Section 1 of 9A double circulatory system
Humans have a double circulatory system: blood passes through the heart twice in each complete circuit of the body.
- so the blood is returned to the heart to be given a boost before travelling around the body.
Section 2 of 9Structure of the heart
The heart is a muscular organ made mostly of cardiac muscle, and it acts as a double pump.
- because it pumps blood at high pressure around the whole body,
- while the right pumps only to the nearby lungs.
| Blood vessel | Carries blood | From / to |
|---|---|---|
| Vena cava | Deoxygenated | Body to right atrium |
| Pulmonary artery | Deoxygenated | Right ventricle to lungs |
| Pulmonary vein | Oxygenated | Lungs to left atrium |
| Aorta | Oxygenated | Left ventricle to body |
| Coronary arteries | Oxygenated | Aorta to the heart muscle |
Section 3 of 9The path of blood
The heart muscle itself needs its own supply of oxygen and glucose, delivered by the coronary arteries. If they become blocked, part of the heart muscle is starved of oxygen, leading to a heart attack.
Section 4 of 9Controlling heart rate and pacemakers
The resting heart rate is controlled by a group of cells in the right atrium wall called the pacemaker.
Section 5 of 9Heart problems and treatments
In coronary heart disease, fatty deposits narrow the coronary arteries.
Section 6 of 9Blood flow and the cardiac cycle
Every heartbeat follows a cycle.
- making the 'lub' sound,
- and forces blood out through the semilunar valves at the start of the aorta and pulmonary artery.
Cardiac output is stroke volume multiplied by heart rate. At rest, an adult's cardiac output is about 5 dm3 per minute, and during hard exercise it can rise to about 20 dm3 per minute. Fit people have a larger stroke volume, so a lower resting heart rate for the same output.
A frequent misconception is that blood on the right of the heart is mixed with that on the left.
- and it is why the double circulation is so efficient: fully oxygenated blood goes to the body without being diluted by deoxygenated blood.
Section 7 of 9Exam-style questions
Explain why the wall of the left ventricle is thicker than that of the right ventricle. [2]2 marks
The left ventricle pumps blood to the whole body [1], so needs to generate a higher pressure than the right ventricle, which only pumps to the lungs [1].
Describe the pathway of blood from the vena cava to the aorta. [4]4 marks
Vena cava to right atrium [1]; right ventricle [1]; pulmonary artery to lungs and then pulmonary vein to left atrium [1]; left ventricle then aorta [1].
State the role of the pacemaker and suggest why a person might need an artificial one. [2]2 marks
The pacemaker controls heart rate by producing electrical impulses [1]. A person whose natural pacemaker is faulty needs the artificial one to keep the heart beating regularly [1].
Section 8 of 9Key terms
| Term | Meaning |
|---|---|
| Atrium | Upper chamber of the heart that receives blood. |
| Ventricle | Lower chamber of the heart that pumps blood out. |
| Valve | Structure that stops the backflow of blood. |
| Aorta | Main artery carrying oxygenated blood to the body. |
| Vena cava | Main vein returning deoxygenated blood to the heart. |
| Pacemaker | Group of cells that controls heart rate. |
| Coronary artery | Artery supplying blood to the heart muscle. |
| Double circulation | System where blood passes through the heart twice per circuit. |