CBSE Class 10 Science 2018 · Paper 31/1 · Question 18(a)

(a) Any two of these: plasma, red blood cells (RBCs), white blood cells (WBCs) and platelets. (b) $\text{lungs}\to \text{pulmonary veins}\to \text{left atrium}\to \text{left ventricle}\to \text{aorta}\to \text{arteries}\to \text{capillaries of the body organs}$. Oxygen-rich blood from the lungs enters the left atrium through the pulmonary veins; the left atrium contracts and passes it into the left ventricle, which contracts and pumps it through the aorta to all the organs of the body. (c) The valves between the atria and the ventricles prevent the backflow of blood: they let blood pass only from the atria into the ventricles, and close when the ventricles contract. (d) An artery has a thick, elastic wall, while a vein has a thin wall. (Or: veins have valves along their length, arteries do not.)