What is the Difference Between Oncotic and Hydrostatic Pressure

The key difference between oncotic and hydrostatic pressure is that oncotic pressure is a form of pressure that is exerted by proteins either in blood plasma or interstitial fluid, while hydrostatic pressure is a form of pressure that is exerted by blood plasma and interstitial fluid on the capillary walls.

Capillary dynamics are very important in the microcirculation that occurs in the blood capillaries. According to Starling’s principle, there are important forces modulating capillary dynamics. These forces are oncotic or colloid osmotic pressure and hydrostatic pressure. The net filtration pressure is determined by the sum of these forces. Oncotic pressure pushes fluid into the blood capillaries, whereas hydrostatic pressure pushes fluid out of the blood capillaries. Therefore, both oncotic and hydrostatic pressure determine the fluid flow into and out of the blood capillaries.

CONTENTS

1. Overview and Key Difference
2. What is Oncotic Pressure
3. What is Hydrostatic Pressure
4. Similarities – Oncotic and Hydrostatic Pressure
5. Oncotic vs Hydrostatic Pressure in Tabular Form
6. Summary – Oncotic vs Hydrostatic Pressure

What is Oncotic Pressure?

The oncotic pressure is a form of pressure that is exerted by proteins either in blood plasma or interstitial fluid. It is also the force that pushes fluid into the blood capillaries. Oncotic pressure mainly depends on blood proteins such as albumin. About 75% of the plasma oncotic pressure is due to albumin. It is also known as colloid osmotic pressure. The oncotic pressure exerted by large proteins in the human plasma has a normal value of 26 to 28 mmHg. Normally, when water molecules in plasma are displaced from a blood vessel, it creates a relative water molecular deficit. Thus, water molecules move back into the circulatory system within the lower venous pressure end of capillaries, creating an oncotic pressure.

Furthermore, oncotic pressure has the opposite effect of hydrostatic blood pressure.  Oncotic pressure causes interstitial fluid movement into the blood capillaries. Usually, the interstitial fluid contains metabolic wastes and CO₂. Hence, oncotic pressure helps the removal of metabolic wastes from tissues.

What is Hydrostatic Pressure?

Hydrostatic pressure is a form of pressure exerted by the blood plasma and interstitial fluid on the capillary walls. In fact, it is the force that pushes fluid out of the blood capillaries. This force helps the movement of fluid from blood capillaries to the interstitial fluid. This pressure facilitates filtration. Moreover, hydrostatic pressure is highest at the arteriolar end and lowest at the venular end. Typically, at the arterial end of the capillaries, hydrostatic pressure is 30 mmHg. As blood moves along the capillary, the fluid moves out through capillary pores into the interstitial space. Therefore, this movement causes a decrease in the pressure exerted by the blood when blood moves along the capillary from the arterial to the venous end.

Furthermore, net filtration is determined by the hydrostatic pressure in blood capillaries and the osmotic pressure of the interstitial fluid. High filtration pressure can be observed when there is a high-pressure difference. This will facilitate proper filtration across blood capillaries.

What are the Similarities Between Oncotic and Hydrostatic Pressure?

• Both hydrostatic and oncotic pressure help the movement of fluid in and out of the blood capillaries.
• Both hydrostatic and oncotic pressure are used in the microcirculation of capillaries.
• They are important forces in Starling’s principle involved in capillary dynamics.
• Dysregulation of both can cause diseases.

What is the Difference Between Oncotic and Hydrostatic Pressure?

The oncotic pressure is a form of pressure exerted by proteins either in blood plasma or interstitial fluid, while hydrostatic pressure is a form of pressure exerted by the blood plasma and interstitial fluid on the capillary walls. So, this is the key difference between oncotic and hydrostatic pressure. Furthermore, oncotic pressure is the force that pushes fluid into the blood capillaries, while hydrostatic pressure is the force that pushes fluid out of the blood capillaries.

The below infographic lists the differences between oncotic and hydrostatic pressure in tabular form for side by side comparison.

Summary – Oncotic vs Hydrostatic Pressure

Oncotic and hydrostatic pressure together determine the fluid flow in and out of blood capillaries. Among them, the oncotic pressure is the force that pushes fluid into the blood capillaries, while the hydrostatic pressure is the force that pushes fluid out of the blood capillaries. Thus, this is the key difference between oncotic and hydrostatic pressure.

Reference:

1. “Oncotic Pressure.” An Overview | ScienceDirect Topics.
2. “Hydrostatic Pressure.” An Overview | ScienceDirect Topics.

Image Courtesy:

1. “2108 Capillary Exchange” By OpenStax College – Anatomy & Physiology, Connexions Web site, Jun 19, 2013. (CC BY 3.0) via Commons Wikimedia
2. “Hydrostatic pressure and protein osmotic pressure along the length of a capillary” By David Nascari and Alan Sved – Own work (CC BY-SA 4.0) via Commons Wikimedia