The main function of red blood cells, or erythrocytes, is the transport of oxygen from the lungs to body tissues. Erythrocytes are tiny disc-shaped structures that are hollowed out on either side. Their small size allows them to squeeze through microscopic blood vessels called capillaries. They number about 5 million per cubic millimeter of blood; in the entire human body, there are about 25 trillion red blood cells.
Red blood cells are formed in the red bone marrow of certain bones, where they produce a substance called hemoglobin. Hemoglobin is a protein pigment that contains iron and that gives red blood cells their color. The hemoglobin in red blood cells combines with oxygen in the lungs, transporting that oxygen to the tissues throughout the body. It also carries carbon dioxide from the tissues back to the lungs, where some of the carbon dioxide is exhaled. Each red blood cell lives only about four months. New red blood cells are constantly being produced in the bone marrow to take the place of old ones.
A single hemoglobin molecule is made of four identical sub-units. Each sub-unit has a heme component, aglobin chain and an iron atom bound to the heme section. Red blood cells are completely lacking in most other common cellular parts, such as a nucleus with DNA, or mitochondria.
Oxygen is able to bind to each of the iron atoms, meaning that a single hemoglobin molecule is able to carry up to four oxygen molecules at its maximum capacity. Interestingly, the structure of hemoglobin makes it such that the more oxygen that is bound to one of the sub-units, the more other oxygen molecules are attracted to the remaining iron atoms. This effect is important to the proper functioning of a red blood cell in oxygen transport.
The ability of oxygen to bind to hemoglobin is effected by many factors. The acidity of the blood (pH) is a primary factor, as is the temperature. Fetal blood has a different ability to bind oxygen (it holds on to the oxygen more tightly). Other chemicals such as hydrogen sulfide, carbon monoxide, hydrogen sulfide and 2,3bisphosphoglycerate also effect the ability of hemoglobin to carry oxygen.
Secondary functions
When erythrocytes undergo shear stress in constricted vessels, they release ATP which causes the vessel walls to relax and dilate so as to promote normal blood flow.
When their hemoglobin molecules are deoxygenated, erythrocytes release S-nitrosothiols which also acts to dilate vessels, thus directing more blood to areas of the body depleted of oxygen.
It has been recently demonstrated that erythrocytes can also synthesize nitric oxide enzymatically, using L-arginine as substrate, just like endothelial cells. Exposure of erythrocytes to physiological levels of shear stress activates nitric oxide synthase and export of nitric oxide, which may contribute to the regulation of vascular tonus.
Erythrocytes can also produce hydrogen sulfide, a signalling gas that acts to relax vessel walls. It is believed that the cardioprotective effects of garlic are due to erythrocytes converting its sulfur compounds into hydrogen sulfide.
Erythrocytes also play a part in the body’s immune response: when lysed by pathogens such as bacteria, their hemoglobin releases free radicals which break down the pathogen’s cell wall and membrane, killing it.
