Discuss the Short, Medium and Long Term Regulation of Arterial Blood Pressure.
Arterial blood pressure (ABP) is defined as the hydrostatic pressure exerted by the blood on the walls of the arteries. A typical blood pressure of a healthy adult is 120mm Hg systolic and 80 mm Hg diastolic. However, this regularly changes in response to the needs of the body (e.g. under exercise blood pressure increases to supply the increased demand for oxygen to the tissues). ABP is dependent on the cardiac output of the heart (CO) and peripheral resistance (PR). Because ABP is proportional to both CO and PR we can use a formula to calculate ABP:.
ABP = CO x PR.
Since cardiac output is equal to heart rate x stroke volume, an increase in either of these factors would lead to an increase in CO, which would lead to an increase in ABP. Likewise an increase in any of the factors that cause peripheral resistance (vasodilation of arterioles, length of blood vessels and blood viscosity) would lead to an increase in ABP.
Blood pressure can change dramatically under certain circumstances. For example, blood pressure would drop if someone were haemorrhaging seriously and losing a lot of fluids, so the body needs to be able to regulate blood pressure to be able to maintain circulation. A number of mechanisms have evolved to regulate ABP over short, medium and long term time periods:.
The baroreceptor reflex, which works on a second to second basis.
Capillary fluid shift mechanism that works over minutes and hours.
Renin-Angiotensin mechanism, a complicated hormonal regulation of ABP working over hours and days.
Baroreceptor Reflex.
Baroreceptors are specially adapted nerve endings sensitive to the stretching of blood vessel walls. They are located in large populations in the aortic arch and the carotid arteries, as well as in smaller numbers in other arteries found in the chest and neck. Baroreceptors are constantly emitting action potentials, but an increase in arterial blood pressure would cause an increase of stretching of the arterial wall, which would stimulate the baroreceptors to increase the frequency of the action potentials.