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The heart is the muscular organ at the centre of the circulatory system. In this lesson you will learn about the double circulatory system, the structure of the heart, and how blood flows through it.
The circulatory system transports substances around the body. It is made up of:
Humans have a closed circulatory system, meaning blood is always contained within blood vessels (it does not flow freely around body cavities).
Humans have a double circulatory system. This means blood passes through the heart twice during one complete circuit of the body.
The two circuits are:
| Circuit | Route | Purpose |
|---|---|---|
| Pulmonary circuit | Heart → lungs → heart | Blood picks up oxygen and releases carbon dioxide |
| Systemic circuit | Heart → body → heart | Blood delivers oxygen and nutrients to cells, and collects waste |
Because blood passes through the heart twice per circuit, it can be pumped at high pressure both to the lungs and to the rest of the body. This ensures:
Exam tip: A common question asks you to explain the advantage of a double circulatory system. The key answer is: blood returns to the heart after visiting the lungs so it can be pumped at high pressure to the body, ensuring efficient delivery of oxygen and glucose to tissues.
The heart is a muscular organ roughly the size of your fist, located slightly to the left of centre in the chest cavity. It is made of cardiac muscle, which contracts rhythmically without tiring.
| Chamber | Position | Function |
|---|---|---|
| Right atrium (RA) | Top right | Receives deoxygenated blood from the body via the vena cava |
| Right ventricle (RV) | Bottom right | Pumps deoxygenated blood to the lungs via the pulmonary artery |
| Left atrium (LA) | Top left | Receives oxygenated blood from the lungs via the pulmonary vein |
| Left ventricle (LV) | Bottom left | Pumps oxygenated blood to the body via the aorta |
Important note on heart diagrams: When you look at a diagram of the heart, the left side appears on the right of the diagram and vice versa. This is because diagrams show the heart as if you are looking at the patient — so their left is your right.
The septum is a thick muscular wall that separates the left and right sides of the heart. It prevents oxygenated and deoxygenated blood from mixing.
Follow this pathway carefully — it is a very common exam question:
graph TD
A["Body"] -->|"deoxygenated blood via vena cava"| B["Right Atrium"]
B -->|"through tricuspid valve"| C["Right Ventricle"]
C -->|"through pulmonary valve"| D["Pulmonary Artery"]
D --> E["Lungs - gas exchange"]
E -->|"oxygenated blood"| F["Pulmonary Vein"]
F --> G["Left Atrium"]
G -->|"through bicuspid valve"| H["Left Ventricle"]
H -->|"through aortic valve"| I["Aorta"]
I --> A
style B fill:#3498db,color:#fff
style C fill:#3498db,color:#fff
style D fill:#3498db,color:#fff
style E fill:#2ecc71,color:#fff
style G fill:#e74c3c,color:#fff
style H fill:#e74c3c,color:#fff
style I fill:#e74c3c,color:#fff
Exam tip: Remember — arteries carry blood Away from the heart and veins carry blood towards (into) the heart. The pulmonary artery is special because it carries deoxygenated blood, and the pulmonary vein is special because it carries oxygenated blood. They are named for the vessel type, not the blood they carry.
Valves are essential structures that prevent the backflow of blood, ensuring it always flows in one direction.
| Valve | Location | Function |
|---|---|---|
| Tricuspid valve | Between right atrium and right ventricle | Prevents backflow from RV to RA |
| Bicuspid (mitral) valve | Between left atrium and left ventricle | Prevents backflow from LV to LA |
| Semilunar valves | At the base of the pulmonary artery and aorta | Prevent backflow from arteries back into ventricles |
The "lub-dub" sound of the heartbeat is caused by the valves closing.
The left ventricle has a much thicker muscular wall than the right ventricle.
Why? The left ventricle must pump blood at high pressure all the way around the body (systemic circuit). The right ventricle only needs to pump blood the short distance to the lungs (pulmonary circuit), so it requires less force.
Exam tip: If asked why the left ventricle wall is thicker, always link it to the distance blood must travel and the pressure needed. A common wrong answer is "because it pumps more blood" — both ventricles pump the same volume of blood per beat.
The coronary arteries branch off the aorta and supply the heart muscle itself with oxygenated blood and glucose.
The heart's natural rhythm is controlled by a group of specialised cells called the sinoatrial node (SAN), located in the wall of the right atrium.
If the natural pacemaker malfunctions, an artificial pacemaker can be implanted — a small electrical device that sends regular electrical impulses to keep the heart beating at the correct rate.
| Key Term | Definition |
|---|---|
| Double circulatory system | Blood passes through the heart twice per circuit |
| Pulmonary circuit | Heart → lungs → heart |
| Systemic circuit | Heart → body → heart |
| Septum | Wall separating left and right sides of the heart |
| Coronary arteries | Blood vessels supplying the heart muscle with oxygen |
| SAN (pacemaker) | Group of cells in the right atrium that controls heart rate |
| Valves | Structures that prevent the backflow of blood |
Exam tip: When describing blood flow, always name the specific blood vessels and chambers in the correct order. Simply writing "blood goes from the heart to the lungs" will not gain full marks — you need to say "blood travels from the right ventricle through the pulmonary artery to the lungs."
A classic six-mark question asks you to compare the structure and function of the left and right ventricles. Examiners expect precise comparative language, not two separate paragraphs.
| Feature | Left Ventricle | Right Ventricle |
|---|---|---|
| Wall thickness | Thick muscular wall (approx. 10–15 mm) | Thin muscular wall (approx. 3–5 mm) |
| Pressure generated | High — enough to pump blood around the entire body | Lower — only needs to reach the nearby lungs |
| Destination of blood | Aorta → systemic circulation (whole body) | Pulmonary artery → lungs only |
| Distance pumped | Several metres through every tissue | Short distance to lungs |
| Oxygenation status | Oxygenated blood | Deoxygenated blood |
Model answer: "The left ventricle has a thicker muscular wall than the right ventricle because it must generate a higher pressure to pump oxygenated blood through the aorta around the entire systemic circulation. The right ventricle pumps deoxygenated blood a shorter distance to the lungs, so a thinner wall generates sufficient pressure. A thicker wall on the right side would force blood through the delicate alveolar capillaries at a pressure high enough to rupture them."
Common mistake callout: Many students write "the left ventricle is bigger" — this loses marks. You must specify that the wall is thicker (more cardiac muscle), not that the chamber itself is larger. In fact the two chambers hold roughly the same volume of blood.
Cardiac output is the volume of blood pumped by the left ventricle per minute. The formula examiners expect you to use is:
Cardiac output (cm³/min) = stroke volume (cm³) × heart rate (beats/min)
Worked example: An athlete at rest has a stroke volume of 80 cm³ and a heart rate of 60 beats/min. During exercise, their stroke volume rises to 120 cm³ and heart rate to 150 beats/min. Calculate the percentage increase in cardiac output.
This shows why training improves cardiovascular performance: larger stroke volume plus raised heart rate multiplies the oxygen delivered to respiring muscle cells per minute.
The cardiac cycle describes one full heartbeat and has three main stages, each lasting around 0.3–0.4 seconds in a resting adult:
graph LR
A["Atrial Systole"] --> B["Ventricular Systole"]
B --> C["Diastole"]
C --> A
Exam tip: If a question gives a pressure-volume graph, remember the AV valves open when atrial pressure exceeds ventricular pressure, and close when ventricular pressure exceeds atrial pressure. The semilunar valves behave similarly with aortic/ventricular pressure.
Grade boundaries reward increasingly specific vocabulary. Use the examples below to audit your own answers.
Edexcel alignment: This content is aligned with Edexcel GCSE Biology (1BI0) specification Topic 8 Exchange and transport in animals / Topic 9 Ecosystems and material cycles — specifically 8.2 Circulatory system, 8.3 Blood. Assessed on Paper 2.