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

What happens to blood after it loses oxygen in the tissues?

1. It becomes a bright red color

2. It turns dark red and returns to the heart

3. It releases carbon dioxide

4. It gains more hemoglobin

The correct answer is: It turns dark red and returns to the heart

After blood loses oxygen in the tissues, it turns dark red and returns to the heart. This change in color occurs because when hemoglobin, the protein in red blood cells, binds to oxygen, it becomes bright red. However, once it releases oxygen to the tissues, it no longer has this attachment. Instead, the hemoglobin changes to a darker shade, resulting in dark red blood. This deoxygenated blood is then transported back to the heart through the veins, where it will be sent to the lungs for reoxygenation. The focus on the sequence of events highlights how blood travels through the circulatory system and emphasizes the importance of oxygen delivery and carbon dioxide removal in maintaining efficient bodily functions.