The Heart and Blood Vessels

The heart is the organ at the centre of the circulatory system, pumping blood around the body to deliver oxygen and nutrients and remove waste products. This lesson covers the structure of the heart, the double circulatory system, and the three types of blood vessel — all as required by the AQA GCSE Combined Science Trilogy specification (8464).

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The Double Circulatory System

Mammals, including humans, have a double circulatory system. This means that blood passes through the heart twice during each complete circuit of the body.

graph LR
    subgraph "Circuit 1 — Pulmonary"
        H1["Right side of heart"] -->|"Deoxygenated blood"| L["Lungs"]
        L -->|"Oxygenated blood"| H2["Left side of heart"]
    end
    subgraph "Circuit 2 — Systemic"
        H2 -->|"Oxygenated blood"| B["Body tissues"]
        B -->|"Deoxygenated blood"| H1
    end

Circuit	Name	Blood Flow	Purpose
Circuit 1	Pulmonary circulation	Heart → lungs → heart	Blood picks up oxygen and releases carbon dioxide
Circuit 2	Systemic circulation	Heart → body → heart	Blood delivers oxygen and nutrients to cells and removes waste

Advantage of a double circulatory system: Blood returns to the heart after visiting the lungs, so it can be pumped at high pressure to the body. This ensures rapid delivery of oxygen and glucose to respiring cells. A single circulatory system (as in fish) delivers blood at lower pressure after passing through the gills.

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Structure of the Heart

The heart is a muscular organ with four chambers:

Chamber	Position	Function
Right atrium	Top right	Receives deoxygenated blood from the body via the vena cava
Right ventricle	Bottom right	Pumps deoxygenated blood to the lungs via the pulmonary artery
Left atrium	Top left	Receives oxygenated blood from the lungs via the pulmonary vein
Left ventricle	Bottom left	Pumps oxygenated blood to the body via the aorta

Blood Flow Through the Heart

graph TD
    VC["Vena cava (from body)"] --> RA["Right atrium"]
    RA -->|"Tricuspid valve"| RV["Right ventricle"]
    RV -->|"Semilunar valve"| PA["Pulmonary artery (to lungs)"]
    PA --> LUNGS["Lungs — gas exchange"]
    LUNGS --> PV["Pulmonary vein (to heart)"]
    PV --> LA["Left atrium"]
    LA -->|"Bicuspid (mitral) valve"| LV["Left ventricle"]
    LV -->|"Semilunar valve"| AO["Aorta (to body)"]

Key Structural Features

Feature	Explanation
Left ventricle wall is thicker than the right	The left ventricle must pump blood all the way around the body (systemic circulation) at high pressure, so it needs a thicker muscular wall to generate a stronger force. The right ventricle only pumps blood the short distance to the lungs.
Valves (tricuspid, bicuspid, semilunar)	Prevent the backflow of blood, ensuring blood flows in one direction only
Septum	A thick muscular wall dividing the left and right sides of the heart, preventing oxygenated and deoxygenated blood from mixing
Coronary arteries	Supply the heart muscle itself with oxygenated blood and nutrients for respiration

Exam Tip: Remember that on a diagram of the heart, left and right are reversed (as if you are looking at someone facing you). The left ventricle appears on the right side of the diagram. Also note that arteries carry blood AWAY from the heart and veins carry blood TO the heart — this is true regardless of whether the blood is oxygenated or deoxygenated.

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The Three Types of Blood Vessel

Feature	Arteries	Veins	Capillaries
Direction of flow	Away from the heart	Towards the heart	Connect arteries to veins
Blood pressure	High	Low	Low (blood slows down)
Wall thickness	Thick, muscular, elastic	Thinner, less muscle	One cell thick
Lumen (internal space)	Small	Large	Very narrow (just wide enough for one red blood cell)
Valves	No (except semilunar in aorta/pulmonary artery)	Yes (to prevent backflow)	No
Oxygen content	Usually oxygenated (except pulmonary artery)	Usually deoxygenated (except pulmonary vein)	Varies — exchange occurs here

graph LR
    A["Arteries"] -->|"Blood flows to"| B["Arterioles"]
    B -->|"Blood flows to"| C["Capillaries"]
    C -->|"Blood flows to"| D["Venules"]
    D -->|"Blood flows to"| E["Veins"]

Why Capillaries Are Adapted for Exchange

• Walls are only one cell thick — this provides a very short diffusion distance, so substances can move in and out of the blood quickly
• Very narrow lumen — red blood cells are squeezed through in single file, bringing them close to the capillary wall for efficient gas exchange
• Huge network — capillaries form dense networks (capillary beds) in tissues, providing a very large total surface area for exchange
• Permeable walls — allow substances like oxygen, glucose, carbon dioxide and urea to pass through

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The Heartbeat

The heart beats automatically without needing signals from the brain (this is called myogenic contraction). The natural resting heart rate is controlled by a group of cells in the right atrium called the natural pacemaker (sinoatrial node, or SAN).

The pacemaker:

• Produces small electrical impulses that spread across the atria, causing them to contract
• The signal then passes to the ventricles, causing them to contract
• This coordinated contraction produces the heartbeat

If the natural pacemaker is faulty, an artificial pacemaker (a small electrical device) can be implanted to regulate the heartbeat.

Exam Tip: The heart rate can be calculated from pulse rate. You can also calculate cardiac output:

$$\text{Cardiac output} = \text{stroke volume} \times \text{heart rate}$$Cardiac output=stroke volume×heart rate

Where stroke volume is the volume of blood pumped per beat, and heart rate is the number of beats per minute.

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Common Mistakes