Health

Thrombin is an enzyme produced when thromboplastin, in the presence of calcium ions, catalyses the conversion of prothrombin to thrombin.

Thrombin then catalyses the conversion of the soluble plasma protein fibrinogen into insoluble fibrin.

The fibrin forms a mesh of fibres across the wound, which traps platelets and red blood cells to form the clot that seals the damaged vessel.

A risk factor is any feature of a person's lifestyle, environment or genetic make-up that increases the probability of developing a disease — it raises the likelihood of cardiovascular disease but does not guarantee that the disease will occur.

Two modifiable risk factors (ones the person can change) are: a diet high in saturated fat or salt, and smoking. (Other acceptable answers: physical inactivity, obesity, high blood pressure.)

One non-modifiable risk factor (one that cannot be changed) is age. (Other acceptable answers: sex, or genetics/family history.)

Atherosclerosis begins when the endothelium, the single layer of cells lining the artery, is damaged. This damage is commonly caused by high blood pressure, toxins from cigarette smoke or high blood glucose.

The damage triggers an inflammatory response: white blood cells, particularly macrophages, move into the artery wall.

Cholesterol carried by low-density lipoproteins (LDL) is then deposited at the damaged site. Macrophages engulf the lipid and form foam cells, building up a fatty streak.

Over time the deposit is reinforced with fibrous tissue and calcium salts to form a hardened plaque, or atheroma, which bulges into the lumen of the artery and narrows it.

The narrowed lumen restricts blood flow and raises blood pressure. The raised blood pressure causes further endothelial damage, so the process is a positive-feedback cycle that accelerates over time.

Cholesterol is transported in the blood inside lipoproteins. Low-density lipoproteins (LDL) carry cholesterol from the liver to the body tissues, where excess cholesterol is deposited in artery walls. This deposition drives atherosclerosis, so a high blood LDL level increases the risk of cardiovascular disease. High-density lipoproteins (HDL), by contrast, carry cholesterol from the tissues back to the liver for removal, so a higher HDL level is protective.

The most important dietary advice concerns the type of fat rather than simply the total amount. Saturated fats raise the level of LDL in the blood, so reducing saturated fat lowers LDL and slows the formation of atheromas. Replacing saturated fats with unsaturated fats lowers the LDL-to-HDL ratio, which is more closely linked to risk than total cholesterol alone.

A person should also be advised to reduce their salt intake. A high-salt diet lowers the water potential of the blood, so more water is retained by osmosis, raising blood volume and therefore blood pressure. High blood pressure damages the endothelium and accelerates atherosclerosis, so cutting salt reduces this risk.

Finally, increasing the intake of fruit and vegetables provides antioxidants such as vitamin C, which can reduce oxidative damage to the endothelium and to LDL. Together, lowering saturated fat and salt while increasing unsaturated fats and antioxidants reduces LDL, lowers blood pressure and so reduces the overall risk of cardiovascular disease.

The data show a positive correlation: as the number of cigarettes smoked per day increases, the incidence of cardiovascular disease increases. This is the strongest type of evidence an observational study can provide, and the very large sample of 50 000 people followed over a long period of 15 years makes the result more reliable and representative, reducing the effect of chance and anomalies.

However, a correlation on its own does not prove causation. It is possible that a confounding variable explains the link: for example, people who smoke heavily may also have poorer diets, take less exercise or drink more alcohol, and any of these factors could increase the incidence of cardiovascular disease independently of the smoking itself. Without controlling for these variables, the study cannot isolate smoking as the cause.

A causal claim becomes far more convincing when there is a plausible biological mechanism. In this case there is one: toxins in cigarette smoke, such as carbon monoxide and nicotine, damage the endothelium of arteries and raise blood pressure, which promotes atherosclerosis and therefore cardiovascular disease. The existence of this mechanism, combined with the large sample and the dose–response relationship (more cigarettes, more disease), strengthens the case for causation considerably.

In conclusion, the student is overstating the evidence by saying the data 'prove' causation, because a single correlational study cannot prove cause and effect on its own. Nevertheless, the large sample, the dose–response pattern and the known biological mechanism together make it highly likely that smoking does contribute to cardiovascular disease. The justified statement is that smoking is strongly associated with, and very probably a cause of, cardiovascular disease, rather than that it is definitively proven.

Anticoagulants, such as warfarin and heparin, work by slowing down the blood clotting cascade — for example by interfering with the production of clotting factors or the action of thrombin. Their main benefit is therefore the prevention of unwanted clots. In a patient with atherosclerosis, a clot (thrombus) can form on a ruptured plaque; if this blocks a coronary artery it causes a heart attack, and if it blocks an artery to the brain it causes a stroke. By reducing the likelihood of such clots, anticoagulants substantially lower the risk of these life-threatening events in high-risk patients, for example those who have already had a heart attack or who have an irregular heartbeat that promotes clotting.

There are, however, significant risks. Because anticoagulants reduce the blood's ability to clot, the main danger is excessive bleeding: a minor injury, a stomach ulcer or surgery can lead to dangerous or even fatal haemorrhage. The dose must be carefully monitored, often with regular blood tests, because the correct level varies between patients and can be affected by diet and other medicines. This makes treatment inconvenient and carries a risk of accidental overdose or underdose.

Weighing these against each other depends heavily on the individual patient. For a high-risk patient — one who has already suffered a thrombosis or has a condition such as atrial fibrillation — the benefit of preventing a fatal clot clearly outweighs the bleeding risk, so anticoagulants are justified. For a low-risk patient with no history of clotting problems, the risk of harmful bleeding may outweigh the small reduction in clot risk, so the drug may not be appropriate and lifestyle measures or other treatments might be preferred.

In conclusion, anticoagulants are a valuable treatment whose benefits outweigh the risks for high-risk patients, where preventing a stroke or heart attack is critical, but they should be prescribed selectively because of the serious bleeding risk. The decision must be based on each patient's individual level of risk, with the dose carefully monitored throughout treatment.

LDL (low-density lipoprotein) carries cholesterol from the liver to the body tissues, where it is deposited in artery walls, increasing the risk of atherosclerosis.

HDL (high-density lipoprotein) carries cholesterol in the opposite direction — from the tissues back to the liver for removal — so a higher level of HDL is protective.

A high intake of salt increases the concentration of sodium ions in the blood plasma. This lowers the water potential of the blood.

Because the blood now has a more negative water potential than the surrounding tissue fluid and cells, water moves into the blood by osmosis, down the water potential gradient. Less water is also lost in the urine because the kidneys reabsorb more to balance the solute load.

The result is an increase in blood plasma volume. A greater volume of blood within the same vessels increases the pressure the blood exerts on the artery walls, so blood pressure rises.

Persistently raised blood pressure increases the shearing force on the endothelium, causing endothelial damage. This is the first step of atherosclerosis, so the formation of atheromas is accelerated, narrowing arteries and increasing the risk of heart attack and stroke.

The data show a positive correlation: as mean daily saturated fat intake increases, the percentage of adults developing coronary heart disease increases. The rise is also more than proportional — incidence climbs from 4.0% at 10 g day⁻¹ to 18.5% at 40 g day⁻¹, with the steepest increase between the 30 g and 40 g groups (an 11.0% to 18.5% jump).

The percentage increase in incidence between the lowest and highest intake groups is calculated as:

percentage increase = (18.5 − 4.0) ÷ 4.0 × 100 = 14.5 ÷ 4.0 × 100 = 362.5%.

So adults in the highest intake group were about 3.6 times more likely to develop CHD than those in the lowest intake group.

The biological link is that saturated fat raises the level of LDL in the blood. LDL deposits cholesterol in artery walls at sites of endothelial damage, building atheromas; these narrow the coronary arteries (atherosclerosis), reducing blood flow to the heart muscle and increasing the risk of coronary heart disease.

It should be noted, however, that this is correlational data: although the trend and a clear biological mechanism support a causal link, confounding variables such as differences in exercise, smoking or total energy intake between the groups would need to be controlled before causation could be firmly established.

Statins work by inhibiting an enzyme (HMG-CoA reductase) that controls cholesterol synthesis in the liver. This lowers the amount of LDL cholesterol in the blood. Because high LDL drives the deposition of cholesterol in artery walls, reducing LDL slows the formation of atheromas and the progression of atherosclerosis. The main benefit is therefore a reduced risk of heart attacks and strokes, and clinical trials show this benefit is substantial in people who already have, or are at high risk of, cardiovascular disease.

However, there are risks and drawbacks. Statins can cause side-effects such as muscle pain and, rarely, muscle damage or liver problems, so patients may need monitoring. They must usually be taken for life to remain effective, which raises issues of long-term cost and patient compliance. For a person at low risk, the absolute reduction in their chance of a cardiovascular event is small, so the side-effects and cost may not be justified; lifestyle changes such as reducing saturated fat and increasing exercise could lower their cholesterol without the drug.

Weighing these factors, the balance depends on the individual's level of risk. For a high-risk patient — for example one who has already had a heart attack or has very high LDL — the reduction in the chance of a fatal event clearly outweighs the modest side-effect risk, so statins are justified. For a low-risk patient, lifestyle measures should be tried first.

In conclusion, statins are a valuable and effective treatment whose benefits outweigh the risks for high-risk patients, but they should be prescribed selectively rather than to everyone, because the small benefit to low-risk individuals may not justify the side-effects and lifelong cost.