How Does Mass Transport Work in the Circulatory System?
Learn how the circulatory system transports blood, how haemoglobin loads and unloads oxygen, and why active tissues receive more oxygen in AQA A Level Biology.
Mass transport works by using the circulatory system to move blood around the body. Blood delivers oxygen and nutrients to cells and carries away carbon dioxide. Haemoglobin (also spelt hemoglobin) in red blood cells transports most of the oxygen.
How blood circulates
Humans have a double circulatory system: blood passes through the heart twice on each complete journey. The right side pumps blood to the lungs, where it takes up oxygen and releases carbon dioxide. The left side pumps oxygenated blood to the rest of the body. Valves keep blood flowing in one direction.
Arteries carry blood away from the heart under high pressure. They branch into smaller vessels and then capillaries, whose thin walls provide a short diffusion distance. Substances move between blood and cells across these walls, often via tissue fluid. Veins return blood to the heart.
How haemoglobin carries oxygen
In the lungs, oxygen diffuses into the blood and binds reversibly to haemoglobin, forming oxyhaemoglobin. Where oxygen concentration is lower, such as in respiring tissues, oxyhaemoglobin releases oxygen for cells to use.
Cooperative binding means that when one oxygen molecule binds to haemoglobin, the next binds more easily. This helps haemoglobin load oxygen in the lungs. In active tissues, increased carbon dioxide reduces haemoglobin’s affinity for oxygen, so more oxygen is unloaded. This is the Bohr effect: affinity means how readily haemoglobin binds oxygen.
For example, exercising muscle respires more rapidly and produces more carbon dioxide. Haemoglobin therefore releases more oxygen in that muscle, where demand is high.
Exam mistake to avoid: do not say haemoglobin releases oxygen because the blood has “run out” of it. Oxygen unloading depends on conditions in the tissues, including oxygen and carbon dioxide levels.
