1.2 Body Fluid Separation
Last time, we talked about different compartments of body fluid. Today let's take a look at how these different compartments of body fluid are separated in our body.
To take a quick review, body fluid was first divided into intracellular fluid (ICF) and extracellular fluid (ECF). ECF was then further divided into interstitial fluid (ISF), plasma and transcellular fluid. Since transcellular fluid is a very small portion that only exists in specific locations, we should focus on ISF and plasma for now.
To get straight on to the point, ICF and ISF are separated by the cell membrane while ISF and plasma are separated by the capillary wall. The diagram below shows the body fluids and the separating barriers.
Diagram 1. Body fluids and the separating barriers
1) Cell membrane
Cell membrane separates ICF from ECF, in specific, the ISF. Cell membrane is found in all cells and surrounds all human cells at its border. Cell membrane separates and protects the cell from the outside environment. In order to do that, cell membrane has a huge role in regulating the entry and exit of molecules and ions to and from the cell.
- Structure: Cell membrane is composed of a phospholipid bilayer with proteins embedded in it.
- Permeability: It shows selective permeability, allowing water and small ions to pass through while blocking large molecules or big ions.
- Transport Mechanism: By passive diffusion, facilitated diffusion, active transport, osmosis and endocytosis/exocytosis.
2) Capillary wall
Capillary wall separates ISF and plasma. Capillaries are the smallest type of blood vessels where blood meets each and every cell. Capillary wall is the wall of the capillary. At the capillary level of the blood vessel, blood provides oxygen and nutrients to the cell and takes carbon dioxide and waste products from the cell. Transport of these molecules and ions occur through the capillary wall.
- Structure: Capillary wall is a very thin layer composed of a single layer of endothelial cell and basal lamina.
- Permeability: Capillary wall is highly permeable as oxygen and nutrients go into the cell while carbon dioxide and waste products exit from the cell. However, it shows selective permeability to large molecules such as proteins.
- Transport Mechanism: By diffusion, osmosis and filtration.
Today we studied that body fluids are separated by cell membrane and capillary wall. Next time, we will take a look at different transport mechanisms that were briefly stated today.
