Blood plasma is the liquid medium of blood. While it's made mostly of water, it also contains many important dissolved chemicals. The difference between blood plasma and blood is that the plasma doesn't contain any cells--red and white blood cells and platelets are components of whole blood, but not of plasma. As the liquid medium in which the cells are bathed in the body, blood plasma has many important chemical characteristics and properties.
Osmotic Pressure
One important chemical characteristic of blood plasma is that it contains sufficient quantities of dissolved proteins, salts and other biomolecules to maintain appropriate osmotic pressure in the body. This means that the plasma must neither draw water out of the cells, nor must it force water into the cells. In their book "Biochemistry," Drs. Mary Campbell and Shawn Farrell explain that for plasma to maintain appropriate osmotic pressure, it must neither be hypertonic--containing more salts and dissolved molecules--nor hypotonic--containing fewer salts and dissolved molecules--to water.
Acidity
Blood plasma is slightly basic, notes Dr. Lauralee Sherwood in her book, "Human Physiology." Acidity and basicity are measured on what is called the pH scale, where most chemicals fall between pH values of 1 and 14, and where chemicals with a pH of 7 are neutral. Basic chemicals have pH values higher than 7. Blood plasma, with a pH of 7.4, is slightly basic. In addition, blood plasma contains buffers, which are chemicals that help to maintain the pH at a constant level. This is a very important chemical property of plasma, because cells die if the plasma surrounding them becomes excessively acidic or basic.
Water-based
One of the most important properties of blood plasma is that it's water-based. Chemicals either dissolve in water or in fatty or greasy substances, but never in both. Many of the biomolecules that must travel through the body in the blood plasma dissolve well in water, but not in grease. These chemicals include proteins, many hormones, sugar and other nutritional molecules, and salt. If plasma weren't water-based, chemicals that cells need to maintain life couldn't travel to the cells dissolved in the blood plasma. Explains Dr. Gary Thibodeau in his book "Anatomy and Physiology," the fact that both the plasma and the interior of the cells are water-based, while the membrane surrounding the cells is fat-based, allows cells to remain as integral units, and allows them control over which substances pass in or out of the cell.
References
- "Biochemistry"; Mary Campbell, Ph.D. and Shawn Farrell, Ph.D.; 2005
- "Human Physiology"; Lauralee Sherwood, Ph.D.; 2004
- "Anatomy and Physiology"; Gary Thibodeau, Ph.D.; 2007
