#2: BLOOD / HEMATOLOGY

Transcription

1 Biology 212: Anatomy and Physiology II. Lab #2: BLOOD / HEMATOLOGY For labs associated with Dr. Thompson s lectures References: Saladin, KS: Anatomy and Physiology, The Unity of Form and Function 8 th (2018). Required reading before beginning this lab: Chapter 18 Blood is one of the most important components of the human body. It transports materials (i.e., oxygen, carbon dioxide, hormones, waste products, nutrients, etc.) throughout the body, and its leukocytes (white blood cells) protect us from toxic materials and infectious agents like bacteria and viruses. The average human body contains roughly 5 liters of blood that is constantly being pumped through our arteries and veins by the heart. Many pathologies of the human body involve blood. For example: a) excess loss of blood results in a life-threating condition called hypovolemic shock. b) anemia can result from excess blood loss or the inability to form red blood cells which deliver oxygen to all tissues of the body. c) An increased risk of heart disease is associated with abnormal levels of cholesterol (carried in the blood as lipoproteins) or triglycerides in the blood. d) Abnormal blood clotting, or hemostasis, is associated with such disorders as stroke, pulmonary embolism, hemophilia, or hemorrhage. e) A number of cancers, collectively called leukemias, develop from the leukocytes or cells which form them. LEARNING OBJECTIVES: Upon completion of this lab exercise students will be able to: Describe the composition of blood plasma Identify the five types of leukocytes typically seen in human blood and perform a differential white blood cell count Safely obtain a sample of blood by finger-stick to measure hematocrit and determine ABO and Rh blood types Understand the relationship between blood group antigens and plasma antibodies Understand the roles of hematocrit, mean corpuscular volume, and mean corpuscular hemoglobin in maintaining normal oxygen delivery Blood is a type of connective tissue with two fractions: Plasma the liquid part of the blood which contains water, ions, proteins, nutrients, antibodies, clotting factors, and many other molecules being transported from one part of the body to another. Formed elements the cells and cell fragments a. Erythrocytes (red blood cells) b. Leukocytes (white blood cells) c. Platelets 1

2 The analysis of blood and its components is very important clinically since the body often changes the composition of blood in response to changes in physiology or disease. In this lab exercise, we will draw a small blood sample from a finger. We will examine the different components of blood as well as look at some of the basic diagnostic tests commonly used in hematology. Working safely with blood In this lab exercise, we will be working with human blood (your own or that of your lab partners). While it is unlikely that someone in this class carries an infectious agent in their blood (bacteria, viruses), it is a universal practice in all laboratories to assume that every blood sample might be contaminated with something harmful. As a result, we will follow the following safety protocols: a) Always wear disposable gloves whenever you are handling blood or any materials that have come into contact with blood (even if it is your own). b) Any object which has (or may have) come into contact with blood must be disposed of in appropriate receptacles for disinfection by trained personnel in the Biology department. These are never discarded in the trash. Your instructor will provide details. c) Sharp items which may be contaminated with blood must be disposed of in special containers to prevent them from wounding and possibly infecting another person. These are never discarded in the trash. Your instructor will provide details. d) Any surface which has (or may have) been contaminated with blood or another body fluid must be disinfected. If you accidentally spill a blood sample, please inform your instructor immediately. e) Even though you have been wearing gloves, wash your hands before leaving the lab. A. BLOOD PLASMA: It is important to understand some of plasma s properties, even though it is not the major focus of this lab. Plasma normally forms about 55-60% of our blood volume. More than 90% of it is water, but plasma contains so many different solutes in so many different concentrations that is has a moderately viscous, or sticky feel. It is normally slightly cloudy and has a light tan or straw-like color, although this too can change according to the solutes it contains. You will be able to examine the appearance of plasma when we measure your hematocrit later in this exercise. Exercise #1: Composition of Plasma Examine Table 18.2 and 18.3 of your Saladin text, Composition of Blood Plasma and Major Proteins of Blood Plasma. Note that its components include many different proteins, nutrients, electrolytes or ions, and waste products although you will not be expected to know the specific concentrations of any of these. It also contains all of the hormones which are being circulated in the blood and which we discussed in the previous lab exercise. 2

3 Besides being the fluid within which solutes are being carried around the body, the plasma serves three other important functions in the homeostasis of the body: 1) It is one of the major reservoirs for water within the body. If another organ or tissue loses water, it can easily withdraw it from the plasma. If another organ or tissue has an excess of water, the extra joins the plasma to be carried to the kidneys where it can be removed. 2) It is one of the most important acid-base buffering systems within the body. It maintains a relatively stable ph of approximately 7.35 to 7.45 even when acidic and basic molecules are constantly entering and leaving it. 3) It helps maintain a consistent temperature within the body. When some part of the body either gains or loses heat (think of breathing cold air, hot or cold drinks, or being in hot or cold water) the body-temperature blood quickly restores that local temperature to the normal range. Questions for discussion from your required reading of Chapter 18: If the plasma forms 55 to 60% of your total blood volume, what is the other 40-45%? Identify at least ten different solutes carried in the plasma and explain their functions to other members of your lab group. What is the normal color of plasma? B. FORMED ELEMENTS: The formed elements of blood consist of erythrocytes, five different types of leukocytes, and platelets. Examine Figure 18.1 and Table 18.6 of your Saladin text and note their appearances. In this exercise, we will be studying the formed elements of blood using the blood smear slides found in your slide boxes (Slide #6). This is a smear, meaning that the blood was smeared across the slide, allowed to dry, then stained. Thus, you see three-dimensional cells - they were not cut. Since these are prepared slides and have been sterilized, you won t be needing gloves for this exercise. Exercise #2: Examination of Erythrocytes By far, erythrocytes are the most numerous of the cells you will see on a blood smear. They are small (7.5 microns in diameter), round, and have an indented center as shown in Figures 18.1 and of your Saladin text. These are red in color because they contain large amounts of the protein hemoglobin to which oxygen and carbon dioxide can bind as they are carried in the blood. 3

4 Obtain a microscope and examine slide # 6. Bring the image into focus first under the smallest objective lens, then successively work your way up to the highest (400x) magnification. Notice that erythrocytes are by far the most numerous cells on the slide, but you should also observe a much smaller number of leukocytes in which the nucleus is present and stains a moderate to deep blue. Look more carefully, and you should also see platelets what appear as small, irregular blue structures about 20% the size of the erythrocytes. Compare what you see to Figure 5.23 in your Saladin text. Note that the erythrocytes do not have nuclei. As shown in Figure 18.6 of your Saladin text, these had nuclei when they were developing as erythroblasts but lost their nuclei just before entering the blood. Without nuclei they have no DNA and are incapable of forming new proteins, so they will only live for a few months before they are removed from the circulation. During their development, however, these cells synthesized large amounts of hemoglobin, so they carry a large fraction of both the oxygen and the carbon dioxide which the blood contains. Questions for discussion from your required reading of Chapter 18: Approximately how many erythrocytes are contained in each milliliter of blood? (Caution: Table 18.1 in your Saladin textbook lists the concentration as number per microliter. There are 1,000 microliters in each milliliter) Discuss with other members of your lab group where erythrocytes are produced Discuss with other members of your lab group where old and damaged erythrocytes are removed from the circulating blood. Discuss with other members of your lab group where erythrocytes obtain their oxygen and where they release it. Exercise #3: Examination of Platelets With slide #6 still under high power, identify the platelets or thrombocytes which appear as small, irregular blue structures scattered between the erythrocytes and leukocytes. These will be much smaller than the other formed elements (1-2 micrometers in diameter). Platelets also lack nuclei since they are actually fragments of a much larger cell called a megakaryocyte which remains in the bone marrow. Questions for discussion from your required reading of Chapter 18: Approximately how many platelets are contained in each milliliter of blood? (Caution: Table 18.1 in your Saladin textbook lists the concentration as number per microliter. There are 1,000 microliters in each milliliter) Explain to other members of your lab group how platelets are produced Explain to other members of your lab group the role of platelets in hemostasis, or blood clotting. 4

5 Exercise #4: Examination of Leukocytes With slide #6 still under high power, identify the different types of leukocytes as noted below. Because there are far fewer of these than erythrocytes, you will only see one or two leukocytes in each microscopic field and will need to move the slide to identify all of them. There are two main groups of leukocytes in human blood, granulocytes and agranulocytes. Examples of these are shown in Figure 18.1, Figure 18.18, and Table 18.6 of your Saladin text. Granulocytes will possess visibly stained granules within the cytoplasm which look like speckles within the cell, while agranulocytes will not. Additionally, granulocytes tend to possess nuclei which are more lobed in appearance and may even make them look like they are multinucleated, while agranulocytes tend to possess more rounded nuclei. A word of caution: even the best hematologist can t accurately identify every single leukocyte in a blood smear. Many are damaged beyond recognition during preparation of the slide, or for some other reason don t show their characteristic appearances. You will drive yourself crazy if you initially try to identify every leukocyte you see. Instead, figure out the characteristics of one specific type of cell to identify, then scan around the slide until you find some cells which have those characteristics. As you get better at identifying cells in this lab exercise, you will find fewer and fewer cells that you can t identify Identify the following granulocytes: 1. Neutrophils are the most common of all the leukocytes (40%-60%) in the blood. As the name ( neutral-loving ) tells you, their granules stain neutrally, neither distinctly blue nor red, and are not as large as the granules of the other two types. The nucleus is quite characteristic, consisting of three or more blue-staining lobes connected by thin segments. You should have no trouble finding many of these on the slide. Neutrophils are phagocytic cells which play an important role in protecting the body against infections by destroying bacteria, fungi, viruses, and even abnormal or cancerous cells. Their numbers will increase dramatically when an individual is sick with a viral or bacterial infection anywhere in the body, and they are able to leave the blood to easily reach other parts of the body. 2. Eosinophils are much less commonly seen in the blood, representing only 4% or less of the leukocytes. The name means eosin-loving. Eosin is the stain which provides a red color, and the granules of these cells (actually large lysosomes) are a distinctive orange or red color. The nucleus typically consists of two large blue-staining lobes connected by a thin segment, but it is often difficult to see because of all the large cytoplasmic granules lying on top of it (remember, this is a smear, not a section). It may take you a minute or two, but you should be able to identify some of these on the slide. Eosinophils are also phagocytic cells like the neutrophils, and they can also leave the blood to get into other tissues of the body. They secrete chemicals which damage multicellular pathogens such as parasites, and they destroy allergens and the antigen/antibody complexes which are formed when the body is fighting an infection. 5

6 3. Basophils, as their name tells you, have granules which love the basic dyes which produce a blue color. These granules are large and stain so darkly that they usually obscure the visibility of the nucleus, which is also blue and shaped like a fat U or S. These granules are actually vesicles containing the chemicals heparin and histamine which the cell releases during inflammation to dilate small blood vessels, make them leak plasma into surrounding tissues, and decrease its clotting. Unlike the other two types of granular leukocytes, these are not phagocytic. Although common throughout the body, basophils don t spend much time in the blood so they are rather hard to find, representing a fraction of 1% of all leukocytes in circulating blood. You will not be expected to identify any of these cells in this class, so don t waste a lot of time trying to find one. However, you should know what they look like in case you happen to run across one. Identify the following agranulocytes: 4. Lymphocytes are relatively common when seen in the blood, 20% to 40% or more of the leukocytes depending on how actively the body is using them to fight infections. You should be able to easily identify these when you see them on the slide. The nucleus is round or slightly indented and stains a uniform dark blue or purple, taking up most of the volume of the cell. The relatively sparse cytoplasm forms a rim around the nucleus and usually stains a medium blue. Since these are agranular leukocytes, don t expect to see any granules in the cytoplasm. While most of the circulating lymphocytes are relatively small, between erythrocytes and other leukocytes in size, they can become very large when fighting an active infection. There are actually two different types of lymphocytes in the blood. However, they look exactly the same with routine staining so you will not be able to tell them apart. Both types are part of the body s immune system and both are much more active in other tissues after they leave the blood. B lymphocytes are responsible for what is called humoral immunity - they release large numbers of antibodies into the extracellular matrix of tissues where they are located. T lymphocytes produce what is called cellmediated immunity by directly attacking invading or abnormal cells. They are also primarily responsible for rejection of transplanted tissues and for destruction of tumors. Like the basophils, lymphocytes are not phagocytic. 5. Monocytes are usually the largest leukocytes you will see in the blood. They typically have a blue or purple nucleus which is slightly indented or even kidney-shaped. There is usually a lot of cytoplasm around the nucleus, which stains a lighter blue or blue-gray. These make up 4% to 8% of the leukocytes in the blood, so you should be able to identify a few of them on the slide. These are the most active phagocytes of all of the leukocytes. When they leave the blood, they form the macrophages which we have been discussing in different tissues and organs throughout this class. 6

7 Questions for discussion from your required reading of Chapter 18: Approximately how many leukocytes (all types) are contained in each milliliter of blood? (Caution: your textbook and Table 18.6 lists the concentration as number per microliter. There are 1,000 microliters in each milliliter) How does this compare to the number of erythrocytes? To the number of platelets? Explain to other members of your lab group where leukocytes are produced. Exercise #5: On a separate sheet of paper, draw from memory a typical erythrocyte, monocyte, lymphocyte, basophil, eosinophil, neutrophil, and platelet. Pay attention to the size of each one, the shape and color of its nucleus (if present), and the number, size and color of its granules (if present). NOTE: The purpose of this is NOT to produce a pretty picture. It is to be sure you understand what each of the formed elements should look like. DO IT FROM MEMORY, using the book as a reference when necessary. Discuss your drawing with other members of your lab group and make any necessary corrections. Exercise #6: Working from memory, discuss and compare the functions of each type of formed element listed above. Exercise #7: Differential White Blood Cell Count Determining the proportions of each type of leukocyte in the blood is a useful tool in detecting and diagnosing diseases. The numbers of each type can dramatically and quickly change in response to such things as infections, inflammations, allergies, or diseases of various organs. It often provides one of the first indications that something is wrong. Again using your blood smear slide (slide #6), start at one edge of the slide and systematically keep track of the numbers of different leukocytes you see as you move the slide in a specific pattern as shown in this diagram. Be sure you are not re-counting the same cells. You can ignore all of the erythrocytes and platelets you see since we are only interested in the leukocytes. Keep moving to new adjacent regions of the slide until you have counted and identified 100 leukocytes. Keep track of the numbers in the table below. (You want a total of 100 when you are done.) Cell Type Neutrophil Eosinophil Basophil Lymphocyte Monocyte Number Observed Total Number of Cells Counted: (this should be 100) 7

8 Questions for discussion from your required reading of Chapter 18: How do the results of your count compare to the expected percentages of each type of cell as listed in Table 18.6 of your Saladin text? What information might it give your doctor if the percentage of neutrophils in your blood was significantly increased? Of eosinophils? Of lymphocytes? Of monocytes? Of basophils? C. PHYSIOLOGY OF THE BLOOD: For the last two parts of this lab exercise, measuring your hematocrit and determining your blood type, you need to obtain samples of your blood from a finger. Be sure you have read and understand the information about Working Safely With Blood at the beginning of this exercise. Although it was part of your required reading assignment before this lab, it would also be a good idea to review, and be sure you understand, the sections in your textbook which describe the ABO and Rh blood types. Pay particular attention to Figures and which describe the antigens found on erythrocytes and how these react with antibodies in the serum if the wrong blood type is present. In order to minimize the number of times you will need to cause your finger to bleed (ideally, just once), be sure you have also read through both Blood Typing and Hematocrit sections below and know what samples you need to obtain before you begin or you may have to stick yourself again. Exercise #8: Obtaining a Blood Sample for Blood Typing and Hematocrit 1) Obtain a packaged alcohol wipe and a couple of pieces of paper towel. 2) Obtain two capillary tubes which contain the anticoagulant heparin, marked with a thin red band at one end (do not use tubes which do not contain heparin). These will draw blood into them by capillary action and the heparin will prevent that blood from clotting. You will also need a small tray of clay ( Critoseal ), used for sealing off one end of each capillary tube after it contains blood, and three or four clean toothpicks. 3) Obtain two glass slides. Use a wax pencil to divide one slide in half. Label one side A and the other side B. Label the other slide Rh. 8

9 4) Obtain small bottles of Anti-A Serum, Anti-B Serum, and Anti-Rh / Anti-D Serum. Place one drop of Anti-A Serum on the half of the slide labelled A and a drop of Anti-B Serum on the half of the slide labelled B. Place a drop of Anti-Rh / Anti-D Serum on the slide labelled Rh, 5) Obtain a Medlance device for lancing your finger. Notice that this is spring-loaded and holds a sharp lancet protected by a plastic tip. Twist the plastic tip to remove it and expose the lancet inside. 6) Use the alcohol swab to scrub the side of either your middle or fourth (ring) finger about halfway down its distal segment. Let the alcohol air-dry for a few seconds. 7) Holding your finger as low as possible, push the lancet device firmly against your finger until the spring-loaded lancet is triggered. A drop of blood should immediately appear - wipe it away with a paper towel. A second drop of blood should flow onto your skin. If not, you may squeeze the finger gently, but if this doesn t work you will probably have to repeat the lancing. 8) Still holding your finger low, place the tip of a heparinized capillary tube into the drop of blood, holding the tube so that the other end points slightly downward away from the finger. Blood will be drawn into the tube, and it is OK if small amounts of air are also drawn in. When the tube is half to two-thirds full (no more), set it flat on your lab table and fill the second capillary tube. Set it flat also. If you are having trouble getting blood, you can skip filling the second tube. 9) Wipe the blood off of your finger with a paper towel and immediately squeeze gently to get another drop to appear on the skin. Touch this drop to one of the drops of antiserum you previously put on a slide, wipe off your finger, and repeat this process with the other two drops of antiserum. Immediately use clean toothpicks to mix each blood sample with its antiserum. 10) Keeping it flat, pick up one of the capillary tubes. Firmly hold a finger against one end and plug the other end by pressing it into the clay, twisting gently, and removing it. Repeat with the other tube. You may leave them in the clay for a few minutes until you are ready to measure your hematocrit, but be careful not to accidently break them. 11) You are done bleeding. Apply pressure to the puncture site for a minute or two with a piece of paper towel - the bleeding should stop. 12) Now would be a good time to properly dispose of contaminated supplies, but don t waste too much time. 9

10 Exercise #9: Determining ABO and Rh Blood Types In the previous exercise you mixed a sample of your blood with serum taken from patients who had formed antibodies against each three different antigens normally found on erythrocytes (the antiserum you put on the slides). You should complete this exercise within only five or ten minutes of that time. Otherwise, the slides will dry out and be useless to you. As you know from your reading of Chapter 18, your erythrocytes carry glycolipid molecules called antigens on their surfaces, and your plasma contains protein molecules called antibodies. As we will discuss when we get to the immune system, each antibody will recognize and bind onto one specific antigen. During fetal development and early childhood, your body learned which antigens are normally present on your cells and does not form antibodies against them, while at the same time your body produces antibodies which will bind onto any antigens which are different than your own. This binding causes the cells to which the antigens are attached to stick together or agglutinate. Since a person does not normally produce antibodies against the antigens on their own erythrocytes but will produce them against antigens which are different, we can use this property of agglutination to determine which of these antigens you do or do not have on your erythrocytes - that is, what blood type you have Blood is classified into specific groups or types depending on which antigens are present or absent on the erythrocytes. The A and B antigens are considered together as the ABO group note that there is no O antigen because humans do not produce serum antibodies which will bind onto that glycolipid on erythrocytes. The Rh antigen is considered its own group. Examine those slides, gently rocking each one back and forth to make the fluid gently flow back and forth. Determine if the erythrocytes remain evenly suspended or if they have agglutinated (clumped) as shown in Figure of your Saladin text. If they agglutinated when mixed with the antiserum for one of the three antigens we are testing, that indicates that you have this specific antigen on your erythrocytes. If they do not agglutinate, you do not have that antigen. For example, if your blood agglutinated with Anti-A Serum but did not agglutinate with Anti-B Serum, you know that your erythrocytes have the A antigen but do not have the B antigen. Remember that there is no O antigen on erythrocytes. Similarly, if your erythrocytes agglutinated with the Anti-Rh Serum you know that your erythrocytes have the Rh antigen. If they did not agglutinate, your erythrocytes do not have that antigen. Draw your results in the circles below. Anti-A Anti-B Anti-Rh 10

11 There are, of course, many different combinations. Your erythrocytes may have only the A antigen, only the B antigen, both the A and B antigens, or neither the A nor the B antigens. As a result, your erythrocytes may have clumped with just one antiserum, two of the antisera, all three of the antisera, or none of the antisera. Based on your results, referring to the first two columns of the chart below: What is your blood type for the ABO group? What is your blood type for the Rh group? ABO Blood Type Antigen(s) Present On Your Erythrocytes Antibodies Present In Your Plasma A A only Anti-B B B only Anti-A AB Both A and B (None) O Neither A nor B Both Anti-A and Anti-B Rh Blood Type Antigen Present On Your Erythrocytes Antibodies Present In Your Plasma Rh-Positive Rh (None) Rh-Negative (none) Anti-Rh Exercise #10: Remember that your immune system must do two things: it must form antibodies against all of the antigens which are not present on your cells, while at the same time NOT forming antibodies against any antigens which your cells DO have. The latter prevents your immune system from attacking and destroying your own cells. Based on the results of the blood typing you just did, therefore, you can also determine which antibodies are present in your own plasma. For example, if your erythrocytes have the A antigen but not the B antigen, you know that your plasma must contain anti-b antibodies but can not contain anti-a antigens which would react with your A antigen and make you very sick. Based on your results, referring to the last two columns of the chart above: Which antibodies against the A or B antigens would be present in your plasma? Would you have antibodies against the Rh antigen in your plasma? 11

12 Exercise #11: Blood Transfusion As you no doubt realize, these antigens (on erythrocytes) and antibodies (in the plasma) determine which types of blood can safely be transfused into another person without causing a strong immune response leading to illness and possibly death. a) The donor s erythrocytes can t carry antigens to which the recipient s plasma contains antibodies, and b) The donor s plasma can t contain antibodies which will bind onto antigens of the recipient s erythrocytes Based on your understanding of these antigens and antibodies, use the chart below to discuss with other members of your lab group which types of blood, including both the ABO and Rh groups, can and can not be safely transfused into an individual with each of the following types of blood. Recipient Blood Type A-positive blood (that means Type A and Rhpositive) B-positive blood AB-positive blood O-positive blood A-negative blood B-negative blood AB-negative blood O-negative blood What types of blood can safely be transfused into that recipient? What types of blood can NOT be safely transfused into that recipient? Question for discussion: Based on that information, explain to other members of your lab group why a person with Type O-negative blood is considered a universal donor, and why a person with Type AB-positive blood is considered a universal recipient. 12

13 Exercise #12: Hematocrit The hematocrit, or packed cell volume, is another common laboratory test to help determine the state of your health. It is very easy to do. The small capillary tube of blood you obtained in Exercise #8 will be centrifuged for a few minutes to pack the formed elements at the bottom and leave the plasma at the top, after which the percentage of each can be easily measured. Since erythrocytes are a very large majority of formed elements in blood and are all approximately the same size, the hematocrit quickly and accurately indicates if you do not have enough of them. This exercise should be done within half an hour of filling the capillary tube. With the help of your instructor, place it into the centrifuge designed for this, being sure it fits firmly into a slot. The clay-plugged end must point outward and touch the rubber rim. Remember the number of the slot containing your tube - they all look alike. When enough tubes are loaded, your lab instructor will properly close the centrifuge and turn it on for four or five minutes. When the centrifuge has stopped, remove your tube and examine it. Observe that there are three separate layers (Figure 18.2 in Saladin). The bottom of the tube now contains the erythrocytes packed tightly together, the top of the tube contains the straw-colored plasma, and there is a very thin layer of leukocytes and platelets between these two. Use the microhematocrit reader (your lab instructor will show you how) to determine the percentages in your blood sample of erythrocytes, of leukocytes plus platelets, and of plasma. The percentage of each one is easily calculated by dividing its height in the capillary tube by the overall height of the blood sample at the top of the plasma layer Record your measurements here: % Eythrocytes % Leukocytes+Platelets % Plasma Obviously, your hematocrit is an important indicator of how much oxygen and carbon dioxide your blood can distribute to all tissues and cells of your body a higher hematocrit means more erythrocytes which can carry these gasses. You should realize, however, that there is another important factor. Since oxygen and carbon dioxide are bound to hemoglobin within the erythrocytes, the amount of hemoglobin in each cell, and thus the total amount of hemoglobin in your blood, are also essential. a) you must have enough total erythrocytes in your blood (hematocrit) b) those erythrocytes must be the right shape and size (mean corpuscular volume) c) each erythrocyte must have enough hemoglobin to carry enough oxygen (mean corpuscular hemoglobin) d) that hemoglobin must be able to bind oxygen in the lungs, carry it to other cells of the body as the blood circulates, then release it for them to use for aerobic metabolism. This produces carbon dioxide. e) the hemoglobin must be able to bind that carbon dioxide, carry it back to the lungs, and release it. This is a prime example of homeostasis: If any one of these is too low or too high, your erythrocytes will not be able to function normally. 13

14 Exercise #13: Examine the structure of hemoglobin as shown in Figure 18.5 in your Saladin text, a) Note that each hemoglobin molecule consists of four protein chains, called globins, two of which are a form called alpha and two of which are a form called beta. b) Note that each globin is attached to a molecule called heme, at the center of which is an iron ion (Fe 2+ ). Oxygen and carbon dioxide bind onto this iron to be circulated throughout the body. Within the capillaries of the lungs, molecules of oxygen diffuse into the blood and bind onto the iron of each heme group. As that blood circulates, it is carried from the lungs to all tissues of the body. In those tissues, the oxygen is released and diffuses out of the erythrocyte, and carbon dioxide takes its place to be carried back to the lungs. Since each hemoglobin molecule contains four heme groups and each iron ion can bind one molecule of oxygen or carbon dioxide, a total of four molecules of oxygen or carbon dioxide are bound to each molecule of hemoglobin. Obviously, the amount of these gasses which can be circulated in your blood depends on both the number of erythrocytes and the amount of hemoglobin in each one. Questions for discussion from your required reading of Chapter 18: How does your hematocrit compare to normal or expected values? How do normal hematocrit values differ between men and women? Why do you think such a difference might exist? Discuss with other members of your lab group some common reasons why your hematocrit might decrease from one day to the next. Discuss with other members of your lab group the disorders called polycythemia and anemia. What are some causes of these? 14