The blood is a complex chemical soup made up of water-based plasma, several types of cells and a wide variety of molecules. It serves many purposes in the body, from transporting oxygen and nutrients to cells to allowing cell-to-cell communication through the use of hormone molecules. For a multicellular organism, blood is essential, as it’s the only way to connect cells far from the lungs to a source of oxygen, and without it, cells couldn’t communicate or coordinate their activities.
Function
Blood cells and molecules serve several important functions, explains Dr. Gary Thibodeau in his book "Anatomy and Physiology." As a combined result of its constituent parts, blood is both a provider of nutrients and a waste management system for the body, carrying oxygen and sugar to the cells, while removing metabolic waste products. It’s also an important part of the immune system because some of the cells and molecules in blood seek out and kill invading pathogens. Cells can communicate with distant body cells by secreting molecules into the blood, and as the blood circulates, the molecules arrive at and deliver messages to their targets.
Types
Cells and molecules in the blood are incredibly varied. Red blood cells are simple, composed of little more than a membrane and hemoglobin, the oxygen-transporting protein, notes Thibodeau. White cells, on the other hand, include large pathogen-destroying T-cells and antibody-producing B-cells. Some molecules of the bloodstream, such as oxygen and carbon dioxide, are very small. Others, such as the hormone insulin, are very large. Still other molecules, such as the blood protein albumin, are so large that they almost resemble cells.
Cell Features
Blood cells are, by and large, independent of one another physically. While organs and other body tissues are made up of large networks of connected cells held together by various biological bonds, blood cells must be able to separate from one another to move, single-file, through the very narrow capillary vessels. Explains Dr. Lauralee Sherwood in her book "Human Physiology," it's not that blood cells don't work together, it's just that they do so purely through blood-borne communication rather than through physical connection.
Molecule Features
The molecules of the bloodstream are also an incredibly varied group. The tiny molecules carbon dioxide and oxygen are not water soluble, so they mix poorly with blood. Oxygen is carried through the bloodstream by red blood cells, while carbon dioxide undergoes a chemical reaction to make it water soluble, explains biochemists Mary Campbell and Shawn Farrell. Other blood molecules, however, dissolve quite well in the blood. Glucose, for instance, which is also called blood sugar, is both ubiquitous and quite soluble in the liquid of blood, called plasma.
Expert Insight
While most people know that the blood carries oxygen to, and carbon dioxide from, the body cells, few realize what an important communication medium blood is. Frequently, physically distant organs must communicate with each other, and blood is the only connection between them. The anterior pituitary gland, for instance, which is a part of the brain, communicates with the gonads of both males and females by secreting hormones into the bloodstream. These hormone molecules, notes Sherwood, travel throughout the body, but are only sensed by the relevant gonad cells, which respond by secreting hormones or initiating actions of their own.
References
- “Anatomy and Physiology”; Gary Thibodeau; 2007
- “Human Physiology”; Lauralee Sherwood; 2004
- “Biochemistry”; Mary Campbell and Shawn Farrell; 2005
