Blood pressure is controlled by three things
1 - How fast the heart beats (heart rate). The pace at which the heart
beats, or heart rate, is counted in heartbeats per minute. Generally,
when heart rate increases, blood pressure rises. When heart rate
decreases, blood pressure drops.
2 - A number of things affect heart rate, including the body's nervous
system; chemical messengers called hormones, body temperature,
medications, and diseases
3 - How much blood the heart pumps with each beat (stroke volume).
The amount of blood pumped out of a ventricle with each heartbeat
is called stroke volume. When you're resting, stroke volume is about
the same as the amount of blood that veins carry back to the heart.
But under stressful conditions, the nervous system can increase stroke
volume by making the heart pump harder.
Stroke volume can also be affected by certain hormones, drugs, and
diseases, as well as increases or decreases in the amount of blood in
the body, called blood volume.
Nice To Know:
You might also hear the term "cardiac output" used to describe the
amount of blood that's pumped through the body. Cardiac output is
simply the amount of blood pumped out of a ventricle in one minute:
Cardiac output = Heart rate x Stroke volume (amount of blood pumped
with each beat)
As cardiac output increases, so does blood pressure. This is why heart
rate and stroke volume are important ways for the body to control
blood pressure.
How difficult it is for blood to travel around the body (peripheral resistance).
The third major component that affects the blood pressure is the caliber or
width of the arteries. Blood traveling in narrower vessels encounters more
resistance than blood traveling through a wider vessel (its harder for water
to pass through a narrow pipe than a wide pipe).
Depending on what a person is doing, the amount of blood the heart pumps
varies enormously. Yet the blood pressure normally remains pretty stable.
That's mainly because the body adjusts the resistance of the arteries, either
widening or narrowing them as appropriate, to prevent the blood pressure
from swinging wildly.
This ability to regulate the width of the blood vessels is called the peripheral
resistance. Most of the resistance to blood flow in the circulation occurs in
the small-diameter arteries called arterioles.
These arterioles are especially important in the immediate regulation of
blood pressure. That's because they contain specialized smooth muscle in
their walls that can relax or contract, allowing the blood vessel to get wider
or narrower.