Control of plasma osmolality

Why is the collecting duct important for urine concentration?

for urine concentration because it determines how much water is reabsorbed before urine is excreted.

Key Reasons:

1. Medullary Osmotic Gradient:

   • The ascending limb of the loop of Henle pumps out NaCl but is impermeable to water.

   • This creates a high solute concentration in the medullary interstitial fluid.

2. Water Reabsorption Depends on Permeability:

   • If the collecting duct is impermeable to water:

     • Water stays in the filtrate.

     • Result: Large volume (~25 L/day) of dilute urine.

   • If the collecting duct is permeable to water:

     • Water leaves the filtrate by osmosis into the concentrated medulla.

     • Result: Small volume of concentrated urine.

3. Regulated by Antidiuretic Hormone (ADH):

   • ADH increases water permeability of the collecting duct by inserting aquaporins (water channels).

   • This allows water reabsorption, concentrating the urine.

   • In the absence of ADH, the collecting duct stays water-impermeable, leading to dilute urine

How are fluid and electrolytes balance controlled?

1. Countercurrent Exchanger Mechanism

• Involves the loop of Henle and vasa recta.

• Maintains a concentration gradient in the medullary interstitium.

• Enables water reabsorption and urine concentration.

2. Antidiuretic Hormone (ADH / Vasopressin)

• Released from the posterior pituitary in response to high plasma osmolality or low blood volume.

• Increases water permeability of the collecting duct by inserting aquaporins.

• Result: More water is reabsorbed, leading to concentrated urine and water retention.

3. Aldosterone

• A mineralocorticoid hormone secreted by the adrenal cortex, stimulated by angiotensin II or low sodium levels.

• Acts on the distal convoluted tubule (DCT) and collecting duct.

• Increases Na⁺ reabsorption (and water follows osmotically) and promotes K⁺ excretion.

• Result: Helps maintain sodium balance, blood volume, and blood pressure.

What are the cell types in the nephron?

1. Proximal Convoluted Tubule (PCT) Cells

• Type: Cuboidal epithelial cells

• Features:

  • Dense microvilli (brush border) → increases surface area for reabsorption

  • Many mitochondria → supports active transport

• Function: Reabsorption of nutrients (glucose, amino acids), Na⁺, water, and secretion of waste products

2. Loop of Henle

• Thin descending limb:

  • Type: Flattened squamous epithelial cells

  • Function: Highly permeable to water, not solutes

• Thick ascending limb:

  • Type: Cuboidal to low columnar cells

  • Function: Actively reabsorbs Na⁺, K⁺, Cl⁻ (impermeable to water)

3. Distal Convoluted Tubule (DCT)

• Type: Cuboidal epithelial cells, fewer microvilli than PCT

• Function: Selective reabsorption of ions (Na⁺, Ca²⁺) and fine-tuning of pH

4. Collecting Duct Cells

• Principal cells:

  • Respond to aldosterone and ADH

  • Involved in Na⁺ reabsorption and water permeability

• Intercalated cells:

  • Involved in acid-base regulation

  • Type A cells secrete H⁺, reabsorb HCO₃⁻ (acidic conditions)

  • Type B cells secrete HCO₃⁻, reabsorb H⁺ (alkaline conditions)

Where is the osmotic gradient created?

Loop of henle