Nephron Structure and Ultrafiltration

Ultrafiltration in the Bowman's Capsule

• Think of a coffee filter holding back coarse grounds while letting the liquid drip through.
• A similar mechanical sorting happens during ultrafiltration, where blood is filtered under high pressure.
• This process occurs at the very start of the nephron, between a knot of capillaries called the glomerulus and the cup-shaped Bowman's capsule.

To force fluid out of the blood, the body creates high hydrostatic pressure. Blood enters the glomerulus through a wide afferent arteriole but must exit through a much narrower efferent arteriole. This "bottleneck" effect builds intense pressure, pushing fluid out of the capillaries. However, the fluid must pass through the basement membrane, a non-cellular layer that acts as a molecular sieve. It separates molecules entirely based on size.

Selective Reabsorption in the PCT

• Why doesn't a healthy person excrete the sugar they consume?
• Once small molecules enter the glomerular filtrate, the body must quickly reclaim the useful ones before they are lost in urine.
• This process, called selective reabsorption, happens almost entirely in the Proximal Convoluted Tubule (PCT).

Exactly 100% of the filtered glucose is reabsorbed back into the blood from the PCT. Because the glucose must often move against its concentration gradient, this process requires active transport. Amino acids and some mineral ions are also reabsorbed here. As these solutes move back into the blood, they alter the osmotic gradient, causing roughly 85% of the filtered water to follow them passively via osmosis.

The PCT is highly adapted to maximise the efficiency of selective reabsorption. The epithelial cells lining the tubule feature tiny folds called microvilli, which vastly increase the surface area available for transport proteins. Furthermore, these cells contain many mitochondria to supply the ATP (energy) required to actively pump glucose against its concentration gradient.

Regulating Water Balance in the Medulla

• When you spend hours sweating in the summer sun without a drink, your body urgently needs to conserve fluid.
• The latter half of the nephron is dedicated to osmoregulation—maintaining the delicate balance of water and salts in your body fluids.
• This involves a coordinated effort between the Loop of Henlé and the collecting duct.

The Loop of Henlé dips down into the medulla. The descending limb is permeable to water but not salts, allowing water to leave by osmosis. The ascending limb is impermeable to water but actively pumps sodium and chloride ions out into the surrounding tissue. This active transport creates a very high salt concentration (a low water potential) deep in the medulla, setting up a steep concentration gradient.

The final adjustment of water volume happens in the collecting duct, controlled by Antidiuretic Hormone (ADH).

• If you are dehydrated: The pituitary gland releases more ADH. The collecting duct becomes highly permeable. Because of the salty medulla created by the Loop of Henlé, large amounts of water move out of the duct by osmosis and back into the blood. You produce a small volume of highly concentrated urine.
• If you are overhydrated: The pituitary gland releases less ADH. The collecting duct becomes less permeable. Little water is reabsorbed, resulting in a large volume of dilute urine.