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A metallic chemical element having atomic number 26, and symbol Fe. Iron-containing enzymes and proteins, often containing heme prosthetic groups, participate in many biological oxidations and in transport.
The iron-containing substance in red blood cells that transports oxygen from the lungs to the rest of the body. It consists of a protein (globulin) and haem (a porphyrin ring with an atom of iron at its center).
Human erythrocytes or red blood cells (RBCs) are the primary cellular component of blood. They are involved in oxygen transport through the body and have distinct cellular features that differ from every other type of cell in the human body. Adult humans have roughly 2–3 × 10^13 (20-30 trillion) RBCs at any given
time, comprising approximately one quarter of the total human body cell
number (women: ~4 to 5 million RBCs/uL of blood; men: ~5 to 6 million
RBCs/uL of blood).
RBCs are disk shaped in that they appear to be "caved in" (concave) or almost flattened in their mid section. This bi-concave shape allows RBCs to bend and flow smoothly through the narrowest blood vessels in the body. Gas exchange with tissues occurs in capillaries, which are tiny blood vessels that are only as wide as one cell. Many RBCs are wider than the capillary, but their shape gives them the flexibility needed to squeeze through into the capillaries.
A typical human RBC has a disk diameter of 6–8 µm and a thickness of 2 µm, being much smaller than most other human cells. These cells have an average volume of about 90 fL with a surface of about 136 μm2. They can swell up to a sphere shape containing 150 fL, without bursting their cell membrane. When their shape does change, it inhibits their ability to carry oxygen or participate in gas exchange, which occurs in spherocytic (sphere shaped) anemia, or sickle-cell anemia.
Although RBCs are considered cells, they lack a nucleus (and nuclear DNA) and most organelles, including the endoplasmic reticulum and mitochondria. RBCs, therefore, cannot divide or replicate like other labile cells of the body and they also lack the components to express genes and synthesize proteins. While most cells have chemotaxic ways to "swim" through the body, RBCs do not, and are carried through the body by the flow and pressure of blood alone.
The most important component of RBCs is their hemoglobin molecules. Hemoglobin is a specialized protein that contains a binding site for the transport of oxygen and other molecules. The RBCs distinctive red color is also due to the spectral properties of the binding of hemic iron ions in hemoglobin. Each human red blood cell contains approximately 270 million of these hemoglobin biomolecules, each carrying four heme groups (individual proteins) and hemoglobin comprises about a third of the total RBC volume. This protein is responsible for the transport of more than 98% of the oxygen, while the rest of it travels as dissolved molecules through the plasma.