Final answer:
Aerobically fit individuals have cardiovascular systems that are more efficient due to physical exercise, which increases heart size and lowers cholesterol levels rather than directly 'thinning' the blood.
Step-by-step explanation:
When comparing an aerobically fit individual to a sedentary individual, generally, the fit person's blood may not necessarily be 'thinner,' but they do experience various cardiovascular efficiency improvements due to regular physical activity.
Regular exercise promotes cardiovascular health by increasing the heart size in athletes, leading to a higher stroke volume which means a larger amount of blood is moved with each beat, allowing for the same amount of blood to be delivered with fewer heartbeats.
This decreased demand on the heart reduces the overall stress on the cardiovascular system. Moreover, exercise can help in lowering overall cholesterol levels by removing low-density lipoproteins (LDLs), and also promotes healthier blood vessels by decreasing the formation rate of arterial plaque and reducing blood pressure.
Notably, hereditary factors like the type of muscle fibers—slow-twitch or fast-twitch—can also influence changes in blood and vascular function due to exercise. Slow-twitch muscle fibers, more abundant in endurance athletes, support better oxygen delivery and aerobic activities, possibly influencing blood characteristics over time.