Renal Physiology and Urine Production: Reabsorption and Secretion

Filtration and Reabsorption

• Filtration Rate: Extremely high, the volume of filtrate surpasses total plasma volume in 30 min.
• Nephron Role: Reabsorb 99% of filtrate.
• Proximal Convoluted Tubule: Major site for reabsorption, facilitated by microvilli (brush border).

Solute and Water Reabsorption

• Active Transport: Requires ATP or gradient energy.
• Passive Transport: Moves down the gradient.
• Water Movement: Via osmosis, follows solutes.
• Protein Return: Small proteins return to blood via pinocytosis.

Secretion and Blood pH Maintenance

• Hydrogen Ions: Secreted to maintain pH and prevent acidity.
• Elimination: Ammonium ion, creatinine, potassium eliminated.
• Reabsorption Paths: Transcellular (through cells) and paracellular (between cells).

Sodium Reabsorption

• Active Transport: Sodium-potassium pumps crucial for sodium reabsorption.
• Obligatory Water Reabsorption: Water follows solutes, particularly sodium.
• Facultative Water Reabsorption: Occurs in collecting duct, regulated by antidiuretic hormone (ADH).

Transport Mechanisms

• Primary Active Transport: Directly uses energy from ATP.
• Secondary Active Transport: Uses gradient energy; involves symporters (same direction) and antiporters (opposite direction).
• Transport Maximum: Upper limit of transport rate.

Water Reabsorption

• 90% Reabsorption: Occurs with sodium chloride and glucose.
• ADH Role: Increases water reabsorption when dehydrated.
• Diabetes Mellitus: Can cause glucosuria due to high blood glucose.

Proximal Convoluted Tubule (PCT) Reabsorption

• Sodium Transporters: Facilitate reabsorption of solutes like glucose, amino acids.
• Buffering Agents: Bicarbonate reabsorbed to buffer blood.
• Electrochemical Gradients: Aid reabsorption of Na+, Cl-, K+, and other ions.

Examples of Transport Processes

• Sodium-Glucose Symport: Uses sodium gradient to transport glucose.
• Sodium-Hydrogen Antiport: Regulates hydrogen ion secretion.

Ammonia and Bicarbonate

• Ammonia Secretion: Deamination of amino acids produces ammonia and bicarbonate in PCT.
• Ammonium Ion Formation: Binds with hydrogen for excretion.
• Symporter Role: Facilitates sodium and bicarbonate reabsorption into the bloodstream.