Medicosis Lecture: Donnan's Equilibrium and Physiological Concepts

Overview

• Continuation of physiology playlist; 24th video.
• Discussion on Donnan's equilibrium (Donnan's effect).
• Explanation in multiple phases for better understanding.

Questions from Previous Video

1. Types of ATP Pumps in the Body

   • Sodium-potassium ATPase: Found in every cell.
   • Calcium ATPase: Found in muscle cells.
   • Hydrogen ATPase: Found in the kidney.
   • Hydrogen-potassium ATPase: Found in the stomach (proton pump) and the kidney.

2. Transport Mechanisms

   • Simple Diffusion: No carrier or ATPase.
   • Primary Active Transport: Sodium-potassium ATPase.
   • Secondary Active Transport: Sodium-calcium exchanger (antiport).
   • Paracellular Route: Simple diffusion.

3. Infusion of Normal Saline

   • To deliver 1 liter to plasma, infuse 4 liters of saline.
   • Plasma is 1/4 and interstitial space is 3/4 of extracellular fluid.
   • Normal saline maintains osmotic balance, preventing fluid shifts to intracellular space.

Donnan's Equilibrium

• Definition: Presence of non-diffusible ions on one side affects distribution of diffusible ions.
• Non-diffusible Ions: Plasma proteins (negatively charged, unable to pass membrane).
• Diffusible Ions: Sodium and chloride, can pass membrane.

Explanation

1. Proteins as Anions: Plasma proteins migrate towards positive electrode (negative charge).
2. Equilibrium Mechanism:
   • Concentration gradient: High to low concentration.
   • Electrical gradient: Positive to negative attraction.
   • At equilibrium: Opposite forces equalize.

Consequences of Donnan's Equilibrium

• Equalities:

  1. Concentration equals electrical gradient for sodium.
  2. Positive equals negative charges in each compartment.
  3. Product of sodium and chloride concentrations is equal in both compartments.

• Inequalities:

  1. Sum of ions is greater in protein compartment than non-protein compartment.
  2. Greater osmotic pressure in protein compartment.
  3. Water moves from non-protein to protein compartment.

Significance

• Water movement supports organ perfusion.
• Without Donnan's equilibrium, organ function would be impaired due to lack of perfusion.

Physiology in One Word

• Physiology: Homeostasis.
• Pathology: Lack of homeostasis.
• Internal Medicine: Hypoperfusion.

Examples of Hypoperfusion

• Heart: Stable/unstable angina, myocardial infarction.
• Brain: TIA, ischemic stroke.
• Kidney: Prerenal azotemia, acute kidney injury.
• Lung: Limited impact due to dual blood supply.
• Other organs: Various ischemic conditions.

Economics Analogy to Donnan's Equilibrium

• Plasma proteins act in self-interest to maintain balance and support organ perfusion.

Final Remarks

• The presence of non-diffusible ions affects ion distribution and water flow essential for perfusion.
• Consideration of both protein and non-protein compartments important for understanding fluid dynamics in physiology.

Homework Question

• Given compartments with specific ions, calculate sodium at equilibrium.

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