Oncotic Pressure

We all know that every cell in our body needs various things for proper functioning, like food and oxygen. All of these requirements are met with the help of blood, which floods and nourishes every cell. This way, even cells 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. What helps to prevent this is known as oncotic pressure. Given below are details regarding oncotic pressure definition and what happens if this pressure goes down.

What is Oncotic Pressure?
As mentioned earlier, 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. What helps to hold the fluid in the capillaries is the plasma proteins that are present. These plasma proteins 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 that are present in kidneys, or more specifically in the renal glomeruli. In this region, the glomeruli are impermeable to the larger protein molecules and so, these protein molecules tend to accumulate in the capillaries, due to which there is an increase in the oncotic pressure. Albumin, thus, has a significant role to play in maintaining the colloid osmotic pressure.

Tissue Oncotic Pressure
There is also another type of oncotic 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 colloid osmotic 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 osmotic pressure. The usual tissue osmotic pressure is said to be around 5 mm of Hg.

Changes in Oncotic Pressure
Any change in the colloid osmotic pressure leads to problems in the body. Did you know that when there is a decrease in the oncotic 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), then as there are less proteins in the blood to contain the fluid, it escapes from the walls of the capillaries and leads to water retention and edema.

This was all about oncotic pressure, how it is maintained and what happens if there are any changes in this pressure. 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.