Blood pressure is the force exerted by the blood upon our blood vessels and this force must be maintained inside the normal parameters for blood pressure.

So, What mechanisms are used by our bodies to keep these normal values?

Autonomous Nervous System.

For blood regulation the autonomous nervous system will send stimulatory signals for the Sympathetic Nervous System and inhibitory signals for the Parasympathetic Nervous System, this will have three consequences.

Most of the arterioles of the systemic circulation will contract increasing thus the peripheral resistance which in time will cause an increase on blood pressure.

The veins will contract making more blood return to the heart, the extra blood will expand, more than usual, the heart walls and this will cause a reactionary stronger than normal heart contraction, this will put more blood into the circulation and this extra blood will increase blood pressure.

The heart will be directly stimulated by Sympathetic nerves to improve its pumping

Baroreceptors.

These are receptors sensitive to stretching, an increase on blood pressure will stretch this receptors and they will send signals to the Central Nervous System which in time will send back signals through the Autonomous Nervous System to reduce blood pressure.

These receptors respond better to changes on pressure than to static high blood pressure.

Baroreceptors are found in the carotid sine and the aortic arch.

Chemoreceptors.

These receptors respond to low O2, high CO2 and high H+.

Their mechanism of action is through an associated baroreceptor system that works exactly as the explained above. The difference here is that the regulation is initiated by changes on chemical substances instead of the stretching of the arterial walls.

Source: Guyton and Hall Textbook of Medical Physiology 13th Ed.