Cardiovascular System and Fluid Exchange

Overview

• Function: Circulation of blood and plasma.
• Plasma: Liquid portion of the blood involved in fluid exchange.
• Fluid Exchange: Occurs at the capillary level.

Capillary Structure

• Endothelial Cells: Line the capillaries, single-cell thickness.
• Pores: Allow movement of water and hydrophilic substances.
• Lipid Solubility: Lipophilic substances diffuse through cell membranes.
• Proteins: Generally do not move easily, albumin is key.

Fluid Dynamics

• Bulk Flow: Movement of water across capillary wall influenced by pressures.
• Starling Forces: Govern fluid movement between intravascular and interstitial spaces.

Starling Forces

1. Capillary Hydrostatic Pressure

   • Exerts pressure on capillary walls.
   • Favors filtration (fluid out of capillary).
   • Higher at arterial end (32 mmHg) than venous end (12 mmHg).

2. Interstitial Hydrostatic Pressure

   • Pressure from interstitial fluid.
   • Typically negligible under normal conditions.
   • Pathological conditions may increase this, favoring absorption.

3. Oncotic Pressure (Osmotic Pressure)

   • Caused by proteins (albumin) in capillaries.
   • Favors absorption (fluid into capillary).
   • Considered constant across capillary.

4. Interstitial Oncotic Pressure

   • Typically negligible due to low protein in interstitial space.
   • Pathological protein presence favors filtration.

Fluid Balance

• Filtration: Fluid leaves capillary at arterial end.
• Absorption: Fluid re-enters capillary at venous end.
• Net Filtration: About 3 liters/day excess fluid moved to interstitial space, picked up by lymphatic system.

Lymphatic System

• Function: Drains excess interstitial fluid back into the circulatory system.
• Pathway: Typically enters through the thoracic duct into left internal jugular vein.

Edema

• Definition: Excess fluid in interstitial space not absorbed.
• Causes:
  • Heart Failure: Increased venous pressure leads to higher capillary hydrostatic pressure, more filtration.
  • Liver/Kidney Failure: Decreased plasma protein leads to reduced oncotic pressure, more filtration.
  • Lymphatic Obstruction: E.g., post-surgical removal of lymph nodes.

Clinical Examples

• Pulmonary Edema: Fluid in lungs, seen in heart failure.
• Pitting Edema: Visible indentations on applying pressure to skin.
• Ascites: Fluid accumulation in peritoneal cavity (common in liver disease).
• Post-Surgical Lymphedema: Due to lymphatic disruption, e.g., post-breast cancer surgery.