A red blood cell that has just picked up oxygen in the lungs must pass back through the heart again before it can deliver that oxygen to a muscle in the leg. Why does blood need to travel through the heart twice to complete one full round trip around the human body?
- The heart only has enough force to pump blood a short distance at a time, so it re-pumps blood repeatedly everywhere it goes
- Humans have a double circulatory system: a pulmonary circuit carries blood to the lungs and back to the heart, then a separate systemic circuit carries the now-oxygenated blood from the heart out to the rest of the body, keeping oxygenated and deoxygenated blood separate
- The first pass through the heart filters waste out of the blood, and the second pass is needed to add oxygen back in
- Blood only passes through the heart twice in people with certain heart conditions; in a typical healthy person it passes through only once per circuit
Why B? And why not the others?
Correct answer: B. Humans have a double circulatory system: a pulmonary circuit carries blood to the lungs and back to the heart, then a separate systemic circuit carries the now-oxygenated blood from the heart out to the rest of the body, keeping oxygenated and deoxygenated blood separate
Humans (and other mammals) have a double circulatory system made of two separate loops that both pass through the heart: the pulmonary circuit sends deoxygenated blood from the heart to the lungs to pick up oxygen and returns it to the heart, and the systemic circuit then pumps that freshly oxygenated blood from the heart out to the rest of the body, including the leg muscle, before it returns deoxygenated again. This keeps oxygen-rich and oxygen-poor blood from mixing and lets the heart re-pressurise the blood between the two circuits, delivering oxygen far more efficiently than a single loop could. The option about limited pumping force is wrong because the heart's structure is built around these two specific, separate circuits rather than generic repeated short-range pumping. The option describing waste filtering is wrong because filtering blood is the job of the kidneys and liver, not a function of either pass through the heart. The option claiming this only happens in people with heart conditions is wrong because the double circulatory system, passing through the heart twice per full round trip, is the completely normal and healthy circulation pattern in every human being.
Source: BBC Bitesize GCSE Biology: Heart and circulation -- the double circulatory system