JAYS QUIZPAD: Kidney Biochemistry and Nephron Functions

Kidney Biochemistry

• The kidney regulates the body's water and electrolyte content.
• It retains substances vital to the body.
• The kidney maintains acid-base balance.
• It excretes waste products, water-soluble toxic substances, and drugs.
• The kidney also has endocrine functions.

Nephron Structure

• A nephron is the functional unit of the kidney.
• It filters blood, reabsorbs useful substances, and secretes waste products.
• The nephron has a glomerulus, proximal tubule, loop of Henle, and distal tubule.

Urine Formation

• Urine is formed through a three-phase process:
  • Glomerular filtration
  • Selective (active) and passive reabsorption
  • Secretion

Glomerular Filtration

• Filtration occurs through the semipermeable walls of glomerular capillaries.
• Hydrostatic pressure from arterial blood drives filtration.
• Approximately 20% of renal plasma flow is filtered each minute (GFR ~ 125 ml/min).
• The result is glomerular filtrate (primary urine) (~ 150 liters).

Reabsorption

• In the proximal and distal tubules, filtrate becomes highly concentrated due to water removal.
• Many low-molecular-weight constituents are reabsorbed actively, including glucose, amino acids (AAs), and ions.

Proximal Tubule

• It reabsorbs 60% of all solutes, 100% of glucose and amino acids, 90% of bicarbonate, and 80-90% of inorganic phosphate and water.

Loop of Henle

• It further concentrates the filtrate:
  • Descending loop: water reabsorption via osmosis (increasing osmolarity, becoming hypertonic).
  • Ascending loop: active transport of NaCl out of the tubule (decreasing osmolarity, becoming hypotonic).

Distal Tubule and Collecting Ducts

• Aldosterone promotes sodium (Na+) ion and water reabsorption.
• Hydrogen (H+) ions are excreted.
• Antidiuretic hormone (ADH) influences urine concentration.

Energy for Transport

• Adenosine triphosphate (ATP) is derived from the oxidative metabolism of glucose, lactate, pyruvate, fatty acids, glycerol, citrate, and amino acids (AAs) absorbed from the blood.

Secretion

• Certain substances, such as urea, creatinine, and drugs, are actively transported into the urine.
• Hydrogen (H+) and potassium (K+) ions are also actively secreted.

Maintenance of Acid-Base Balance

• Tubule cells absorb carbon dioxide (CO2) from the blood.

• CO2 is hydrated to form carbonic acid (H2CO3), which dissociates into bicarbonate (HCO3-) and hydrogen (H+) ions.

• Hydrogen ions are actively pumped into the urine.

• Bicarbonate ions re-enter the bloodstream.

• Glutamine metabolism helps maintain acid-base balance, producing ammonia (NH3).

• Ammonia, along with other substances, helps regulate blood pH and balance.

Endocrine Functions of the Kidney

• Hormones like erythropoietin, calcitriol, and renin-angiotensin system are generated by the kidney.

• Erythropoietin (EPO): A hormone produced mainly by the kidney (and to a lesser extent by the liver) that stimulates red blood cell production in bone marrow. Released in response to low oxygen levels (hypoxia) in the blood.

• Calcitriol: A form of vitamin D produced in the kidney. Critical for calcium homeostasis. Kidney activity is regulated by the hormone parathyrin.

• Renin-Angiotensin System:

  • Renin, an enzyme, converts angiotensinogen to angiotensin I.
  • Angiotensin converting enzyme (ACE) in the lungs converts angiotensin I to angiotensin II.
  • Angiotensin II boosts blood pressure, and stimulates the release of aldosterone.
    • Aldosterone promotes sodium (Na+) and water retention, increasing blood volume.
  • Renin release is triggered by decreased blood pressure or blood volume, or reduced renal perfusion.

This quiz covers the intricate processes involved in kidney biochemistry and the structure and function of nephrons. Topics include urine formation, glomerular filtration, and the kidney's regulatory roles in maintaining homeostasis. Test your knowledge on how kidneys filter blood and form urine.