Summary of The Loop of Henle: Structure and Function

## Introduction The Loop of Henle is a U-shaped segment of the nephron that follows the proximal convoluted tubule and precedes the distal convoluted tubule. Its primary role is to help concentrate the renal medulla and influence water and solute handling in the nephron. This material breaks down structure, permeability properties, transport mechanisms, and physiological consequences in clear steps for efficient learning. ## Nephron components (quick review) > **Definition:** A nephron is the functional unit of the kidney consisting of the renal corpuscle (glomerulus plus Bowman's capsule), the proximal convoluted tubule (PCT), the Loop of Henle (descending and ascending limbs), and the distal convoluted tubule (DCT). - Typical counts: one kidney ≈ 1.2 million nephrons, two kidneys ≈ 2.4 million nephrons. ## Anatomy of the Loop of Henle ### Structure - Descending limb: extends from the PCT down into the renal medulla. - Ascending limb: returns from the medulla toward the cortex and connects to the distal convoluted tubule. ### Regional context - The loop dips into the renal medulla where interstitial osmolality changes with depth. - Typical interstitial osmolality along the medulla (examples used for teaching): 300 mOsm/kg near cortex, increasing stepwise to 1200 mOsm/kg deep in the medulla. ## Permeability and transport properties ### Descending limb - Highly permeable to water via aquaporin-1 channels (always present). - Relatively impermeable to most solutes (salts do not cross here significantly). > **Definition:** Aquaporin-1 are membrane channels that permit rapid, passive water movement across cell membranes. ### Ascending limb - Impermeable to water. - Contains transporters that move Na+, K+, and Cl- from tubular fluid into the interstitium: the primary transporter is the Na+-K+-2Cl- cotransporter located on the apical membrane of thick ascending limb cells. Basolateral channels/transporters then move ions into the interstitial space. > **Definition:** The Na+-K+-2Cl- cotransporter moves sodium, potassium, and two chloride ions from the tubular lumen into tubular cells, contributing to interstitial solute accumulation. ## How the loop changes filtrate composition (stepwise) 1. Filtrate enters the loop isotonic with plasma (about 300 mOsm/kg) after PCT reabsorption because water and solute reabsorption were balanced. 2. As filtrate descends: water leaves passively through aquaporin-1 into the increasingly hyperosmotic medullary interstitium, so filtrate osmolality rises (becomes more concentrated). 3. At the bend and ascending limb: water cannot follow (ascending limb is water-impermeable); active transport of Na+, K+, and Cl- from lumen into interstitium reduces filtrate osmolality (filtrate becomes more dilute) while increasing medullary interstitial osmolality. ## Important transport detail - Apical: Na+-K+-2Cl- cotransporter brings Na+, K+, 2Cl- into tubular cells from the lumen. - Basolateral: separate channels and transporters move Na+ and Cl- into the interstitium; some K+ recycles back to the lumen. ## Table: Descending vs Ascending limb | Feature | Descending limb | Ascending limb | |---|---:|---:| | Water permeability | High (aquaporin-1) | Low/none | | Solute permeability | Low | High (active transport) | | Primary movement | Water out of lumen | Na+, K+, Cl- out of lumen | | Effect on filtrate | Becomes concentrated | Becomes dilute | ## Mechanistic consequences (compact view) - The descending limb concentrates tubular fluid by losing water. - The ascending limb dilutes tubular fluid by actively removing solutes without water following. - Result: differential movement of water and solutes along the loop changes both filtrate composition and medullary interstitial osmolality.