How Does Blood Get Back to the Heart?
You learned about the arteries and arterioles that deliver nutrients and oxygen to the cells of your body. But the story doesn't end in the cell. When the cell uses the oxygen and nutrients, it produces wastes such as carbon dioxide. So the body has vessels called veins to carry those wastes away from the cells that produce them. In this section you investigate the vessels that carry the wastes away from the cells.
Veins and venules are the vessels that return blood from the capillaries back to the heart. For every major artery carrying blood to an area, there is a vein taking blood back out of that area. The blood that carries waste materials from cells flows through the veins. The veins close to the heart are the largest of the veins in your body. The walls of large veins are thicker than the walls of the smaller veins. But they are not nearly as thick as the walls of arteries that have the same diameter. The smallest veins called venules have very soft and thin walls.
The walls of veins can be thinner and less elastic than the walls of arteries, because the blood is under much less pressure in veins than in arteries. The pressure created by the squeezing of the heart pushes blood through arteries, arterioles, and capillaries. But that pressure is almost completely gone by the time the blood enters the venules. Then how does the blood get back to the heart?
Find someone whose veins on the top of the hand are easy to see. Boys sometimes have more obvious veins than girls do. Ask your partner to swing and shake one hand down at his or her side for 30 seconds. What happens to the blood in these veins? Now ask the person to raise the same hand high in the air for 30 seconds. What happened to the blood? Explain your observations.
Varicose veins are those squiggly veins that some people have on the backs of their legs, often just below the knee. Varicose veins form when blood flows backwards and overloads the veins by stretching out the walls. Those veins lying outside muscle groups, closest to the skin, are the most likely to become varicose.
Figure 5.1 The veins form a network of vessels that return blood to the heart.
You learned that blood moves through vessels because of a pressure difference. The pressure difference that causes blood to move through vessels is like the pressure that moves water through a garden hose. It's also like the pressure difference that forces air out of a balloon. You learned that the squeezing of the blood by the heart generates the pressure that moves blood through arteries. This pressure causes the arteries to stretch. Then when the heart relaxes, the recoil of the stretched arteries keeps the blood moving. But the pressure generated by the heart is all used up pushing blood through mile of tiny arterioles and even tinier capillaries. So what pushes the blood through veins?
Figure 5.2 Veins have thinner, less elastic walls than arteries. The walls of veins can expand to hold more blood, or collapse when they hold little blood.
Gravity or Valves
There are two important mechanisms that return blood to the heart. One mechanism is gravity. Some blood ends up in veins above the heart, such as the blood in the head. Blood above the heart flows back to the heart just like water in a stream flows down a mountain. But gravity makes the problem of getting blood back to the heart worse when the blood ends up below the heart, such as the blood in the feet. To take care of this problem, veins have something that arteries don't. Veins have one-way valves.
Veins have flaps of tissue that work as valves allowing blood to flow in one direction only-toward the heart. But we still haven't discovered what causes the blood to flow up in the veins from below the heart. The answer is the muscles of your body working with the one-way valves. As the muscles of your body contract and relax to move your limbs they also squeeze your veins. Whenever veins are squeezed the blood in them moves. Because of the one-way valves, the blood can only keep moving toward the heart.
About 60 percent of your blood is in your veins when you are resting. Remember that the walls of the veins are thinner than the walls of the arteries. The thinner walls of veins can expand more than the thicker walls of arteries. So your veins can hold a larger volume of blood than your arteries can. Unlike arteries, veins collapse when they are empty.
Activity 5-1: The Direction of Blood Flow
How does blood flow through your blood vessels? Does it flow back and forth or in one direction only? In this activity you recreate the procedures and experiments of William Harvey. In the 17th century Dr. Harvey discovered evidence that blood flows in one direction only through blood vessels. He showed that blood moves away from the heart in arteries and then back to the heart in veins. William Harvey used a tourniquet to trap blood in the surface veins of the arm and hand. This procedure helped him explore the one-way flow of blood. You will use a much safer method to explore blood flow.
- Clock or watch
- Activity Report
Step 1 Raise one arm and hand as shown in Figure 5.3. Hold it above your head for four minutes. Keep your other arm relaxed at your side.
Do you notice any tingling or throbbing in the ends of your fingers? Is there any change in the warmth of your hand? Is there any difference between the feelings in your two arms and hands? Describe the differences between them to your lab partner. Have your lab partner record these differences on your Activity Reports.
Figure 5.3 Hold one arm above your head.
Step 2 After four minutes, place your two hands in front of you. Compare the backs of your two hands and note any differences in color and visibility of blood vessels. Trace around each hand and draw in the pattern of blood vessels.
Step 3 Shake your left hand vigorously for about 30 seconds or until you can easily see the vessel pattern on the back of your hand.
Step 4 Immediately place your left hand on the table surface palm-Side up. Place the index finger of your other hand over a prominent vessel at your wrist. This vessel is a vein. Arteries are deeper. Press firmly as you move your finger about 3cm toward the elbow. You should notice that the blood follows your finger.
Step 5 Repeat step 4 moving your finger away from the elbow. Be sure to note any differences in color and size of the vessel immediately behind the path of your finger. You should notice that the blood does not follow your finger. There are valves located in the vein that prevent the backward flow of blood.
Step 6 Now let your lab partner follow Steps 1 to 5 while you record the observations your partner experiences.
Step 7 What did you learn about the flow of blood in this activity? Complete the questions on the Activity Report. Then write a summary of what you have learned about the one-way flow of blood in your body.
Figure 5.4 English scientist Dr. William Harvey drew these diagrams of his experiments on blood circulation. They were first published in the year 1653.
Veins and Muscle Contraction
Veins appear blue because a yellow pigment in the skin makes the dark red blood in the vessels look bluish.
Airplane test pilots who pull their planes out from steep dives can experience increased gravity that makes blood pool in their feet. Unless they wear pressurized suits called G-suits they can pass out. A G-suit increases pressure on the legs and feet, and counteracts the effects of gravity.
Since veins are especially flexible, gravity causes the blood to pool in the feet and legs when you are standing. For this reason soldiers that have to stand at attention for long periods of time must keep their leg muscles contracting and relaxing even though they are not walking around. If they don't, the blood pools in the veins of the feet and legs. If too much blood pools in the feet and legs, the brain doesn't get enough blood and a person can pass out.
You may get a warning from your body before you faint. You can feel unsteady and become giddy or dizzy. Your vision may blur and you may start to sweat. You can feel cold or clammy and may not think straight. If this starts happening to you or someone else, remember you need to increase blood flow to the brain. There's a very good way to increase the blood flow to the brain. Lie flat and prop the feet up so they are higher than the head. This position lets blood from the legs move up to the heart thanks to gravity. However, that is not the only action you should take in a fainting situation. After a person who has fainted or is feeling faint is lying down safely, you or someone should get help right away.
Strenuous exercise can increase the amount of blood the heart pumps out from 5 liters (1.3 gallons) per minute to 30 liters (8 gallons) per minute.
Figure 5.5 While the heart powers blood through the arteries, the veins depend on muscles contracting (squeezing) to move blood upward to the heart. Valves in the veins keep blood from flowing backwards.
The drawing in Figure 5.5 shows that when leg muscles contract they squeeze the veins and push the blood towards the heart. The drawing also shows that veins have valves that prevent the back flow of blood. These are like the valves in the siphon pumps. They let fluid pass in one direction only. Exercise helps your circulation because when you contract your leg muscles, you empty your leg veins. The leg veins fill from the capillary beds in your feet when the muscles relax.
- The soldiers who guard Buckingham Palace in London, England do not move when they are on guard. They are known for their ability to stand still for very long periods of time. Some guards have fainted after only 30 minutes on duty. Why do you think this happened? What happened? How could the guards have prevented it?
- Why is it more important for leg veins to have valves than neck veins?
A Closed Circuit
You learned how the heart pumps blood to the body. Now you've learned how the blood returns from the body to the heart. All these vessels working together with each other and the heart form a closed circuit. There is usually a good balance within that closed circuit. Each minute, the amount of blood the heart pumps to the arteries equals the amount of blood returned by the veins to the heart. Here's another way to explain that. The amount of blood the right side of the heart pumps to the lungs is the same as the amount of blood the left side of the heart pumps to the body. Or the simplest way to explain it is what comes into the heart must flow out of the heart.
The amount of blood flowing through the heart changes depending on the oxygen needs of the body cells. During exercise the veins return more blood to the heart and more blood is pumped out to deliver oxygen to the cells.
At rest your 5 liters of blood is distributed in your circulatory system as follows:
veins and venules 60 percent
arteries and arterioles 15 percent
pulmonary 12 percent
(in lung capillaries)
heart 8 percent
capillaries 5 percent
Figure 5.6a The amount of blood entering the heart per beat is the same as the amount of blood leaving the heart per beat. In this example, each ventricle pumps 70ml of blood with each heartbeat.
Figure 5.6b The amount of blood pumped per minute is the cardiac output.
Percentages of Blood Compare the percentages of blood found in different parts of your circulatory system listed in the Did You Know? above. Make a circle graph showing the data. Remember that you have 5 liters of blood in your body. Now calculate how many liters of blood are in each part of your circulatory system while you are at rest.
- At rest, your heart pumps only 20 to 25 percent of the total blood volume in your body. What is the purpose of the other 75 to 80 percent of the blood? Where is this blood?
- In which blood vessels does blood travel the most slowly? Why would the blood travel more slowly in those vessels?
Pretend you are a drop of blood. What differences would you feel going to the heart through a vein than going from the heart through an artery?
- In the Arteries and Arterioles section you learned about arteries. In this section you learned about veins and compared the structure and function of an artery and vein. List four differences.
- Explain how blood gets back to your heart from your feet when you are walking. What factors help the return of venous blood to the heart? What makes it more difficult to return venous blood back to the heart?
- When you are resting, a greater percentage of your total blood volume is in your veins than when you are exercising. How is this possible?
- How is blood output from the heart related to blood return?
- Describe William Harvey's famous experiment. Include what he did and the conclusions he reached.