Just as a plumbing system carries water through a series ofpipes to different parts of a house, the circulatory system

carries blood through a series of blood vessels to different partsof the body. Blood is a type of connective tissue containing bothdissolved substances and specialized cells. Blood collects oxygenfrom the lungs, nutrients from the digestive tract, and wasteproducts from tissues. Blood helps to regulate factors in thebody’s internal environment, such as body temperature. Inaddition, components in blood help to fight infections. Blood caneven form clots to repair damaged blood vessels.

Blood PlasmaThe human body contains 4 to 6 liters of blood, which is about 8percent of the total mass of the body. As Figure 37–7 shows,about 45 percent of the volume of blood consists of cells, whichare suspended in the other 55 percent—a straw-colored fluidcalled Plasma is about 90 percent water and 10percent dissolved gases, salts, nutrients, enzymes, hormones,waste products, and proteins called plasma proteins.

Plasma proteins, which perform a variety of functions, aredivided into three groups: albumins, globulins, and fibrinogen.Albumins and globulins transport substances such as fattyacids, hormones, and vitamins. Albumins also help to regulateosmotic pressure and blood volume. Some globulins fight viraland bacterial infections. Fibrinogen is the protein responsiblefor the ability of blood to clot.

plasma.

Key Concepts• What is the function of each

type of blood cell?• What is the function of the

lymphatic system?

Vocabularyplasmahemoglobinlymphocyteplateletlymph

Reading Strategy: Asking Questions Beforeyou read, rewrite the headingsin the sections as how, why, orwhat questions about blood andthe lymphatic system. As youread, write brief answers to theheading questions.

Whole Blood Sample Sample Placed in Centrifuge

Plasma

White blood cells

Red blood cells

Platelets

Blood Sample ThatHas Been Centrifuged

! Figure 37–7 Blood consists ofplasma, blood cells, nutrients,hormones, waste products, andplasma proteins. InterpretingGraphics When a whole bloodsample is placed in a centrifuge, asshown below, what is the result?

37–2 Blood and the Lymphatic System

1 FOCUSObjectives37.2.1 Describe blood plasma.37.2.2 Explain the functions of red

blood cells, white bloodcells, and platelets.

37.2.3 Describe the role of the lym-phatic system.

Vocabulary PreviewHave students preview new vocabu-lary by skimming the section andlisting the highlighted, boldfaceterms. They should leave space tomake notes as they read the section.

Reading StrategySuggest that students preview sectioncontent by studying the figures andreading the captions. They shouldwrite down any unfamiliar terms andtry to find the meanings as they read.

2 INSTRUCT

Blood PlasmaUse Community ResourcesHave students contact their localchapter of the American Red Cross tolearn about blood banks and blooddrives in their community. Suggestthat they try to find out who can andcannot donate blood, how donationsare made, and what happens to thedonated blood. Urge students toshare what they learn with the class.

Circulatory and Respiratory Systems 951

Answer to . . . Figure 37–7 The cellular portion andthe plasma portion of blood separate.

Section 37–2

SECTION RESOURCES

Print:• Teaching Resources, Lesson Plan 37–2,

Adapted Section Summary 37–2, AdaptedWorksheets 37–2, Section Summary 37–2,Worksheets 37–2, Section Review 37–2

• Reading and Study Workbook A,Section 37–2

• Adapted Reading and Study Workbook B,Section 37–2

• Investigations in Forensics, Investigation 10

Technology:• iText, Section 37–2• Transparencies Plus, Section 37–2

Tim

eSaver

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Blood CellsThe cellular portion of blood consists of red blood cells, whiteblood cells, and platelets. Red blood cells transport oxygen,white blood cells perform a variety of protective functions, andplatelets help in the clotting process. Platelets are actuallyfragments of cells derived from larger cells in bone marrow.

Red Blood Cells The most numerous cells in the blood are thered blood cells, or erythrocytes (eh-RITH-roh-syts). Redblood cells transport oxygen. They get their color fromhemoglobin. is the iron-containing protein thatbinds to oxygen in the lungs and transports it to tissues through-out the body where the oxygen is released.

Red blood cells, like those shown in Figure 37–8, areshaped like disks that are thinner in the center than along theedges. These cells are produced from cells in red bone marrow.As these cells gradually become filled with hemoglobin, theirnuclei and other organelles are forced out. Thus, mature redblood cells do not have nuclei. Red blood cells circulate for anaverage of 120 days before they are worn out from squeezingthrough narrow capillaries. Old red blood cells are destroyedin the liver and spleen.

White Blood Cells White blood cells, or leukocytes (LOO-koh-syts), do not contain hemoglobin. They are much less com-mon than red cells, which outnumber them almost 1000 to 1.Both white and red blood cells are produced from the samepopulation of blood-forming stem cells found in the bone mar-row. Unlike red blood cells, however, white blood cells containnuclei. They may live for days, months, or even years.

White blood cells are the “army” of the circulatorysystem—they guard against infection, fight parasites, andattack bacteria. There are many types of white blood cells, andthey perform a wide variety of important functions. Some protectthe body by acting as phagocytes, or “eating cells,” that engulf anddigest bacteria and other disease-causing microorganisms. Somewhite blood cells react to foreign substances by releasing chemi-cals known as histamines. These chemicals increase blood flowinto the affected area, producing redness and swelling that areoften associated with allergies. Other white blood cells, known as

are involved in the immune response. B lympho-cytes produce antibodies. Antibodies are essential to fightinginfection and help to produce immunity to many diseases.T lymphocytes help fight tumors and viruses. You will learnmore about lymphocytes in Chapter 40.

White blood cells are not confined to the circulatory system.Many white blood cells are able to slip out of capillary walls,travel through the lymphatic system, and attack invadingorganisms in the tissues of the body. In many ways, white bloodcells are the first lines of defense when the body is invaded bydisease-causing organisms.

lymphocytes,

Hemoglobin

! Figure 37–8 Red bloodcells transport oxygen. Whiteblood cells fight invasions offoreign substances, cells, andorganisms. Red blood cells and asingle white blood cell are shown inthis scanning electron micrograph.

(magnification: 2342!)

For: Links on blood cells

Visit: www.SciLinks.orgWeb Code: cbn-0372

NSTA

952 Chapter 37

Blood CellsBuild Science SkillsCalculating Help students appreci-ate the relative proportions of redand white cells in blood. Point outthat a milliliter of blood containsabout 5 million red blood cells(about 5.2 million cells in males andabout 4.7 million cells in females)and that red blood cells outnumberwhite blood cells about 1000 to 1.Ask: How many white blood cellsare there in a milliliter of blood?(About 7000) If a milliliter of bloodwas found to have 20,000 whiteblood cells, what might explainthis increase? (An infection)

Use Community ResourcesArrange for interested students tovisit a clinic or hospital laboratorywhere they can observe blood countsbeing performed. If possible, have alab technician or supervisor explainwhy blood counts are performed andwhat can be learned from them.Other topics the technician or super-visor might address include theimportance of accuracy in bloodcounts and the safety precautionsthat must be taken when handlingblood samples. Have students reportto the class on what they learn.

NSTA

Download a worksheeton blood cells for students to com-plete, and find additional teachersupport from NSTA SciLinks.

Inclusion/Special NeedsHelp students grasp the material on blood cellsby helping them organize the details in a com-pare/contrast table. Use headings for type ofblood cell, number, size, shape, functions, pres-ence/absence of nucleus, source, and life span.Then, call on volunteers to fill in the table with arow for red blood cells and a row for whiteblood cells. Have students save their tables touse as study guides.

Advanced LearnersChallenge students to find out how hemophiliais inherited and why it occurs primarily in males.Have them focus on hemophilia in the family ofQueen Victoria of England. They should find afamily tree showing which of Victoria’s descen-dants had the disease or carried the defectivegene. Ask volunteers to explain to the class howhemophilia is inherited, using the family tree toillustrate their explanation.

37–2 (continued)

UNIVERSAL ACCESS

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DemonstrationUse a microprojector to display pre-pared slides of red blood cells, whiteblood cells, and platelets. Have stu-dents make a sketch of eachstructure. Then, call on students todescribe the size, shape, and generalappearance of each structure.

Use VisualsFigure 37–9 Check students’ com-prehension of the figure by havingthem identify each of the following inthe drawings: red blood cells (red),platelets (pink), and fibrin filaments(blue). Then, ask: What is the mainrole of platelets in blood clotting?(The production of clotting factors suchas thromboplastin) What role do cal-cium and vitamin K play in bloodclotting? (They help convert pro-thrombin to thrombin.)

Circulatory and Respiratory Systems 953

Like an army with units in reserve, the bodyis able to increase the number of white bloodcells dramatically when a “battle” is underway. Asudden increase in the white blood cell count isone of the ways in which physicians can tell thatthe body is fighting a serious infection.

Platelets and Blood Clotting Blood isessential to life. An injury can cause the body tolose this essential fluid. Fortunately, blood has aninternal mechanism to slow bleeding and beginhealing. A minor cut or scrape may bleed for afew seconds or minutes, but then it stops. Cleanit up with soap and water, cover it with a band-age, and it begins to heal. Have you ever won-dered why the bleeding stops?

The answer is that blood has the ability toform a clot. Figure 37–9 summarizes the process.

Blood clotting is made possible byplasma proteins and cell fragments calledplatelets. There are certain large cells in bonemarrow that can break into thousands of smallpieces. Each fragment of cytoplasm is enclosed ina piece of cell membrane and released into thebloodstream as a

When platelets come into contact with theedges of a broken blood vessel, their surfacesbecome very sticky, and a cluster of plateletsdevelops around the wound. These platelets then release proteins called clotting factors.The clotting factors start a series of chemicalreactions that are quite complicated. In onereaction, a clotting factor called thromboplastin(thrahm-boh-PLAS-tin) converts prothrombin,which is found in blood plasma, into thrombin.Thrombin is an enzyme that helps convert thesoluble plasma protein fibrinogen into a stickymesh of fibrin filaments. These filaments stopthe bleeding by producing a clot. Figure 37–10shows the tangle of microscopic fibers in anactual blood clot.

What are platelets?

platelet.

Break in Capillary Wall

Clumping of Platelets

Clot Forms

! Figure 37–9 Blood clotting is madepossible by a number of plasma proteins and cellfragments called platelets. Calcium and vitamin Kaid in converting prothrombin into thrombin.

" Figure 37–10 Strands of fibrin trap blood cells, forming a net that prevents bloodfrom leaving a damaged blood vessel. Using Analogies How is a blood clot similar to a screened-in porch?

(magnification: 3000!)

Red blood cell shapesNormal red blood cells are disk-shaped, butsome people have red blood cells with abnor-mal shapes, such as spherical, oval, or sickleshapes. People with spherical red blood cellshave an inherited disorder called spherocytosis.Because of their shape, the red cells becometrapped and destroyed by the spleen, leading toanemia. Treatment may include removal of thespleen, which corrects the anemia but not the

abnormal shape of the red cells. People withoval red blood cells have the inherited disorderelliptocytosis. This sometimes causes mild ane-mia but usually requires no treatment. Peoplewith sickle-shaped red blood cells have sicklecell disease, another inherited disorder and themost serious of the three disorders. Sickle celldisease can cause severe anemia, organ dam-age, deformities, and even death.

FACTS AND FIGURES

Answers to . . . Platelets are cell frag-

ments that form a cluster around awound and release clotting factors.

Figure 37–10 It is similar in forminga mesh barrier that prevents bloodfrom leaving a damaged vessel (or thatkeeps insects out).

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Blood Clotting Problems If the wound is small, within afew minutes the mesh of platelets and fibrin seals the wound, andbleeding stops. Most of the time, this clotting reaction works sowell that we take it for granted. However, if one of the clottingfactors is missing or defective, the clotting process does not workwell. Hemophilia is a genetic disorder that results from a defectiveprotein in the clotting pathway. People with hemophilia cannotproduce blood clots that are firm enough to stop even minorbleeding. They must take great care to avoid injury. Fortunately,hemophilia can be treated by injecting extracts containing themissing clotting factor.

The Lymphatic SystemAs blood circulates, some fluid leaks from the blood into thesurrounding tissues. This isn’t an altogether bad thing. A steadyflow of fluid helps to maintain an efficient movement of nutrientsand salts from the blood into the tissues. However, more than 3liters of fluid leak from the circulatory system into surroundingtissues every day! If this leakage continued unchecked, the bodywould begin to swell with fluid—not a very pleasant prospect.

Fortunately, the interrelationship between two body systemsdoes not allow this to happen. A network of vessels,nodes, and organs called the lymphatic system collectsthe fluid that is lost by the blood and returns it back tothe circulatory system. The fluid is known as (LIMF).lymph

Blood TransfusionsAlthough the first successful transfusions of humanblood were carried out in the 1820s, many recipientshad severe reactions to the transfused blood, andseveral died. Today we know why. We inherit one offour blood types—A, B, AB, or O—which are deter-mined by antigens on our blood cells. Antigens aresubstances that trigger an immune response. Peoplewith blood type A have A antigens on their cells,those with type B have B antigens, those with ABblood have both A and B, and those with type O haveneither A nor B antigens.

When blood types match, the transfusion is suc-cessful. However, transfusions are successful in somecases even when the blood types of the donor andthe recipient do not match. Use the table to answerthe questions that follow.

1. Drawing Conclusions Which blood type issometimes referred to as the “universal donor”?Which is known as the “universal recipient”?

2. Drawing Conclusions In a transfusion involvingthe A and O blood types, does it make a differencewhich blood type belongs to the recipient andwhich to the donor?

3. Applying Concepts Write a brief explanationfor the results in the chart using information aboutphenotypes and genotypes in blood group genes.(Hint: Review Section 14–1 if needed.)

A B AB

A

B

AB

O

!

X

X

!

Blood Transfusions

X

!

X

!

!

!

!

!

Blood Typeof Donor

Blood Type of RecipientO

X

X

X

!

X = Unsuccessful transfusion ! = Successful transfusion

954 Chapter 37

The LymphaticSystemBuild Science SkillsApplying Concepts Tell studentsthat both malaria and sickle cell ane-mia are diseases that damage redblood cells. Ask: Why do peoplewith malaria or sickle cell anemiaoften have enlarged spleens?(Damaged red blood cells are removedby the spleen. It becomes enlargedwhen there are many damaged cells.) Lymphatic System Facts

The thymus is the primary lymphoid organand the site where lymphocytes acquire theirantigen receptors. It attains its full size by agetwo, then decreases in size until puberty, afterwhich it almost disappears. Secondary lym-phoid organs include the spleen and tonsils.Most antibodies are made in the spleen. Itconsists of white pulp, where T cells are stimu-lated by antigen, and red pulp, where old red

blood cells are removed. The red pulp alsomanufactures new red blood cells when toomany are lost during illness. Tonsils are smallpieces of lymphatic tissue in the throat. Theirprimary role is to trap and destroy bacteriathat enter the respiratory tract. One set of ton-sils protrudes from each side of the pharynxbehind the mouth. Another set, commonlycalled adenoids, is located higher up in thepharynx behind the nose.

FACTS AND FIGURES

Tell students that the first successfulblood transfusions were performed inthe early 1800s by a British physiciannamed James Blundell. At least half ofBlundell’s patients benefited from thetransfusions, but many others hadsevere reactions to the transfusedblood, and most of these died. In1900, an Austrian physician, KarlLandsteiner, discovered the ABOblood group. Knowledge of ABOblood types explained Blundell’sresults and, in general, why sometransfusions are successful and othersare not.Answers1. Type O is sometimes referred to asthe “universal donor.” Type AB issometimes referred to as the “univer-sal recipient.”2. Yes, because people with type Ablood can safely receive type Oblood, but people with type O bloodcannot safely receive type A blood.3. Student answers should reflect anunderstanding of blood groups andgenetics. Suggested answer: Peoplewho have type O blood do not pro-duce blood antigens and thereforecan donate blood to any of the otherblood groups safely. Because the alleles for type A and B are codomi-nant, people with type AB blood canreceive blood from all groups.

37–2 (continued)

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Lymph collects in lymphatic capillaries and slowly flows intolarger and larger lymph vessels. Like large veins, lymphvessels contain valves that prevent lymph from flowingbackward. Ducts collect the lymph and return it to thecirculatory system through two openings in the superiorvena cava. The openings are under the left and right claviclebones just below the shoulders. Figure 37–11 shows thelymphatic system.

Along the length of the lymph vessels are small bean-shaped enlargements called lymph nodes. Lymph nodesact as filters, trapping bacteria and other microorganismsthat cause disease. When large numbers of microorgan-isms are trapped in the lymph nodes, the nodes becomeenlarged. If you have ever had “swollen glands,” youactually had swollen lymph nodes.

Lymph vessels do not merely return excess fluid to thecirculation. They also play a very important role in nutrientabsorption. Lymph vessels lie near the cells that line theintestines, where they absorb fats and fat-soluble vitaminsfrom the digestive tract and carry them to the blood. Lymphmoves through the lymphatic system under osmotic pressurefrom the blood and is pushed along by the contractions ofnearby skeletal muscles. It is important that there is asteady flow of lymph. Edema, a swelling of the tissues due tothe accumulation of excess fluid, can occur when lymphaticvessels are blocked due to injury or disease.

In addition to the lymph vessels and lymph nodes, thethymus and spleen also have important roles in thelymphatic system. The thymus is located beneath thesternum. Certain lymphocytes called T cells mature in thethymus before they can function in the immune system.T cells are the cells that recognize foreign “invaders” inthe body. The spleen helps to cleanse the blood andremoves damaged blood cells from the circulatory system.The spleen also harbors phagocytes that engulf anddestroy bacteria and other microorganisms.

Thymus

Superiorvena cava

Lymphnodes

Heart

Thoracicduct

Spleen

Lymph vessels

! Figure 37–11 The lymphaticsystem collects and returns fluid that leaksfrom blood vessels. The spleen is an organwhose main function is to destroy damagedred blood cells and platelets. Certain whiteblood cells called T lymphocytes, or T cells,mature in the thymus gland, which produceshormones that promote their development.

1. Key Concept List themain function of red blood cells,white blood cells, and platelets.

2. Key Concept Describe therole of the lymphatic system.

3. What types of materials aredissolved in plasma?

4. Explain how blood clots.

5. Critical Thinking InferringSometimes lymph nodes must besurgically removed. Althoughmore lymph vessels eventuallygrow, what result would youexpect to see immediately aftersurgery?

37–2 Section AssessmentConstructing a Concept MapConstruct a concept map thatshows the components ofblood. Include informationabout the functions of thedifferent components. (Besure to include the differenttypes of white blood cells.)

Circulatory and Respiratory Systems 955

If your class subscribes to the iText,use it to review the Key Concepts inSection 37–2.

Use VisualsFigure 37–11 Have students com-pare the distribution of lymphaticvessels with the distribution of bloodvessels, shown in Figure 37–3. Then,ask: How are the lymphatic and cir-culatory systems related? (Thelymphatic system collects fluid thatseeps into the tissues from blood andreturns it to the circulatory system.)Point out that the vein where thisfluid is returned to the blood is calledthe superior vena cava. Have stu-dents find the superior vena cava inthe figure. Also have them locate theclusters of lymph nodes. Ask: If youhave a sore throat because of aninfection, which lymph nodesmight become swollen? (The lymphnodes in the neck)

3 ASSESS

Evaluate UnderstandingCall on students at random to nameeach component of the blood. Callon other students to describe thefunction of each component.

ReteachUsing the chalkboard or a trans-parency, work with students to createa concept map of the functions ofblood based on information in theopening paragraph of the section.Then, have students identify theparts of the blood involved in eachfunction.

Students’ concept maps shouldshow that blood consists of plas-ma, platelets, red blood cells, andwhite blood cells, of which thereare five main types: neutrophils,eosinophils, basophils, monocytes,and lymphoctyes.

37–2 Section Assessment1. Red blood cells carry oxygen; white blood

cells fight infection; platelets help blood toclot.

2. Its role is to collect fluid lost by blood andreturn it to the circulatory system.

3. Gases, salts, nutrients, enzymes, hormones,waste products, and plasma proteins

4. When platelets come in contact with the

broken edges of a blood vessel, their sur-faces become sticky. A cluster of plateletsdevelops around the wound. The plateletsrelease clotting factors, which start reactionsthat produce a blood clot.

5. Areas without lymph nodes and vesselswould retain excess fluid and becomeswollen immediately after surgery.

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