Lecture on Proximal Convoluted Tubules

Introduction

• Continuation from a previous video on glomerular filtration.
• Focus on the proximal convoluted tubules (PCT).

Glomerular Filtration Recap

• Filtration of water, electrolytes (sodium, potassium, chloride, calcium, magnesium), nutrients (glucose, amino acids, vitamins), lipids, and small proteins (e.g., insulin, hemoglobin).
• Glomerular filtration membrane filters many substances into Bowman's capsule.

Osmolality

• Defined as the number of particles per kilogram of solvent.
• Blood osmolality is about 300 milliosmoles per liter.
• Filtration into PCT maintains similar osmolality (300 milliosmoles).

Tubular Processes

• Tubular Secretion: Movement of substances from blood into kidney tubules, requires ATP.
• Tubular Reabsorption: Movement from kidney tubules into blood, can be active or passive.

Sodium-Potassium ATPase

• Pumps 3 sodium out of cells, 2 potassium into cells.
• Requires ATP (primary active transport).
• Maintains low sodium concentration inside cells.

Transport Mechanisms

• Sodium-Glucose Co-Transporter: Transports sodium and glucose into cell (secondary active transport).
• Sodium-Amino Acid Co-Transporter: Similar mechanism for amino acids, also lactate.
• 100% reabsorption of glucose, amino acids, and lactate under normal conditions.

Bicarbonate Reabsorption

• Complex process involving carbonic anhydrase.
• Converts CO2 and water into carbonic acid, dissociates into bicarbonate and protons.
• Bicarbonate is reabsorbed into the blood (90% reabsorption).

Water Reabsorption

• Water follows sodium into the blood through osmosis (obligatory water reabsorption).
• 65% reabsorption of both sodium and water in PCT.

Paracellular Transport

• Movement of calcium, magnesium, and potassium between cells into the blood.
• Also includes some chloride reabsorption.

Lipid and Urea Reabsorption

• Lipids and urea cross cell membranes into the blood.

Small Protein Reabsorption

• Insulin and hemoglobin can be reabsorbed through receptor-mediated endocytosis.

Parathyroid Hormone (PTH) Effect

• Inhibits sodium-phosphate co-transport, leading to phosphate excretion.

Secretion of Waste Products

• Ammonium Production: From glutamine, helps in pH regulation.
• Drug Excretion: Organic acids/bases, some drugs (e.g., penicillin, morphine) are secreted actively, requiring ATP.

Conclusion

• Tubular reabsorption and secretion are crucial processes in kidney function.
• Secretion is generally an active process.
• Next topic will cover the loop of Henle.