Nephron - The Functional Unit of Filtration in the Kidney

Overview

• Nephron: Functional unit of the kidney, responsible for filtration.
• Kidney Structure: Contains millions of nephrons located in the renal pyramids, including the renal medulla and renal cortex.
• Kidney Model: Visual aid showing the placement and structure of nephrons, including the loop, capsule, and collecting duct.

Main Regions within Nephron

• Renal Cortex: Located above the dotted line in the diagram.
• Renal Medulla: Located below the dotted line in the diagram.

Structural Components of the Nephron

• Bowman’s Capsule: Beginning of the nephron where filtration occurs.
• Renal Artery: Branches deliver blood to the nephron, entering the Bowman’s capsule and forming the glomerulus.
• Glomerulus: Coiled section of artery within Bowman’s capsule, responsible for filtration due to increased pressure.

Filtration Process

• Filtration: Blood plasma (about 20%) exits the glomerulus into Bowman’s capsule to become filtrate.

Reabsorption Process

• Efficiency: Filtration removes numerous substances which are then selectively reabsorbed.
• Reabsorption Sites: Proximal convoluted tubule to medulla, distal convoluted tubule, and collecting duct.
• Selective Reabsorption: Water, nutrients, and electrolytes are reabsorbed back into the bloodstream.
• Importance of Reabsorption: Prevents dehydration by reclaiming essential fluids and substances.

Nephron Pathway

1. Proximal Convoluted Tubule: Near Bowman’s capsule; primary site for reabsorbing two-thirds of water and nutrients.
2. Nephron Loop (Loop of Henle): Dips into medulla, crucial for water reabsorption through osmosis in the descending limb and active ion transport in the ascending limb.
3. Distal Convoluted Tubule: Reabsorption site, particularly in presence of ADH.
4. Collecting Duct: Collects urine from multiple nephrons; permeable to water in presence of ADH.

Regulating Reabsorption

• Osmolarity: Medulla's saltiness attracts water via osmosis.
• ADH (Anti-Diuretic Hormone): Released by the pituitary gland; increases water reabsorption by making the distal convoluted tubule and collecting duct permeable to water.
• Without ADH: Excess water remains in the urine, creating dilute urine.
• With ADH: More water is reabsorbed to conserve body fluids, creating concentrated urine.

Summary of Kidney Function

• Filtration: Blood filtrate is created in the Bowman’s capsule.
• Proximal Convoluted Tubule: Reabsorbs two-thirds of water and vital nutrients.
• Nephron Loop (Descending Limb): Water reabsorption driven by osmotic gradient.
• Nephron Loop (Ascending Limb): Active transport of ions to create the osmotic gradient.
• Distal Convoluted Tubule and Collecting Duct: Site of regulated reabsorption influenced by ADH.
• Final Pathway: Urine travels from the collecting duct to the ureter and bladder for excretion.

Recap Exercise

• Task: Explain the process of filtration, reabsorption in the nephron, and the regulation by ADH.
• Use the Diagram: Visualize and explain using the provided nephron diagram to reinforce understanding.