Blood vessels and the double-circulatory system
The double-circulatory system
Blood travels around the body in , passing through the heart twice on each trip. This is the double-circulatory system.
- The : the right side of the heart pumps deoxygenated blood to the lungs, where it picks up oxygen, then back to the heart.
- The : the left side of the heart pumps oxygenated blood to the body, including the working muscles, then back to the heart.
Why it matters for exercise: oxygenated blood is pumped out to the body at high pressure, separate from deoxygenated blood, so the working muscles get the oxygen they need quickly. A cyclist's leg muscles depend on it.
Key terms
Arteries, capillaries and veins
| Arteries | Capillaries | Veins | |
|---|---|---|---|
| Direction | Away from the heart | Between arteries and veins, through the tissues | Towards the heart |
| Pressure | High | Low | Low |
| Wall | Thick, muscular and elastic | Very thin: one cell thick | Thin |
| Valves | No | No | Yes, to stop blood flowing backwards |
Capillaries are where oxygen passes from the blood into the muscles, and carbon dioxide passes back. Their thin walls let gases pass through easily.
Remember
Arteries go Away from the heart.
Key terms
Red blood cells
Red blood cells carry oxygen from the lungs to the working muscles. They contain , which joins with oxygen in the lungs and releases it in the muscles. They also carry some carbon dioxide back to the lungs to be breathed out.
Why it matters: the more red blood cells and haemoglobin, the more oxygen reaches the muscles, so an endurance athlete such as a rower can keep working aerobically for longer.