(i) Veins have valves to prevent the backflow of blood.
Arteries do not have valves as they carry blood away from
the heart and valves would only hinder the flow of blood from the
heart to other parts of the body.
Arteries --> no valves --> blood flows in spurts, high
pressure, very fast --> blood unlikely to backflow --> valves
unneccessary
Arteries --> thick elastic muscular walls --> strong
stretching and recoiling --> pushes blood forward at high
pressure --> blood unlikely to backflow --> valves
unneccessary
Veins --> low blood pressure, slow --> blood likely to
backflow --> valves needed
Veins --> thin walls; blood unable to move forward on its own
--> dependant on skeletal muscles for blood movement -->
blood likely to backflow --> valves needed
(ii) Capillaries have walls made up of only a single layer of
cells called the endothelium, which is partially permeable to
enable substances to diffuse quickly through capillary walls.
Capillaries have many branches, providing a large surface area for
faster/efficient/rapid exchange of substances
between blood and tissue cells.