Lesson 3.2 - Arteries and Veins

Look at the diagram of the circulatory system, heart, urinary system, and kidney on page 187 of your textbook. 

Diagrams in colour often show arteries as red and veins as blue.  If you look at your arm, you may be able to see blue blood vessels but not any red ones.  So what gives? 

Is blood really blue and red?  Are arteries always coloured red in pictures and veins coloured blue when making diagrams?

Human blood is never blue!  Blood that is rich in oxygen (oxygenated) is a brighter red than blood that is low in oxygen (deoxygenated).  Blood that comes from your lungs to your heart is loaded with oxygen and low in carbon dioxide.  Your heart pumps this bright red blood to all parts of your body through arteries.

The body cells take the oxygen from the blood, and the blood picks up carbon dioxide waste from the cells.  The blood turns a less bright colour and returns to the heart through veins.  It is still red-not blue.  If the blood is always red, why do we see these blue vessels in our arms?  The answer is that the blood vessels are viewed through the skin.  The skin is not transparent, and the poor transmission of the light makes the blood vessels appear blue regardless of whether they are arteries or veins.

Now, look carefully at the diagram of the heart on page 187. 

Are all arteries coloured red?  Are all veins coloured blue?  No, the pulmonary artery from the right ventricle is coloured blue.  The veins from the lungs leading to the left atrium are coloured red.  Here is the rationale for the colouring: 

  • Blood vessels that carry oxygenated blood are coloured red. 
  • Blood vessels that carry deoxygenated blood are coloured blue.

How can you tell veins from arteries?  Arteries always carry blood away from the heart; veins always carry blood to the heart.  

An easy way to remember this is blood flowing Away from the heart is flowing through an Artery.

Now, answer the following questions.

 

Question 1. What is the colour of blood that flows through the veins?

Question 2. What is the colour of blood that flows through the arteries?

Question 3. In a diagram that has coloured blood vessels, are red vessels arteries and blue vessels veins?

Question 4. Why do the blood vessels in your arm appear blue?

Question 5. How can you tell if a blood vessel is an artery or a vein?

 

Check your answers with those that follow.

 

 

 

 

 

 

 

 

 

 

 

 

Answers to Questions:

Question 1. What is the colour of blood that flows through the veins?

Blood that is flowing through the veins is red; it is not as bright a red as blood flowing through the arteries.

Question 2. What is the colour of blood that flows through the arteries?

Blood that is flowing through the arteries is red; it is a brighter red than blood flowing through the veins.

Question 3. In a diagram that has coloured blood vessels, are red vessels arteries and blue vessels veins?

No, arteries are not always coloured red and veins are not always coloured blue.

Question 4. Why do the blood vessels in your arm appear blue?

The blue colour is due to blood vessels viewed through the skin.  The skin is not transparent and the poor transmission of the light makes the blood vessels appear blue regardless of whether they are arteries or veins.

Question 5. How can you tell if a blood vessel is an artery or a vein?

In a diagram of blood vessels, you have to know if the blood is flowing away from the heart or towards the heart to tell if it is an artery or a vein.  Arteries always carry blood away from the heart, veins always carry blood towards the heart.

 


Go to the next page to continue Lesson 3.