Bodily Defenses Image Map

Blood Flow

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

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 flow.
  • 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 blood flow.
  • In any one capillary, this resistance is an advantage because the slowed blood flow has more time for gas exchange to occur.
  • When an arteriole dilates, the diameter almost doubles. When a vessel's diameter increases, the blood flow increases by four times the original amount. 
  • 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. 
  • Under 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 all. This puts a tremendous amount of stress on the heart and can lead to heart failure. 
  • 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 to death.

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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The blood vessel on the right allows 4 times as much blood flow as the vessel on the left.  You can calculate this by using the formula for the area of a circle, which is Area = pi x radius˛.

A normal cardiac artery with little build up of fats and cholesterol


A cardiac artery with severe  build up of cholesterol that is reducing the diameter of the vessel.