Red blood cells (erythrocytes) survive for around four months in the human body. The reason for their relatively short life is that although they can make their own energy without using the oxygen they carry, they cannot make protein. Without protein they cannot repair themselves.
Following some biochemical changes to the cell wall, water is soon able to pass into the red blood cell, transforming their shape from a disc to a sphere. The speed with which they travel through the blood vessels is then greatly reduced.
Another type of cell within the blood called a phagocyte then engulfs these slow-moving spherical erythrocytes in a similar way to an amoeba. A larger form of phagocyte called a macrophage which is present in the circulation, lymph nodes, spleen, red bone marrow and certain other places then mop up the phagocytes.
Once this absorbtion has taken place, the old fragile erythrocytes are broken down. The erythrocyte proteins are broken down into their constituent amino acids and are stored ready to be used for the manufacture of new proteins. The iron from the haemoglobin is removed and sent for storage in the bone marrow, where eventually it will be turned into new haemoglobin although a small proportion is stored in specialised cells within the circulatory system for immediate use to correct such things as iron levels in the bloodstream.
Now although the protein and iron from the red blood cells is constantly recycled in this way with hardly any loss, the same cannot be said of the what's left of the haemoglobin.
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