Learning Objectives
By the end of this section, you will be able to:
Discuss the structure and function of erythrocytes (red blood cells) and hemoglobin
- Describe the anatomy of erythrocytes
- Explain the composition and function of hemoglobin
- Discuss the various steps in the lifecycle of an erythrocyte
The erythrocyte, commonly known as a red blood cell (or RBC), is by far the most common formed element: A single drop of blood contains millions of erythrocytes and only thousands of leukocytes (Figure 18.3.1). Specifically, males have about 5.4 million erythrocytes per microliter (µL) of blood, and females have approximately 4.8 million per µL. In fact, erythrocytes are estimated to make up about 25 percent of the total cells in the body. They are small cells, with a mean diameter of 7–8 micrometers (µm). The primary function of erythrocytes is to pick up oxygen from the lungs and transport it to the body’s tissues, and to pick up carbon dioxide at the tissues and transport it to the lungs.
Shape and Structure of Erythrocytes
As an erythrocyte matures in the red bone marrow, it extrudes its nucleus and most of its other organelles. During the first day or two that it is in the circulation, an immature erythrocyte, known as a reticulocyte, will still typically contain remnants of organelles. Reticulocytes should comprise approximately 1–2 percent of the erythrocyte count and provide a rough estimate of the rate of RBC production. Abnormally low or high levels of reticulocytes indicate deviations in the production of these erythrocytes. These organelle remnants are quickly shed, so circulating erythrocytes have few internal cellular structural components. They lack endoplasmic reticula and do not synthesize proteins.
The erythrocytes’ function of transporting blood gases is complimented by their structure, such as their lack of organelles, particularly mitochondria, their biconcave shape, and the presence of a flexible cytoskeletal protein element called spectrin. Since erythrocytes lack mitochondria and must rely on anaerobic metabolism, they do not utilize any of the oxygen they are transporting as they deliver it to the tissues. Erythrocytes are biconcave disks; that is, they are plump at their periphery and very thin in the center (Figure 18.3.2). Since they lack most organelles, there is more interior space for the presence of the hemoglobin molecules that, as you will see shortly, transport gases. The biconcave shape also provides a greater surface area across which gas exchange can occur, relative to its volume; a sphere of a similar diameter would have a lower surface area-to-volume ratio. In the capillaries, the oxygen carried by the erythrocytes can diffuse into the plasma and then through the capillary walls to reach the cells, whereas some of the carbon dioxide produced by the cells as a waste product diffuses into the capillaries to be picked up by the erythrocytes. Capillary beds are extremely narrow, slowing the passage of the er