by Erich Rosenberger M.D.

Created on: December 27, 2007   Last Updated: February 25, 2009

Red blood cells, known also as RBCs, have several important roles to play in our bodies. The primary function of red blood cells is to carry oxygen from the lungs to the tissues around your body. As a secondary function, they are also a key player in getting waste carbon dioxide from your tissues to your lungs, where it can be breathed out. When red blood cells stop functioning properly, you can rest assured that many things are going to go wrong in your body.

In order to properly understand the function of a red blood cell, you have to understand something about the structure. A typical RBC is about 6-8 micrometers in diameter, about the same as the width of a spider web strand. An RBC is biconcave in shape. Think of it like a miniature donut, only the hole in the center doesn't poke all the way through. This small shape and physical structure allows the RBC to squish in to the small capillaries where your blood vessels are the smallest. Without this ability to flex, they would easily get stuck and cause obstructions in your circulation.

The oxygen carried in your red blood cells is stored in a special protein known as hemoglobin. There are several different types of hemoglobin and the exact structure of this important protein is quite complicated, so this explanation will be something of a gross oversimplification. 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. Again, the details of this effect involve a lot of complex chemistry, but this effect it 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 (there's a mouth-full,

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