What is Oncotic Pressure

Dr. Sumaiya Khan Apr 21, 2019
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Oncotic pressure is the pressure that helps in keeping blood inside the capillaries. Read on to know about its definition, types and what happens if there is a change in this pressure...
We all know that every cell in our body needs various things, like food and oxygen for proper functioning. All of these requirements are met with the help of blood, which floods and nourishes every cell, even those present at the tips of our extremities get nourishment.
However, have you wondered how this blood manages to stay in the capillaries and manages to reach every cell in the body? The normal blood pressure is around 120/80 mm of Hg. With blood flowing at this kind of pressure, most of it should leak out of the capillaries, but it doesn't. This is because of oncotic pressure.

What is Oncotic Pressure?

Oncotic pressure is the pressure that help to ensure that the fluid remains in capillaries and does not leak out. The epithelium lining the capillaries is thin and so, capillaries are highly permeable. Plasma proteins in the capillaries help to hold the fluid in them. These have the capacity to pull water and so, help to maintain it inside the capillaries.
Oncotic pressure is also known as colloid osmotic pressure because it is exerted by proteins. Of the different kinds of proteins, nearly 70% of the pressure is exerted by albumin.
Plasma oncotic pressure is said to be around 25-30 mm of Hg and is denoted by the symbol 'π'. However, this pressure tends to increase along the length of the capillary and is higher in areas that have capillaries whose net filtration rate is higher.
This can be seen in capillaries present in kidneys, or more specifically in the renal glomeruli. Here, the glomeruli are impermeable to larger protein molecules and so, these protein molecules tend to accumulate in the capillaries, leading to an increase in the oncotic pressure. Albumin, thus, has a significant role to play in maintaining this pressure.

Tissue Oncotic Pressure

There is also another type of pressure which is known as tissue oncotic pressure. The colloid osmotic pressure present in tissues and in the capillaries are interconnected. If the capillary wall lining is less permeable, then there will be higher concentration of proteins in the tissues, leading to higher tissue pressure.
On the contrary, if there is higher capillary permeability, then there will be more filtration, leading to a decrease in the tissue proteins, which in turn, decreases the tissue pressure. The usual tissue osmotic pressure is said to be around 5 mm of Hg.

Changes in Pressure

Any change in the colloid osmotic pressure leads to problems in the body. Did you know that when there is a decrease in this pressure, edema sets in? How, you may ask.
Well, when a person suffers from a protein deficiency malnutrition disease like Kwashiorkor or if he is losing proteins in his urine (proteinuria), there are lesser proteins in the blood to contain the fluid. This fluid escapes from the walls of the capillaries and leads to water retention and edema.
Colloid osmotic pressure is essential for maintaining proper blood circulation in the body. So, if there are signs like edema and water retention in the body, then there are chances that the oncotic pressure is low, pointing towards the possibility of deficiency of proteins in the body.
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