Why does the heart automatically adjust the flow of blood to match activity levels?

A. Extreme activity damages blood cells and the body needs replacements.

B. The heart requires more oxygen when exercising.

C. It builds up a higher level of oxygen in the blood.

D.  Organs and muscles require more blood under stress

The correct answer is: B. The heart requires more oxygen when exercising.

Diagram of the heart showing the coronary arteries that supply blood to the heart muscle
Diagram of the heart showing the coronary arteries that supply blood to the heart muscle

The heart is the organ that pumps blood around the body to all the tissues. The heart automatically adjusts the flow of blood to match a person’s activity level because the heart requires more oxygen when exercising.

When you exercise your heart has to work harder to pump blood through your blood vessels. The blood vessels carry the blood around your body to your tissues, especially to your muscle tissues. The heart is also a muscle that needs more oxygen during this time so that it is able to pump harder.

Several physiological events happen to increase the blood flow and therefore oxygen flow to the muscles, including to the heart muscle. We would not be able to exercise if these events did not happen.

Adrenaline and noradrenaline are two hormones that are released when you exercise. These hormones act to increase the respiratory rate and heart rate. The amount of blood flowing out of the heart each minute increases with an increased heart rate.

Vasodilation of blood vessels also occurs which increases blood flow through arteries, arterioles, and capillaries. This means more oxygen can be offloaded to skeletal muscles where it is needed. Vasodilation is the process by which the lumen (space) of the blood vessel increases so as to carry more oxygen.

The circulatory system

The human circulatory system consists of the heart and several blood vessels that form a closed system. Blood is pumped around the body through the blood vessels to the various tissues.

The blood vessels are the tubes that are responsible for transporting the blood around the body. The heart is a very muscular organ that contracts to pump blood into the blood vessels.

The heart like all organs of the body also needs to have oxygen and nutrients. It therefore also has a blood supply in the form of coronary arteries that carry blood to the heart muscle.

When we breathe in we take in oxygen into the lungs. Oxygen diffuses from the air sacs of the lungs into the blood where it attaches to hemoglobin on the blood cells.

Blood cells carry oxygen and blood plasma carries various nutrients and hormones. This oxygenated blood goes from the lungs to the left side of the heart where it enters the left atrium.

The blood moves into the left ventricle and then the heart contracts to push the blood into three arteries that leave the left side of the heart. The oxygenated blood is carried from arteries that become smaller vessels called arterioles.

These arterioles enter capillaries which then become venules. Nutrients and oxygen diffuse from the blood cells into the cells of the body in the capillary bed. At the same time, carbon dioxide and wastes move from the body cells into the blood that then moves into the venules.

This deoxygenated blood then passes from venules into veins. The veins carry the blood back to the right side of the heart. The big veins carrying deoxygenated blood are the vena cavae which enters the right side of the heart.

This blood is then pumped to the lungs where the carbon dioxide diffuses into the air sacs of the lungs. We then breathe the carbon dioxide out.

Control of blood flow

Several factors influence the blood flow in the circulatory system. These factors include cardiac output, blood volume, blood viscosity, blood volume, compliance and the length and diameter of the blood vessel.

Cardiac output is the measurement of blood flowing from the ventricles. In other words, it is the amount of blood coming out of the heart each minute. Cardiac output is increased by hormones such as adrenaline and noradrenaline released during exercise. This then causes blood flow to increase.

The viscosity of blood is how thick it is. The more viscous the blood the lower the flow rate will be since it can’t move as easily or as fast.

This should not change much unless a person is ill. Compliance is the ability of something to expand in size. A vein is a vessel that has great compliance while an artery is not as compliant.

The length of a blood vessel impacts the resistance with longer vessels having greater resistance than shorter vessels. Blood flow is slower when resistance is higher. Once a person is an adult the length of vessels should not change.

However, the diameter of blood vessels can change. The diameter of the vessels has a big impact on blood flow. Chemical and nervous signals can either cause the diameter to increase (vasodilate), or decrease (vasoconstrict). In fact, vasodilation increases blood flow while vasoconstriction decreases blood flow.

Exercise and blood flow

Exercise triggers several events in the body that lead to increases in blood flow. Various hormones such as epinephrine and norepinephrine trigger changes in the rate of respiration and heartbeat.

The respiratory rate increases to bring more oxygen into the body. The heart rate increases to pump more blood around the body to the muscles where it is needed. The increased heart rate pumps increased blood to the lungs.

The respiratory muscles need this extra blood flow so they can contract faster to pull more oxygen into the lungs from outside, by breathing faster. Furthermore, this extra oxygen is also needed by the heart muscle so that the heart can beat faster.

Cardiac output increases with exercise. In addition, vasodilation of blood vessels to skeletal muscles occurs. At the same time, vasoconstriction of blood vessels to the kidneys occurs.

This serves to direct the highest blood flow to the skeletal muscles where it is needed. Vasodilation of arterioles causes more blood to flow into the capillaries of the skeletal muscles. The result of this increased blood flow is increased oxygen delivery to the tissues.

Reference

  1. JG Betts, et al. (2017). Human Anatomy and Physiology. Houston: USA, Rice University.
  2. Editors of Encyclopedia Britannica (2018). Heart. Retrieved from Encyclopedia Britannica.
  3. MF Oliver, BE Mathews, ME Rogers (2018). Circulatory system. Retrieved from Encyclopedia Britannica.
  4. RJ Korthuis (2011). Skeletal muscle circulation. California, USA: Morgan & Claypool Publishers.
  5. C Rye, R Wise, V Jurukovski, J DeSaix, J Choi, and Y Avissar (2017). Biology. Rice University.

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