Red blood cells, or as it is often called erythrocytes constitute approximately 99% of the blood cells. The other 1% being a mixture of white blood cells and platelets. Red blood cells are obtained in a cellular differentiation process which involves several cell types. The precursors to erythrocytes are called reticulocytes. These are precursor cells that differentiate to red blood cells by expelling their nucleus.
Erythrocytes do not possess nuclei and also do not possess mitochondria. Therefore, it does not divide nor it is able to synthesize ATP aerobically. Cancer that affect directly red blood cells is not possible. It usually affects the precursor cells to erythrocytes such as occurs in leukemia.
Red blood cells are not spherical in shape but are biconcave in structure. This is a crucial fact for its normal functioning in squeezing into the blood capillaries. In spherocytosis a condition in which blood cells have spherical structure this situation can lead to hemolysis of red blood cells easily due to its spherical structure.
The importance of red blood cells stem from the fact that they are the carrier of oxygen in the blood to the various tissues of the body. Erythrocytes contain the important molecule of hemoglobin which is the carrier of oxygen in the blood. Hemoglobin is a protein molecule which is composed of a heme portion and a globin portion. The globin portion is a protein molecule that is composed of 4 polypeptide chains that are attached to the heme portion. The heme portion is made of 4 pyrrole rings that are attached to a central iron metal ion that can be oxidized or reduced intermittently.
In addition to the function of hemoglobin in carrying oxygen in the blood, the molecule NO or nitric oxide which is a known vasodilator binds to hemoglobin before it exerts its effect on the blood vessels. It is released in the blood arteries causing dilatation of these arteries the thing which increases blood flow to the body tissues.
Red blood cells live approximately 120 days before they age and die. The amount of red blood cells and hence hemoglobin in the body of males is slightly higher than that in females. This is so due to the effect of the hormone testosterone which is the male sex hormone which stimulates red blood cells formation. Testosterone is present in much lower concentration in females, the thing that can make their red blood cells content lower than males.
After the life cycle of red blood cells is ended it undergoes a series of events which include its destruction by macrophages in the spleen and the liver. The content of the destroyed red blood cells escape to the extracellular space causing among other things hyperkalemia and bilirubinemia if destroyed in large quantities as occurs in hemolysis.
The hemoglobin molecule is further degraded enzymatically to several metabolic products. These include amino acids from the globin portion of the hemoglobin molecule. These amino acids can be either reused to form new proteins or can be metabolized in the liver in various ways such as glucose generation via gluconeogenesis, or they can be used to generate krebs cycle intermediates which can be used to generate ATP molecules.
The heme portion of hemoglobin is degraded to the pigment bilirubin which is then secreted to the gallbladder and from there to the small intestine. Iron metal ions are also liberated from red blood cells upon hemolysis. It is then attached to the carrier protein of transferrin which transports the metal to the various tissues of the body especially the liver. There it binds to the protein ferritin.
In states of excessive destruction of red blood cells such as occurs in hemolysis due to genetic defect in red blood cells hemoglobin molecule or in the enzymes of red blood cells the large content of red blood cells accumulate in the blood causing hyperkalemia due to excessive amount of potassium that is liberated from the eryhtrocytes. In addition , hyperbilirubinemia occurs as well due to the accumulation of the pigment bilirubin in the blood.