What is oncotic pressure and hydrostatic pressure?


What is oncotic pressure and hydrostatic pressure?

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Let me start with the easy one:

Hydrostatic pressure: as the word says, hydro-water and static-stationary pressure. This is the pressure exerted by the fluid (liquids and gases) on it’s container walls when the fluid is completely stationary.

This is different than dynamic pressure of a fluid (pressure due to motion if the fluid). Imagine unit Street blowing a balloon, the pressure that you feel when you blood into the balloon is a combination of the static and the dynamic pressure. The dynamic pressure is because if the moving air. When you stop blowing into the balloon, you feel a slight drop in pressure on it’s walls. That’s because there’s no dynamic pressure anymore, the issue that the balloon walls feel is the static pressure of the air inside pushing on it’s walls.

In the case of a blood vessel, the hydrostatic pressure is the pressure exerted by the blood(or the interstitial fluid -fluid between cells) on the blood vessel walls.

Oncotic pressure: this is a type of osmotic pressure. Osmotic pressure is because of the tendency of molecules to want to spread out evenly. If there are too many molecules of the same thing very closely packed inside a volume, they would want to spread out evenly. This is very similar to how you can smell perfume as soon as the bottle is opened even without wind. The perfume is densely packed inside the bottle. But once the lid is opened, it is free to spread into the surroundings. And when the perfume molecules are trying to spread out evenly, they move (or atleast try to move if there was a barrier stopping it). The pressure those molecules exert against that barrier is osmotic pressure. The greater the difference in the concentration of molecules inside and outside the barrier, the greater the pressure.

In the context of blood vessels, the oncotic pressure is the pressure due to the uneven concentration of proteins on either side of the blood vessel. The side with more concentration of proteins (the interstitial fluid – outside the blood vessel) creates a pressure on the blood vessel wall (which is acting as a barrier for the proteins to spread out into the blood to reduce their concentration in the interstitial fluid).

So, conclusively, the pressure due to blood in the walls (outwards) is hydrostatic pressure. The pressure due to proteins on the blood wall (inwards direction) is the oncotic pressure.

The total net force on the blood vessel walls is determined by which of these pressures is greater. Also, if the hydrostatic pressure is greater than oncotic pressure, things should want to move outside the blood vessels, if oncotic pressure is greater, things would want to move into the blood vessel. This is how things are transported into and outside the blood vessel walls.