Questions and Answers
What happens to ADH secretion by the posterior pituitary when the body is overhydrated?
It decreases, making the collecting ducts relatively impermeable to water.
What is the primary role of aldosterone in the body?
To increase blood volume and blood pressure
What is the effect of ANP on blood sodium levels?
It reduces blood sodium levels, thereby decreasing blood volume and blood pressure.
What happens to the collecting ducts when ADH secretion is decreased?
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What stimulates the release of aldosterone into the blood?
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What is the relationship between sodium and water in the body?
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What is the primary mechanism of sodium reabsorption in the renal tubules?
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What is the purpose of sodium reabsorption in the renal tubules?
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What is the transport maximum (Tm) of a substance?
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What happens when the transport maximum of a substance is exceeded?
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What is the main difference between the reabsorptive capabilities of the Proximal Convoluted Tubule (PCT) and other segments of the renal tubules?
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What is the primary function of the transport proteins in the renal tubules?
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Why are there plenty of transporters for substances like glucose?
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What is the consequence of uncontrolled diabetes mellitus on the renal tubules?
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What percentage of sodium and water is reabsorbed by the PCT?
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What happens to solutes in the descending limb of the nephron loop?
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Where is most of the glucose and amino acids reabsorbed?
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What is the function of anti-diuretic hormone (ADH)?
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What percentage of the originally filtered NaCl is reabsorbed in the DCT and collecting duct?
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What is the function of aquaporins in the collecting duct?
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What percentage of uric acid is reabsorbed in the proximal tubule?
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What is the name of the part of the nephron where permeability changes dramatically?
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What is the primary function of tubular secretion in the kidney?
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What is the effect of elevated blood volume or blood pressure on ANP release?
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Which of the following substances is NOT primarily reabsorbed at the DCT?
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What is the primary site of excretion for most substances?
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What is the effect of parathyroid hormone (PTH) on calcium reabsorption?
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Which of the following is NOT a function of tubular secretion?
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What is the effect of decreased blood pH on renal tubule cells?
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What is the primary function of atrial natriuretic peptide (ANP) in the kidney?
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What is the primary function of the countercurrent multiplier in the kidney?
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What is the result of the countercurrent exchanger in the vasa recta?
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What is the effect of increased solute concentration in the ascending limb of the nephron loop?
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What is the purpose of the medullary osmotic gradient?
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What is the relationship between the countercurrent multiplier and the medullary osmotic gradient?
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What is the result of the positive feedback cycle in the countercurrent multiplier?
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What is the purpose of the vasa recta in the kidney?
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What is the effect of increased water reabsorption from the descending limb of the nephron loop?
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Study Notes
Urine Formation - Tubular Reabsorption
- Sodium reabsorption is almost always active and via the transcellular route
- Reabsorption of sodium provides energy and means for reabsorbing almost every other substance, including water and ions
- There is a transport maximum (Tm) for nearly every substance that is reabsorbed using a transport protein in the membrane
Transport Maximum
- The Tm reflects the number of transport proteins in the renal tubules available to ferry a particular substance
- When transporters are saturated, the excess is excreted in urine
- This occurs in individuals who become hyperglycaemic due to uncontrolled diabetes mellitus
Reabsorptive Capabilities of the Renal Tubules and Collecting Ducts
- Proximal Convoluted Tubule (PCT):
- Most active "reabsorbers"
- Reabsorbs all of the glucose and amino acids in the filtrate and 65% of the sodium and water
- Reabsorbs nearly all of the uric acid and about half of the urea, but both are later secreted back into the filtrate
- Nephron Loop:
- Permeability of the tubule epithelium changes dramatically
- Water can leave the descending limb of the nephron loop but not the ascending limb
- Solutes are reabsorbed both actively and passively in the ascending limb
- Distal Convoluted Tubule (DCT) and Collecting Duct:
- Reabsorption is fine-tuned by hormones
- Only a small amount of filtered load is subject to this fine tuning
- Roles of hormones:
- Anti-Diuretic Hormone (ADH): regulates water reabsorption in the collecting ducts
- Aldosterone: fine-tunes reabsorption of remaining sodium
- Atrial Natriuretic Peptide (ANP): reduces blood sodium content
- Parathyroid Hormone (PTH): increases reabsorption of calcium
Urine Formation - Tubular Secretion
- Tubular secretion moves selected substances from the peritubular capillaries through the tubule cells into the filtrate
- The urine eventually excreted contains both filtered and secreted substances
- Important for disposing of substances that are tightly bound to plasma proteins, eliminating undesirable substances, and controlling blood pH
Regulation of Urine Concentration and Volume
- The countercurrent multiplier depends on actively transporting solutes out of the ascending limb
- The vasa recta acts as a countercurrent exchanger, preserving the medullary gradient
- Countercurrent exchange does not create the medullary gradient but preserves it
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