Heart Disease and Risk Factors (Unit 2: Biological Systems and Disease)

Welcome to this crucial chapter! This topic is incredibly important because heart disease, specifically Coronary Heart Disease (CHD), is a major non-communicable illness globally. As Biology students, you need to understand the underlying biological process (how the arteries get blocked) and the specific lifestyle factors that dramatically increase a person's risk. Don't worry if the medical terms sound complicated; we will break them down step-by-step!

The goal of this section is not just to memorise risk factors, but to understand the biological mechanisms by which these factors cause disease.

1. The Biological Basis of Heart Disease: Atheroma

The core mechanism leading to most heart disease involves the damage and blockage of arteries, particularly the coronary arteries (the arteries that supply oxygen and nutrients directly to the heart muscle itself).

1.1 What is Atheroma?

Atheroma refers to the presence of lipid-containing material (fatty substances, including cholesterol) deposited within the walls of the arteries. This process is called Atherosclerosis.

Analogy: Think of a healthy artery as a clean, wide water pipe. Atheroma is like gunk building up inside the pipe, making the walls stiff and the hole much narrower. This buildup is often called plaque.

Step-by-Step Atheroma Formation:

  1. Damage occurs to the inner lining (endothelium) of the artery wall (often due to factors like high blood pressure or toxins from smoking).
  2. This damage triggers an inflammatory response.
  3. Lipids (fats and cholesterol) from the blood accumulate at the site of damage, along with white blood cells (macrophages).
  4. A fibrous cap forms over this fatty core, creating the hard plaque known as atheroma.
  5. The plaque hardens and thickens the artery wall, leading to two major problems: loss of elasticity and narrowing of the lumen (the space inside the vessel).
1.2 Serious Consequences of Atheroma

The presence of atheroma increases the risk of three critical, life-threatening conditions:

1. Increased risk of Aneurysm:

  • As the atheroma stiffens the artery wall, it weakens the wall structure.
  • The high pressure of the blood can cause the weakened section of the artery to balloon outwards. This ballooning is called an aneurysm.
  • If the aneurysm ruptures (bursts), it leads to massive internal bleeding, which is often fatal.

2. Increased risk of Thrombosis:

  • The surface of the atheroma plaque is rough, unlike the smooth lining of a healthy artery.
  • This rough surface can trigger the formation of a blood clot inside the vessel, known as a thrombus.
  • This rapid formation of a clot is called thrombosis.
  • The thrombus can quickly block the artery entirely, cutting off blood flow immediately.

Quick Review: Atheroma Links

Atheroma (plaque) $\rightarrow$ Weakens wall $\rightarrow$ Aneurysm (ballooning/rupture)
Atheroma (plaque) $\rightarrow$ Creates rough surface $\rightarrow$ Thrombosis (clot formation/blockage)

2. Myocardial Infarction (Heart Attack)

Atheroma directly leads to the condition most people call a "heart attack." The biological term for a heart attack is Myocardial Infarction (MI).

2.1 What causes a Myocardial Infarction?

MI is caused by an interruption of blood flow to the cardiac muscle (myocardium).

If a blood clot (thrombus) completely blocks one of the small coronary arteries, the heart muscle cells supplied by that artery are deprived of oxygen and glucose.

  • Lacking oxygen for respiration, these cells cannot produce ATP.
  • The cardiac muscle tissue quickly dies (infarction).
  • This death of muscle tissue prevents the affected area of the heart from contracting properly, leading to a heart attack.

Did you know? The severity of a heart attack depends on which artery is blocked and how much cardiac muscle tissue is starved of oxygen.

3. Key Risk Factors Associated with Coronary Heart Disease

Risk factors are aspects of a person's lifestyle, environment, or inherited traits that make them statistically more likely to develop a disease. The syllabus requires you to understand four key lifestyle risk factors for CHD:

3.1 Diet and High Blood Cholesterol

Your diet directly influences the lipid levels in your blood, specifically cholesterol.

The Mechanism:

  • Consuming a diet high in saturated fats leads to increased levels of Low-Density Lipoproteins (LDL), often called 'bad cholesterol', in the blood.
  • High levels of LDL cholesterol contribute significantly to the lipid-containing material that forms the atheroma plaque inside the artery walls.
  • Therefore, a diet high in saturated fat raises blood cholesterol, accelerating the process of atherosclerosis and increasing the risk of blockage and thrombosis.

Accessibility Tip: Remember that LDL is the Lousy/Lethal cholesterol that causes buildup, while HDL is the Healthy cholesterol that helps remove fat from tissues.

3.2 High Blood Pressure (Hypertension)

Blood pressure is the force exerted by the blood against the walls of the arteries.

The Mechanism:

  • Chronically high blood pressure (hypertension) puts immense physical stress on the artery walls.
  • This excessive force damages the delicate inner lining (endothelium) of the artery.
  • This damage acts as an ideal site for the initial accumulation of lipids and the beginning of atheroma formation.
  • The heart muscle itself also has to work harder against high resistance, leading to thickening (hypertrophy) and eventual exhaustion of the heart muscle.
3.3 Cigarette Smoking

Cigarette smoke contains thousands of chemicals, several of which directly damage the cardiovascular system.

The Mechanism:

  • Carbon Monoxide (CO): CO combines readily with haemoglobin, reducing the blood's ability to transport oxygen. This means the heart has to pump faster and harder to deliver sufficient oxygen, increasing stress.
  • Nicotine: This chemical stimulates the production of the hormone adrenaline, which causes vasoconstriction (narrowing of arteries) and increases heart rate and blood pressure.
  • Chemicals and Toxins: Many chemicals in smoke directly damage the endothelium (inner lining) of arteries, dramatically speeding up atheroma formation.
  • Smoking also increases the risk of blood clotting (thrombosis).
3.4 Other Risk Factors

While the syllabus highlights the four major lifestyle factors, it's worth noting other biological factors that often interact with these:

  • Genetics: Some individuals inherit a predisposition for high cholesterol or high blood pressure.
  • Lack of Exercise: Physical activity helps lower blood pressure and improves the ratio of HDL ('good') to LDL ('bad') cholesterol. Inactivity increases risk.

4. Evaluating Data and Reducing Risk

A key skill you must demonstrate is the ability to analyse, interpret, and evaluate data related to these risk factors and the incidence of CHD.

4.1 Understanding Correlation vs. Causation

Epidemiological studies (studies of disease incidence) often show a correlation between a risk factor (like smoking) and a disease (CHD). Remember a core biological principle:

Correlation between variables does not necessarily mean that there is a causal relationship.

However, scientists use evidence (like the biological mechanisms listed above) to support the claim that high blood pressure *causes* (is causally related to) atherosclerosis, not just that they happen to occur together.

4.2 Reducing the Risk

Understanding these mechanisms allows us to develop effective public health advice:

  • Dietary Change: Reducing intake of saturated fats lowers blood cholesterol levels, slowing atheroma formation.
  • Cessation of Smoking: Stopping smoking reduces immediate stress on the heart and allows damaged endothelium to begin repairing itself.
  • Medication: Drugs (like statins for cholesterol or anti-hypertensives for blood pressure) directly treat the biological abnormalities caused by the risk factors.

Don't worry if this seems tricky at first; understanding the 'pipes are getting clogged and stressed' analogy is a great starting point!

Key Takeaway Summary

CHD is caused by atherosclerosis (atheroma plaque buildup) in the coronary arteries.

  • Atheroma (lipid-rich plaque) leads to artery narrowing and stiffness.
  • Atheroma increases risk of Aneurysm (bursting) and Thrombosis (clotting).
  • A Myocardial Infarction occurs when a thrombus blocks a coronary artery, causing heart muscle death due to oxygen starvation.
  • Major Risk Factors (Diet, Cholesterol, Smoking, High BP) increase CHD incidence by either damaging the artery lining or increasing lipid deposition, thus accelerating atheroma formation.