ANATOMY AT A GLANCE

The haematological system consists of the blood and the sites of blood production (especially red bone marrow and lymphatic tissue). The main components of blood are shown in Figure 3.1. Different types of blood cell all develop from their common origin, the pluripotent stem cell. Red blood cells (RBCs), for example, follow a pathway from the pluripotent stem cell via the myeloid stem cell stage, develop into a proerythroblast, then a reticulocyte when they expel their nucleus to finally become a mature RBC or erythrocyte. The pathways followed by other types of blood cell are shown in Figure 3.2.

Figure 3.1 Composition of blood.

Figure 3.2 Formation of white blood cells (eucopoiesis).

PHYSIOLOGY YOU NEED TO KNOW

MAIN FUNCTIONS OF BLOOD

Transport medium for metabolic requirements (e.g. oxygen, nutrients, hormones) and waste products of metabolism (e.g. carbon dioxide).

Maintains homeostasis by facilitating regulation of body function, e.g. continual exchange of constituents across capillary wall between blood and interstitial fluid, regulates body temperature by distributing heat around the body.

Protection: ingest and destroy (phagocytosis) foreign particles, antibody production mediated by different types of white blood cells, clotting mechanism to prevent bleeding.

CONSTITUENTS OF BLOOD

Erythrocytes (RBCs) are biconcave, circular discs with no nucleus. They are packed with haemoglobin whose main function is to carry oxygen and some carbon dioxide. A haemoglobin molecule consists of a protein called globin and a pigment called haem, which contains iron. The more oxygen is attached to the molecule, the redder the colour appears. Four oxygen molecules combine with each haemoglobin molecule to form oxyhaemoglobin which can readily disassociate back into oxygen and haemoglobin when required.

Leucocytes (white blood cells). There are three main types:

1. Granulocytes

Neutrophils; phagocytose microbes

Eosinophils; phagocytose antigen/antibody complexes and attack parasitic worms.

Basophils intensify inflammatory response by developing into mast cells and releasing histamine, serotonin and heparin.

2. Lymphocytes

B Lymphocytes secrete antibodies to attack antigens. Some B cells serve as memory B cells for future defensive actions against the same antigen.

T Lymphocytes exist in various forms and attack antigens. Memory T cells can recognize the same antigen and subsequently trigger a powerful defensive response.

3. Monocytes differentiate into macrophages which can phagocytose invading organisms.

Platelets contain a range of active chemicals and play an important role in stopping haemorrhage by adhering to the injured area. They are derived from the fragmentation of cells called megakaryocytes. They also adhere together to form platelet plugs which help occlude damaged blood vessels and prevent blood loss. This mechanism takes place alongside the clotting mechanism in blood which converts the soluble plasma protein fibrinogen into strands of insoluble fibrin, forming a clot which, together with the platelet plug and localized vasoconstriction, all combine to stop bleeding from severed blood vessels.

Plasma is the straw-coloured liquid component of blood which is mostly water but which also contains a range of important solutes (sodium, potassium, etc.) and plasma proteins such as albumin, fibrinogen and gamma globulin antibodies.

Blood Groups. Erythrocytes contain on their surface complex molecules known as isoantigens or agglutinogens which act as antigens. These isoantigens occur in characteristic combinations which give rise to a complex series of different blood groups. The original discovery of this system led to the following simple model based upon four possibilities:

Type A antigens	Group A
Type B antigens	Group B
No antigens	Group O
A and B antigens	Group AB

Plasma also contains antibodies or agglutinens which will react with certain antigens:

Anti-A-antibody (plasma) reacts with Antigen A (erythrocyte).

Anti-B-antibody (plasma) reacts with Antigen B (erythrocyte).

A person does not possess the antibody that will react with the antigen they display on their own erythrocytes. Grouping and cross matching for blood transfusion ensures that a person only receives blood that is compatible with their own antibodies so that a potentially fatal antigen–antibody reaction does not occur. Blood group O is known as the universal donor because it does not display any antigens which can provoke an antigen–antibody reaction. Research has shown that there are many different types of blood beyond the basic four types originally described in the ABO system, making grouping and cross matching an essential but time-consuming laboratory procedure.

ANAEMIA (P374)

PATHOLOGY: Key facts

The term anaemia indicates a reduction in the oxygen carrying capacity of the blood and it occurs because of a reduction in the numbers of erythrocytes or a decrease in the concentration of haemoglobin. This results from the body not producing enough erythrocytes or losing/destroying erythrocytes faster than they can be replaced.

1. Inadequate erythrocyte production (erythropoiesis) (p375)

• Iron deficiency anaemia produces smaller than normal erythrocytes with reduced quantities of haemoglobin. It can not only be caused by a poor diet, but also by chronic blood loss (menstruation, peptic ulcers), inadequate absorption of iron in the bowel or extra demand for iron exceeding dietary intake as in pregnancy.

• Pernicious anaemia is caused by a lack of absorption of vitamin B₁₂ or a deficiency in the diet (notably in vegans) leading to fewer erythrocytes than normal being formed which are, however, bigger than normal (megaloblastic anaemia).

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