Anatomy and Physiology: Cells, Bells!

Cells, Bells!

My first introduction to white blood cells was that old sci-fi classic, Fantastic Voyage. (I recommend it, but to have the most fun, you should keep an eye out for the mistakes!) One of the final scenes involves the bad guy being eaten by a white blood cell! You may not have learned, however, that while there is only one type of red blood cell (RBC), that there are many white blood cells (WBCs), not to mention platelets! All of these cells are called formed elements, and they make up about 45 percent of blood volume.

In The Bones, I mentioned that the red bone marrow was the site of blood cell production in a process called hemopoiesis, which is summarized in Figure 10.1. In truth, not all blood cells are made in the red bone marrow, because while the B-lymphocytes are made in the bone (B for bone), the T-lymphocytes are made in the thymus (T for thymus). All of the blood cells come from the same stem cell—the pluripotent hematopoietic stem cell, its friends call it a hemocytoblast—in a model of differentiation.

This figure may be a bit frightening, but try to look for the commonalities. From the one stem cell (the hemocytoblast) there are five cells into which it can develop. Note that the first cells all have the suffix “-blast,” and that the three of the five cells have, in their name, a clue as to the final cell (for example, a lymphoblast will ultimately become a lymphocyte). The second step, with the exception of the early erythrocyte, all have similar names (for example, promonocytes ultimately become monocytes). Look for such strands as you follow the diagram, and you will see that it is not so frightening after all!

Now I Know My RBCs

RBCs make up 99.9 percent of all formed elements, that's a nearly a 1000 to 1 ratio! RBCs give blood its red color. I always tell my students that erythrocytes, or red blood cells (RBCs), are on a suicide mission. From the earlier diagram you may remember that the last step of RBC formation is the loss of the nucleus (as well as ribosomes, mitochondria, and most of the other cell organelles). That it is a very important event, for it says a tremendous amount about that cell.

Without a nucleus (and its DNA cargo) or ribosomes, an RBC cannot make protein, and so it is trapped with its current complement of proteins. That means it can do very little to react to its environment, and it is completely incapable of repairing itself. The loss of mitochondria means it can process glucose only without oxygen (an irony, considering how much oxygen is in the RBC): Producing only 2 ATP versus 36, the RBC must have very low energy needs.

An RBC's role is basically as a floating sack of hemoglobin, which is the protein that carries oxygen. The iron in the hemoglobin causes the cells to turn more red in the presence of oxygen, just the way rusting iron turns red! Without a nucleus, RBCs are incapable of dividing, so they are on a suicide mission, working until they are destroyed by phagocytic cells, or in the spleen.

RBCs are a beautiful example of differentiation. Here we have a cell that does only one thing, lives for 120 days before it is broken down, and must constantly be replaced. The single-minded purpose of the cell is linked to the hemoglobin, or Hb, which makes up 95 percent of the proteins in the cell. The lack of the nucleus allows room for more hemoglobin molecules, which means that the cell can carry more oxygen.

So what's the big deal? After all, why can't we just carry the O₂ in the plasma, as we do the other molecules? The answer is due in part to the fact that O₂ is very poorly soluble in water (1.5 percent), and in part to the structure of the hemoglobin molecule itself. I discussed the quaternary structure of proteins. A hemoglobin molecule is roughly x-shaped, with two alpha chains (each at a diagonal), and two beta chains (each also at a diagonal); each of these four chains is a tertiary structure (see Figure 10.2).

In the midst of each chain is a heme group, which contains an iron ion (Fe²⁺). Each heme group can hold one O2 molecule, so each Hb molecule therefore carries four oxygen molecules; all this adds up to a tremendous amount considering we have about 280 million Hb molecules in every RBC! This arrangement allows the hemoglobin in the RBC to be far more efficient at carrying oxygen than plasma. In addition to carrying about 98.5 percent of the O₂, hemoglobin carries about 23 percent of the CO₂; the combination of CO₂ and Hb is called carbaminohemoglobin. The remaining CO₂ is carried as ions (see The Digestive System) in the plasma, or the cytoplasm of the RBC.

Do you remember surface area to volume ratio (SA/V)? The important idea to remember is that a smaller cell has a larger SA/V. RBC is the star of SA/V, with a SA/V of about 1.5 million to one! The shape of an RBC is a bit weird; it's shaped like a biconcave disk (refer to Figure 10.3). Think of the shape of a bagel or doughnut, but instead of a hole, the center is merely pinched in (like a bialy). This shape has a far greater SA/V ratio than a sphere of the same volume. In fact, the total surface area of all 25 trillion RBCs in the average adult's body—260 million in a single drop!—is equivalent to 3,800 m², or 2,000 times larger than that of our skin (about 1.9 m²)!