The heart has two pumps consisting of two chambers, each pumping blood throughout the body. Blood enters a chamber, called the atrium, which then pumps blood into a chamber called the ventricle, which then pumps blood to the rest of the body. The left side of the heart pumps blood into the all of the body except the lungs. The pressures generated in this side of the heart are measured as your blood pressure - typically 120/80 mm Hg. This means that the peak pressure (systolic pressure) is enough to raise a column of blood 120 mm of mercury high, or the equivalent of almost 120 cm of water high. As the heart gets ready to pump again, the pressure in the systemic circulation typically falls to 80mm Hg (diastolic pressure). These are the measurements taken when you have your blood pressure taken.

Blood pressure has long associations with breathing. I have a copy of a book published in the 1920s called "The Cure of High Blood Pressure with Respiratory Exercises." Quiet breathing slows the heartbeat and reduces blood pressure. The beneficial effects of treating hypertension with biofeedback, yoga and relaxation are all well documented. If we are mounting a "fight of flight" response our sympathetic system is activated; our blood pressure and our breathing rate go up. Our blood vessels are constricted, thus increasing the pressure that the heart needs to pump the blood against, if the blood is to travel through these blood vessels. A similar phenomenon also occurs in the brain. If we breathe rapidly sufficient to lower arterial carbon dioxide levels, the lowering of arterial carbon dioxide levels leads to constriction of the blood vessels to the brain. The oxygen supply to the brain is reduced correspondingly. If we slow our breathing, we are able to consciously reduce our sympathetic drive, thus lowering our blood pressure. Cognitive and behavioural therapies, many of which use breathing reeducation, are able to reduce the stress response not only temporarily, but on a long-term basis.