Excretory Products and Their Elimination - Urine formation, Osmoregulation

Ultrafiltration: Occurs in the Malpighian corpuscle. Blood is filtered through three layers: the endothelium of glomerular blood vessels, the epithelium of Bowman's capsule (podocytes), and the basement membrane between these two layers. The resulting fluid is called glomerular filtrate.

Glomerular Filtration Rate ($GFR$): The amount of filtrate formed by the kidneys per minute. In a healthy individual, $GFR$ is approximately $125 \text{ mL/min}$, which totals $180 \text{ L/day}$.

Selective Reabsorption: Nearly $99\%$ of the filtrate is reabsorbed by the renal tubules. The Proximal Convoluted Tubule ($PCT$) reabsorbs nearly all essential nutrients and $70-80\%$ of electrolytes and $H_2O$.

Tubular Secretion: Cells of the tubule secrete substances like $H^+$, $K^+$, and ammonia into the filtrate to maintain ionic and acid-base balance ($pH$) of body fluids.

Counter-Current Mechanism: The Henle's loop and vasa recta play a significant role in concentrating urine. The flow of filtrate in the two limbs of Henle's loop and blood flow in vasa recta occur in opposite directions, creating a gradient from $300 \text{ mOsmolL}^{-1}$ in the cortex to $1200 \text{ mOsmolL}^{-1}$ in the inner medulla.

Regulation by ADH: Antidiuretic Hormone (Vasopressin) from the neurohypophysis facilitates water reabsorption from the distal parts of the tubule ($DCT$ and collecting duct), preventing diuresis.

Renin-Angiotensin-Aldosterone System ($RAAS$): A fall in glomerular blood pressure activates Juxtaglomerular ($JG$) cells to release Renin. Renin converts Angiotensinogen to Angiotensin I and then to Angiotensin II (a powerful vasoconstrictor). Angiotensin II also stimulates the adrenal cortex to release Aldosterone, leading to $Na^+$ and $H_2O$ reabsorption.

Atrial Natriuretic Factor ($ANF$): Released by the heart atria in response to increased blood flow/pressure; it acts as a check on the $RAAS$ by causing vasodilation and decreasing blood pressure.