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Renal Physiology: Tubular Handling of Sodium

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Terms in this set (20)

  • Functions of the nephron
    Filtration (glomerular filtration), Reabsorption (tubular function returning substances to blood), and Secretion (tubular function removing unwanted substances).
  • What is tubular reabsorption?
    The process of moving substances from the tubular fluid back into the blood via peritubular capillaries.
  • Active reabsorption
    Requires energy (ATP) to move solutes against electrochemical gradients, e.g., sodium-potassium ATPase pump and secondary active transport like glucose reabsorption.
  • Passive reabsorption
    Occurs by osmotic differences, concentration gradients, electrical gradients, or diffusion without energy use.
  • Role of peritubular capillaries in reabsorption
    They receive reabsorbed substances from the tubular epithelium and return them to the blood circulation.
  • Transport maximum (Tm)
    The maximum rate at which a substance can be actively reabsorbed or secreted due to carrier protein saturation.
  • What happens when filtered solute exceeds Tm?
    Carrier proteins become saturated, excess solute is not reabsorbed and is excreted in urine.
  • Is there a transport maximum for sodium?
    No, because aldosterone can increase the number of sodium carriers in distal tubules and collecting ducts.
  • Glucose transport maximum
    Approximately 375 mg/min; above this, glucose appears in urine due to saturation of reabsorption capacity.
  • Filtered load formula
    Filtered load = GFR x plasma concentration of the substance.
  • Renal threshold for glucose
    The plasma glucose concentration (~200 mg/100 ml) at which glucose begins to appear in urine before Tm is reached.
  • Effect of increased GFR or plasma glucose on glucose reabsorption
    Increases filtered load, potentially exceeding Tm and causing glucosuria.
  • How does uncontrolled diabetes mellitus affect glucose handling?
    Elevated blood glucose increases filtered load beyond Tm, leading to glucose in urine.
  • Primary active transport example in renal tubules
    Sodium-potassium ATPase pump actively transports sodium out of tubular cells.
  • Secondary active transport example
    Glucose reabsorption coupled with sodium transport using the sodium gradient.
  • Factors influencing passive reabsorption
    Osmotic gradients, electrical gradients, and time available for reabsorption (affected by GFR).
  • Role of aldosterone in sodium reabsorption
    Inserts more sodium carrier proteins in distal tubules and collecting ducts to increase sodium reabsorption.
  • What is tubular secretion?
    The process by which unwanted substances are actively secreted from blood into tubular fluid.
  • Relationship between GFR and tubular flow rate
    Decreased GFR reduces tubular flow rate, allowing more time for reabsorption.
  • Why is there no Tm for sodium?
    Because aldosterone can regulate and increase sodium carrier proteins dynamically.