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Circulation And The Heart

by e-Tutor

In this lesson you will explore the road blood takes to circulate our body. You will learn also about the different parts of the heart and how it works.

Lesson Problem

How does blood flow through the human heart?


Study Guide

It may be hard to believe, but your heart is the strongest muscle in your body. It has to be strong to move blood through your body to other muscle cells, your brain cells, skin cells, and nerve and bone cells all day and all night long for your entire life. The heart is located under the ribcage and lies between the two lungs. A human heart is roughly the size of your clenched fist. The average adult heart beats about 70 times per minute.

The circulatory system has three major components which are blood, blood vessels, and the heart. Blood contains oxygen, nutrients, and wastes. Blood vessels transport blood throughout the body. The heart is the main component of the circulatory system. The heart's job is to transport oxygenated blood to the body's cells and carry away waste materials. Your heart is a muscular organ, about the size of a clenched fist, that pumps the blood. A fibrous tissue or sac, called the pericardium, encloses and protects the heart.

The heart is made up of four chambers. The upper half is composed of the right and left atria, and the lower half is composed of the right and left ventricles. Make sure to note that right and left refer to the body's right and left side, so when you are looking at a diagram the sides are actually reversed. A muscular wall, called the septum, separates the right and left chambers. Valves separate the atria from the ventricles.


To stay alive, your heart must pump constantly. Your heart muscle keeps pumping all through your life. It pumps more than two billion times during a normal lifetime. Pumping of blood is produced by alternate contractions of the atria and ventricles. When the heart contracts, blood is pumped out of the heart. When the heart relaxes, blood fills the heart.

Because your heart works like a pump, the blood is actually pushed through the arteries in spurts. Your heartbeat is actually the sound of the heart pumping and the valves within the heart opening and closing to allow the blood to flow through. You can determine the number of times your heart beats in a minute by having someone listen to your heart through a stethoscope or through a simple hollow tube, such as an empty paper towel roll.

You can also measure your heart at work by taking your pulse in areas where your arteries are close to the skin. By placing your first two fingers (not your thumb) of your hand on one of these areas, you can feel the spurting, or surging, action of the blood as the heart pumps. The number of times you feel this surge in a minute is your pulse. Generally, the smaller the heart, the faster the pulse. A young child's heart beats from 100 to 120 times a minute. By the time you reach adulthood, your pulse will measure form 70 to 90 times per minute. Adults that are in good condition can have a heart rate between 50 and 60 beats per minute.

Since your heart is a muscle, it needs exercise just as your other muscles do. When you exercise, the heart works faster and harder to supply oxygen and food to the rest of your body. This exercise, in turn, strengthens the heart and keeps it fit. Heart-healthy exercise increases your pulse rate, and your heart gets a workout. Good exercise for the heart involves an activity which increases the pulse rate for about twenty minutes at a time.


A system of blood vessels carries the blood through the body. These vessels include arteries, veins and capillaries. Some of your arteries and veins are as big around as your thumb; others are almost too small to see. Arteries have thick walls and a smaller inside diameter than do veins. The walls of arteries are muscular and elastic, allowing them to expand and contract easily. The walls of veins are thin and contain less muscle tissue than arteries. The walls of capillaries are only one cell layer thick, and cannot be seen without a microscope. These three types of vessels and the heart make up the circulatory system.

Your blood vessels are a seemingly endless stream of hollow, soft, elastic tubes. It is estimated that if an adult's vessels were laid end-to-end, they would stretch over 12,400 miles. That is more than four times the distance across the United States.

The blood vessel system carries blood from the heart to all parts of the body. Blood leaves the left side of the heart through the main artery, called the aorta. When the heart relaxes, the artery contracts. This contraction exerts a force that pushes blood through. The aorta then branches off into increasingly smaller arteries until it reaches tiny capillaries. Through the capillary walls, blood cells deliver oxygen and food to other cells in exchange for carbon dioxide and other wastes. The blood then begins its return trip to the right side of the heart through the veins. Veins have cup-like valves that prevent backflow of blood, which keeps blood flowing toward the heart. Most veins carry dark red blood that is low in oxygen. The veins in your skin appear blue because of the color of your skin and blood vessels. Blood in the veins has much lower pressure than blood in the arteries. Therefore, blood moves more rapidly through the arteries. Blood returns to the heart through two major veins, the superior vena cava, which transports blood returning from the arms and head, and the inferior vena cava, which returns blood from the lower part of the body. Learn more about the specific vessels that transport blood .

Entering the right side of the heart, the blood travels from the heart, to the lungs, by way of the pulmonary artery. In the lungs, waste gases are exchanged for oxygen. From the lungs, blood enters the left side of the heart. Also, as blood passes the digestive tract, food is picked up to be delivered to cells throughout the body. As the blood travels through your body, a series of one-way gates, or valves, allow it to flow in only one direction. There are valves throughout your circulatory system and also in your heart.


Perhaps you have had your blood pressure taken during a health checkup. Blood pressure is a measure of the pressure of blood in the arteries. Every time the heart beats, the pressure inside the arteries increases. When the heart relaxes between beats, the pressure decreases. Blood pressure is expressed in millimeters of height of a column of mercury. When the human heart contracts, also known as the systole reading, normal blood pressure in the arteries range from 110 to 150 millimeters of mercury. Normal pressure is 60 to 80 millimeters when the heart relaxes, or during the diastole reading. Blood pressure varies from one person to another, and even within the same person. It goes up when you are excited and down when you are asleep. Continued high or low blood pressure readings may indicate poor health. Blood pressure may be affected by changes in the amount of blood, clogging of blood vessels, and rate of heartbeat.


Entering the right side of the heart, the blood travels from the heart, to the lungs, by way of the pulmonary artery. In the lungs, waste gases are exchanged for oxygen. From the lungs, blood enters the left side of the heart. Also, as blood passes the digestive tract, food is picked up to be delivered to cells throughout the body. As the blood travels through your body, a series of one-way gates, or valves, allow it to flow in only one direction. There are valves throughout your circulatory system and also in your heart.

Let's follow a droplet of blood through the blood vessels. When blood leaves the left ventricle of the heart, it goes through a valve and into the aorta. This vessel is the largest artery in the body. Soon after leaving the heart, the aorta branches, so the drop of blood can move into smaller arteries leading to the head or arms. We will assume that this drop of blood travels down toward the legs. The large artery splits in order to carry blood down to each leg. This drop of blood goes into one of these branches. From here, the blood travels in smaller and smaller arteries until it reaches the very end of the big toe. The blood vessel here is so small it cannot be seen by the naked eye. This tiny vessel is a capillary. At this point, the blood nourishes the cells next to the capillary and takes away the waste products.

Now that the blood has given up its food and oxygen it must return to the heart. Blood leaves the capillary and moves into a larger blood vessel, called a vein. This drop of blood joins more blood going back to the heart. The blood vessels become larger and larger, until they finally all join. One large blood vessel receives all blood from the lower part of the body. This blood vessel is called the inferior vena cava. There is another large blood vessel that collects blood from the head and arms, called the superior vena cava. Each of the vena cavas separately enters the right atrium of the heart. Contraction of the right atrium forces blood into the right ventricle. The tricuspid valve, between the right atrium and the right ventricle, is very important. This valve prevents the blood form flowing back up into the right atrium. The valve only opens in the direction of the ventricle. When blood pushes against the valve it is forced closed, preventing blood form leaking back into the atrium. Contraction of the right ventricle forces blood through the pulmonary valve and into the pulmonary artery. Blood travels through the pulmonary artery to the lungs, and into the lung capillaries. There it picks up oxygen and loses carbon dioxide by diffusion. The oxygen-rich blood then travels through the pulmonary veins to the left atrium of the heart. Blood is pumped from the left atrium to the left ventricle, through the bicuspid valve. Oxygen-rich blood is pumped out of the left ventricle through the aortic valve, and into the aorta. The aorta is the largest artery in the body. It branches into smaller arteries, which connect to capillaries. When the left ventricle contracts, it exerts more force than any other heart chamber. The oxygen-rich blood in the aorta is pumped throughout the body. The blood gradually loses oxygen to body cells and gains carbon dioxide by diffusion. To complete the cycle, the blood returns to the right atrium through the large veins called vena cavas.


Where does the energy come from that keeps the heart beating? The pacemaker, or SA node, is where the heartbeat originates. It is a result of rhythmic impulses that come from cells in the heart itself. The pacemaker is located in the inner wall of the right atrium. The tissue creates electrical impulses through changes in cell membrane properties, without outside stimulation. The heartbeat impulse spreads from there throughout the heart muscle. Nerves outside the heart control the rate of heartbeats.


Directions: Complete each of the questions below, using what you have learn from the lesson and resources.

A. Complete

B. True or False. Write the word TRUE or FALSE, not T or F!!!

1. Capillaries are the largest kind of blood vessels. ___________

2. Blood is blue when it has a low amount of oxygen in it. ___________

3. The aorta is a major artery that carries oxygen rich blood to the body ___________ except for the lungs.

4. Some white blood cells make antibodies that recognize foreign material, ___________ while others surround and destroy a bacteria.

5. The major organ in the circulatory system is the heart. ___________

C. The heart is composed of chambers, valves and vessels. Using the diagram below, label as many parts as you can.

Extended Learning

A. Complete

1. Blood passes by the ______________________________ to pick up nutrients from digesting food, and is filtered by the _____________________ to remove toxins and waste products of metabolism.

2. ______________________ makes up the liquid part of the blood, about 90% of which is water, and the other part is dissolved substances.

3. Blood is said to travel through the body in two loops; one loop contains ________________ __________________ blood, and the other loop contains ________________ __________________ blood.

4. Blood vessels called _____________________________ are large vessels that carry blood away from the heart.

5. Name three things the circulatory system carries to or away from cells: ___________________________________________________________________________

6. Clotting of blood at the point of a break in a blood vessel is accomplished by cells called _____________________ which stop the flow of blood.

7. Substance such as gases and nutrients can be exchanged easily through blood vessels called _____________________________ because they are tiny and have thin walls.

B. Pretend that you are a drop of blood.

Describe, in writing, your journey through the human body. Be sure to include the vessels and structures that you travel through.

Start your trip in the left ventricle and follow the path through the body and all the way back into the left ventricle. Share your journey with members of your family.