Final answer:
Capillaries have the highest number and total cross-sectional area among blood vessels, allowing for efficient exchange between blood and tissues.
Step-by-step explanation:
The type of blood vessel that has the highest number and total cross-sectional area is the capillaries. The vast network of capillaries in the human body greatly exceeds that of any other type of blood vessels, both in terms of their number and combined cross-sectional area. As blood moves from larger vessels such as arteries into capillaries, the total cross-sectional area increases significantly, which allows for the efficient exchange of substances between the blood and the cells.
When considering a major artery with a cross-sectional area of 1.00 cm² branching into smaller arteries, each with a cross-sectional area of 0.400 cm², the overall cross-sectional area increases and as a result, the average blood velocity is reduced. This decrease in velocity is crucial in capillary beds, as it facilitates the exchange of nutrients and waste materials with tissues. Considering blood flow continuity (Q = Av, where Q is blood flow, A is cross-sectional area, and v is velocity), when an artery with a cross-sectional area of 1.00 cm² branches into 18 smaller arteries, each with an average cross-sectional area of 0.400 cm², the total cross-sectional area is now 7.2 cm² (18 x 0.400 cm²). This implies that for a constant flow rate, the velocity must decrease by a factor proportional to the increase in the total cross-sectional area.