Chapter 28 Blood disorders
Red blood cells (erythrocytes) perform two functions:
• They transport oxygen from the lungs to the tissues of the body: they are able to perform this task because of the haem-based structure they contain (haemoglobin; see Chapter 13 ‘Vitamins and minerals’, p. 96).
• They catalyse the reversible reaction between carbon dioxide and water, thus enabling the removal of large amounts of carbon dioxide from the body tissues: erythrocytes contain the enzyme carbonic anhydrase, which makes it possible for the blood to transport carbon dioxide in the form of bicarbonate ions (HCO3−) (see Figure 18.3, p. 135).
Anaemias
Iron-Deficiency Anaemia (Microcytic Anaemia)
• Usually discovered with a full blood count (Chapter 41 ‘Scientific tests’, p. 331) and is one of the most common types of anaemia.
• Can vary from mild, with no signs or symptoms, to severe, in which the patient experiences extreme fatigue and weakness.
For reasons for iron deficiency, see Chapter 13 ‘Vitamins and minerals’ (p. 97).
Pernicious Anaemia
• Autoimmune type of anaemia: due to the patient producing antibodies against the intrinsic factor needed for efficient absorption of vitamin B12.
Leukaemia
Abnormal growth of the cells in the bone marrow that develop into white blood cells.
Treatment
Treatment
• Underlying Cause Porphyria
• Any iron in the body that is not associated can create free radicals (see Chapter 7 ‘Free radicals’, p. 41) because it attaches itself to any protein it comes into contact with. Liver cirrhosis is a result of this. Iron must therefore be carefully controlled.
• Excess iron usually not a problem in a healthy individual because iron that is absorbed from the duodenum binds immediately to a carrier molecule called transferrin, which transports the iron to storage as ferritin. The transferring molecule binds very strongly with the cell membranes of erythroblasts (precursors to red blood cells) in bone marrow, ensuring that at no time is free iron released.
• The body has no means of actively excreting iron (although about 0.6 mg is lost in the faeces every day and approximately 1.3 mg per day in menstruation). Regulation of the amount in the body is done by regulating uptake.
Haemostasis
Haemostasis (the arrest of bleeding) occurs because of the creation of an abnormal environment in the blood vessels.
Normal Events in Haemostasis
• The process of repairing a blood vessel starts with vasoconstriction and the formation of a platelet plug at the site of an injury.
• This is then stabilized by the formation of a fibrin network, which is the end result of the coagulation cascade (Figure 28.1).
• The Different Pathways Involved in Clotting
Two different pathways are involved in the clotting cascade (Figure 28.1):
• The extrinsic pathway: activated by tissue factor (a protein, released from damaged tissue, that triggers the clotting cascade), which is expressed by cells during injury.
• Formation of a Thrombosis
A thrombosis is a blood clot in the wrong place. A thrombosis forms due to:
• Abnormalities of the vessel wall: an atheromatous plaque in an artery wall (see Figure 27.2, p. 205).
Antiplatelet Drugs
Antiplatelet drugs are preferred to anticoagulants as they are less likely to cause excess bleeding.
Aspirin
• Aspirin is commonly used, as it is cost effective and easy to administer. A loading dose is initially required.
• Aspirin inactivates a key enzyme involved in the platelet synthesis pathway. This is an irreversible reaction as the platelets have no nucleus. Recovery from taking aspirin for an antiplatelet effect is 10 days, the time it takes to produce more platelets.
Figure 28.2 Various haemostatic medications, demonstrating the natural derivatives of modern medications.
• Adverse Efects
• Allergy reaction to aspirin. Not given to some one with a known allergy to aspirin.
Note: this applies to herbs and foods containing salicylates.
• Possibility of renal failure in patients with renal artery stenosis and patients taking ACE inhibitors.
Note: it is not a good idea to give aspirin to some one with a disease such as haemophilia or any type of sickle-cell anaemia. Omega 3 fish oils are also considered a problem in these cases.
Anticoagulants
• Heparin
• Heparin is a large, highly charged molecule with very little ability to cross the blood–placenta barrier. As warfarin can cross the placenta, patients on warfarin treatment have to use heparin when pregnant.
• Warfarin
• Is modelled on dicoumarol the natural anticoagulant derived from coumarin (see Figure 28.2 and Chapter 21 ‘Phenols’, p. 157). Acts as a vitamin K antagonist.
• When the vitamin K acts on a cofactor it is recycled by reductase reactions. If this processes is inhibited the active vitamin K is not able to act on the factors in the cascade pattern.
• Warfarin therapy is monitored very carefully because it can sometimes prove difficult to balance. Much of the warfarin administered to a patient binds to plasma proteins, but can be easily displaced, releasing higher levels of free, unbound warfarin into the bloodstream than desirable (see Chapter 16 ‘How do drugs get into cells’ p. 125). The converse is true when too little warfarin is administered.
