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The circulatory system is the organ system responsible for transporting substances around the body in the blood. For AQA GCSE Biology, you need to understand the structure of the heart, how it pumps blood, the difference between the two circulation loops, and the role of key blood vessels. The human circulatory system is described as a double circulatory system because the blood passes through the heart twice during each complete circuit of the body.
Humans have a double circulatory system, meaning the blood flows through two separate circuits:
graph LR
A[Right Side of Heart] -->|Pulmonary artery| B[Lungs]
B -->|Pulmonary vein| C[Left Side of Heart]
C -->|Aorta| D[Body — Organs and Tissues]
D -->|Vena cava| A
| Circuit | Name | Path |
|---|---|---|
| Circuit 1 | Pulmonary circulation | Right ventricle pumps deoxygenated blood to the lungs via the pulmonary artery. Blood picks up oxygen and returns to the left atrium via the pulmonary vein. |
| Circuit 2 | Systemic circulation | Left ventricle pumps oxygenated blood to the body via the aorta. Blood delivers oxygen and nutrients, picks up carbon dioxide, and returns to the right atrium via the vena cava. |
In a single circulatory system (like a fish), blood passes through the heart only once per circuit. In a double circulatory system, blood returns to the heart after visiting the lungs, allowing the heart to pump it out to the body at higher pressure. This means blood flows faster and more efficiently to the organs and tissues, delivering oxygen and nutrients more rapidly. This is essential for active, warm-blooded mammals.
Exam Tip: A common 6-mark question asks you to describe the pathway of blood through the heart and lungs. Practise writing this out in order, naming every chamber and vessel. Use the mnemonic: "Right side pumps to lungs, left side pumps to body."
The human heart is a muscular organ located in the chest (thorax), slightly to the left of centre. It is about the size of a clenched fist.
Blood vessels (from left to right above the heart):
Note on orientation: Heart diagrams are drawn as if you are facing the patient — so what is labelled the "right atrium" appears on the left of the page, and vice versa. Blue shading = deoxygenated blood, red = oxygenated blood. The left ventricle wall is visibly thicker because it must generate enough pressure to push blood around the entire body.
| Structure | Function |
|---|---|
| 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 |
| Septum | A thick muscular wall that separates the left and right sides of the heart, preventing oxygenated and deoxygenated blood from mixing |
| Valves | Prevent the backflow of blood, ensuring blood flows in one direction only |
| Blood Vessel | Blood Type | Direction |
|---|---|---|
| Vena cava (superior and inferior) | Deoxygenated | From the body TO the right atrium |
| Pulmonary artery | Deoxygenated | From the right ventricle TO the lungs |
| Pulmonary vein | Oxygenated | From the lungs TO the left atrium |
| Aorta | Oxygenated | From the left ventricle TO the body |
Exam Tip: Students often confuse the pulmonary artery and pulmonary vein. Remember: the pulmonary artery carries deoxygenated blood (away from the heart to the lungs) and the pulmonary vein carries oxygenated blood (to the heart from the lungs). These are the only artery and vein in the body where this is reversed from the usual pattern.
The heart beats continuously without tiring. The cardiac cycle has three main stages:
| Stage | What Happens |
|---|---|
| 1. Atrial contraction (atrial systole) | Both atria contract, pushing blood down into the ventricles through the atrioventricular (AV) valves |
| 2. Ventricular contraction (ventricular systole) | Both ventricles contract, pushing blood out of the heart through the semilunar valves into the pulmonary artery and aorta. AV valves close to prevent backflow. |
| 3. Relaxation (diastole) | The heart muscles relax. Blood flows into the atria from the vena cava and pulmonary vein. Semilunar valves close to prevent blood flowing back into the ventricles. |
graph TD
A[Blood enters atria] --> B[Atria contract — blood pushed into ventricles]
B --> C[Ventricles contract — blood pushed out of heart]
C --> D[Heart relaxes — cycle repeats]
D --> A
The heart has its own natural pacemaker — a group of cells in the wall of the right atrium called the sinoatrial node (SAN). The SAN produces small electrical impulses that spread across the atria, causing them to contract. The impulses then pass to the ventricles (via the atrioventricular node), causing them to contract shortly after. This ensures the atria always contract before the ventricles.
If the natural pacemaker is faulty, an artificial pacemaker (a small electronic device) can be surgically implanted to send electrical impulses and regulate the heartbeat.
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