Blood pressure is dependent upon the heart rate, strength of contraction blood output per heartbeat and the resistance to blood flow of the blood vessels. The contraction or relaxation of smooth muscle in the blood vessel walls causes narrowing, vasoconstriction, and widening, vasodilation of the arteries and arterioles. Vasoconstriction increase the resistance to blood flow and therefore raises the blood pressure while vasodilation causes a drop in pressure. The control of smooth muscle contraction in the arterioles is controlled by the autonomic nervous system from the vasomotor centre in the medulla oblongata of the brain. The action of the vasomotor centre is regulated by nerve impulses arising from the pressure sensors or baroreceptors in the walls of the aorta and the carotid arteries, the aortic and carotid bodies. Increase in the baroreceptor impulse output causes the vasomotor centre to send out more impulses along the parasympathetic nerve fibres to the arterioles causing relaxation of the smooth muscle resulting in vasodilation and reduction in blood pressure. When the baroreceptors reduce their nerve impulse output the vasomotor centre sends out more impulses along the sympathetic nerve fibres which cause constriction of the smooth muscle of the arterioles resulting in an increase in pressure. This is a good example of a feedback system. As well as having this pressure detection system, there are sensors in the carotid and aortic bodies for CO₂ level and pH. These are detected by chemoreceptors which feed information to the vasomotor centre. An increase in CO₂ level or a drop in pH will cause an increase in blood pressure through vasoconstriction. The CO₂ can also directly affect the smooth muscle in the arteriole wall causing arterioles to dilate in areas which are highly active and producing large amounts of CO₂ or lactic acid. This increases the blood flow to the region thereby increasing oxygen release to the tissue. This combination of effects of vasoconstriction in the general body circulation and vasodilation in the tissue responsible for extra CO₂ release will result in faster blood flow to the tissue and easier unloading of oxygen to it. Other factors affect the blood pressure, for example stress, emotional upset and release of adrenalin from the adrenal glands.