The Basics: Fluid

Let’s start out with the basics and talk about fluid in the body.  Water makes up 60% of our body weight! 40% of that fluid is intracellular fluid (ICF, or fluid inside the cells of your body) and 20% is extracellular fluid (ECF, or fluid outside of the cells of your body, located in the interstitial space or vascular space).  This water can move freely between the compartments depending on solutes in that compartment.  We’ll talk more about those later!

It’s important for us to go over some concepts pertaining to the movement of fluid between cells.

Osmosis: the diffusion of water through a semipermeable membrane from an area of lower solute concentration to an area of higher concentration, to equalize the two concentrations.  So basically, this means that the water will move from the compartment with the most water to the compartment with the least water until there are equal amounts of water in both.

Osmolarity/osmolality: a measure of solute concentration, refers to a solution outside of the body (IV solution) or within the body.  This is a measure of the amount of electrolytes in the fluid.

Oncotic (osmotic) pressure: the osmotic pressure exerted by colloids (proteins).  This pressure is a force exerted by solutes to hold the water in that compartment.  It will also “pull” water into the compartment.

Hydrostatic pressure: a mechanical type of pressure exerted by a stationary fluid within a closed system.  This pressure “pushes” the fluid into a compartment.

I am going to give you some practice questions with each post.  Keep track of them and I will release the correct answers in my last post.  They won’t be too tricky.

Fluids comprise what percentage of body weight?

What is oncotic pressure?

What is hydrostatic pressure?


Carroll, R. G. (2009). Anatomy and physiology review: Body fluid compartments and cellular function. In Black, J. M., & Hawks, J. H. (Eds). Medical-surgical nursing. Clinical management for positive outcomes, (Vol 1., 8th Ed), (pp. 120-126). St. Louis, MO: Elsevier Inc.

McCarthy, M. & Olsen, K. (2011). Clients with fluid imbalances [PowerPoint Slides].


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