The trillions of cells that compose the human body rely on red blood cells, a.k.a. erythrocytes to supply them with oxygen and help direct the elimination of waste carbon dioxide gas. Red blood cells have a unique structure and properties that allow them to fulfill these crucial missions. Red blood cells (RBCs) are the primary cells in human blood. They are biconcave discs, having a depressed center on both sides. These depressed centers allow the cells to have more cell membrane surface which can be exposed to diffusing oxygen while transiting the lungs. This structure also allows them to be more flexible when negotiating tight passages.
RBCs are about 7.8 micrometers in diameter (A micrometer is 1/1,000,000 of a meter). They have a flexible nature that allows them to bend and bounce back their original shape. This comes in handy when they must squeeze through the minute capillary alleyways between cells in the tissues.
Hemoglobin
Unlike most cells, mature RBCs of humans don’t have nuclei, mitochondria, or other organelles. Instead, they are packed full of a special substance called hemoglobin (Hgb), a complex molecule composed of protein and iron. Hgb is responsible for picking up oxygen, which diffuses across membranes in the small vessels of the minute lung sacks called alveoli. Hgb holds onto oxygen until it reaches areas of the body where it is in low concentration and then releases it to diffuse into local tissues.
RBCs are passive in nature, being swept along by the blood. They sustain their meager energy needs by a form of anaerobic respiration. Since they don’t have mitochondria, there are no worries they might gobble up the oxygen they are charged with transporting.
Management of Carbon Dioxide
RBCs also contain an enzyme called carbonic anhydrase which takes carbon dioxide and water and catalyzes the creation of a stable molecule called bicarbonate (HCO3-). Bicarbonate dissolves much better than carbon dioxide in the fluid part of blood and great quantities can be transported this way without needing to be in the small red blood cells.
Bicarbonate is an important buffer for blood, keeping it at a stable pH of about 7.4. Buffers can be thought of as chemical “shock absorbers.” They keep the fluid of the blood from being too acidic or basic, maintaining an environment where cellular machinery works best. When the bicarbonate dissolved in the blood arrives in the capillaries of the lungs it is converted back to gaseous carbon dioxide by RBCs and then diffuses out into the lungs where it is breathed out.
Structural Abnormalities of RBCs
Sometimes structural changes occur in RBCs that indicate a possible pathological condition. For instance, one may observe enlarged RBCs with folate or vitamin B12 deficiency (macrocytic anemia). Small RBCs may be observed with iron deficiency or chronic blood loss (microcytic anemia). Sickle cell anemia is a condition where a genetic flaw leads to defective hemoglobin that changes shape under certain conditions. This shape change induces the cell to lose its original biconcave disc confirmation and take on a more sickled appearance.
Sources
Kumar. Clinical Medicine, 6th ed., Elsevier, (2005)
Beers, The Merk Manual, 18th ed., Merk Research Laboratories (2006)
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