitrogenous waste: ammonia (most toxic, requires abundant water), urea (less toxic), and uric acid (least toxic, requires minimal water).

Ammonotelism is the excretion of ammonia (e.g., bony fishes). Ureotelism is the excretion of urea (e.g., mammals). Uricotelism is the excretion of uric acid (e.g., birds, reptiles).

The human excretory system consists of a pair of kidneys, a pair of ureters, a urinary bladder, and a urethra.

Each kidney contains about one million nephrons, which are the structural and functional units. A nephron has two parts: the glomerulus and the renal tubule.

The renal tubule begins with the Bowman's capsule, followed by the Proximal Convoluted Tubule (PCT), a hairpin-shaped Henle's loop, and the Distal Convoluted Tubule (DCT).

Urine formation involves three steps that occur in different parts of the nephron: glomerular filtration, reabsorption, and secretion.

GFR is the volume of filtrate formed by the kidneys per minute. In a healthy adult, it is approximately $125 \text{ ml/minute}$ or $180 \text{ litres/day}$.

Nearly 99 percent of the filtrate is reabsorbed by the renal tubules. Substances like glucose, amino acids, and $\text{Na}^+$ are actively reabsorbed, while nitrogenous wastes and water are reabsorbed passively.

The PCT is the main site for reabsorption of essential nutrients, electrolytes, and 70-80 percent of water. It also helps maintain pH by secreting $\text{H}^+$ and ammonia.

The descending limb is permeable to water but impermeable to electrolytes, concentrating the filtrate. The ascending limb is impermeable to water but allows transport of electrolytes, diluting the filtrate.

Conditional reabsorption of $\text{Na}^+$ and water occurs in the DCT. The collecting duct allows reabsorption of large amounts of water to produce concentrated urine.

The flow of filtrate in Henle's loop and blood in vasa recta in opposite directions helps maintain an osmolarity gradient (from $300$ to $1200 \text{ mOsmolL}^{-1}$) in the medulla, facilitating water reabsorption.

ADH, or vasopressin, is released from the neurohypophysis and increases water reabsorption from the DCT and collecting duct, thereby preventing excessive water loss (diuresis).

A fall in GFR activates the Juxtaglomerular Apparatus (JGA) to release renin. This triggers the formation of angiotensin II, a vasoconstrictor that increases blood pressure and stimulates aldosterone release for $\text{Na}^+$ reabsorption.

ANF is released by the heart's atria in response to high blood pressure. It causes vasodilation, which decreases blood pressure and acts as a check on the renin-angiotensin mechanism.

Micturition is the process of releasing urine. It is a neural reflex initiated by the stretching of the urinary bladder walls, leading to the contraction of bladder muscles and relaxation of the urethral sphincter.

Lungs remove $\text{CO}_2$, the liver eliminates bile pigments and cholesterol, and the skin excretes salts, urea, and lactic acid through sweat.

Uremia is the accumulation of urea in the blood, treated by hemodialysis. Renal calculi are kidney stones, and glomerulonephritis is the inflammation of the glomeruli.