Blood pressure: Encyclopedia II - Blood pressure - Physiology

Blood pressure - Physiology

The mean blood pressure in the arteries supplying the body is a result of the heart pumping blood from the veins back into the arteries.

The mean blood pressure value is determined by the volume of blood the heart is pumping per minute, termed cardiac output, versus the resistance of the 20,000 to 30,000 arterioles, termed total peripheral resistance, through which the blood must flow to reach the capillaries and then veins.

The up and down fluctuation of the arterial blood pressure results from the pulsatile nature of the cardiac output. The pulse pressure is determined by the interaction of the stroke volume versus the volume and elasticity of the major arteries.

The larger arteries, including all large enough to see without magnification, are low resistance (assuming no advanced atherosclerotic changes) and conduit vessels with high flow rates but producing very little pressure drop. For instance, about 5 mmHg mean pressure decrease in the blood flow traveling from the heart all the way to the toes is typical, assuming the individual is supine (horizontal with respect to gravity).

Blood pressure - Regulation of blood pressure

The endogenous regulation of blood pressure is not completely understood. Currently, three mechanisms of regulating blood pressure have been well-characterized:

• Baroreceptor reflex: Baroreceptors in various organs can detect changes in blood pressure, and adjust the mean arterial pressure by altering both the force and speed of the heart's contractions, as well as the total peripheral resistance.

• Renin-angiotensin system (RAS): This system is generally known for its long-term adjustment of blood pressure. This system allows the kidney to compensate for loss in blood volume or drops in blood pressure by activating an endogenous vasoconstrictor known as angiotensin II.

• Aldosterone release: This steroid hormone is released from the adrenal gland in response to either high serum potassium levels or if angiotensin II is present. This hormone increases the excretion of potassium by the kidneys, while increasing sodium retention. Since sodium is the main ion which determines the amount of fluid in the blood vessels by osmosis, aldosterone will increase fluid retention, and indirectly, blood pressure.

These different mechanisms are not necessarily independent of each other, as indicated by the link between the RAS and aldosterone release. Currently, the RAS system is targeted pharmacologically by ACE inhibitors and angiotensin II receptor antagonists. The aldosterone system is directly targeted by spironolactone, an aldosterone antagonist. The fluid retention may be targeted by diuretics; however, the antihypertensive effect of diuretics is not due to its effect on blood volume. Generally, the baroreceptor reflex is not targeted in hypertension because if blocked, individuals may suffer from orthostatic hypotension and suffer from fainting.

Adapted from the Wikipedia article "Physiology", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki