Final answer:
A closed circulatory system is under pressure to maintain a constant blood flow, which is essential for efficiently supplying oxygen and nutrients to the body tissues and removing waste. The pressure facilitates the rapid distribution of these vital substances, supporting the metabolic needs of larger, more active organisms.
Step-by-step explanation:
The purpose of a closed circulatory system being under pressure is To maintain a constant blood flow. This is crucial because closed circulatory systems enable the efficient transportation of oxygen, nutrients, and waste throughout the body. By maintaining blood under pressure, it ensures that all tissues receive an adequate supply of oxygen and nutrients necessary for metabolism and removes waste effectively. Furthermore, the pressure facilitates rapid distribution, particularly important in active organisms that require a lot of oxygen and energy for movement. Therefore, options B, to minimize the need for oxygen, and D, to slow down blood circulation, are not correct as these would be counteractive to the primary function of the circulatory system.
Throughout evolutionary history, different organisms have developed varying respiratory and circulatory structures to meet their metabolic needs. Larger organisms, such as humans, need a more complex system since the diffusion of gases is not efficient over long distances due to their size. Thus, closed circulatory systems, seen in vertebrates like mammals, birds, and fish, support higher levels of activity and larger body sizes.