Could somebody please explain hydrostatic and oncotic pressure?

Tharem · Aug 15, 2008

RNRabbit2be75 said:
I'm having a difficult time understanding this... Thanks in advance.

From inside the capillaries, think of hydrostatic pressure as the 'pushing force', pushing the fluid out of the capillaries. It's the result of the actual pressure of the fluid on the capillary walls.

Oncotic pressure, also called 'colloid osmotic pressure', is the 'pulling force', pulling fluids from the surrounding tissue into the capillaries. It's the result of a difference in the concentration of solutes in the fluid inside the capillaries as opposed to outside them, because water will naturally seek a state of balance in the concentration of solute (particles).

As fluid leaves the capillaries as a result of hydrostatic pressure, albumin and other large proteins cannot pass through the capilary walls. This results in a greater concentration of solutes inside the capillaries as opposed to outside of them, and the oncotic pressure rises, pulling more water into the capillaries in order to balance the solute concentration.

Whenever hydrostatic pressure is greater than oncotic pressure, fluid will leave the capillaries, whenever the oncotic pressure is greater than the hydrostatic pressure fluid will enter the capillaries.

Daytonite, BSN, RN · Aug 15, 2008

Hydrostatics is a physics concept and has to do with equilibrium of water and other liquids. In the case of the human circulation you are talking about an equilibrium between water and colloids (the other stuff floating around in the liquid of our bodies).

I am quoting you one paragraph from page 6 of Fluid & Electrolyte Balance: Nursing Considerations, 4th edition, by Norma M. Metheny. You may need to read this paragraph several times and think about it before the light bulb finally turns on.

Quote
"Normal movement of fluids through the capillary wall into the tissues depends on two forces:
hydrostatic pressure
(exerted by pumping of the heart) and
oncotic pressure
(exerted by nondiffusible plasma proteins, primarily albumin). At the arterial end of the capillary, fluids are filtered through its wall by a hydrostatic pressure that exceeds the oncotic pressure exerted by plasma proteins. In contrast, because oncotic pressure is greater than hydrostatic pressure in the venous end of the capillary, fluids re-enter the capillary here."

The technical definitions of the two terms are:

Hydrostatic pressure - the pressure exerted by a fluid within a closed system [as in the circulatory system of the body which includes the capillaries]. Osmotic pressure is the hydrostatic pressure required to stop the flow of water.

Oncotic pressure - the pressure exerted by the small, nondiffusible particles suspended in a solution or fluid [the composition of the fluids found throughout the human body]. These particles (colloids) are unable to pass through a semipermeable membrane themselves.

On one hand you have the pumping action of the heart exerting hydrostatic pressure which results in the sucking and pulling of water out of the tissues as blood is being pumped through the arteries. But at some point in the circulation (arteries-capillaries-veins) the blood looses some of that pressure as it gets farther and farther away from the heart and the colloids in the surrounding tissues (that's the oncotic pressure) begins to have more control of the movement of water out of the blood and the tissues. In the end, it's all a balanced system.

In pathological conditions this can get way out of hand as in congestive heart failure.

bsugar888 · Aug 15, 2008

Thanks. I also understand better now!