As the blood vessels become stiff, the peripheral resistance increases (peripheral resistance is the resistance offered by the blood vessel to the flow of blood) and flexibility of vessel increases due to elastic property of the vessel. Therefore, elasticity is a critical factor to maintain blood vessel diameter. In normal arteries, according to the speed and volume of the blood, the diameter of the blood vessel changes accordingly and optimum peripheral resistance is maintained which is essential for proper flow of blood. As the people aging, the elasticity and thickness of blood vessels increases so that, low ability of older people to respond to changes in blood pressure.
The aging of the people may cause blood vessels with embolus and atherosclerosis. Therefore, the capillary exchange efficiency will be greatly decreased.
Thus, the options (d), (c) and (b) are incorrect.
As the blood vessels become stiff, the peripheral resistance increases (peripheral resistance is the resistance offered by the blood vessel to the flow of blood) and flexibility of vessel increases due to elastic property of the vessel. Therefore, elasticity is a critical factor to maintain blood vessel diameter.
Therefore, the correct option is – A. Something that is able to resist and be flexible.