Cardiovascular Disease

Most of the time our hearts and blood vessels do a pretty good job at delivering oxygenated blood to various body tissues. Sometimes though, damage occurs in an artery which can lead to it becoming blocked. If a complete blockage occurs, body tissues might not receive enough oxygen and start to die - that’s how a heart attack happens.

Atherosclerosis

Atherosclerosis a process in which the artery lining becomes hard due to the build-up of fatty material in the artery wall. It is a problem because it can lead to the artery becoming completely blocked, cutting off blood circulation and leading to events like a heart attack or stroke. It occurs in the following stages:

• The inner lining of the artery wall (the endothelium) becomes damaged due to high blood pressure (hypertension).

• This triggers an inflammatory response - white blood cells (such as macrophages) move towards the site of damage and accumulate with lipids (fats) which are circulating in the blood. The accumulation of these substances underneath the artery wall leads to the formation of fatty streaks in the endothelium.

• The fatty streaks develop into an atheroma, causing the lumen of the artery to become narrower and restricting blood flow. This causes blood pressure to increase further.

• The hardening of arteries, caused by the development of atheromas, is referred to as atherosclerosis.

Cardiovascular disease

The formation of an atheroma in an artery causes partial blockage of an artery. If the atheroma ruptures, it can trigger a blood clot to form which may completely prevent blood flow. The stages of this process occur as follows:

• The atheroma ruptures and bursts through the endothelium of the artery, damaging the artery wall.

• This triggers blood clotting (thrombosis) at the site of damage.

• The blood clot can cause a complete blockage within the artery.

• This prevents blood flow in the artery which means that oxygen isn’t delivered to the tissues downstream of the artery.

• The cells within the tissue cannot carry out aerobic respiration and will start to die.

If the blockage happens within a coronary artery, the blockage will lead to a heart attack. If it occurs in a blood vessel leading to the brain it will cause a stroke and if it happens in an artery in the legs in can cause deep vein thrombosis (DVT).

Blood Clotting

Fragments in our blood, called platelets, clump together at the site of a wound to prevent the excessive loss of blood when we injure ourselves. It also prevents microorganisms from entering our body when the skin’s protective barrier is broken. However, when this process happens inside arteries it can be dangerous as it has the potential to completely restrict blood flow within the vessel.

The process of blood clotting (thrombosis) takes place in the following stages:

• An enzyme called thromboplastin is released from damaged blood vessels. It requires calcium ions in order to function.

• Thromboplastin catalyses the formation of prothrombin into thrombin, which is itself an enzyme.

• Thrombin catalyses the formation of fibrinogen into fibrin. Fibrinogen is a soluble protein that is dissolved in the blood. Fibrin is insoluble and forms fibres which tangle together platelets and red blood cells to form a blood clot.

Risk factors for CVD

Things which increase our risk of developing cardiovascular disease can be grouped into lifestyle factors (things which result from our behaviour and are under our control) and non-lifestyle factors (things that we were born with and cannot control). Some of these factors are interlinked - for example, the genes you have may mean that you are more susceptible to high blood pressure, which is also affected by your diet.

Lifestyle factors

• Smoking

  • The nicotine in cigarette smoke makes platelets more sticky, increasing the chance of a blood clot forming which could lead to blockage in the arteries.

  • Carbon monoxide in cigarette smoke binds to haemoglobin in red blood cells, reducing the amount of oxygen being transported in the blood. This means the heart has to pump faster to get the same amount of oxygen delivery, which increases blood pressure.

  • Smoking also reduces levels of antioxidants in the blood. Antioxidants are molecules in our body which can protect the arteries from damage. The reduced levels of antioxidants in smokers can increase the likelihood of endothelial damage and atheroma formation.

• High blood pressure

  High blood pressure puts more strain on the artery walls and means they are more likely to suffer damage. This leads to the process of atherosclerosis which in turn can lead to blood clotting and cardiovascular disease. A diet high in salt, high alcohol consumption, inactivity and stress can increase blood pressure.

• Diet

  • A diet high in saturated fats and low in unsaturated fats can increase the risk of CVD. Consumption of saturated fats increases blood cholesterol levels which increase the likelihood of atheroma formation.

  • A diet high in salt can increase the risk of CVD by increasing blood pressure.

• Lack of exercise

  A sedentary lifestyle can increase the chances of CVD by increasing blood pressure.

Non-lifestyle factors

• Age

  The risk of CVD increases with age (heart attacks are much more uncommon in people under the age of 40). This is because the fatty deposits which accumulate in our arteries build up over time. In fact, atherosclerosis is happening even in young children. It’s only when the individual becomes middle-aged that the hardening of arteries can become problematic.

• Genetics

  Some people are unlucky enough to inherit certain alleles which increase cholesterol production or make them more susceptible to high blood pressure. Likewise, other people can inherit beneficial alleles which will reduce their risk of CVD by dampening down cholesterol production or giving them slightly lower blood pressure.

• Gender

  Men are at a much greater risk of developing CVD than women due to lower oestrogen levels. Oestrogen increases the levels of ‘good’ cholesterol (HDLs) which removes the amount of cholesterol circulating in the blood. This reduces the risk of atheroma formation.

Good and bad cholesterol - HDLs and LDLs

Not all fats are created equally and the different types of fat can have very different effects on our bodies. Saturated fats, which contain only single C-C bonds, are more unhealthy and are found in animal-based foods such as cheeses, butter and beef burgers. Unsaturated fats, which contain at least one double C=C bond, are healthier and are typically found in plant-based foods such as olive oil, nuts and oily fish.

Saturated fats increase the levels of a molecule called ‘low-density lipoprotein (LDL)’ in the blood. Lipoproteins are like little packages of lipids surrounded by protein and function to transfer cholesterol between the liver (where it is made and broken down) and the body tissues. LDLs take cholesterol from the liver and deposits it in the blood, where it will circulate until it is needed by our cells. The other type of lipoprotein, called high-density lipoprotein (HDL) transports cholesterol from the bloodstream and returns it to the liver where it can be destroyed. A high intake of unsaturated fats increase HDL levels in our blood. This means that a high amount of LDLs and low levels of HDLs can have the effect of increasing cholesterol levels, which make us more susceptible to cardiovascular disease.

Our cells need cholesterol because it is used to make sex-hormones (such as oestrogen), to synthesise bile and as a component of plasma membranes. The problem is that our liver already synthesises enough cholesterol, then on top of that we consume way more than our bodies know what to do with. Therefore, it is important that we have a high ratio of HDL to LDLs by eating more unsaturated fats to saturated fats. High levels of HDLs will ensure that excess cholesterol is taken back to the liver to be broken down and removed from our body.

Obesity Indicators

Doctors use measurements such as the body mass index (BMI) and waist-to-hip ratio to determine whether someone is packing on the pounds and needs to lay off the biscuits. People who are identified in the overweight and obese categories will be more at risk of developing CVD and will be advised by their doctor to eat healthier and to carry out more exercise.

Body mass index (BMI) - a person’s BMI is calculated by dividing their body mass by their height squared. The problem with calculating a BMI is that it doesn’t take into account whether their body mass consists of fat or muscle. Using BMI, a bodybuilder could be classified as morbidly obese even though they have little fat on their body and are perfectly healthy.

Waist-to-hip ratio - the waist-to-hip ratio is calculated by dividing an individual’s waist size by their hip size. It takes into account where fat is accumulating on the body and can be a more accurate prediction of whether a person has excess weight.

Treatments for CVD

• Antihypertensives - beta-blockers and vasodilators are two types of drug which act as antihypertensives since they work by lowering blood pressure. A lower blood pressure means there is a lower risk of endothelial dysfunction (damage to the artery wall). The side-effects of antihypertensives include heart palpitations, fainting, headaches and drowsiness.

• Statins - these are drugs which work by reducing cholesterol levels by lowering the LDL concentration in the blood. Side-effects of statins include muscle pain, nausea, headaches, increased risk of diabetes and problems with the digestive system.

• Anticoagulants - these are substances which reduce blood clotting. The risks include excessive bleeding if the person injures themselves, allergic reactions, tissue swelling and osteoporosis.

• Platelet inhibitors - these are also substances which reduce blood clotting which work by preventing platelets from sticking together. The side-effects include rashes, nausea, liver dysfunction, diarrhoea and excessive bleeding if an injury occurs.

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