For gas exchange to occur in the lungs and the rest of
the body's tissues, blood must flow continuously through the tissues.
The heart pushes
blood through the tissues and provides a constant force
for blood flow to occur.
The heart provides enough
force to propel the blood through the arteries and veins in the body.
The arteries entering tissues, called arterioles, can constrict (become more narrow)
or dilate (become relaxed and less narrow) to change the amount of blood
flowing to an area. If an arteriole constricts, less blood is available
for the tissues it supplies. If an arteriole dilates, more blood reaches
the tissues it supplies.
Why is it useful for the arteries to change size?
Can you think of situations where certain tissues may
need more or less blood flow?
Blood pressure is a measure of the force needed for blood
to move through the tissues.
- This pressure depends on the amount of
blood in the body, the diameter of the blood vessels, and how hard the heart is pumping blood.
- Resistance in the circulatory system
is caused by the blood rubbing against the walls of the blood vessels
as it flows through them. This rubbing produces a force, called
resistance, opposite the blood
- A large vessel is less resistant than a small
blood vessel because relatively less blood rubs against the walls of the
blood vessel, while a small blood vessel is more resistant because it
has a smaller area for the blood to flow through. This means
that more blood rubs against the walls of the vessel and it slows
- In any one capillary, this resistance is an advantage
because the slowed blood flow has more time for gas exchange
When an arteriole dilates, the diameter almost doubles. When a vessel's
diameter increases, the blood flow increases by four times the original
- This is like the difference between a water hose with a 1/2"
inner diameter and a 1" inner diameter water hose. Under the same
pressure, the 1" hose will have 4 times the flow of the smaller hose.
With decreased resistance, more blood can flow to the tissues that need
more nutrients and gas exchange. For example, under conditions of moderate exercise, blood
flow to skeletal muscles can increase by up to 10 times, because the arterioles
in that area become dilated and supply more blood to the muscles.
conditions of little resistance, the heart does not have to work as hard
to move blood into the tissues.
Is it better for the heart to work less to move blood
through the tissues? Why?
What happens to blood flow if the heart is not working hard enough?
When the blood pressure in the body is elevated, the heart must work very hard to provide adequate blood flow to the tissues.
- Many people have blood pressure that is too high
(called hypertension.) One
known cause of hypertension is atherosclerosis. Atherosclerosis is a condition in which the walls of the blood
vessels become thick and stiff, reducing their flexibility
and ability to dilate.
- Eventually fats and cholesterol build up and reduce the
diameter of the blood vessels, making it very difficult to for blood to
flow through the vessel. In some cases, blood is unable to flow at
puts a tremendous amount of stress on the heart and can lead to heart
- When an artery supplying the brain or heart is blocked, tissue
damage can occur very quickly and if not treated immediately, is
permanent and can lead
The nicotine in cigarettes and chewing tobacco increases
blood pressure because it causes the arterioles to constrict, while increasing
heart rate. Both stress the heart. See Hazards.
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