Blood is made up of many different components. For instance, blood is red because of the red blood cells, which are responsible for delivering oxygen to the tissues. There are also white blood cells, which are part of the immune system and serve the function of searching for and killing invading pathogens. These two cell types are suspended in water, which makes up much of the liquid component of blood. Finally, blood contains many different so-called plasma proteins, which serve a variety of functions in the body.
Maintain Osmotic Pressure
As blood passes through the arteries to the tissues, it moves into smaller and smaller vessels. The smallest of these vessels, called capillaries, are so narrow that blood cells pass through in single file. Capillaries are tiny and numerous enough that each cell in the body has one or more nearby, and the liquid component of blood leaks out of each capillary to bathe nearby cells in nutrient-rich fluid. Of course, if liquid continually leaked out of the blood vessels and never re-entered, blood pressure would drop and death would quickly ensue. The job of certain plasma proteins, called albumins, is to draw fluid back into the capillaries for return to the heart. The proteins produce what is called osmotic pressure, according to the textbook "Human Physiology." Osmotic pressure is simply a physical force that acts on liquid, pulling it from around the cells, where there are not plasma proteins exerting a force on it, to the insides of the capillaries where the plasma proteins are located.
Transport Molecules
Some plasma proteins are responsible for transporting certain molecules in the bloodstream. Since blood is water-based, only compounds that can dissolve in water are able to dissolve in, and be transported by, blood. Biochemists Mary Campbell, Ph.D., and Shawn Farrell, Ph.D., explain that while some molecules have their own transporters--oxygen, for instance, can't dissolve in blood but is transported by red blood cells--other molecules must catch rides on passing plasma proteins. The waste molecule carbon dioxide is one such example. It is produced by every metabolically active cell and must reach the lungs for exhalation via the bloodstream, but it's not water-soluble and therefore often uses plasma proteins as a means of transportation.
Immunity
The immune system is complex and is made up of many different components. Plasma proteins called immunoglobulins--also called antibodies in common parlance--are produced by certain white blood cells, according to Dr. Thomas Pollard and William Earnshaw, Ph.D., in their textbook "Cell Biology." The major purpose of immunoglobulins is to mark invading pathogens for destruction so that cells of the immune system can kill and digest them.
Blood Clotting
Fibrinogen is a plasma protein that serves an important role in clotting. If tissue is damaged and bleeding ensues, damaged cells send chemical signals and initiate a cascade of events that culminate in the production of a blood clot or, if the damage is to the skin, a scab. During this reaction cascade, fibrinogen is converted to long, fibrous strands of protein, which are then called fibrin, and these strands form a delicate net that captures cells from the bloodstream to form a patch. As the fibrin and cell patch begins to solidify, it produces a clot or scab, which stops the bleeding.
References
- "Human Physiology"; Lauralee Sherwood, Ph.D.; 2004
- "Biochemistry"; Mary Campbell, Ph.D. and Shawn Farrell, Ph.D.; 2005
- "Cell Biology"; Thomas Pollard, M.D. and William Earnshaw, Ph.D.; 2007
