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Red blood cells (erythrocytes) shown above in a stained slide. Note how they stain darker at the edges than in the middle reflecting their biconcave shape.
Erythrocytes in a peripheral blood smear derive their reddish colour from the protein haemoglobin, and usually appear round or oval with a pale-staining centre region.
They are biconcave in shape, as seen arrowed in this scanning electron microscopic image.
This shape increases the cell's surface area and facilitates
diffusion of O2 and CO2 into or out of the cell. Likewise, the lack of nuclei and
organelles contribute to increased haemoglobin content and gas-carrying capacity. Cell
shape is maintained by a cytoskeleton composed of several proteins (spectrin, actin,
etc.), but normal erythrocytes must be very flexible. They become deformed when flowing
through capillaries and narrow slits in the spleen (see section on the lymphatic system).
As mentioned above red blood cells (RBC's) or erythrocytes are highly specialised for the transport of oxygen. The nucleus is lost in the process of red blood cell formation in the bone marrow. This process is known as haemopoeisis. During this process all of the internal cell organelles are also degraded thus allowing the RBC to carry more haemoglobin.
As you can see by the above photomicrograph red blood cells in life are highly deformable and are able to squeeze through minute gaps.
All red blood cells have a limited life span of around 100 to 120 days and must therefore be continuously replaced. Mature red blood cells are unable to synthesise new enzymes to replace those lost during normal cell metabolic processes due to their lack of inner organelles.
It is probable that as the RBC's age they become less deformable due to diminishing efficiency of ion pumping mechanisms and as a result they are no longer able to pass through the filtering system of the spleen and are thus removed by being literally eaten up by white blood cells in a process known as phagocytosis.
Aged RBC's are removed by the spleen, liver and the bone marrow. Although it appears that the spleen is most active their relative importance to each other under normal circumstances is uncertain. Certainly we know that individuals who have lost their spleen are able to function normally with few limitations.
RBC's have a specialised cytoskeleton in order to maintain their shape. This composes of an actin/spectrin network held together by another protein call ankyrin. A condition known as Hereditary Spherocytosis is caused by an abnormal arrangement of the internal cytoskeleton of RBC's. In this condition the ankyrin binding of spectrin is absent. As a result the cell membrane is not adequately braced and is too easily deformed. Individuals who suffer the condition have spherical RBC's which are abnormally fragile and do not resist osmotic pressure.
This slide shows a blood sample from a
person with hereditary spherocytosis
Haemoglobin (Hb) is a red coloured protein pigment found within RBC's. It is formed during haemopoesis mature red cells are unable to produce further supplies of haemoglobin. It consists of two basic parts
The picture on the right shows the haemoglobin molecule.
The main function of haemoglobin is oxygen uptake in the lungs, carriage in the RBC's and release in other tissues. The process is dependent on the partial pressure of oxygen at these sites. Haemoglobin also carries CO2 as carbaminohaemoglobin.
Haemoglobin is also a powerful buffering agent in the maintenance of blood pH levels. All of these functions are discussed in greater detail in the appropriate sections on respiration and fluid and electrolyte balance.
The most common blood disorder is anaemia in which an inadequate haemoglobin supply causes weakness, pallor and sometimes, breathlessness.
It may be the result of impaired RBC functioning or increased RBC destruction.
The most common cause is iron deficiency. Iron is essential for the formation of haemoglobin and a deficiency in the diet means that cells formed in the bone marrow are pale-staining (hypochromic) and smaller than normal (microcytic).
Excessive red cell destruction usually occurs if the red cells produced are structurally abnormal in some way and are therefore liable to damage in their passage around the body. Any such cells are removed prematurely and in excess by the spleen resulting in haemolytic anaemia this is often due to a genetic abnormality such as Hereditary Spherocytosis or Sickle Cell Anaemia.
This slide shows a blood sample taken taken from a patient
suffering from a condition known aa cold aglutinin disease. The red cells are clumped
together. This condition is associated with Raynaud's Disease and mycoplasma infection.
This page last updated Wednesday, 28 April 1999 20:44 +0100
[Introduction] [White Blood Cells] [Platelets] [Plasma] [Haemopoiesis] [Anaemia]
