[box type=”download”] Normal values Knowledge of normal adult values for Hb, haematocrit, platelet and white cell counts Red blood cells Haemoglobin as the key component Erythropoietin (EPO) as the renally-produced stimulant to production Role of red marrow and spleen in production and sequestration[/box]
Normal values table
Red blood cells (erythrocytes) are biconcave discs ∼8 μm wide, and uniquely have no nucleus (cannot repair themselves).
Lifespan of only 100–120 days.
The shape and flexibility of allows them to deform easily and pass through capillaries.
Red cells are formed by a process called erythropoiesis.
They originate from committed stem cells (CFU-E; colony forming unit-erythroid) in the bone marrow of the adult, and liver and spleen of the fetus.
The glycoprotein hormone erythropoietin (EPO) increases the number of committed stem cells and promotes production of red cells.
Erythropoietin is produced mainly by the kidneys in adults, and liver in the fetus.
The key stimulus for increased erythropoietin is low O2 (hypoxia).
Stem cells differentiate into erythroblasts (early normoblasts), which are relatively large (∼15 μm) and nucleated.
As differentiation proceeds, the cells shrink and haemoglobin is synthesized, which requires iron, folate and vitamin B12.
In the late normoblast the nucleus breaks up and disappears.
The young red cell shows a reticulum on staining, and is called a reticulocyte. As it ages, the reticulum disappears and the characteristic biconcave shape develops.
Normally 1–2% of circulating red cells are reticulocytes.
This increases when erythropoiesis is enhanced (e.g. by hypoxia).
About 2 × 1011 red cells are produced from the marrow each day.
The spleen holds a reserve of red cells that can be released following blood loss.
Red cells are destroyed by macrophages in the liver and spleen after ∼120 days.
The haem group is split from haemoglobin and converted to biliverdin and then bilirubin.
The iron is conserved and recycled via transferrin, an iron transport protein, or stored in ferritin.
Bilirubin is a brown–yellow compound which is excreted in the bile.
An increased rate of haemoglobin breakdown results in excess bilirubin, which stains the tissues (jaundice).
Red cells have surface antigens that form the basis of blood groups (ABO and Rh).
White blood cells
[box type=”download”] Appreciation of their key role in immunity and defence against infection Refer to pathology curriculum for additional required knowledge[/box]
White blood cells (leucocytes) defend the body against infection by foreign material, and the white cell count increases greatly in disease.
Three main types are present in blood: granulocytes, lymphocytes and monocytes.
Granulocytes are further classified as neutrophils (neutral-staining granules), eosinophils (acid-staining granules) and basophils (basic-staining granules).
All contribute to inflammation by releasing mediators.
Neutrophils have a key role in the innate immune system, and migrate to areas of infection (chemotaxis) within minutes, where they destroy bacteria by phagocytosis (engulfing them).
They are a major component of pus.
Neutrophils live for ∼6 h in blood, longer in tissues.
Eosinophils are less motile but longer lived, and phagocytose larger parasites.
They are increased in allergic disease, to which they contribute by releasing inflammatory mediators.
Basophils release histamine and heparin as part of the inflammatory response and are similar to tissue mast cells.
Lymphocytes originate in the bone marrow but mature in the lymph nodes, thymus and spleen before returning to the circulation.
Most remain in the lymphatic system.
Lymphocytes are critical components of the immune system and are of three main forms:
B cells which produce γ-globulins (immunoglobulins, antibodies),
T cells which coordinate the immune response,
and natural killer (NK) cells which kill infected or cancerous cells.
Monocytes are phagocytes but larger and longer lived than granulocytes.
After formation in the marrow they circulate in the blood for ∼72 h before entering tissues to become macrophages, which unlike
granulocytes can also dispose of dead cell debris.
Macrophages form the reticuloendothelial system in liver, spleen and lymph nodes.