What is elastic recoil in the arteries? Why do they do this?
Elastic Recoil in any tissue (including arteries) refers to the inherent resistance of a tissue to changes in shape, and the tendency of the tissue to revert to its original shape once deformed.
The elasticity in arteries gives rise to Windkessel Effect which helps to maintain a relatively constant pressure in the arteries despite the pulsating nature of the blood flow.
During a systole the arteries expand and recoil when blood pressure (BP) falls during a diastole. Now, the rate of blood entering these elastic arteries exceeds that leaving them due to the peripheral resistance there is a net storage of blood during systole which discharges during diastole.
Peripheral Resistance is a resistance offered by the circulatory system to the flow of the blood. So, you see when the blood flow is restricted it will lead to blood reaching late to the heart and hence its shortage during systole.
The elastic recoil of the arteries allows the artery to expand as normal but then exert an inward force to create blood pressure. The artery is slowly returning back to it's original shape which is continually 'maintaining' pressure. (because it's continually pressing inwards... the blood pressure is gradually lowering), this is why blood pressure fluctuates for a normal person between 70-120 mmHg.
All this simply means that if the heart contractions were sole basis of Blood Pressure then it would drop from maximum to minimum within seconds but due to Peripheral Resistance & Elastic recoil the blood flow is controlled and pressure is maintained leading to the blood pressure of 70-120 mmHg.
Arteries ,esp those that are close to heart are made of elastin. This makes them elastic and spongy. Hence, elastic recoil.