Blood pressure in the arteries must be regulated to ensure an adequate supply of blood, and hence oxygen, to the organs. If arterial blood pressure is too low, not enough blood reaches body tissues. If it is too high, it may damage blood vessels and organs. Rapid changes in blood pressure trigger compensatory responses from the nervous system within seconds. These autonomic nervous responses do not involve the conscious parts of the brain. Longer-term changes are largely regulated by hormones that affect the volume of fluid excreted by the kidneys. Hormonal responses work over several hours.
Heavy bleeding or a sudden change in posture may cause a rapid change in blood pressure, to which the nervous system immediately responds. Baroreceptors (stretch receptors in the walls of the major arteries) detect pressure changes and send signals along sensory nerves to the brain. An autonomic response adjusts the heart rate, volume of blood pumped, and arterial diameter to restore normal pressure.
Blood pressure is controlled in the long term by the action of hormones. The kidneys respond to low blood pressure by secreting renin. This hormone is converted into angiotensin, which constricts arteries and raises blood pressure. The adrenal glands, hypothalamus, and heart also respond to high or low pressure by secreting aldosterone, vasopressin, and natriuretic hormone, respectively. These hormones alter the amount of fluid excreted by the kidneys, which affects the volume of blood in the body and hence the blood pressure.
From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.