Cardiac Output
The primary mechanism by which the body lowers blood pressure is cardiac output. Sensors within our cardiovascular system called baroreceptors constantly measure our blood pressure and send messages to the heart and brain. Within seconds the brain sends messages to the heart to increase or decrease cardiac output. By decreasing the number of heartbeats, as well as the force of each, the body can decrease blood pressure. The opposite can occur as well, for example during the fight-or-flight-response, wherein the adrenal glands release adrenaline and noradrenaline, signaling the heart to temporarily raise blood pressure by increasing the forcefulness and number of heartbeats, which increases blood pressure.
Arteries and Veins
Arterioles can constrict or dilate to manipulate blood pressure. This expanding or narrowing lowers or raises blood pressure by increasing or decreasing resistance to blood flow. Veins also contribute to the regulation of blood pressure. For example, when veins expand, more blood is retained and less blood is returned to the heart. This causes less blood to be pumped into arteries, thereby lowering blood pressure. When less blood is retained in veins, more blood is returned to the heart to be pumped out, increasing blood pressure.
Total Blood Volume
The kidneys can also regulate blood pressure by varying the amount of urine produced. Urine is made when water is removed from our blood. When more urine is made, total blood volume decreases, thus lowering blood pressure. Lower urine production results in more blood volume and consequent higher blood pressure. Moreover, manipulating hormones such as angiotensin II, aldosterone, and vasopressin allows the kidneys to retain either salt or water to change blood volume. Whereas cardiac output and artery and/or vein constriction or dilation takes only seconds, urine fluctuations take hours or days to affect blood pressure.
