(i) Veins are thin walled and collapsible. Their lumen is comparatively wide. They take deoxygenated blood from various parts of the body back to heart. To prevent the backflow of blood, valves are present at intervals.
Arteries are thick walled and non-collapsible. Their lumen is narrow. As the blood in arteries flows with jerks the latter do not have valves.
(ii) Since blood is supplied to different organs of the body by ventricles, their wall is thicker than those of auricles because a thick wall exerts more pressure on the blood.
(iii) The opening of right ventricle into the pulmonary artery is guarded by semilunar valves which allow the flow of blood from the right ventricle into the pulmonary artery, but not in the reverse direction. Hence blood from the right ventricle enters the pulmonary artery, but cannot go back from pulmonary artery to right ventricle.
(iv) The left auricle contains oxygenated blood, because it receives blood from the lungs where blood is oxygenated.
(v) The red blood corpuscles cannot divide because they lack nuclei.
(vi) Pulmonary artery carries deoxygenated blood from the right ventricle to the lungs for oxygenation.
(vii) Auricles are collecting chambers for the blood, so they are thin walled to accommodate large volume of blood.
(viii) Left ventricle has to pump blood all over the body, under pressure, through the dorsal aorta. Hence, more muscular.
