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This lesson covers the structure of the heart, the double circulatory system and how the heart functions as a pump, as required by the Edexcel GCSE Combined Science specification (1SC0). You need to label the chambers, valves and major blood vessels, explain double circulation and understand cardiac output.
Humans have a double circulatory system. This means blood passes through the heart twice on each complete circuit of the body:
graph TD
A["Right atrium"] -->|"Through tricuspid valve"| B["Right ventricle"]
B -->|"Through pulmonary valve"| C["Pulmonary artery → Lungs"]
C -->|"Gas exchange: O₂ in, CO₂ out"| D["Pulmonary vein → Left atrium"]
D -->|"Through bicuspid (mitral) valve"| E["Left ventricle"]
E -->|"Through aortic valve"| F["Aorta → Body tissues"]
F -->|"O₂ delivered, CO₂ collected"| G["Vena cava → Right atrium"]
G --> A
A double circulatory system maintains high blood pressure throughout the body. Blood pressure drops as blood passes through the lungs, so the left side of the heart gives the blood a second "boost" to push it at high pressure through the systemic circuit. This ensures efficient delivery of oxygen and nutrients to all body cells.
Exam Tip: The right side pumps to the lungs (pulmonary circuit) and the left side pumps to the body (systemic circuit). A common error is to mix these up. Think: Right = lungs, Left = body (L for Long journey).
The heart is a muscular organ about the size of a fist, located in the chest cavity between the lungs. It is made of cardiac muscle, which contracts rhythmically without tiring.
| Chamber | Position | Receives blood from | Pumps blood to |
|---|---|---|---|
| Right atrium | Top right | Vena cava (body) | Right ventricle |
| Right ventricle | Bottom right | Right atrium | Pulmonary artery (lungs) |
| Left atrium | Top left | Pulmonary vein (lungs) | Left ventricle |
| Left ventricle | Bottom left | Left atrium | Aorta (body) |
Valves prevent the backflow of blood, ensuring one-way flow through the heart:
| Valve | Location | Prevents backflow from |
|---|---|---|
| Tricuspid valve | Between right atrium and right ventricle | Ventricle → atrium |
| Bicuspid (mitral) valve | Between left atrium and left ventricle | Ventricle → atrium |
| Pulmonary valve (semilunar) | Between right ventricle and pulmonary artery | Artery → ventricle |
| Aortic valve (semilunar) | Between left ventricle and aorta | Artery → ventricle |
When the ventricles contract, pressure forces the atrioventricular valves (tricuspid and bicuspid) shut and opens the semilunar valves. The "lub-dub" heart sounds are caused by these valves closing.
| Vessel | Blood type | Connection |
|---|---|---|
| Vena cava (superior and inferior) | Deoxygenated | Body → right atrium |
| Pulmonary artery | Deoxygenated | Right ventricle → lungs |
| Pulmonary vein | Oxygenated | Lungs → left atrium |
| Aorta | Oxygenated | Left ventricle → body |
Exam Tip: The pulmonary artery carries deoxygenated blood (the only artery that does). The pulmonary vein carries oxygenated blood (the only vein that does). Examiners love to test whether you know these exceptions.
The left ventricle has a much thicker muscular wall than the right ventricle.
Why? The left ventricle must generate enough pressure to pump blood all the way around the body (systemic circuit), which is a much longer distance than from the right ventricle to the lungs (pulmonary circuit). A thicker wall produces a more powerful contraction.
The coronary arteries branch off the aorta and supply the heart muscle itself with oxygenated blood and glucose for aerobic respiration. The heart muscle needs a constant supply of energy because it never stops beating.
If a coronary artery becomes blocked (e.g. by a build-up of fatty deposits), the heart muscle downstream is starved of oxygen. This can cause a heart attack (myocardial infarction).
The heart beats automatically thanks to a group of cells called the sinoatrial node (SAN) — the natural pacemaker. It is located in the wall of the right atrium.
graph TD
A["SAN (pacemaker) in right atrium"] -->|"Electrical impulse"| B["Atria contract (atrial systole)"]
B -->|"Signal reaches AVN"| C["Short delay"]
C -->|"Signal passes to ventricles"| D["Ventricles contract (ventricular systole)"]
D --> E["Blood pumped into aorta and pulmonary artery"]
E -->|"Heart relaxes (diastole)"| F["Chambers refill with blood"]
F --> A
Cardiac output is the volume of blood pumped by the heart per minute. It is calculated as:
Cardiac output=stroke volume×heart rate
| Term | Definition | Typical resting value |
|---|---|---|
| Stroke volume | Volume of blood pumped per beat | ~70 mL |
| Heart rate | Number of beats per minute | ~70 bpm |
| Cardiac output | Volume of blood pumped per minute | ~4900 mL/min (~5 L/min) |
Exam Tip: Make sure you can rearrange this equation. If given cardiac output and heart rate, stroke volume = cardiac output ÷ heart rate. Always include units in your answer.
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