Most tissues of the body are fixed in position. Some tissues such as muscles are able to contract to provide a force to perform work; blood is an exception to this because it is constantly in motion to function properly. Blood consists of a liquid component called plasma, which suspends three types of blood cells, each with its specialized functions.
Types
There are three types of blood cells. Erythrocytes, also known as red blood cells, are shaped rather like donuts, except instead of a hole they are indented in the middle. This allows them to be flexible when passing through tiny blood vessels called capillaries. Erythrocytes are filled with a protein called hemoglobin. White blood cells, known also as leukocytes, are cells of the immune system, and there are several subtypes of these as well, with specific functions in fighting off infections. Finally, platelets, or thrombocytes, are the smallest of the three types.
Functions
The red blood cells are designed to help with the transport of oxygen throughout the body. They serve some other functions as well, such as helping to control the acidity of the blood. Their hemoglobin binds oxygen for transport to the tissues. White blood cells have a variety of functions related to immunity. They can sense markers of infection sent out by infected cells and migrate to that part of the body. Once there, they can attack the invading organisms and signal other immune cells. Platelets function to stop bleeding by forming plugs at sites of broken blood vessels.
Development
Blood cells are made in the bone marrow. Stem cells are cells that are not yet commited to becoming one particular type of cell. Instead they can divide to make a number of different types of cells. According to Nature Reviews, there are two types of stem cells in the bone marrow responsible for the development of blood cells. First, lymphoid stem cells can divide to make a special kind of white blood cell called the lymphocyte. Second, myeloid stem cells can divide to make all other types of blood cells: white, red and platelets.
Problems
Problems with blood cells will affect the function of that cell. In the condition known as anemia, the red blood cell counts are too low. Not enough oxygen is delivered to the body and the person will feel fatigued. The human immunodeficiency virus destroys a type of lymphocyte so that the immune system cannot fully respond to invading organisms; the result is full-blown AIDS and potentially lethal infections. Likewise, if platelet counts get too low or platelets do not function correctly, the body is susceptible to abnormal bleeding.
Tests and Studies
A doctor's office or hospital can run various tests on the blood if there is a suspected problem. Two of the most common are the CBC and peripheral smear. A CBC or complete blood count measures the numbers of red blood cells, white blood cells and platelets in a given volume of blood. It also gives the characteristics of the red blood cells, such as the average concentration of hemoglobin. A peripheral smear spreads the cells out on a microscope slide to look for problems in the cells.
References
- "Robbins & Cotran Pathologic Basis of Disease";Vinay Kumar; 8th Ed. 2009
- "Physiology"; Linda S. Costanzo; 4th Ed 2009
- Nature Reviews: Dendritic-cell development from haematopoietic precursors.
- Lab Tests Online: Blood Smear
