CV Physiology: Hydrostatic and Oncotic Pressures

Overview of Pressures

• Transcapillary fluid exchange is influenced by:
  • Two hydrostatic pressures
  • Two oncotic pressures

Types of Pressures

• Capillary Hydrostatic Pressure (PC)
• Tissue (Interstitial) Hydrostatic Pressure (Pi)
• Capillary (Plasma) Oncotic Pressure (C)
• Tissue (Interstitial) Oncotic Pressure (i)

Capillary Hydrostatic Pressure (PC)

• Drives fluid out of the capillary (filtration)
• Highest at arteriolar end, lowest at venular end
• Pressure drop along capillary: 15-30 mmHg
• Influenced more by venous pressure (PV) than arterial pressure (PA)
• Increased by precapillary vasodilation, decreased by vasoconstriction
• Equation: Derived from a model with series-coupled pre- and postcapillary resistance
• Typical resistance ratio (post-to-precapillary) is 0.2

Tissue (Interstitial) Hydrostatic Pressure (Pi)

• Determined by interstitial fluid volume and tissue compliance
• Low compliance (brain, kidney) leads to large pressure changes with volume
• High compliance (skin, muscle, lung) allows expansion with small pressure changes
• Interstitial pressure normally near zero

Capillary Plasma Oncotic Pressure (C)

• Determined by plasma proteins, mainly albumin (~70% contribution)
• Typical value: 25-30 mmHg
• Increases along capillary due to filtration
• Affected by protein permeability and reflection coefficient (σ)
• Continuous capillaries have high σ (>0.9); discontinuous/fenestrated have low σ

Tissue (Interstitial) Oncotic Pressure (i)

• Depends on interstitial protein concentration and capillary wall reflection coefficient
• Higher capillary permeability increases interstitial protein concentration
• Interstitial oncotic pressure is about 5 mmHg
• Functions to oppose filtration and promote reabsorption

Key Points

• Reflection coefficient (σ): Measures capillary permeability to proteins
• Hydrostatic Pressure Gradient: Decreases with increased interstitial pressure, reducing filtration
• Oncotic Pressure Influence: Higher plasma oncotic pressure opposes filtration

Revised 11/05/2023

Note: For detailed explanations, refer to the provided links in the source document.