From the Guidelines
Hydrostatic pressure and oncotic pressure are two distinct forces that regulate fluid movement across capillary membranes, with hydrostatic pressure pushing fluid out of blood vessels and oncotic pressure pulling fluid back into the bloodstream. The key difference between these two pressures lies in their direction and source: hydrostatic pressure is the physical pressure exerted by fluid against vessel walls, primarily driven by cardiac output and vascular resistance, whereas oncotic pressure is a pulling force created by plasma proteins, mainly albumin, that helps retain fluid within the blood vessels 1.
Key Characteristics of Hydrostatic and Oncotic Pressure
- Hydrostatic pressure is highest at the arterial end of capillaries and decreases along the capillary length, with values approximately ranging from 35 mmHg at the arterial end to lower values at the venous end.
- Oncotic pressure, approximately 25 mmHg, remains relatively constant throughout the capillary and is crucial for maintaining fluid balance within the vascular space.
- Conditions that alter either hydrostatic or oncotic pressure can lead to fluid imbalances, such as edema, which is seen in heart failure due to increased hydrostatic pressure or in liver failure due to decreased oncotic pressure from reduced albumin production 1.
Clinical Implications
Understanding the difference between hydrostatic and oncotic pressure is vital for managing conditions involving fluid imbalances. For instance, in heart failure with preserved ejection fraction, determining the sources of edema, whether from increased hydrostatic pressure or decreased oncotic pressure, guides the treatment approach, as outlined in recent expert consensus decision pathways 1. This distinction also informs the appropriate use of intravenous fluids and diuretics in clinical practice, aiming to restore the balance between these pressures and alleviate symptoms such as dyspnea and edema.
From the Research
Definition of Hydrostatic and Oncotic Pressure
- Hydrostatic pressure refers to the force exerted by a fluid at equilibrium at any point of the fluid due to the force of gravity 2, 3, 4, 5, 6
- Oncotic pressure, also known as colloid osmotic pressure, is the pressure exerted by proteins (such as albumin) in a fluid, which helps to retain fluid in the blood vessels 2, 3, 4, 5, 6
Difference between Hydrostatic and Oncotic Pressure
- The main difference between hydrostatic and oncotic pressure is the direction of fluid movement they promote: hydrostatic pressure pushes fluid out of the blood vessels, while oncotic pressure pulls fluid into the blood vessels 2, 3, 4, 5, 6
- Hydrostatic pressure is influenced by the weight of the fluid and the force of gravity, whereas oncotic pressure is influenced by the concentration of proteins in the fluid 4, 5, 6
Role of Hydrostatic and Oncotic Pressure in Fluid Movement
- The balance between hydrostatic and oncotic pressure determines the movement of fluid across the microvascular wall: when hydrostatic pressure is higher than oncotic pressure, fluid moves out of the blood vessels, and when oncotic pressure is higher, fluid moves into the blood vessels 2, 3, 4, 5, 6
- Changes in hydrostatic and oncotic pressure can affect fluid absorption and reabsorption in various parts of the body, such as the kidneys and lungs 3, 6