Circulatory System in Animals (AQA A-Level Biology): Revision Notes
Cardiovascular Disease
Risk factors for cardiovascular disease
Several factors independently raise the likelihood of developing cardiovascular disease. When multiple risk factors occur together, they create a disproportionately greater risk than the sum of individual factors. The main risk factors include smoking, high blood pressure, elevated blood cholesterol, and poor dietary choices.
Understanding risk factors is crucial because they often work together in ways that multiply rather than simply add to overall disease risk.
Smoking
Smoking dramatically increases cardiovascular disease risk, with smokers being two to six times more likely to develop heart disease compared to non-smokers. Tobacco smoke contains two particularly harmful components that directly affect the cardiovascular system.
Carbon monoxide binds easily and irreversibly to haemoglobin in red blood cells, forming carboxyhaemoglobin. This reduces the blood's oxygen-carrying capacity, meaning the heart must work harder to supply adequate oxygen to body tissues. During exercise, this insufficient oxygen supply can cause chest pain (angina) or, in severe cases, lead to a myocardial infarction (heart attack).
Nicotine stimulates the production of adrenaline, which increases heart rate and raises blood pressure. This puts additional strain on the cardiovascular system and elevates the risk of coronary heart disease and stroke. Nicotine also makes platelets in the blood more "sticky", increasing the likelihood of thrombosis and subsequent strokes or heart attacks.
Stopping smoking is the single most effective way to improve life expectancy and reduce cardiovascular risk.
High blood pressure
High blood pressure can result from genetic factors that cannot be modified through lifestyle changes. However, lifestyle factors such as chronic stress, poor diet, and lack of exercise can worsen high blood pressure in susceptible individuals.
High blood pressure increases cardiovascular disease risk through several mechanisms:
Three Key Mechanisms of High Blood Pressure Damage:
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The heart must work harder to pump blood into arteries that already have elevated pressure, making the heart more susceptible to failure over time.
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Higher pressure within arteries increases the risk of developing an aneurysm (weakening of the arterial wall) which may burst and cause dangerous bleeding.
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To withstand increased pressure, arterial walls tend to thicken and harden, which restricts blood flow and reduces the efficiency of circulation.
Blood cholesterol
Cholesterol is an essential component of cell membranes and must be transported through the blood as it cannot dissolve in plasma. It travels as tiny particles called lipoproteins, which combine lipids and proteins.
There are two main types of cholesterol-carrying lipoproteins:
High-density lipoproteins (HDLs) remove cholesterol from tissues and transport it to the liver for breakdown and excretion. HDLs help protect arteries against heart disease and are often called "good" cholesterol.
Low-density lipoproteins (LDLs) transport cholesterol from the liver to tissues throughout the body, including arterial walls. When LDLs infiltrate artery walls, they contribute to atheroma formation, which can lead to heart disease. LDLs are often referred to as "bad" cholesterol.
Remember the difference: HDL = "Healthy" cholesterol that removes cholesterol from arteries, while LDL = "Lousy" cholesterol that deposits cholesterol in arterial walls.
Diet
Several dietary factors influence cardiovascular disease risk both directly and indirectly:
High salt intake raises blood pressure, which then increases cardiovascular disease risk through the mechanisms described above.
High saturated fat consumption increases LDL cholesterol levels in the blood, leading to higher overall cholesterol concentrations and greater risk of atheroma formation.
Conversely, foods acting as antioxidants (such as vitamin C) reduce cardiovascular disease risk. Non-starch polysaccharides (dietary fibre) also provide protective effects against heart disease.
Combined effects of risk factors
When multiple risk factors occur together, they create a multiplicative rather than additive effect on disease risk. For example, a person with four or five risk factors faces a disproportionately higher risk than someone with just one or two factors.
Research Finding: Combined Risk Factor Effects
Data from studies of American men show that the combination of smoking, high blood pressure, and elevated cholesterol creates significantly higher heart attack risk than any single factor alone. The effects are particularly pronounced in individuals who smoke and have high blood pressure, where the risk curve rises steeply compared to non-smokers with normal blood pressure.
Understanding probability vs certainty
Risk factors increase the probability of developing cardiovascular disease but do not guarantee that someone will develop it. Heavy smokers with high blood pressure and elevated cholesterol may never develop heart disease, while some individuals with fewer risk factors might still be affected. Risk factors simply make disease more likely to occur.
Think of risk factors as loading dice rather than determining outcomes - they increase the chances of an unfavourable result but don't make it inevitable.
Remember!
Key Points to Remember:
- Multiple risk factors combine to create disproportionately greater cardiovascular disease risk than individual factors alone
- Smoking reduces oxygen transport through carbon monoxide and raises blood pressure through nicotine stimulation
- High blood pressure forces the heart to work harder and increases aneurysm risk while causing arterial wall thickening
- HDL cholesterol protects against heart disease by removing cholesterol from tissues, while LDL cholesterol contributes to atheroma formation
- Dietary factors like high salt and saturated fat increase risk, whilst antioxidants and fibre provide protection