How Does Blood Get From the Heart to All Parts of the Body?
Blood circulates around the body in a system of blood vessels. Remember that arteries take blood from the heart and veins bring blood back to the heart. In this section you will investigate how the blood moves through the arteries to get to the cells all over the body.
The blue whale is the largest animal that has ever lived on Earth. An adult blue whale is so large that its heart is the size of a compact car. You could crawl on your hands and knees through its aorta.
Let's begin the journey blood takes from the heart through the body to the cells. The vessels that take blood from the heart to body cells are called arteries. Arteries bulge when they get blood from the heart during systole (squeeze). They have thick, elastic walls that help to squeeze and push the blood along its way. Arteries take blood to every organ in the body. Arteries branch from larger arteries to smaller ones called arterioles. Most arteries are located deep inside muscles or close to bones to protect the vessels from injury. About 15 percent of the body's blood is in the arteries at any one time.
The biggest artery is the aorta, which is the big artery that comes out of the left ventricle. Arteries that branch from the aorta supply the entire body with blood. The very first branches are the coronary (KOHR-uhn-ayr-ee) arteries. The word coronary comes from a word that means crown. The main coronary arteries circle the top of the heart like a crown. Your heart is a very hard-working muscle that needs a steady supply of oxygen and nutrients, which it gets from the coronary blood in the arteries. These coronary arteries receive up to 5 percent of the cardiac output. Have you ever heard of someone having a “coronary” or heart attack? A heart attack occurs when a coronary artery gets clogged and cannot deliver oxygen to the heart muscle. Figure 3.1 is a drawing of where some of the coronary arteries are located.
Figure 3.1 shows a surface view of the heart with the left and right coronary arteries. Find the aortic valve. The left coronary artery and the right coronary artery leave the aorta just above the aortic valve. They take blood to the muscle cells making up the walls of the ventricles. This blood carries a lot of oxygen. The heart muscle cells are working all the time so the heart needs a lot of oxygen.
Figure 3.1 Your heart has the same needs as the rest of your body for food nutrients, gas exchange, and waste removal. To do this the heart has its own system of blood vessels including the coronary arteries.
If you turn back to Figure 2.20, you can remind yourself of how the aorta leaves the heart and makes a big arch as it turns to take blood to the body below the head and arms. Look again in Figure 2.20 at the arteries that leave the right ventircle to go to the lungs. These arteries are called the pulmonary arteries. It takes less force to push blood through the lungs than it does to push it all around the rest of the body. So, the pulmonary arteries are not as large and thick as the aorta. The blood coming from the right ventricle to the lungs is at a lower pressure than the blood coming from the left ventricle to the rest of the body.
Why does more blood go to muscles of the ventricles during diastole than during systole?
The pressure inside arteries is high. So if you cut an artery, blood spurts out. This high pressure results when the ventricles pump blood into the arteries during systole. The high pressure tends to stretch the walls of the arteries-sort of like filling a balloon with water. At the end of systole, the stretched walls of the arteries return to their normal size. As they do, they squeeze the blood inside. This pushes the blood through the arteries even during diastole while the heart fills.
Why does the pressure in arteries need to be high? Well, think about water pipes. If a truck hits a fire hydrant, the water may spurt 12 meters (40 feet) into the air. The pipe connected to the fire hydrant is called the water main. The water main is like an artery. Water in the water main is under high pressure. This high pressure forces water all over the city. In a similar way, blood passing through the aorta is under high pressure. The high pressure distributes blood to all the cells of the body.
Figure 3.2 Your blood vessel system has many branches varying in size.
Taking Your Pulse
Taking your own pulse helps you understand how your heart and arteries work. Use a clock or watch with a second hand to take your pulse.
- Find your wrist pulse using the second and third fingers of either hand. Look at Figure 3.3.
- Press lightly to feel the blood make the artery expand as a result of each heartbeat. This is your pulse.
- Sit comfortably and remain quiet. Determine your pulse by counting the number of pulses during a 60 second period.
- Record your results. Compare your pulse rate with those of your classmates. You also can take your pulse in the neck by finding your carotid artery just to the side of your larynx or voice box.
Figure 3.3 You can feel your pulse on your wrist, near the base of your thumb. Calculate your heart rate by counting the number of beats per minute.
Activity 3-1: Blocked Arteries
What does a diet containing too much fat and cholesterol (kuh-LES-ter-ahl) do to your arteries? Too much cholesterol can cause atherosclerosis (ATH-uh-roh-skluhr-OH-sus). Atherosclerosis is a disease that blocks the arteries. But how do doctors know that cholesterol can cause atherosclerosis? What do blocked arteries look like? Why are they dangerous? In this activity you will learn about atherosclerosis. The activity helps you find out what causes it and how to prevent it.
- Clear rubber tubing or toilet paper rolls
- Cotton or clay
- Resources 1 and 2
- Activity Report
Step 1 Read Resource 1. Examine the arteries shown in the photographs.
Step 2 Make a sketch of the artery under item number 1 of the Activity Report as you examine each photograph. Label each sketch.
Step 3 In your group discuss the following questions.
- Why is atherosclerosis considered a danger to health?
- What evidence suggests that a high-fat, high-cholesterol diet is linked to atherosclerosis?
- Should you be concerned about your diet at your age? Why or why not? At what age should you become concerned?
- What are two things you can do to prevent atherosclerosis?
Step 4 Design and build models of a healthy artery and an unhealthy artery. Compare and contrast the two arteries. Demonstrate how blood flow in the unhealthy artery is reduced and/or blocked.
Step 5 Discuss with your group how a coronary bypass operation is performed. Use the diagram on Resource 2. Use the scissors as a scalpel. Then use glue or staples to stitch the blood vessel into position.
Why is cutting an artery dangerous?
Arteries start out large as the aorta. But they branch again and again to reach all parts of the body. As they branch, the arteries get smaller and smaller. Arterioles are the smallest and narrowest arteries. At the ends of the smallest arteriole there is a bed of tiny vessels called capillaries. This is where materials and gases are exchanged, because the capillaries are very thin and very leaky. Every cell of the body is close to a capillary. The cell exchanges oxygen (O2), nutrients, and wastes such as carbon dioxide (CO2) with the blood passing through that capillary.
Figure 3.4 Arterioles have a much smaller diameter than arteries.
As blood flows from the aorta through smaller arteries to arterioles and finally to capillaries, its pressure is steadily falling. There are two reasons why blood pressure falls as it flows from the heart to the capillaries. The first reason may surprise you. The total cross-sectional area of the capillaries is greater than the cross-sectional area of the huge aorta. Think of a garden hose leading to sprinklers. When there is only one sprinkler on the hose, its pressure is high. But as more and more sprinklers are added, their pressure goes down.
The other reason blood pressure falls as it flows towards the capillaries is that the resistance to flow is greater the smaller the vessel. Think about drinking milk through a tiny straw in comparison to a bigger tube. You have to suck harder when you use the tiny straw because the resistance to flow is higher.
The smallest arterioles have rings of muscle around them. These muscles control the diameter of the arteriole and therefore how much blood flows into each capillary bed.
Figure 3.5a The arteriole is completely open when the rings of muscle are relaxed.
Figure 3.5b When the rings of muscle contract the arteriole becomes narrower and less blood flows through.
Figure 3.5c Sometimes the rings of muscle can squeeze so tightly that very little blood flows through. This situation can occur if more blood is needed in one part of the body than another. This is how arterioles help direct the flow of blood.
What do you think happens to the arterioles in your leg muscles when you are running? Explain.
If you are sick or frightened your face may get pale. What's happening in your body?
If you are embarrassed you may blush. What's happening in this situation inside your body?
Pretend you are a drop of blood. Describe any differences in passing through a healthy artery and passing through an unhealthy, atherosclerotic artery.
Arterioles are “downstream” from arteries and “upstream” from capillaries. The position of the arterioles helps in several ways. Being between the arteries and the capillaries, the arterioles act together to keep the pressure high in arteries. And they act to control the distribution of blood to capillary beds so that blood is directed to where it is needed most. Individual arterioles control how much blood enters capillaries downstream from them. If all the arterioles in the body opened at once, pressure inside the arteries would drop too low to distribute blood to all parts of the body.
- Describe three characteristics of arteries.
- Why doesn't blood stop flowing in your arterioles when the heart relaxes between beats?
- Describe two characteristics of arterioles. Explain how they work.
- What happens during a heart attack?
- What is atherosclerosis? How can you prevent it?
- Explain the role of blood pressure in moving blood from the heart through arteries to the cells of the body.