Intracellular Fluid: Definition & Composition

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  • 0:01 What is Intracellular Fluid?
  • 0:37 Composition of…
  • 1:00 Osmosis & Intracellular Fluid
  • 2:12 Electrolytes in…
  • 3:06 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 find the definition of intracellular fluid. In addition, the composition of intracellular fluid and how water moves in and out of the cell will be provided. Finally, the relationship of electrolytes and intracellular fluid will be explained.

What is Intracellular Fluid?

Human cells are bathed in fluids both inside the cell and out. In fact, the water that is inside the cell makes up about 42% of the total body weight. The fluid inside the cell is called intracellular fluid (ICF). All the fluid outside of the cell is called extracellular fluid (ECF) and is separated from the intracellular fluid by a semipermeable membrane that surrounds the cell. This membrane allows fluid to flow in and out, but prevents unwanted molecules or materials from getting in.

Composition of Intracellular Fluid

Intracellular fluid contains water and dissolved solutes and proteins. The solutes are electrolytes, which help keep our body functioning properly. An electrolyte is an element or compound that, when dissolved in fluid, breaks up into ions. There are a lot of electrolytes inside the cell, but potassium, magnesium, and phosphate have the greatest concentrations.

Osmosis and Intracellular Fluid

When water moves inside and outside the cell, the process is called osmosis. Osmotic pressure is the force that pulls fluid from one compartment to another. The level of osmotic pressure remains approximately equal between the ICF and the ECF compartments.

Osmotic pressure can also be defined as the attraction of water to solutes/electrolytes. If there is a decrease of water in the cell, electrolytes will move into the cell to make the water move back into the cell. The opposite is also true - an increase in water in the cell will cause electrolytes to move out of the cell causing water to leave the cell.

Have you ever eaten pizza and become extremely thirsty afterward? What's happening is the sodium from the pizza is accumulating in your ECF causing water to exit the cells and dilute it. The cell sensors signal the brain that the cells are dehydrating and you get the signal to drink fluid.

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Then again, the opposite holds true. If you have too much water in the body the brain is also signaled by the same cell sensors. The brain then signals the kidneys to get rid of fluid by producing urine. Preferably when you're not asleep.

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