RBCs are specialized bags of hemoglobin

Red blood cells are designed to pick up oxygen from the lungs and release it into the tissues of the body. This is their major function. The only reason that they can pick up and carry oxygen around is because they contain a protein within them called hemoglobin.

If red blood cells (RBCs, or erythrocytes) have to carry oxygen, yet it is the hemoglobin inside them that carries the oxygen, you might expect that RBCs should be loaded with hemoglobin. In fact, they are! They load themselves up so much with hemoglobin to carry oxygen that their nuclei get in the way. So, RBCs don't have any nuclei! 1/3 of their entire volume is due to hemoglobin!

Let's discuss the production of RBCs for a moment. First of all, this process is called erythropoiesis. Hemocytoblasts and RBC precursors have nuclei. But, as cell division continues toward producing the RBC, a cell called a normoblast is made. The normoblast continues to manufacture hemoglobin (at this point, it looks like a normal cell, but somewhat red). Then, the normoblast ejects its nucleus. The cytoplasm continues to make hemoglobin, even without a nucleus, because the instructions from the nucleus are still in the cytoplasm, even though the nucleus is gone. Eventually, enough hemoglobin is made and the cell is released into the blood as an RBC.

The odd-looking shape of the RBC actually allows lots of surface area to exist on this cell. That is another important feature of the RBC because it helps there be more room on the RBC for gas exchange across its membrane. The RBCs are rather small, averaging about 8 mm in diameter; a small size also helps them increase their surface area.

Red blood cells can't last forever. They have no nucleus. They have to squeeze through the teeniest of blood vessels all day long! The odds are against them-- either they will run out of materials they need because they have no nucleus to make more, or they will squeeze through one too many teeny blood vessels and burst. They can only last so long.

How long? On average, a red blood cell will survive 120 days (that's about 4 months... which is pretty good, considering their difficulties). I hope that number impresses you, because it impresses me! Your book notes that over 120 days, each RBC wll travel through the entire body about 75,000 times! Wow!

Regulation of RBC production

We have to be able to obtain enough oxygen to carry out our functions. When we can't seem to get enough oxygen due to problems in our RBCs, that condition is called anemia.

Have you ever travelled to somewhere with a higher elevation and felt winded for a little while? I used to live in Tucson, AZ, which is at an elevation of 3000 feet. Some people who visited me had a hard time with the change in altitude for a couple of days. That's because at higher elevations there is less oxygen in the air... so to get enough oxygen, you actually need more RBCs in your body. Obviously, my visitors could increase their RBC number after a couple of days. How?

A drop in oxygen levels in one's body triggers the release of erythropoietin. (How one's body detects changing oxygen levels will be discussed when we study the respiratory system). Erythropoietin is released from the kidneys and liver, and it triggers erythropoiesis to occur. Within a couple days, new RBCs are in the blood, and a person's RBC level increase. The problem then becomes, how do we stop making RBCs? When is enough enough?

Just like with the other hormones we studied, there is a negative feedback loop to control erythropoiesis. As oxygen levels return to normal, the kidneys (and liver) stop making erythropoietin. The extra RBC production declines as long as oxygen levels remain normal.

Your book has a section on "dietary factors" affecting red blood cell production. One of those factors is iron. You'll see the importance of dietary iron as you read the hemoglobin web page. The other factors that your book focuses on are two B vitamins: vitamin B₁₂ and folic acid. Typically, we have little trouble obtaining enough of these vitamins in our diets. These vitamins are important when cells are dividing a lot-- and RBCs are constantly being made.

Obviously, one must understand hemoglobin to understand what red blood cells do. So, go to that page and read up on it next!