Control of plasma osmolality

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Why is the collecting duct important for urine concentration?

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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

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How are fluid and electrolytes balance controlled?

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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.

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1
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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

2
New cards

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.

3
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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)

4
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Where is the osmotic gradient created?

Loop of henle