Blood pressure is controlled by
a) the elasticity of the arteries
The walls of the arteries are stretched each time the heart contracts and pumps blood into them and they rebound each time the heart relaxes. This elasticity keeps the blood flowing around the body between beats of the heart.
b) the flow of blood
The flow of blood adapts to meet changing bodily needs and we may give different blood pressure readings in different conditions. If we have eaten a large meal the arteries supplying the stomach and intestines with blood are dilated to provide extra supplies while arteries in other areas of the body automatically narrow. If we are stressed or frightened, the arteries serving the skin and limbs are dilated due to the action of the stress hormone adrenaline. If we take exercise the flow of blood increases up to five times the normal rate to accommodate our needs.
c) resistance to the flow of blood
If the arteries (and smaller blood vessels leading from them called arterioles) become narrow for any reason, resistance to the flow of blood becomes greater. Yet the heart continues to pump blood through the body at the same rate – it just has to work harder and increase blood pressure to drive the blood through narrower channels.