Which of the following has no muscular wall?

Arteries are best defined as vessels that:

The diameter of capillaries for $RBC$ to pass through should be:

Assertion : When there is a fall in the blood pressure due to loss of blood volume,this is compensated by vasoconstriction of veins.
Reason : Veins hold the extra amount of blood which can be shifted to the arteries as required.

Which statement is true for veins?

Which of the following is another characteristic of the circulatory system in animals where the volume of blood is limited?