Final answer:
Veins have thin walls and large lumens allowing for low resistance blood flow; the larger diameter and presence of valves aid the return of blood to the heart, contributing to human physiology.
Step-by-step explanation:
The vessels described in the question are veins. Veins have thin walls and a large lumen diameter, which makes it easier for blood to return to the heart. These characteristics of veins allow for a low resistance to blood flow when they are distended. Understanding the circulatory system and the role each type of blood vessel plays is critical for grasping basic human physiology.
The pressure in veins is much lower compared to arteries due to various factors. One, blood has already passed through the capillaries, where the pressure drops. Two, veins have a larger diameter, and according to the principle of blood flow resistance (R = 1/r4), a larger diameter means less resistance. Three, veins have valves that help maintain the direction of blood flow towards the heart, counteracting the effects of gravity, especially in the extremities. Finally, in the case of venoconstriction, the smooth muscle in vein walls constricts and the lumen becomes more rounded, further decreasing resistance and assisting with the flow of blood back towards the heart.