Other physical characteristics of blood include temperature of about 38°C (100.4°F), a pH range of 7.35 to 7.45 (which makes blood slightly alkaline, similar to the pH of ocean water) and a 0.85 to 0.90 percent concentration of salt (NaCl, sodium chloride). Blood constitutes about 8 percent of the total body weight. The blood volume of an average-sized male is between 5 and 6 liters (5 to 6 quarts). The average-sized female has 4 to 5 quarts.

Despite its simple appearance, blood is a complex liquid that performs a number of critical functions:

1. It transports oxygen from the lungs to all cells of the body.
2. It transports carbon dioxide from the cells to the lungs, from where it is then exhaled.
3. It transports nutrients from the digestive organs to the liver and to the cells.
4. It transports waste products from the cells to the kidneys, lungs, and sweat glands.
5. It transports enzymes to various cells.
6. It regulates body pH through buffers and amino acids.
7. It plays a role in the regulation of normal body temperature, because it contains a large volume of water, which is an excellent heat absorber and coolant.
8. It regulates the water content of cells, principally through dissolved sodium ions.
9. It prevents body fluid loss through the clotting mechanism in response to an injury.
10. It protects against toxins and foreign microbes by transporting various components of our immune system. The first line of defense against anything that gets into our body that doesn't naturally belong there is our immune system, and one type of immune cell is called a natural killer cell. Our blood transports the natural killer cells and other cells that make up our immune system.

Microscopically, blood is composed of two portions: plasma, which is a liquid containing dissolved substances; and formed elements, which are cells and cell-like bodies suspended in the plasma.

Source: Principles of Anatomy and Physiology (third edition, p. 442), Tortora & Anagonstakos, Harper & Row, New York, 1981.