Urine Formation and Kidney Function

Urine Formation

Three Main Activities in the Nephron:

• Filtration: Creates filtrate.
• Selective Reabsorption: Reabsorbs necessary substances.
• Tubular Secretion: Secretes additional waste products.

1. Filtration

• Location: Occurs between the glomerulus and Bowman's capsule.
• Process:
  • Nonselective, passive process.
  • Water and solutes smaller than proteins are forced through capillary walls.
  • Plasma (without proteins) is pushed through.
  • Blood cells and proteins remain in the capillaries due to their size.
• Filtrate Collection: Collected by the Bowman's capsule and sent to the proximal convoluted tubule.
• Concentration: Filtrate in the Bowman's capsule has the same concentration as the plasma in the blood.
• Filtrate is essentially plasma minus the formed elements (cells, large proteins).

2. Selective Reabsorption

• Location: Proximal convoluted tubule.
• Process:
  • Reabsorbs materials from the filtrate back into the peritubular capillaries (back to the blood).
  • Approximately 65% of water is reabsorbed.
  • All nutrients (glucose, amino acids) are reabsorbed.
• Glucose in Urine:
  • The presence of glucose in urine can be a sign of diabetes.
• Reabsorption Mechanisms:
  • Some reabsorption is passive, but most is active.
  • Cells in the proximal convoluted tubule have many mitochondria to power active transport.
• **Non-Reabsorbed Materials (Nitrogenous Wastes): ** * These remain in the filtrate and become part of the urine.
  • Urea (from deamination of proteins in the liver).
  • Uric acid (from nucleic acids).
  • Creatinine.
  • Ammonia.

3. Tubular Secretion

• Location: Distal convoluted tubule.
• Process:
  • Reabsorption in reverse; materials move from the peritubular capillaries into the tubules via active transport.
  • Hydrogen ions: Balance pH levels.
  • Water: Control hydration or dehydration (hormone-controlled).
  • Sodium ions: Control blood pressure (hormone-controlled).
  • Excess substances (vitamins, hormones, antibiotics like penicillin) are secreted.
• Difference Between PCT & DCT:
  • Proximal Convoluted Tubule (PCT): Primarily sends materials back to the blood.
  • Distal Convoluted Tubule (DCT): Primarily takes materials from the blood into the filtrate.

Water Content in Urine

• Water movement: Occurs in all areas of the nephron (except one impermeable area in the loop of Henle).
• Hydration Status:
  • Hydrated: More water moves into the tubules, resulting in dilute urine (lower concentration of solutes).
  • Dehydrated: Less water moves into the tubules, resulting in concentrated urine (higher concentration of solutes).
• **Water Movement Process (Three Steps): ** 1. Sodium Ions: Actively pumped in the direction required.
  2. Chlorine Ions: Follow due to charge attraction.
  3. Water: Moves by osmosis due to the hypertonic environment created by ion movement.

Comparison of Plasma to Urine

Nutrients

• Glucose:
  • Plasma: Present.
  • Urine: Should be absent (0).
• Amino Acids:
  • Plasma: Present.
  • Urine: Should be absent (0).
• Lipids:
  • Plasma: Present.
  • Urine: Should be absent (0).
• Proteins:
  • Plasma: Present.
  • Urine: Should be absent (0).
• Almost all nutrients are returned to the blood via active transport

Nitrogenous Wastes

• Nitrogenous wastes are concentrated in the urine for efficient removal.

Electrolytes

• Urine contains some salts, but plasma is not stripped of electrolytes like it is with nitrogenous wastes.
• More electrolytes are lost through sweat than through urine.