Blood, clotting and atherosclerosisEdexcel A-Level Biology B: Revision notes

Blood consists of plasma, a pale yellow liquid that is mostly water and carries dissolved glucose, amino acids, mineral ions, hormones, urea, carbon dioxide (mainly as hydrogencarbonate ions) and plasma proteins, and blood cells: erythrocytes, leucocytes and platelets (cell fragments).

Functions:

• transport of oxygen, carbon dioxide, nutrients, wastes, hormones and heat
• defence against pathogens, by phagocytes, lymphocytes and by clotting
• formation of tissue fluid and lymph: at the capillaries, fluid leaves the plasma (without most proteins and cells) as tissue fluid, and some of it drains into lymph vessels.

Erythrocytes are biconcave discs with no nucleus, giving a large surface area and room for haemoglobin.

Leucocytes all have nuclei and are classed as:

• Neutrophils: nucleus with 2 to 5 lobes and pale granular cytoplasm; the most common; phagocytes that engulf and digest bacteria.
• Eosinophils: usually a two-lobed nucleus and red-staining granules; they help to defend against parasites and are involved in allergic responses.
• Monocytes: the largest, with a kidney-shaped nucleus; they leave the blood, become macrophages and phagocytose pathogens and debris.
• Lymphocytes: a large round nucleus filling most of the cell, with little cytoplasm; they produce antibodies (B cells) or destroy infected cells (T cells).

Platelets are fragments of larger cells, with no nucleus; they function in clotting.

Clotting seals a damaged vessel, stops blood loss and prevents entry of pathogens. The sequence is:

1. Platelets stick to the damaged vessel wall and form a plug.
2. Platelets and damaged tissue release clotting factors, including thromboplastin.
3. Thromboplastin (with calcium ions) catalyses the conversion of prothrombin (an inactive plasma protein) to its active form, thrombin.
4. Thrombin, an enzyme, converts soluble fibrinogen into insoluble fibrin.
5. Fibrin forms a mesh of fibres across the wound that traps red blood cells and platelets, forming the clot.

This is a cascade: each step produces more of the next, so a small stimulus gives a large, rapid response.

Atherosclerosis is the build-up of fatty deposits in artery walls. The stages are:

1. The endothelium lining the artery is damaged, for example by high blood pressure or by chemicals in tobacco smoke.
2. An inflammatory response occurs. White blood cells, mainly monocytes that become macrophages, move into the artery wall and take up cholesterol from LDL (low-density lipoprotein), forming fatty streaks.
3. Smooth muscle cells and fibrous tissue build up with the lipid, forming a hard plaque (atheroma), which may be calcified.
4. The plaque narrows the lumen, increases blood pressure and makes the artery wall less elastic.
5. The plaque can rupture, exposing the damaged wall; this triggers clotting and a thrombus (a blood clot in a vessel) forms.

A narrowed or blocked artery reduces blood flow and oxygen supply:

• in a coronary artery: angina (chest pain on exertion) or a myocardial infarction (heart attack) when the blood supply to part of the cardiac muscle is cut off and the cells die
• in an artery to the brain: stroke
• a weakened artery wall may stretch to form an aneurysm, which can burst
• high blood pressure, as the artery is narrowed and less elastic.

A thrombus can break away and block a smaller vessel elsewhere (an embolus).

Factors that raise the risk of atherosclerosis:

• high blood pressure, which damages the endothelium
• smoking: carbon monoxide and nicotine damage the endothelium and raise blood pressure
• high blood LDL cholesterol, linked to a diet high in saturated fat
• obesity, lack of exercise and diabetes
• age, being male and a family history (genetic factors) that cannot be changed.

Risk is expressed as a correlation: people with these factors are statistically more likely to develop the disease, but having a factor does not mean someone will definitely get it.