4. Urine Formation

• What are the three processes involved in urine formation?

  Glomerular filtration, tubular reabsorption, secretion
• How is the urinary excretion rate mathematically expressed?

  Excretion rate = Filtration rate - Reabsorption rate + Secretion rate
• What is the initial fluid filtered from the glomerular capillaries into Bowman’s capsule like?

  It is virtually free of protein
• What happens to most substances in plasma during glomerular filtration?

  They are freely filtered except for proteins
• How does filtered fluid change as it passes through the tubules?

  It is modified by reabsorption and secretion
• What is the renal handling of substances in urine formation?

  • Panel A: Freely filtered, not reabsorbed or secreted
  • Panel B: Freely filtered, partly reabsorbed
  • Panel C: Freely filtered, fully reabsorbed
  • Panel D: Freely filtered, not reabsorbed, secreted
• What is the excretion rate for a substance that is freely filtered but not reabsorbed or secreted?

  It is equal to the filtration rate
• What happens to the excretion rate of a substance that is freely filtered and partly reabsorbed?

  It is less than the filtration rate
• What is the excretion rate for substances that are fully reabsorbed?

  It is zero
• What is the excretion rate for substances that are freely filtered and secreted?

  It is calculated as filtration rate plus secretion rate
• How do the rates of filtration, reabsorption, and secretion affect urine excretion?

  Their combination determines the excretion rate
• What types of substances are typically handled by the kidneys?

  • Waste products (e.g., creatinine)
  • Electrolytes (e.g., sodium, chloride)
  • Nutritional substances (e.g., amino acids, glucose)
  • Organic acids and bases
• What are the three basic renal processes?

  Filtration, reabsorption, and secretion
• Why is tubular reabsorption more important than tubular secretion?

  It is quantitatively more important in urine formation
• What role does secretion play in renal function?

  It determines amounts of potassium and hydrogen ions
• Which substances are poorly reabsorbed and excreted in large amounts?

  Urea, creatinine, uric acid, and urates
• What happens to foreign substances and drugs in the kidneys?

  They are poorly reabsorbed and secreted into tubules
• Which electrolytes are highly reabsorbed by the kidneys?

  Sodium ions, chloride ions, and bicarbonate ions
• What happens to amino acids and glucose in the kidneys?

  They are completely reabsorbed and not excreted
• How is the rate of sodium filtration affected by excess sodium in the body?

  It usually increases with less reabsorption
• How do changes in filtration and reabsorption affect renal excretion?

  Even slight changes can lead to large changes
• What would happen to urine volume with a 10% increase in GFR?

  It would raise urine volume 13-fold
• Why is a high GFR advantageous for the kidneys?

  It allows rapid removal of waste products
• How often can the entire plasma volume be filtered by the kidneys?

  About 60 times each day
• What is the approximate GFR in liters per day?

  About 180 L/day
• What is the total plasma volume in liters?

  About 3 liters
• How does a high GFR help control body fluids?

  It allows precise control of volume and composition