The Heart and Circulatory System

The human heart is a muscular organ with four chambers that pumps blood around the body.

Chambers

• Right atrium — receives deoxygenated blood from the body via the vena cava
• Right ventricle — pumps deoxygenated blood to the lungs via the pulmonary artery
• Left atrium — receives oxygenated blood from the lungs via the pulmonary vein
• Left ventricle — pumps oxygenated blood to the body via the aorta

Key Features

• The left ventricle has a thicker muscular wall than the right ventricle because it pumps blood at high pressure around the entire body (the right ventricle only pumps blood to the nearby lungs)
• Valves prevent the backflow of blood:
  • Atrioventricular (AV) valves — between the atria and ventricles
  • Semi-lunar valves — at the base of the aorta and pulmonary artery
• The septum is the muscular wall separating the left and right sides, preventing oxygenated and deoxygenated blood from mixing
• Coronary arteries supply the heart muscle itself with oxygenated blood and nutrients

Humans have a double circulatory system — blood passes through the heart twice in one complete circuit:

Circuit 1: Pulmonary Circulation

$\text{Right ventricle} \xrightarrow{\text{pulmonary artery}} \text{Lungs} \xrightarrow{\text{pulmonary vein}} \text{Left atrium}$

• Blood is pumped to the lungs to pick up oxygen and release carbon dioxide

Circuit 2: Systemic Circulation

$\text{Left ventricle} \xrightarrow{\text{aorta}} \text{Body} \xrightarrow{\text{vena cava}} \text{Right atrium}$

• Oxygenated blood is pumped to the body's organs and tissues

Advantages of Double Circulation

• Blood returns to the heart after visiting the lungs, so it can be pumped at high pressure to the body
• This delivers oxygen and nutrients to tissues more efficiently than a single circulatory system

There are three main types of blood vessel:

Arteries

• Thick walls with muscle and elastic tissue to withstand high pressure
• Elastic tissue stretches and recoils with each heartbeat, helping to maintain blood flow
• Blood flows in pulses corresponding to heartbeats

Veins

• Thinner walls than arteries because blood pressure is lower
• Have valves to prevent blood flowing backwards
• Wider lumen than arteries to reduce resistance to flow
• Skeletal muscles help squeeze blood through veins

Capillaries

• Extremely thin walls — only one cell thick, allowing efficient diffusion of substances
• Form dense networks in tissues (capillary beds) to maximise the surface area for exchange
• Substances exchanged: oxygen, carbon dioxide, glucose, amino acids, waste products

Blood is a tissue made up of four main components:

Haemoglobin and Oxygen Transport

In the lungs (high oxygen concentration): $\text{Haemoglobin} + \text{Oxygen} \rightarrow \text{Oxyhaemoglobin}$

In the tissues (low oxygen concentration): $\text{Oxyhaemoglobin} \rightarrow \text{Haemoglobin} + \text{Oxygen}$

Coronary heart disease is caused by a build-up of fatty deposits (atheroma/plaque) in the coronary arteries.

How It Develops

1. Fatty deposits build up on the inner walls of coronary arteries
2. The lumen (space inside the artery) narrows
3. Blood flow to the heart muscle is restricted
4. The heart muscle receives less oxygen and glucose
5. This can cause angina (chest pain) or a heart attack (if blood supply is completely blocked)

Treatments

Artificial Hearts

• Used as a temporary measure while waiting for a heart transplant
• Or as a permanent solution when a transplant is not possible
• Advantages: No need for donor; no rejection by immune system
• Disadvantages: Risk of blood clots; need for power supply; infection risk; limited lifespan of device

Heart Transplants

• A donor heart replaces the diseased heart
• Advantages: Can restore full heart function
• Disadvantages: Donor shortage; risk of rejection (immune system attacks the donor heart); lifelong immunosuppressant drugs needed

Question: Explain why the left ventricle has a thicker muscular wall than the right ventricle. (3 marks)

Solution:

The left ventricle pumps blood to the entire body through the systemic circulation, which requires blood to travel a much longer distance than the pulmonary circuit. To achieve this, the left ventricle must generate a higher pressure to push blood through all the body's organs and tissues. A thicker muscular wall allows the left ventricle to contract with greater force to create this high pressure. The right ventricle only needs to pump blood a short distance to the nearby lungs, so it requires less force and has a thinner wall.

• Remember the exception: The pulmonary artery carries deoxygenated blood (away from the heart to the lungs), and the pulmonary vein carries oxygenated blood (from the lungs to the heart). Don't assume arteries always carry oxygenated blood.
• When describing the heart, always specify which chamber or vessel you're referring to — don't just say "the blood goes to the lungs."
• For 6-mark questions on CHD, include the cause (fatty deposits narrowing the lumen), the effect (reduced blood flow → reduced oxygen supply), and a treatment with advantages and disadvantages.

• The heart has four chambers: two atria (receive blood) and two ventricles (pump blood out).
• Humans have a double circulatory system: pulmonary (heart ↔ lungs) and systemic (heart ↔ body).
• Arteries have thick walls and carry blood at high pressure; veins have valves; capillaries are one cell thick for exchange.
• Blood contains red blood cells (oxygen transport), white blood cells (defence), platelets (clotting), and plasma (transport of dissolved substances).
• Coronary heart disease is caused by fatty deposits narrowing coronary arteries; treated with stents, statins, or surgery.