Ultrafiltration, Selective Reabsorption, Loop of Henle & Osmoregulation

1. Ultrafiltration

Location

Ultrafiltration occurs in the cortex of the kidney, specifically across the specialized barrier between the glomerulus (a tight knot of capillaries) and the Bowman’s capsule.

The Mechanism of High Hydrostatic Pressure

Blood enters the glomerulus via the afferent arteriole and leaves via the efferent arteriole.

• The diameter of the afferent arteriole is wider than that of the efferent arteriole.

• This structural bottleneck creates a high hydrostatic pressure within the glomerular capillaries, forcing fluid and small molecules out of the blood plasma.

The Three-Layered Filtration Barrier

To enter the Bowman's capsule space, the fluid must pass through three distinct layers, which act as a molecular sieve (filtering out substances with a molecular mass > 69,000 Mr:

1. Capillary Endothelium: Contains many small pores called fenestrations. This allows fluid and solutes to pass through but blocks blood cells (erythrocytes and leukocytes) and platelets.

2. Basement Membrane: A meshwork of collagen fibers and glycoproteins. It is the primary selective filter that prevents large proteins (like albumin) from leaving the blood due to their size and negative charge.

3. Podocytes (Epithelial layer of Bowman's capsule): Specialized cells with finger-like projections called pedicels that wrap around the capillaries. Major gaps between these projections (filtration slits) ensure fluid can easily pass into the lumen of the nephron.

2. Selective Reabsorption

Location and Purpose

Selective reabsorption happens in the Proximal Convoluted Tubule (PCT) in the cortex. Its goal is to reclaim 100% of useful nutrients (glucose, amino acids) and a high percentage of water and salts back into the blood, preventing them from being wasted in urine.

Cellular Adaptations of PCT Epithelial Cells

• Microvilli: Form a dense brush border on the luminal membrane to exponentially increase surface area for transport proteins.

• Co-transporter Proteins: Specialized carrier proteins embedded in the membrane to move molecules across.

• Many Mitochondria: Provide a continuous supply of ATP required for active transport.

The Process Step-by-Step

1. Sodium-Potassium Pumps: Na+ ions are actively transported out of the PCT cells and into the tissue fluid/blood capillaries via Na+/K+ ATPase pumps on the basal membrane. This lowers the Na+ concentration inside the cell.

2. Co-transport: Due to the established concentration gradient, Na+ ions passively diffuse from the filtrate into the PCT cell through facilitated diffusion carrier proteins. As they do, they are co-transported alongside glucose or amino acids against their respective concentration gradients.

3. Facilitated Diffusion: Glucose and amino acids accumulate inside the cell and then move down their concentration gradients out of the cell into the blood via facilitated diffusion.

4. Osmosis: The movement of solutes into the blood lowers the water potential (Psi) of the blood plasma. Consequently, water moves by osmosis from the filtrate, down its water potential gradient, through the PCT cells and into the blood.

3. The Loop of Henle

Location and Purpose

The Loop of Henle spans from the cortex down into the medulla. Its primary job is to create a very low water potential (high salt concentration) in the tissue fluid of the medulla. This enables water to be reabsorbed later in the collecting duct. It functions as a countercurrent multiplier.

Step-by-Step Mechanism

• The Ascending Limb:

  • This limb is strictly impermeable to water.

  • Near the top, Na+ and Cl- ions are actively pumped out into the surrounding tissue fluid of the medulla.

  • This lowers the water potential of the medullary tissue fluid but leaves the fluid inside the ascending limb dilute.

• The Descending Limb:

  • This limb is highly permeable to water but impermeable to ions.

  • As the filtrate flows down into the increasingly salty medulla, water continuously moves out of the descending limb via osmosis into the medullary tissue fluid, where it is carried away by the vasa recta capillaries.

  • This causes the filtrate inside the descending limb to become progressively more concentrated, reaching its lowest water potential (highest salt concentration) at the very tip/hairpin bend of the loop.

4. Osmoregulation

Mechanism of Action: ADH and the Collecting Duct

Osmoregulation is the homeostatic control of water potential in the blood plasma. It is managed by negative feedback involving the hypothalamus and Antidiuretic Hormone (ADH).

Scenario A: Dehydration (Low Blood Water Potential)

1. Detection: Osmoreceptors in the hypothalamus lose water by osmosis and shrink, triggering nerve impulses.

2. Coordination: The hypothalamus signals the posterior pituitary gland to secrete more ADH into the blood.

3. Effect on Collecting Duct: * ADH binds to specific receptors on the cell surface membranes of the collecting duct cells.

   • This triggers an intracellular signaling cascade that causes vesicles containing aquaporins (water channel proteins) to fuse with the luminal membrane via exocytosis.

   • The wall of the collecting duct becomes highly permeable to water.

4. Response: Water leaves the filtrate by osmosis down the steep water potential gradient established by the Loop of Henle into the hypertonic medulla.

5. Outcome: A small volume of highly concentrated (hypertonic) urine is produced, preserving water in the body.

Scenario B: Overhydration (High Blood Water Potential)

1. Detection: Osmoreceptors in the hypothalamus absorb water, expand, and stop sending impulses.

2. Coordination: The posterior pituitary gland secretes less/no ADH.

3. Effect on Collecting Duct: Aquaporins are removed from the luminal membrane via endocytosis, making the collecting duct walls impermeable to water.

4. Response: Water cannot leave the filtrate as it travels through the medulla.

5. Outcome: A large volume of dilute (hypotonic) urine is produced, expelling excess water.