How does blood volume change with body weight?

Blood volume directly relates to body weight because larger bodies need more blood to deliver oxygen and nutrients efficiently. On average, blood makes up about 7–8% of total body weight. For example, a 70 kg adult has roughly 5–5.5 liters of blood. When body weight increases due to muscle mass, total blood volume rises proportionally because muscles require strong oxygen delivery. However, in obesity, the increase in blood volume is smaller because fat tissue has a lower blood supply than muscle.

Athletes often have higher blood volume because endurance training stimulates the body to produce more plasma and red blood cells, improving oxygen transport and temperature regulation. On the other hand, dehydration or significant blood loss reduces blood volume, leading to lower blood pressure and potential circulatory problems.

In medical practice, knowing a person’s estimated blood volume helps determine safe limits for blood donation or fluid therapy. Children and smaller individuals have less absolute blood volume, though it still represents a similar percentage of body weight. In summary, blood volume scales with lean body mass, not just total weight, ensuring each tissue receives proper circulation.