Main Difference – Hydrostatic vs Oncotic Pressure
Arteries carry oxygenated blood and nutrients to the metabolizing tissues of the body. This oxygenated blood travels through the capillary network inside the tissues. The exchange of fluids in the blood capillaries is called microcirculation. Hydrostatic and oncotic pressure are the two types of driving forces that are involved in the movement of fluids during microcirculation. The main difference between hydrostatic and oncotic pressure is that hydrostatic pressure is the force that pushes the fluid out of blood capillaries whereas oncotic pressure is the force that pushes the fluid into the blood capillaries. The overall interaction between hydrostatic pressure and oncotic pressure is described by Starling’s principle.
Key Areas Covered
1. What is Hydrostatic Pressure
– Definition, Mechanism, Function
2. What is Oncotic Pressure
– Definition, Mechanism, Function
3. What are the Similarities Between Hydrostatic and Oncotic Pressure
– Outline of Common Features
4. What is the Difference Between Hydrostatic and Oncotic Pressure
– Comparison of Key Differences
Key Terms: Arterial End, Blood Capillaries, Colloid Osmotic Pressure, Fluid, Hydrostatic Pressure, Microcirculation, Oncotic Pressure, Proteins, Venular End
What is Hydrostatic Pressure
Hydrostatic pressure refers to the force exerted by the fluid inside the blood capillaries against the capillary wall. It aids the movement of fluid from the blood capillaries to the interstitial fluid. The highest hydrostatic pressure of the capillaries can be identified near the arteriole end. The lowest hydrostatic pressure occurs at the venular end. Hydrostatic pressure at the blood capillaries is caused by the pumping pressure of the heart. Capillary network is shown in figure 1.
Figure 1: Capillary Network
The net filtration pressure is determined by the hydrostatic pressure inside the blood capillary and the osmotic pressure of the interstitial fluid. If the pressure difference is high, high filtration pressure can be observed. At the arterial end of capillaries, the hydrostatic pressure is 30 mmHg while osmotic pressure is 25 mmHg.
What is Oncotic Pressure
Oncotic pressure refers to the force exerted by albumin and other proteins in the blood vessels. Since it is generated by large molecules, oncotic pressure is also called colloid osmotic pressure. Generally, 20 mmHg pressure is generated by large proteins inside the blood capillaries. Albumin contributes around 75% of the plasma oncotic pressure. Oncotic pressure causes the movement of interstitial fluid into the capillaries at their venular end. Interstitial fluid contains metabolic wastes and carbon dioxide from the tissue cells. Hence, oncotic pressure causes the removal of wastes from the tissues. It also helps to maintain the fluid balance in the body. Capillary exchange is shown in figure 2.
Figure 2: Capillary Exchange
The increase of the protein component in the interstitial fluid drops the oncotic pressure. This reduces the fluid movement into the blood capillaries, causing edema. Edema is caused by the oncotic pressures of less than 11 mmHg. The excess protein in the interstitial fluid is removed by the flow of lymph.
Similarities Between Hydrostatic Pressure and Oncotic Pressure
- Both hydrostatic and oncotic pressure are involved in the movement of fluid in and out of the blood capillaries.
- Both hydrostatic and oncotic pressures are used in microcirculation.
Difference Between Hydrostatic and Oncotic Pressure
Definition
Hydrostatic Pressure: Hydrostatic pressure refers to the force that is exerted by the fluid inside the blood capillaries against the capillary wall.
Oncotic Pressure: Oncotic pressure refers to the force that is exerted by albumin and other proteins in the blood vessels.
Mechanism
Hydrostatic Pressure: Hydrostatic pressure pushes fluids out of the blood capillaries.
Oncotic Pressure: Oncotic pressure pushes fluids into the blood capillaries.
Occurrence
Hydrostatic Pressure: Hydrostatic pressure occurs as a result of the pressure of blood inside the capillaries.
Oncotic Pressure: Oncotic pressure occurs due to proteins such as albumin, globulins, and fibrinogens inside the blood capillaries.
Amount
Hydrostatic Pressure: Hydrostatic pressure is around 30 mmHg.
Oncotic Pressure: Oncotic pressure is around 20 mmHg.
Type
Hydrostatic Pressure: Hydrostatic pressure is a type of fluid pressure.
Oncotic Pressure: Oncotic pressure is a type of colloid pressure.
Occurrence
Hydrostatic Pressure: Hydrostatic pressure occurs at the arterial end of blood capillaries.
Oncotic Pressure: Oncotic pressure occurs at the venular end of blood capillaries.
Role
Hydrostatic Pressure: Hydrostatic pressure increases filtration.
Oncotic Pressure: Oncotic pressure prevents fluid from leaving blood capillaries.
Importance
Hydrostatic Pressure: Hydrostatic pressure aids the supply of nutrients to the tissues of the body.
Oncotic Pressure: Oncotic pressure helps to remove metabolic wastes from the tissues.
Conclusion
Hydrostatic pressure and oncotic pressure are two types of forces involved in the movement of fluid in the blood capillaries. Due to the pumping pressure of the heart, high hydrostatic pressure occurs in the arterial end of the blood capillaries, causing the movement of fluid from blood to the interstitial fluid. At the venular end, large proteins generate a colloid pressure inside the blood capillaries. This causes the movement of fluid into the blood capillaries from the interstitial fluid. The main difference between hydrostatic pressure and oncotic pressure is their mechanism and role.
Reference:
1. “Capillary Hydrostatic Pressure.” TutorVista.Com, Available here.
2. “Osmotic pressure and oncotic pressure.” Deranged Physiology, 27 June 2015, Available here.
Image Courtesy:
1. “Capillaries” By National Cancer Institute, National Institutes of Health – (Public Domain) via Commons Wikimedia
2. “2108 Capillary Exchange” By OpenStax College – Anatomy & Physiology, Connexions Web site, Jun 19, 2013. (CC BY 3.0) via Commons Wikimedia
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