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Interstitial Fluid: Definition, Pressure & Composition

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  • 0:01 What is Interstitial Fluid?
  • 0:54 Composition
  • 1:47 Movements of Fluids & Solutes
  • 4:00 Affect of Presssure
  • 5:02 Lesson Summary
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Lesson Transcript
Instructor: Nadine James

Nadine has taught nursing for 12 years and has a PhD in Nursing research

In this lesson, you will learn about the fluid compartments of the body, specifically the interstitial fluid compartment. We'll explain the composition and the pressures related to interstitial fluid using examples.

What is Interstitial Fluid?

The human body is mostly made up of fluid. In order to talk about all this fluid, scientists have divided it up into categories. Fluid in our bodies pretty much have to be inside our cells or outside of them. So there are two main fluid compartments (areas) in the body, intracellular and extracellular. The intracellular (IC) compartment contains the fluid that bathes the inside of the cells of the body. The extracellular (EC) compartment is the fluid that lies outside of the cells. The extracellular compartment is further divided into two areas - intravascular (fluid inside the blood vessels) and interstitial (fluid outside the blood vessels). This last type of fluid is the focus of this lesson.

Composition

The interstitial fluid bathes the outside of the blood vessels and makes up about 75% of the total amount of EC fluid (plasma is the other 25%). Age will decrease the amount of fluid the body holds in the two major compartments. As children age to puberty, the fluid levels go down. A young adult has about 15% of his or her total body fluid as interstitial fluid, but the percentages will continue to decline with age.

Interstitial fluid contains water and dissolved solutes and proteins. The solutes are sugar, salts, acids, hormones, neurotransmitters, wastes and electrolytes. An electrolyte is an element or compound that breaks up into ions when dissolved in fluid and are essential to maintaining healthy body functions. The most common electrolytes are sodium, chloride, calcium, and bicarbonate.

Movement of Fluids and Solutes

Do the body's fluids stay in one place in the body or do they move? It is very important that the fluids in the two compartments remain fairly constant. But they do move inside and outside cells. There is a cell membrane that decides what can go into and out of the cells. It is semipermeable, meaning some things can pass freely through the membrane while others cannot.

Think of a window screen; it lets in air and some dust but keeps out leaves and bugs. The screen represents a semipermeable membrane. The cell membrane allows some molecules and fluids in but keeps others out. So how do the two compartments maintain fluid levels? How do solutes come in or out of the cell?

Think of an amusement park. The park only allows so many people on a ride. There is a gate that keeps the right amount of people through to get on the ride. The rest of the people are outside the gate. When the ride is over, new people can come through the gate. When the cell needs a particular molecule or ion in the cell, it is permitted to enter the cell, just like opening the gate to the amusement ride. This cellular process is called facilitated diffusion (which is a type of transportation of molecules or ions in and out of a cell).

Facilitated Diffusion
facilitated diffusion

Facilitated diffusion (FD) does not require energy because the solutes move along the concentration gradient (the process of solutes moving from an area of higher to lower numbers of solutes). But FD does require a helper called a carrier. The carrier is usually a protein that has the permission to cross the semipermeable membrane of the cell. So in our amusement park example, the carrier would be the person who opens the gate to let you get on the ride.

But what about water? How does it move in and out of the cell? The movement of water across a semipermeable cell membrane is called osmosis. When either side of a membrane has more water, it naturally moves across the membrane to the area with less water until both sides area equal. An area with less water means there are more solutes. Water moves to the area with more solutes until both water and solutes are equalized.

Osmosis
Osmosis

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