Question

question 11
the general function of the valves of the heart is to:
prevent backflow of blood
keep the blood moving
prevent friction
help pump blood
question 12
veins:
branch into smaller vessels called arterioles
transport oxygen - rich blood
often have valves to prevent the backflow of blood
operate under high pressure
carry blood away from the heart
question 13
veins are able to constrict and increase venous return because of their:

Explanation:

For Question 11, heart valves prevent blood from flowing backward. For Question 12, veins often have valves to prevent back - flow of blood as they carry blood to the heart under low pressure and oxygen - poor blood (except pulmonary veins).

Answer:

Question 11: prevent backflow of blood
Question 12: often have valves to prevent the backflow of blood