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:
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.
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.
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.
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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:
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.
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.
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