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Why is blood important to life?

This unit explores all the parts of your circulatory system. The unit begins with a look at blood and the heart. You will then learn about the different kinds of blood vessels and how your body controls blood flow. The unit concludes with a discussion of how to keep your heart and blood vessels strong and healthy.

A City of Cells

Think of your body as a city of cells. Each cell is like a house. A house needs fuel, energy, water, and raw materials. A house must also have a sewer system and garbage pickup to get rid of wastes. Your body must take in needed materials and get rid of waste materials just like a house does.

What makes a delivery system efficient? Imagine you are in charge of a package delivery service. What things might make your job harder? What things might make your job easier? What would be the most important parts of a successful delivery company? Write a paragraph to explain your ideas, and include any lists, diagrams, or drawings that help your explanation.

Pipelines, electrical cables, phone lines, and automobiles transport materials to and from buildings, apartments, and houses in a city. In your body, cells are served by blood in the circulatory (SUHR-kuelah-tor-ee) system. Blood flows through a series of tubes in your body called vessels (VES-uhls). The blood brings food, water, and oxygen to each cell “house.” The blood also removes wastes produced by your cells, including carbon dioxide gas, through a series of vessels. The carbon dioxide is carried to your lungs where you breathe it out. At the center of your network of blood vessels is your heart-the power pump of your circulatory system.

Figure 1.1 Your circulatory system is like the streets of a city, with lots of traffic flowing through the streets to and from different destinations.

Your heart pumps blood through a network of blood vessels. The network and your heart together are called a “circulatory” system. It's called a circulatory system because the blood circulates around and around. You also know that your blood carries oxygen to all of the cells in your body. The blood gets the oxygen from your lungs, which is a part of your breathing machine called the respiratory system. What you might not know is that your heart is really two pumps in one. It pumps the blood through two connected circuits. One circuit goes to the lungs, and one circuit goes to the rest of the body. So a map of the circulatory system is really more like a figure eight than a circle. You can demonstrate the map of the circulatory system by making the model in Activity 1-1: Pathway of Blood through Your Body.

Activity 1-1: Pathway of Blood through Your Body


In this activity you will learn about all the parts of your circulatory system and what they do. Let's start by building a model that can serve as your guide to the parts of the circulatory system and how they fit together.


  • Paper cups (4)
  • Straw
  • Glue
  • Paper towels
  • Colored pencils, pens, or paint (blue and red)
  • Tape
  • Balloon (white)
  • Colored thread (blue and red)
  • Colored yarn (blue and red, 2 pieces 20\;\mathrm{cm} each)
  • Lima beans (3 or 4)
  • Scissors
  • Ruler
  • Activity Report


First, you are going to build a model heart. The heart is two pumps side by side. Each pump has two chambers. In both pumps, blood enters the upper chamber and leaves the lower chamber. So you will have four blood vessels attached to your heart model. Now follow Steps 1 to 6 as you build your model heart.

Figure 1.2 Use a pencil to carefully make a hole in each cup. You will place straws in the holes in the heart model. The straws will represent blood vessels.

Step 1 Place the open ends of two paper cups together. Secure the cups together with tape. Do the same thing with the other two cups.

Step 2 Stand the two sets of cups side by side. Each cup represents a heart chamber.

Step 3 Carefully poke a hole in the side of each cup as shown in Figure 1.2 above,

Step 4 Cut a straw into four equal pieces. Color or paint two of the pieces blue and the other two pieces red, (You'll find out what the colors mean later.)

Step 5 Insert and glue one of the blue straws into opening B. Insert and glue a red straw into opening C.

Step 6 Stick a piece of blue yarn into the open end of the blue straw attached to the cups. Stick a piece of red yarn into the open end of the red straw attached to the cups. The straws and yarn represent blood vessels corning to and leaving the heart

Now you have two halves of what will be your model of the heart. The straws and yarn represent the system of blood vessels through which the heart pumps blood. Remember that this model resembles a figure eight rather than a simple circle. Half of the figure eight is the lung circuit where blood picks up oxygen. The other half of the figure eight is the body circuit where blood gives oxygen to all the cells of the body. Now you know the significance of the blue and red colors. Blue represents vessels carrying blood after it gives oxygen to cells. Red represents vessels carrying blood with a full load of oxygen. You can use this information in completing the following steps to finish your model.

Step 7 Inflate a white balloon to about 10\;\mathrm{centimeters} (4\;\mathrm{inches}) in diameter and tie it off. The balloon represents the lungs.

Step 8 Glue blue and red threads on the surface of the inflated balloon, or use pens to draw blue and red lines. The threads (or colored lines) represent the tiniest blood vessels where the blood picks up oxygen from the air in the lungs.

Step 9 Glue the free ends of the blue yarn to the surface of the balloon that has the tiny blue vessels. Glue the free ends of the red yarn to the surface of the balloon that has the tiny red vessels. You have completed the part of the model that represents the pump that moves blood to your lungs and back to the heart. This part of your completed model should look like the one in Figure 1.3.

Step 10 Now finish your two-pump model of the heart by making a model of the pump that moves the blood to your body cells. Insert and glue the other red straw into opening D. Insert and glue the other blue straw into opening A.

Step 11 Stick a piece of red yarn into the open end of the second red straw. Stick a piece of blue yarn into the open end of the second blue straw.

Figure 1.3 This is how the two halves of your model heart should look.

Step 12 Obtain three or four lima beans to represent body cells. Cut about 10 to 12 pieces of thread, each about 3\;\mathrm{centimeters} long. Half of the pieces should be red. The other half should be blue. Glue one end of several red and blue threads on the surface of each bean.

Step 13 Attach the free end of the red threads to the red yarn. Attach the free ends of the blue threads to the blue yarn. Your completed model should look like the one in Figure 1.4.

Step 14 Be sure you can explain to someone the path that a drop of blood would take in flowing through your model. Draw a picture of your model on your Activity Report. Then write an explanation of how the blood would flow through your model.

Step 15 Write your name and the date on your completed model.

Step 16 Check with your teacher for cleanup instructions and to find out where to store your model.

Figure 1.4 Blood carries oxygen from the lungs to the heart and from the heart to the cells. Blood carries carbon dioxide from the cells to the heart and from the heart to the lungs. This is a two-pump, figure eight model of the heart.


Blood is the fluid that circulates through your body. Blood is a liquid, but it functions like an organ in your body. An organ is a part of a living organism that has a specific function, such as the heart or brain. Blood serves many functions or has many jobs.

  • Blood transports oxygen, food nutrients, wastes, and heat.
  • Blood transports chemical messages called hormones throughout your body.
  • Blood has special cells that protect you against infection.
  • Blood has a clotting system to keep you from bleeding to death.

By doing all of these jobs, blood helps the body maintain homeostasis (hoh-mee-oh-STAY-sis). It helps keep conditions in your body in balance and functioning normally.

Did You Know? Plants such as tall trees don't have hearts to pump fluids. But gases, water, and nutrients also move through plants. Water evaporates through tiny holes in a plant's leaves. As the water evaporates, more water is pulled upward from the roots toward the top of the plant. The water flows through special elongated cells that join end to end to form vessels.

You have about five liters of blood in your body. Your blood contains red blood cells, white blood cells, and platelets. Blood is also made up of a straw-colored fluid called plasma (PLAS-muh). Plasma is about 90\;\mathrm{percent} water. Plasma also contains salts, nutrients, wastes, proteins, and other substances including hormones (HOHR-mohns). Hormones are chemicals that stimulate cells to respond in certain ways. Blood carries all these substances along with oxygen and carbon dioxide to and from your body cells.

Did You Know? Blood is heavier and stickier than water. Your blood makes up approximately 8\;\mathrm{percent} of your total body weight. Calculate how much the blood in your body weighs. Then calculate how many liters of blood will weigh that much. Assume blood weighs the same as water, or 1\;\mathrm{milliliter} =1\;\mathrm{gram}.

Figure 1.5 When a test tube of blood is spun around in a centrifuge, the heavier blood cells settle to the bottom. The lighter plasma rises to the top.

Figure 1.6 Here are some red blood cells seen with a high-powered microscope. The shape of a red blood cell maximizes its surface area. Its surface area determines how rapidly it can exchange O_2 and CO_2.

Red Blood Cells

Red blood cells are doughnut-shaped cells with a flat, filled center. One milliliter (\mathrm{ml}) of blood may contain 5\;\mathrm{million} red blood cells. Two thousand red blood cells lined up next to each other would cross the middle of a dime.

Did You Know? A lobster's blood is blue rather than red. It has no iron-containing hemoglobin to carry oxygen around its body. Instead, it has a blue pigment called hemocyanin (hee-moh-SY-ah-nihn), which contains copper instead of iron.

Red blood cells contain hemoglobin (HEE-muh-glow-bihn), which is a reddish protein that carries oxygen. The red color comes from iron, which is a mineral that is part of hemoglobin. Blood appears bright red in color when oxygen is attached to the iron in hemoglobin. Blood appears dark red when no oxygen is attached to the iron in hemoglobin.

A mature red blood cell has no nucleus (NOO-klee-us). The nucleus is the information and control center found in most other cells. Without a nucleus, the red blood cell has room to carry lots of hemoglobin. The flexible, flattened shape of a red blood cell lets it bend and squeeze through narrow capillaries that are the tiny blood vessels that supply your body's cells.

Figure 1.7 If you could place 2,000 red blood cells side by side, they would fit across the middle of a dime. There are about 25 trillion red blood cells floating in the bloodstream of a human adult. That is the number 25 followed by 12 zeros!

Red blood cells are made in the bone marrow. Bone marrow is the spongy material inside bones. In the marrow each immature red blood cell has a nucleus. The nucleus tells the red blood cell to make hemoglobin. The red blood cell loses its nucleus when it leaves the bone marrow to begin flowing through the vessels of the circulatory system. The new red blood cell is now like a bag of hemoglobin. A red blood cell circulates in the blood for about 120 days. After 120 days the parts of a red blood cell, including the iron in its hemoglobin, are recycled. They are reused by the bone marrow to make new red blood cells.

Did You Know? If blood did not carry away wastes, the body would poison itself with its own waste products.

Each red blood cell contains 200 to 300\;\mathrm{million} molecules of hemoglobin. Hemoglobin is a complex protein containing iron. Iron allows hemoglobin to bind to oxygen. The binding process makes it possible for the red blood cells to carry oxygen from the lungs to the cells of the body. When the blood reaches the tiny blood vessels called capillaries, oxygen is released from the hemoglobin. The oxygen passes through the thin capillary walls to reach the body cells. At the same time, carbon dioxide from body cells passes back through the capillary walls. The carbon dioxide enters the blood and is carried back to the lungs. Hemoglobin transports about 23\;\mathrm{percent} of the carbon dioxide back to the lungs. The rest of the carbon dioxide travels back to the lungs in the plasma.

Figure 1.8 If you use a powerful microscope to look at a cross-section of a blood vessel, you might see red blood cells inside. The photograph (right) shows across-section of a blood vessel. Red blood cells are inside.

Explain why a person living on a mountain at high altitude has a greater number of red blood cells than a person living at sea level.

What Do You Think?

Some athletes use a procedure called “blood doping.” Weeks before an important event they have some of their blood withdrawn and placed in cold storage. Their bodies make new red blood cells to replace the ones that were taken away. The athletes have their stored blood transfused back into their bodies just before the event. This increases the number of red blood cells in their blood and the amount of oxygen they are able to take up from each breath. Why do you think an athlete might do this? Do you think blood doping should be an illegal procedure? Why or why not?

Did You Know?

  • The body produces 200\;\mathrm{billion} red blood cells daily.
  • White blood cells live two weeks.
  • Red blood cells live four months.
  • When watching TV, it takes 35\;\mathrm{seconds} for blood to make a complete trip through the body. When exercising, it only takes 10\;\mathrm{seconds.}

Diseases in Red Blood Cells

Sometimes things go wrong with red blood cells. For example, they can make abnormal hemoglobin. People whose blood cells make abnormal hemoglobin usually have a faulty hemoglobin gene. A gene is a part of the cell that contains information about making a protein such as hemoglobin. Genes are inherited or passed on from parent to child. One kind of faulty gene produces a type of hemoglobin that tends to form crystals. The sharp crystals make the red blood cells sickle-shaped and fragile. The fragile cells break easily, leaving the person without enough healthy red blood cells. This problem causes a disease known as sickle-cell anemia (uh-NEE-mee-uh).

Anemia is the name for a condition in which a person doesn't have enough red blood cells or enough hemoglobin. A person with anemia tires easily. An anemic person may have other symptoms, such as dizziness, a headache, and drowsiness, too. These symptoms occur because there is too little hemoglobin to carry enough oxygen for the body's needs. Medical doctors can check for anemia with a blood test that measures the volume of red blood cells in the blood. The blood test is called the hematocrit (hee-MAT-oh-crit). If a person's hematocrit is too low he or she can take iron supplements to help the bone marrow produce more red blood cells. A normal hematocrit is about 40. That means that when 100\;\mathrm{ml} of blood is centrifuged, the packed cell volume at the bottom of the tube is 40\;\mathrm{ml} and the plasma on top is 60\;\mathrm{ml}, as you saw in Figure 1.5.

White Blood Cells

White blood cells are produced in the bone marrow, spleen, tonsils, and lymph nodes. You will investigate the lymphatic system and lymph nodes a little later. Now it's important to understand that lymph nodes are only one place white blood cells are produced. A white blood cell has a nucleus and is bigger dun a red blood cell. Another difference is that there are many fewer white blood cells than red blood cells in the blood. There is only one white blood cell for every 700 red blood cells in your bloodstream.

Figure 1.9 The red blood cell at the left is a sickle cell. The red blood cell on the right is a healthy cell.

Figure 1.10 A high-powered microscope shows a white blood cell (bottom left). The cells, above right and above left, are red blood cells.

White blood cells work hard to keep you healthy. They spend most of their time circulating with the blood around the body in search of unwanted intruders in the body. Unlike red blood cells, white blood cells can move on their own. They can travel easily to parts of the body where they are needed to fight infection. White blood cells can travel against the current of blood. They can even leave the blood vessel system to attack infections in other parts of the body. The white blood cells attack unwanted organisms, such as bacteria, and unwanted materials, such as a splinter, in different parts of your body.

White blood cells fight off infection in several ways. One way they fight infection is called phagocytosis (fay-go-sy-TOH-sis), A phagocyte (fAY-go-site) is a type of white blood cell that can change its shape and wrap around unwanted or foreign substances. Phagocytosis occurs when a phagocyte “eats up” solid substances such as a virus or bacterial cell. White blood cells also fight infection by releasing germ-fighting proteins called antibodies. Antibodies in your blood can act as either permanent or temporary germ fighters.

Some white cells can “remember” how to fight off infections that they have fought before. So if germs enter your body a second or third time, the white blood cells may kill the germ even before you feel any symptoms. This ability to fight repeat infections is called immunity (ih-MYOON-ih-tee). Vaccinations are an example of this process at work. Vaccinations prevent you from getting certain diseases. A vaccination is an injection of weak or dead virus particles or bacterial cells.

Figure 1.11 The cell membrane of a phagocyte can surround a particle and pull it into the cell. This process is called phagocytosis.

Figure 1.12 This lymph node is swollen due to trapped microorganisms.

The weakened virus or bacteria cause your body to produce antibodies against them. But the virus or bacteria are so weak that they don't give you the symptoms of the illness. The antibodies produced from a vaccination are permanent antibodies. That means they stay in your body to fight off the same intruder if it returns. Some viruses, such as colds and flu, change so quickly that it is impossible to produce one vaccination that stimulates production of effective antibodies.

Most white blood cells live for a few days unless they are fighting an infection. But white blood cells that are attacking infection might live only a few hours. When you are sick, your body makes more white blood cells to battle the infection. White blood cells can increase from 5,000 to 25,000 per cubic millimeter of blood during an infection.

Have you ever gone to the doctor with painful, swollen glands? If you have an infection in your throat, the lymph nodes in your neck may become swollen and tender. Lymph nodes are usually the size and shape of a small bean and can hardly be noticed underneath the skin. White blood cells in the lymph nodes attack foreign particles like bacteria and viruses. When the white blood cells in the lymph nodes attack the intruding bacteria or virus the nodes swell and become tender or even painful. That's how you know where attacks on bacteria or viruses are probably taking place. Common lymph nodes that swell are in the groin and in the scalp behind the ears or at the hairline. The disease called mumps is an example of swollen lymph nodes.

In what other ways does your body keep germs and dirt out?

What other protectors does your body have?

Diseases in White Blood Cells

Sometimes white blood cells show an abnormal increase in number. This increase of white blood cells is called leukemia (loo-KEE-mee-uh), a form of cancer. In any cancer cell division is out of control. So a leukemia patient has too many white blood cells. The white blood cells that are produced in a leukemia patient also may not function normally. When the white blood cells are not able to work normally, the patient's body can't fight infection. Mononucleosis (mah-noh-noo-clee-OH-sis) is another disease of the white blood cells. A virus causes mononucleosis. The mononucleosis virus causes an increase in abnormal white blood cells. The disease also affects the liver and can cause a sore throat and swollen tonsils. Have you ever heard mononucleosis called “the kissing disease?” The virus that causes the disease can live in the throat and mouth of people who have had the disease without any symptoms. The virus is very contagious. A contagious disease is a disease that can be transferred from person to person by contact such as kissing. Mononucleosis is common among young people. The treatment is plenty of rest and a healthy diet.

What can doctors tell about your health from a blood test?

What Do You Think?

Donations of blood and body organs are needed to save the lives of injured or sick people. Blood can be stored for only a few weeks. So new blood is constantly needed. Organs of healthy people who die prematurely could save the lives of others. At the present time not enough donations of blood and body organs are made to meet the need. How do you think the medical profession could educate people about the importance of donations of blood and organs?

Blood Types

Sometimes when someone has lost blood either through an accident or an operation, he or she needs a blood transfusion. A blood transfusion is a process in which blood that has been donated is transferred into the body of someone in need of blood. But blood transfusions are successful only when blood types are compatible. Certain proteins in the cell membranes affect the compatibility. These proteins in the blood cell membranes give people different blood types. The proteins involved in blood typing are called protein A and protein B. Individuals with protein A have blood type A. Those with protein B have blood type B. People with both proteins A and B have blood type AB. People who lack proteins A and B have type O blood.

Protein Blood Type
A + B AB
Neither A nor B O

Did You Know? Blood transfusion is the transfer of blood from one person to another. The Inca people of Peru tried some early blood transfusions hundreds of years ago. Sometimes the transfusions worked, and sometimes they didn't. No one knew why until 1901. An Austrian doctor discovered that certain proteins in the cell membranes of blood cells give people different blood types.

What happens if you give a transfusion of type B blood to someone whose own blood is type A? That person's immune system sees the B protein as a foreign material and attacks the transfused cells as though they were intruders. This transfusion reaction can be very serious, even fatal. In case of emergencies, you should know your blood type and carry it with you on a medical information card.

Blood Platelets

Everyday you do something that ruptures small blood vessels and causes bleeding. Sometimes you can see the bleeding. But sometimes the bleeding is inside your body and you can't see it. Although blood has to be fluid to move through your body, there must be a way to plug the leaks and keep you from losing too much blood from your circulatory system.

Did You Know? Minor cuts may scare you. But they really don't amount to much and usually stop by themselves. Putting pressure on a cut for ten minutes allows blood to clot. Usually, this is all you need to do. Your body does the rest. Remember to keep the healing cut clean and keep it covered with a bandage or gauze.

Bleeding from a major blood vessel can be life threatening. A person only has about 4 or 5\;\mathrm{liters} of blood. So it is very important to control the bleeding and get medical help. To stop the bleeding from a large wound, grasp the sides of the wound, firmly squeeze them together, and apply pressure. Cover the wound with a pad, dressing, or piece of cloth. Get someone else to call for help while you apply pressure.

Plugging blood leaks is a job for blood platelets in the blood. Blood platelets are smaller than red blood cells. Your blood has more platelets than white blood cells. But it has fewer platelets than red blood cells. You learned that white blood cells are produced in the bone marrow. Certain cells in the bone marrow also make platelets. But platelets are not cells. Platelets are little fragments of cells that contain chemicals needed to clot the blood. Platelets are named that because they look like little oval plates. They stay in the bloodstream about one week-unless they are put to use.

Figure 1.13 Platelets gather at the site of bleeding to form a mesh-like plug.

Blood clotting stops bleeding when a blood vessel breaks. Remember that blood starts to leak out when a small vessel breaks. The vessel constricts, or gets narrower, as the blood starts to leak out. Constricting slows the flow of blood leaking out of the vessel. The platelets come into contact with the damaged part of the vessel. As they contact the vessel, the platelets change. They become sticky and start to stick together. More platelets stick and eventually they form a plug. The platelets also release chemicals that start a series of events to finish the process of blood clotting. The clotting process involves a protein called fibrinogen (fy-BRIN-oh-jin), Fibrinogen is a protein circulating in the blood. Fibrinogen changes into protein fibers, called fibrin, at the clot. The protein fibers form a mesh, or net, that traps blood cells and stops the bleeding. Have you ever noticed a scab on your skin after an injury? That scab was the mesh formed from fibrin. A scab forms a barrier against germs and provides the base for new tissue to begin growing.

Hemophilia: A Blood Clotting Disease

Some people have a disease called hemophilia (hee-moh-FEEL-ee-uh). The blood in people who have hemophilia takes longer than normal to clot. People with the disease are called hemophiliacs. Their bodies cannot make certain blood proteins called clotting factors. As a result, they tend to bleed internally after a fall or accident. Hemophiliacs receive transfusions of blood plasma with concentrated clotting factors to get the needed clotting factors they are unable to make. Genetic engineers are looking for cures since faulty genes that are inherited cause hemophilia. Clotting factors are now being made in laboratories by genetic engineering techniques. Artificial clotting factors will make life much easier and safer for hemophiliacs.

What is a bruise? Why does it change color?

Activity 1-2: Composition of Blood


What do you know about blood? What makes up your blood? How does your blood help your body to maintain homeostasis? In this activity you make a model that represents your blood. This model helps demonstrate the composition of blood and how it functions in your body.


  • 2 beakers, 1000\;\mathrm{ml} (milliliter) or clear plastic containers
  • 3 containers, one of which is at least 500\;\mathrm{ml} (milliliter) in capacity
  • Red beans, dried
  • White beans, dried, about twice the size of the red beans
  • Split peas, dried, about half the size of the red beans
  • Salt
  • Yellow food coloring
  • Raw egg
  • Water
  • Small pieces of paper towel
  • 2 graduated cylinders, 25 or 50\;\mathrm{ml} (milliliter)and 500\;\mathrm{ml}
  • Activity Report

Procedure A: Modeling the Solid Portion of Blood

Step 1 Using graduated cylinders measure the following amounts of dried beans or peas. Then, place the correct amount of each material into a separate container:

A. The red beans represent red blood cells. Measure 425\;\mathrm{ml} of red beans. Label a container “Red Blood Cells.” Place the 425\;\mathrm{ml} of red beans into the container marked “Red Blood Cells.”

B. The split peas represent platelets. Label a second container “Platelets.” Measure 22\;\mathrm{ml} of split peas and place them into the container labeled “Platelets.”

C. The white beans represent white blood cells. Label a third container “White Blood Cells.” Measure 3\;\mathrm{ml} of white beans and place them into the container labeled “White Blood Cells.”

Step 2 Answer Questions 1 and 2 on the Activity Report.

Step 3 Label a 1000-\mathrm{ml} beaker “Solid Components of the Blood.” Place all of the beans and the peas into the 1000-\mathrm{ml} container labeled “Solid Components of the Blood.”

Procedure B: Modeling the Liquid Portion of Blood

Step 1 Using graduated cylinders measure the following amounts of solids and liquids. Then, place the correct amount of each solid or liquid into the correctly marked container.

A. Measure 550\;\mathrm{ml} of water to represent the water in plasma. Then, place the 550-\mathrm{ml} of water into a second clean 1,000-\mathrm{ml} container. Do not use the 1,000-\mathrm{ml} container labeled “Solid Components of Blood.”

B. Label a paper cup “Proteins and Fats.” Place a raw egg into the paper cup. The egg white represents the proteins and the yolk represents the fats in plasma.

C. Label a second paper cup “Minerals, Nutrients, and Wastes.” Salt represents minerals and nutrients. Add a pinch of salt to the paper cup labeled “Minerals, Nutrients, and Wastes.”

D. Yellow food coloring represents wastes. Add one drop of yellow food coloring to the salt to represent the wastes in plasma.

Step 2 Answer Question 3 on the Activity Report.

Step 3 Now beat the egg well, so the yolk (fat) and egg white (protein) are mixed. Add the beaten egg to the 1000-\mathrm{ml} container of water. Then add the salt and yellow food coloring to the 1000-\mathrm{ml} container of water and beaten egg. All of these materials mixed together represent the liquid portion of blood. Label this container “Blood Plasma.” DO NOT MIX THE CONTENTS OF THE TWO, 1,000-\mathrm{ml} CONTAINERS REPRESENTING THE SOLID AND THE LIQUID COMPONENTS OF BLOOD TOGETHER. THAT WOULD MAKE A MESS!

Step 4 Complete Questions 4-6 on the Activity Report.

Step 5 Follow cleanup directions given by your teacher.

Procedure C: What can you learn from your model?

Step 1 To perform all of its functions, the composition of the blood must remain within certain narrow ranges. This is a state of homeostasis. Your model represents normal values for the various components of the blood. Describe on your Activity Report how your model simulates blood.

Step 2 Describe what you could do with your model to simulate each of the following conditions. Then after each description explain how a person would function if his or her blood changed that way.

  • Following massive blood loss a person's body can replace plasma volume quicker than the body can produce new red blood cells. The resulting condition is called anemia.
  • Leukemia is a cancer of the white blood cells. Cancer is a state of uncontrolled cell division.
  • Most plasma proteins are made in the liver. Alcoholism can destroy the ability of the liver to make these proteins.
  • A disease that destroys your kidneys makes it impossible for waste products to be removed from your plasma.
  • Platelets are necessary for the blood clotting process to function effectively. Blood takes longer to clot in hemophiliacs.
  • If you eat a very fatty meal, lots of fat enters your blood via your lymphatic vessels.

Blood Impressions What is your impression of how blood looks and works? Paint or draw a picture of blood. Show all of the parts that make up blood. Make sure people will be able to learn something about the parts of blood and their functions from your picture.

Maintaining Homeostasis

Remember that homeostasis is the maintenance of a constant internal environment in your body so that all of your cells can function effectively. So how do all the parts of the blood help the body maintain homeostasis? Each part of the blood helps respond to the many different changes that take place both inside and outside of your body. The blood helps your body work efficiently by decreasing or increasing the exchange of food nutrients, wastes, and gases. The body can make more white cells to fight infection. The body also makes enough platelets to keep you from bleeding to death. You aren't aware of most of the things your blood does for you because it all works automatically without having to think about it. However, you can help your blood do its job better by avoiding the risk factors you will investigate in Section 7 of this unit.

To do its job, your blood needs to be able to move through your body. Your blood could not do this without the power of the heart behind it. In the next section, you will find out more about the heart and how it works.

  • How can your circulatory system help you regulate your body temperature?
  • An accident victim in the emergency room needs a blood transfusion. Her blood type is A. Which blood type(s) are compatible with hers?

Artificial Blood What is artificial blood? Why are scientists having such a hard time making it? Do research on the Internet or in the library on artificial blood. Locate articles about artificial blood. Share them with your class. What do you think might be some benefits of artificial blood?

Figure 1.14 Your lymphatic system naturally recycles and cleanses fluid from body tissues.

Lymphatic System

Although your blood circulates in a closed system of blood vessels, some fluid leaks out. Most of these leaks occur around the tiniest blood vessels where the exchange of food nutrients and gases takes place. Have you ever been stung by a bee? The area around the sting usually becomes hot, red, and swollen. The bee venom stimulates an increased blood flow to the area making it hot and red. The bee venom also causes the tiny blood vessels to become more leaky. When the fluid leaks out from the vessels into the spaces between the cells, the area swells with fluid. This kind of swelling caused by the accumulation of fluids in spaces outside of blood vessels and between cells is called edema (eh-DEE-muh).

Where does fluid go that leaks out of the blood vessels? Extra fluid travels back to the heart through another network of vessels called the lymphatic system. Your lymph nodes are part of the lymphatic system. Lymph nodes play important roles in defending your body against infection. You will learn more about them when you study the immune system. Lymphatic vessels carry no blood. The lymphatic vessels carry a thin, watery fluid called lymph. Lymph is made up of water, salts, food nutrients, waste products, white blood cells, proteins, and other chemicals. After you eat a meal that contains fats, your lymph turns milky white. The lymph is carrying the digested fats from your intestines to your blood. When your body is not absorbing fats from your intestines, the lymph is a pale yellowish fluid.

Lymph vessels go wherever blood vessels go. The lymph vessels pick up the fluids that leak out of blood vessels and fill the spaces between your body cells. The lymphatic system recycles this extra fluid back into your circulatory system. Small lymphatic vessels empty into bigger ones that eventually merge into a single large vessel. That large vessel is called the thoracic duct. The thoracic duct empties into a vein at the base of your neck returning fluids to the circulatory system.

There are some important differences between the lymphatic system and the blood circulatory system. Remember that the circulatory system is a closed circuit. The lymphatic system is not a closed circuit. Also, remember that the heart pumps the blood through the circulatory system. The heart does not pump the lymph through vessels. Instead, your muscles help to move the lymph through the lymph vessels. As the muscles contract and relax, they squeeze the lymph vessels. When the muscles squeeze the vessels the vessels move the lymph that is inside them.

Your school circulates information. Your body circulates blood. Compare and contrast the methods of delivery between your school and your body. What kinds of information circulate? How is information best absorbed? What methods are the more efficient? Which are the most effective? How do students contribute to the flow of information? Is a school like your body-does the system travel in only one direction? Make a chart of different ways of circulating different kinds of information. In writing, compare your school's information circulation system to your circulatory system.

Review Questions

  1. What are five differences between red blood cells and white blood cells?
  2. How do red blood cells carry oxygen? What happens if red blood cells aren't the right shape or there aren't enough of them?
  3. What is in plasma? What do the things in plasma do?
  4. If the doctor discovers your platelet count is low, what might you have to be careful of? Why?
  5. Describe two functions of the lymphatic system.

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