OCR General Certificate of Secondary Education (GCSE) Biology Practice Exam

What does the blood returning to the lungs contain in high amounts?

1. Oxygen

2. Carbon dioxide

3. Glucose

4. Urea

The correct answer is: Carbon dioxide

The blood returning to the lungs contains a high amount of carbon dioxide because it is the result of cellular respiration. During this process, cells use oxygen to convert glucose into energy, creating carbon dioxide as a waste product. This carbon dioxide is then transported back to the lungs through the bloodstream, where it can be expelled from the body. In contrast, blood that has just returned from the lungs is typically rich in oxygen, which is why it's less likely to be the right choice here. Glucose is primarily utilized by cells for energy and would not be present in high concentrations in the blood returning from the body to the lungs. Urea, a waste product from the breakdown of proteins, is also present in the blood but is not the primary component being returned to the lungs. Therefore, focusing on the carbon dioxide content accurately captures the primary role of venous blood flowing back to the respiratory system.