Nephron Function and Urine Formation: A Detailed Look

The Nephron: Structure and Function

The nephron is the functional and anatomical unit of the kidneys. It is responsible for filtering the blood and reabsorbing essential substances.

Nephron Parts:

1. Bowman's Capsule: The end of the nephron is cup-shaped and surrounds a network of capillaries called the glomerulus. The glomerulus is connected to an afferent arteriole (input) and an efferent arteriole (output).
2. Proximal Convoluted Tubule: Named for its serpentine appearance.
3. Loop of Henle: A narrowed area of the proximal tubule, with a descending and an ascending branch.
4. Distal Convoluted Tubule: A continuation of the loop of Henle, it connects with the collecting duct.

The collecting duct empties into the renal pelvis.

Urine Formation

Urine formation takes place in three stages:

1. Glomerular Filtration: This results from the passage of blood from the capillaries of the glomerulus to Bowman's capsule. Blood cells and most plasma proteins are not filtered. The filtrate contains glucose, amino acids, vitamins, salts, water, and other nutrients, as well as waste products like urea, and a smaller proportion of uric acid and creatinine. The filtration rate is approximately 125 ml/min.
2. Tubular Reabsorption: Reabsorption occurs by passive diffusion or active transport. Most of the fluid is reabsorbed along the nephron tubules. At the level of the proximal convoluted tubule, water, glucose, vitamins, urea, amino acids, chloride ions, etc., are reabsorbed. In the loop of Henle, water is reabsorbed in the descending branch, and sodium and chlorine in the ascending branch. At the distal convoluted tubule, water and sodium continue to be absorbed (approximately 124 ml is reabsorbed in total).
3. Tubular Secretion: The resulting fluid passes to the collecting ducts, where water is still reabsorbed. The collecting duct carries urine into the renal pelvis and then to the ureters. From there, urine travels to the urinary bladder, storing up to 0.5 L. Daily urine production is approximately 1.5 L.

Other Mechanisms of Excretion

• Turtles and some lizards eliminate salt through salt glands located near the eyes. Crocodiles eliminate salt through glands in the tongue.
• Freshwater Fishes: The concentration of liquids in their blood and tissues is much higher than the surrounding water. Therefore, water tends to enter their body through porous surfaces such as the eyes, gills, and mouth cavity. Consequently, they produce a large amount of very dilute urine and never drink water.
• Marine Fishes: Their tissue liquids and blood have a lower concentration than seawater, so internal water tends to exit through the gills, mouth cavity, and eyes. Therefore, they produce a small amount of highly concentrated urine. They drink plenty of water. Excess salt is eliminated with water through the gills, requiring energy expenditure.
• Seabirds: They ingest large quantities of salt in food and drinking water. To compensate, they have salt glands above the eyes and beak, eliminating highly concentrated drops.

Creatinine is a waste substance that comes from the degradation of creatine, a molecule involved in muscle metabolism.