Tubular Reabsorption in Urine Formation
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Questions and Answers

What happens to ADH secretion by the posterior pituitary when the body is overhydrated?

  • It is inhibited, causing the collecting ducts to become more permeable to sodium.
  • It increases, making the collecting ducts more permeable to water.
  • It remains unchanged, having no effect on the collecting ducts.
  • It decreases, making the collecting ducts relatively impermeable to water. (correct)

    • What is the primary role of aldosterone in the body?

  • To stimulate the release of ADH
  • To reduce blood sodium levels
  • To inhibit the reabsorption of sodium in the distal portion of the DCT
  • To increase blood volume and blood pressure (correct)

    • What is the effect of ANP on blood sodium levels?

  • It has no effect on blood sodium levels.
  • It increases blood sodium levels, thereby increasing blood volume and blood pressure.
  • It reduces blood sodium levels, thereby decreasing blood volume and blood pressure. (correct)
  • It increases blood sodium levels, but only in response to high extracellular potassium concentration.

    • What happens to the collecting ducts when ADH secretion is decreased?

    <p>They become more impermeable to water.</p> Signup and view all the answers

    What stimulates the release of aldosterone into the blood?

    <p>Decreased blood volume or blood pressure, or high extracellular potassium concentration.</p> Signup and view all the answers

    What is the relationship between sodium and water in the body?

    <p>Water follows sodium if aquaporins are present.</p> Signup and view all the answers

    What is the primary mechanism of sodium reabsorption in the renal tubules?

    <p>Primary active transport</p> Signup and view all the answers

    What is the purpose of sodium reabsorption in the renal tubules?

    <p>To provide energy for the reabsorption of other substances</p> Signup and view all the answers

    What is the transport maximum (Tm) of a substance?

    <p>The maximum amount of the substance that can be reabsorbed per unit time</p> Signup and view all the answers

    What happens when the transport maximum of a substance is exceeded?

    <p>The substance is excreted in the urine at a higher rate</p> Signup and view all the answers

    What is the main difference between the reabsorptive capabilities of the Proximal Convoluted Tubule (PCT) and other segments of the renal tubules?

    <p>The PCT is the most active 'reabsorber' of all substances</p> Signup and view all the answers

    What is the primary function of the transport proteins in the renal tubules?

    <p>To ferry particular substances across the membrane</p> Signup and view all the answers

    Why are there plenty of transporters for substances like glucose?

    <p>Because they need to be retained by the body</p> Signup and view all the answers

    What is the consequence of uncontrolled diabetes mellitus on the renal tubules?

    <p>Increased loss of glucose in the urine</p> Signup and view all the answers

    What percentage of sodium and water is reabsorbed by the PCT?

    <p>65%</p> Signup and view all the answers

    What happens to solutes in the descending limb of the nephron loop?

    <p>They are not reabsorbed at all</p> Signup and view all the answers

    Where is most of the glucose and amino acids reabsorbed?

    <p>Proximal convoluted tubule</p> Signup and view all the answers

    What is the function of anti-diuretic hormone (ADH)?

    <p>Inhibit diuresis</p> Signup and view all the answers

    What percentage of the originally filtered NaCl is reabsorbed in the DCT and collecting duct?

    <p>25%</p> Signup and view all the answers

    What is the function of aquaporins in the collecting duct?

    <p>Facilitate water reabsorption</p> Signup and view all the answers

    What percentage of uric acid is reabsorbed in the proximal tubule?

    <p>Nearly all</p> Signup and view all the answers

    What is the name of the part of the nephron where permeability changes dramatically?

    <p>Nephron loop</p> Signup and view all the answers

    What is the primary function of tubular secretion in the kidney?

    <p>To eliminate undesirable substances and maintain blood pH</p> Signup and view all the answers

    What is the effect of elevated blood volume or blood pressure on ANP release?

    <p>ANP release is stimulated</p> Signup and view all the answers

    Which of the following substances is NOT primarily reabsorbed at the DCT?

    <p>Glucose</p> Signup and view all the answers

    What is the primary site of excretion for most substances?

    <p>Proximal convoluted tubule (PCT)</p> Signup and view all the answers

    What is the effect of parathyroid hormone (PTH) on calcium reabsorption?

    <p>PTH increases calcium reabsorption</p> Signup and view all the answers

    Which of the following is NOT a function of tubular secretion?

    <p>Reabsorbing essential nutrients</p> Signup and view all the answers

    What is the effect of decreased blood pH on renal tubule cells?

    <p>Renal tubule cells increase bicarbonate secretion</p> Signup and view all the answers

    What is the primary function of atrial natriuretic peptide (ANP) in the kidney?

    <p>Regulating blood pressure through sodium reabsorption</p> Signup and view all the answers

    What is the primary function of the countercurrent multiplier in the kidney?

    <p>To generate a high osmotic gradient in the medulla</p> Signup and view all the answers

    What is the result of the countercurrent exchanger in the vasa recta?

    <p>Preservation of the medullary osmotic gradient</p> Signup and view all the answers

    What is the effect of increased solute concentration in the ascending limb of the nephron loop?

    <p>Increased solute concentration in the descending limb</p> Signup and view all the answers

    What is the purpose of the medullary osmotic gradient?

    <p>To vary urine concentration</p> Signup and view all the answers

    What is the relationship between the countercurrent multiplier and the medullary osmotic gradient?

    <p>The countercurrent multiplier increases the medullary osmotic gradient</p> Signup and view all the answers

    What is the result of the positive feedback cycle in the countercurrent multiplier?

    <p>Increased solute concentration in the descending limb</p> Signup and view all the answers

    What is the purpose of the vasa recta in the kidney?

    <p>To preserve the medullary osmotic gradient</p> Signup and view all the answers

    What is the effect of increased water reabsorption from the descending limb of the nephron loop?

    <p>Increased solute concentration in the descending limb</p> Signup and view all the answers

    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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    Description

    Learn about the process of tubular reabsorption in urine formation, including the reabsorption of sodium, nutrients, water, and ions, and how it is regulated by hormonal signals.

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