Nephron Structure and Function

Overview

This lecture explains the structure and function of the nephron, focusing on ultrafiltration, selective reabsorption, and urine formation.

Nephron Structure and Function

• The nephron is the functional unit of the kidney responsible for blood filtration and urine formation.
• Each kidney contains about 1 million nephrons, found within the medulla.
• Major parts: renal capsule (Bowman's capsule), proximal convoluted tubule (PCT), loop of Henle, distal convoluted tubule (DCT), and collecting duct.
• Nephrons are surrounded by capillaries to facilitate exchange.

Ultrafiltration in the Renal Capsule

• Blood enters the nephron via the afferent arteriole, which forms the glomerulus (a bundle of capillaries).
• High hydrostatic pressure in the glomerulus forces small molecules and water into the renal capsule, forming glomerular filtrate.
• Large proteins and blood cells remain in the blood because they cannot pass through the filtration layers.
• Filtration barriers: capillary endothelium, basement membrane, and gaps between podocytes.

Selective Reabsorption in the Proximal Convoluted Tubule

• About 85% of filtrate is reabsorbed at the PCT.
• PCT cells have microvilli (large surface area) and many mitochondria (energy for active transport).
• Sodium ions are actively transported out of PCT cells, lowering sodium inside and allowing diffusion from filtrate.
• Sodium cotransporter proteins bring glucose into PCT cells with sodium; glucose then diffuses into the blood.
• All glucose is reabsorbed; none should be found in healthy urine.

Loop of Henle and Water Reabsorption

• The loop of Henle has an ascending limb (active sodium transport) and descending limb (water leaves by osmosis).
• Sodium is actively pumped out of the ascending limb, lowering water potential in the medulla.
• Water leaves the descending limb due to the low water potential, and is reabsorbed into the blood.
• Longer loops of Henle enable greater sodium pumping and more water reabsorption (important for desert animals).

Distal Convoluted Tubule and Collecting Duct

• Filtrate entering the DCT is dilute due to sodium removal in the loop of Henle.
• Water is further reabsorbed by osmosis in the DCT and collecting duct due to the medulla's low water potential.
• Remaining filtrate in the collecting duct becomes urine.

Summary of Urine Composition

• Urine contains water, dissolved salts, urea, and small waste substances.
• Proteins, blood cells, and glucose should not be present in healthy urine.

Key Terms & Definitions

• Nephron — Functional unit of the kidney for filtration and urine production.
• Glomerulus — Cluster of capillaries in the nephron where filtration occurs.
• Ultrafiltration — Process forcing small molecules and water out of blood under high pressure.
• Proximal Convoluted Tubule (PCT) — Tubule segment where most reabsorption occurs.
• Loop of Henle — U-shaped nephron segment maintaining sodium gradients for water reabsorption.
• Distal Convoluted Tubule (DCT) — Tubule segment for further water and ion reabsorption.
• Selective Reabsorption — Process of reclaiming valuable substances from the filtrate back into the blood.
• Podocytes — Cells lining the renal capsule aiding filtration.
• Basement Membrane — Filtration barrier in the glomerulus.

Action Items / Next Steps

• Watch the next video on osmoregulation and kidney homeostasis.
• Practice questions on nephron structure and function.