Because the flow speed in your capillaries is much less than in the aorta, the total cross-section area of the capillaries considered together must be much larger than that of the aorta. Given the flow speeds noted, the total area of the capillaries considered together is equivalent to the cross-section area of a single vessel of approximately what diameter

Diameter of capillaries is 30-40 µm in diameter while on the other hand, the diameter of aorta is 4 cm.

Explanation:

The diameter of capillaries is 30-40 µm in diameter while on the other hand, the diameter of aorta is 4 cm which is the main reason for higher flow speed of the blood in the aorta as well as more blood is transported in the aorta. If the total area of the capillaries considered together is equivalent to the cross-section area of a single vessel of aorta then the flow speed of both is similar due to similar area.

d. organisms have cells with different shapes and functions.

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jk but i am serious about the part that most people cant answer it beacuse you did not mention the section you are talking about. but it would if you added some detail and were a bit more specific.