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Blood, Lymph, and Immunity Joann Colville

CHAPTER OUTLINE .»: Function Introduction Lymphatic Structures Plasma THE Cellular Components of Blood Function Red Blood Cells Immune Reactions Immunization: Protection Against Disease White Blood Cells THE LYMPHATICSYSTEM Lymph Formation Characteristics LEARNING OBJECTIVES • List and describe the functions of blood • List the functions of the • Describe the composition of • Describe the structure and functions of the lymph nodes, • Describe the characteristics of mature erythrocytes , thymus, tonsils, and GALT Describe the structure of the molecule and • List the functions of the immune system explain the fate of hemoglobin following intravascular and • Differentiate between specific and nonspecific immune extravascular reactions • Give the origin of thrombocytes and describe their • Differentiate between -mediated and characteristics and functions • List the components involved in cell-mediated immunity • Listthe types of leukocytes and describe the functions of each and explain the role of each • Describe the formation of lymph fluid and its circulation List and describe the classes of immunoglobulins through the lymphatic system • Differentiate between active and passive immunity

BLOOD vessels of the cardiovascular system. It has three main func- tions: transportation, regulation, and defense. INTRODUCTION Blood. Say the word, and some people cringe, others faint, Function and if you believe authors Bram Stoker, Stephen King, and 1. Blood is a transport system. Christopher Moore (all authors of vampire books), others • It carries , nutrients, and other essential com- drool. But no matter what you think about blood, animals pounds to every living cell in the body. Oxygen is need it. In this section of the chapter, we'll explore the red, carried by hemoglobin in the red blood cells (RECs). sticky, salty fluid that is blood and find out how it keeps Nutrients and other essential compounds are dissolved animals alive and healthy. in the blood plasma. Blood is actually classified as a connective . It is the • It carries the waste products of cellular metabolism, fluid connective tissue that flows all over the body in the primarily , away from the cells to the

220 Chapter 9 Blood, Lymph, and Immunity 221

waste disposal organs that excrete them from the body, most often the and the kidneys. TABLE 9-1 Clotting Factors • It transports from endocrine glands to target Factor Number Common Name organs. • It transports white blood cells (WBCs) from the I Fibrinogen marrow, where they are produced, to the tissues where II Prothrombin they will do their work. III Tissue factor • It transports platelets to the site of damage to a blood IV Calcium vessel walL Once there, the platelets will form a clump V Proaccelerin to try to prevent blood from escaping out of the VI (Va) Accelerin vesseL VII Proconvertin 2. Blood is a regulatory system. VIII Antihemophilic factor A • It aids in regulation of body temperature. Body tem- IX Christmas factor, antihemophilic factor B perature regulators are located in the brain and are X Stuart-Prower factor partially influenced by the temperature of the blood XI Plasma thromboplastin antecedent that passes through or over them. XII Hageman factor It aids in tissue fluid content. The normal state of the XIII stabilizing factor body () is a state in which, within very

narrow limits, the composition of body tissue fluid is .•~£. maintained as constant as possible. If an animal is low in tissue fluid () because of vomiting, diar- rhea, profuse sweating, or some pathological condition hormones, nutrients, and drugs are either dissolved or sus- that causes it to lose fluid, some of the plasma (not the pended. The liquid portion of blood is called plasma. The cells and larger molecules) will leave the blood- cellular portion of blood is made up of red blood cells stream and enter the body tissues in an effort to com- (erythrocytes), white blood cells (leukocytes), and platelets pensate for the fluid loss. This leaves less plasma in the (thrombocytes) (Figure 9-1). bloodstream, and the cells become more concentrated (The term is used to denote blood con- (hemoconcentration). If an animal has too much body tained in the cardiovascular system or a sample that con- fluid, like after subcutaneous fluids are administered, tains plasma and all its cellular components. Peripheral the excess fluid will enter the bloodstream. This extra blood is whole blood that flows through the blood vessels fluid in the plasma dilutes the cells (hemodilution). that carry blood to and from the heart and lungs. Peripheral It aids in regulation of blood pH (acid-base balance). blood can be drawn into a syringe or tube for laboratory Normal blood pH falls in a range of 7.35 to 7.45, with analysis. the ideal being 7.4 (slightly alkaline). See Chapter 10 for a more detailed description of blood pH. Blood must PLASMA be maintained within this narrow range for the animal Plasma is the fluid portion of a blood sample. It makes up to remain in a state of homeostasis. Therefore, the pH from 45% to 78% of depending on the species must remain slightly alkaline in an effort to buffer the of the animal and the size of its red blood cells. In species acidic waste products of cellular metabolism that it with small red blood cells, such as goats and cats, plasma carries. The pH of arterial blood is slightly more alka- makes up a larger percentage of the blood sample. In species line than that of venous blood (blood that has picked with larger red blood cells, like dogs, plasma makes up a up the waste products). smaller portion of the blood sample. To show how this 3. Blood is a defense system. works, let's say you had two identical glasses and you put • White blood cells provide defense from foreign invad- 100 BBs (to represent small red blood cells) in one and 100 ers into the body through (ingesting the marbles (large red blood cells) in the other. Then if you filled invaders) or through their involvement in immunity. the glasses the rest of the way with water, you'd be able to • In addition to the platelets that are found in blood, put more water in the glass with the BBs. This glass would there are 13 clotting factors found in blood that are be like an animal with small red blood cells. Red blood cells necessary for blood to clot. These factors are activated make up the next largest component of blood after plasma. when a wall is damaged. The clotting The white blood cells and platelets make up a small portion process is very complex, and if one clotting factor is of the total volume of the blood sample. missing, blood will not clot. The clotting factors are Plasma is about 93% water. It contains many substances listed in Table 9-1. dissolved or suspended in it. Plasma such as , globulins, and fibrinogen make up the majority of Composition these substances. The gases most abundant in plasma are Blood is a fluid in which cells and cell fragments are oxygen, carbon dioxide, and nitrogen. , amino acids, suspended; and compounds such as oxygen, , metabolic wastes, and electrolytes such as sodium, potas- 222 Chapter 9 Blood, Lymph, and Immunity

Plasma (percentage by weight) Proteins ~ ~ 7% ~ Percentage ~ Percentage by by volume body weight ~ Water ~ ~ 91% ~ C- =:>

0utrients=:> Plasma Other solutes 2% ¢ 55% C§"ste Prod~

Gases Formed elements C -=::> (number per cubic mm) ~ ----- ~ Platelets ~ 200-500 thous. ¢

Figure 9-1 Composition of blood. Values are approximate for blood components in normal adult dogs.

sium, calcium, magnesium, chloride, and ions The hydration of the patient can also be a factor in plasma are also found in plasma. When systemic drugs are admin- color. In a dehydrated patient all the constituents of plasma, istered to an animal, they are carried to their site of action including , are more concentrated. This could result by plasma. Frequently, these drugs have to attach themselves in deeper yellow-colored plasma. to a transport plasma protein such as albumin to make themselves soluble in the plasma. A whole blood sample appears red because of the many red blood cells suspended in the plasma. The blood cells make it impossible to see through the sample. If the blood cells are removed from the sample, the remaining plasma will appear transparent and varying degrees of yellow. The f an animal has eaten just prior to a blood sample being intensity of the yellow color of plasma is determined by the Idrawn, the plasma may appear cloudy due to from the concentration of hemoglobin breakdown products in digested food suspended in the plasma. This condition is the plasma. Hemoglobin is the protein in red blood cells referred to as postprandial lipemia (postprandial means that gives them their red color and enables them to carry after eating, and lipemia means fat in the blood), and it large amounts of oxygen. When worn out, red blood cells can make the plasma or unsuitable for laboratory analysis, depending on the analytical method used. For this are removed from circulation and the hemoglobin is broken reason, blood samples for laboratory analysis should be down and released. The breakdown products of hemoglobin drawn before an animal is fed or a couple of hours after include bilirubin, which is yellow (more on this when we feeding. discuss hemoglobin later). Chapter 9 Blood, Lymph, and Immunity 223

• CLINICAL APPLICATION !fi.~Anti~~~g~l~nt~, Pi-~sma,~nd Serum

emember the blood clotting factors that were mentioned Rearlier? These factors need to be present in sufficient quantities for blood to clot. If we want to prevent blood from clotting, we add something to it that ties up one of the clotting Serum Plasma factors. Substances that tie up clotting factors and prevent (liquid minus (liquid minus blood cells blood from clotting are called anticoagulants. (Coagulation blood cells) 55% and clotting is another word for dotting.) If an anticoagulant is added to a elements) blood sample in a tube or syringe, the blood will not clot. One of the most common anticoagulants is ethlyenediaminetet- (leukocytes) Clot raacetic acid or EDTA. EDTA prevents clotting by tying up (blood cells calcium, clotting factor number IV. No calcium, no clot. enmeshed Red blood cells in fibrin) If anticoagulant is added to a blood sample as it is drawn (erythrocytes) 45% from an animal, the sample will not clot because all the clotting factors are not present. If the blood sample is then centrifuged (spun at a high speed), the fluid that rises .v: to the top of the tube is plasma (see figure) If no anticoagulant is added to a blood sample as it is Difference between blood plasma and blood serum. Plasma is drawn from an animal, the blood will clot. If the clotted blood whole blood minus cells; serum is whole blood minus the cells and is centrifuged, the fluid that rises to the top of the tube is clotting elements. Plasma is prepared by centrifuging anticoagulated blood; serum is prepared by allowing blood to clot. (From Thibodeau called serum. When blood clots, one of the dissolved plasma GA, Patton KT: Anatomy & physiology, ed 5, St Louis, 2003, Mosby.) proteins-fibrinogen-is converted to insoluble fibrin, which precipitates out of solution as a meshwork of tiny fibers (hence its name) and helps make up the framework of the the sample has been centrifuged, the plasma or serum can clot. Removing fibrinogen from plasma by allowing it to clot be' drawn off and analyzed or frozen for analysis at a converts plasma to serum. later date. Whole blood cannot be frozen, because blood Many of the diagnostic clinical chemistry tests performed cells rupture easily during the freezing and thawing on a patient sample are run on either plasma or serum. After processes.

CELLULAR COMPONENTS OF BLOOD A Word About Stains The cells suspended in plasma fall into three categories Throughout this chapter, you'll read about cells and struc- (Figure 9-2): tures that stain specific colors. These colors are based on the Red blood cells (erythrocytes), which carry oxygen from use of common stains. There are many hematol- the lungs to the cells and tissues of the body. ogy (blood) stains with many different names but they all Platelets (thrombocytes), which help prevent leaks have similar characteristics. Many of these stains from damaged blood vessels. are Romanovsky-type stains that are a combination of White blood cells (leukocytes). There are five types of basic blue and acidic red dyes dissolved in methyl alcohol. white blood cells that can be differentiated on a stained Cellular structures that are basic (alkaline) will stain blue; blood smear. Three of these have granules in their cyto- structures that are acidic will stain red. These stains are plasm and are named by how these granules stain using called polychromatic stains because they stain more than standard hematology stains. These cells are called the one color. . They are (red granules), baso- Wright's stain is the most widely used hematology stain. phils (blue granules), and (granules that The basic (alkaline) part of a Wright's stain is methylene stain neither blue nor red). The other two types of white blue. The acidic part is . Other Romanovsky-type stains blood cells don't have granules in their , so they that are available and used to a lesser degree are Giemsa, are called . The agranulocytes are the Leishman's, Wright-Giemsa, and May-Grunwald stains. and the . Each cell type has Modified Wright's stains are also available. They offer faster specific functions within the body. We discuss each of staining times but don't stain some cellular structures as well them in more detail shortly. as Wright's stain. 224 Chapter 9 Blood, Lymph, and Immunity

Red blood cells Platelets yellow fat cells that have replaced some of the active red (thrombocytes) (erythrocytes) marrow. In a mature animal most of the red . iifi is found at the ends oflong (e.g., femur, tibia, humerus, 4' =! and ulna) and flat bones (e.g., hip bones, sternum, and ribs). Y'f!ib ~ ~f1 The rest is all yellow bone marrow. If the need arises, the and spleen have a limited capacity to participate in 00 hematopoiesis but not to a life-sustaining capacity. All blood cells have a common ancestor. Scattered about White blood cells (leukocytes) the bone marrow is a population of cells called the pluripo- tent stem cells (PPSCs) also known as the hematopoietic Granular leukocytes stem cells. That's a complicated name, but it means these cells are primitive cells (stem cells) that have lots of potential (pluripotent). A PPSC has the potential to develop into any one of the blood cells. Which one is determined by the chem- ical or physiological stimulus that acts upon the . Each type has its own set of stimuli required to activate a PPSC to start down the path of development for that specific type of blood cell. Once a PPSC has been stim-

,.<1:-1;- ulated to produce a specific blood cell, it is an irreversible Agranular leukocytes process. That is, it can't partially develop into a then change its mind and become a neutrophil. Hematopoiesis is a one-way street. For example, one of the stimuli for red blood cell production is produced in the kidney. When these specialized kidney cells detect decreased oxygen levels in the blood (a condition called ), they perceive this as a need for more red blood cells to carry oxygen. In response to this perceived need, they Figure 9-2 The cellular components of blood. Red blood cells produce a called (EPO), which is (erythrocytes), white blood cells (leukocytes), and platelets carried by blood to the bone marrow. There, it will stimulate (thrombocytes) constitute the cellular components of blood. (From some of the PPSC cells to begin their development into red Thibodeau GA, Patton KT:Anatomy & physiology ed 5, St Louis, blood cells. 2003, Mosby.) Other stimuli will be necessary to complete the produc- tion of the various blood cells. This process involves numer- Hematopoiesis ous cell divisions, so that one PPSC eventually becomes Hematopoiesis is a general term for the production of all many cells. For example, one PPSC will eventually become blood cells. Blood cells are not immortal, so they are con- many red blood cells when stimulated properly. In a similar stantly being produced, functioning for a specific time period fashion, there are different stimuli for the production of and then dying of old age. Some blood cells, especially leu- platelets and each . Without these specific kocytes, leave circulation to fight off foreign invaders in the stimuli, PPSCs will not develop into any mature blood cells. tissues, where they can be killed in action. For an animal to With the exception of some lymphocytes, the develop- remain healthy, it needs a sufficient number of all its blood ment of blood cells takes place entirely in the red bone cells all the time. As blood cells die or are destroyed, they marrow. If you were to look at the bone marrow under a need to be replaced. For this reason, hematopoiesis is a , you would see all blood cell lines with cells in continuous process. all stages of development, from very immature cells to fully In an early fetus, most hematopoiesis takes place in the functional and mature cells. Under normal conditions, the liver and spleen. As the fetus develops, the bone marrow bone marrow has reserve pools of mature blood cells and gradually takes over production of the blood cells. In a doesn't release these cells into circulation until they are newborn animal most of the blood cell production is occur- needed. The size of the pool varies by cell type, and there ring in the active bone marrow. This is called the red bone must be some stimulus from the body to release the cells. marrow based on its gross appearance. At this young age, But remember that hematopoiesis is a continuous process, the red bone marrow is found in nearly all bones because so there is constant stimulation and release of cells to replace there is a great demand for blood cells as the animal grows those that die or are killed. and matures. Certain pathological conditions will stimulate a massive The older the animal gets, the less its needs a high blood release of cells. For example, an invading can cell production rate, so some of the red bone marrow is result in a large number of neutrophils being released from converted to inactive, yellow bone marrow, again named for the bone marrow and traveling to the site of the to its gross appearance. Yellow bone marrow is composed of try to control the infection by killing the . If Chapter 9 Blood, Lymph, and Immunity 225 the infection is severe enough, the bone marrow may deplete its reserve of mature neutrophils and will have to start :;~ TEST YOURSELF sending out immature neutrophils. That's like sending a child to do an adult's work. It is just not as effective. 1. What are the main functions of blood? Another interesting thing about PPSCs is that they are 2. What is one of the most common causes of hemocon- self-perpetuating. When a PPSC is stimulated to begin pro- centration, and how can it affect blood cell counts in peripheral blood? duction down a cell line, it undergoes a mitotic division. The 3. What is the most abundant component of plasma? result of that division is that one cell continues to develop 4. What are the three main categories of cellular blood into a blood cell, while the other cell goes back into the stem components? cell pool. 5. What is the difference between red bone marrow and yellow bone marrow? Blood Storage 6. What is the difference between plasma and serum? An animal's body can control its circulating volume ofblood 7. How does one cell population, the pluripotent stem cells, at any given time. When an animal is at rest, it doesn't need give rise to all the different blood cells? as much oxygen to fuel its muscles, so in a way, it has "extra 8. What is the total blood volume of a 675-pound (lean blood." The spleen is a spongelike organ in the abdomen body weight) animal? How about a 3-pound animal? capable of holding a lot of blood. When the body doesn't need all of its blood circulating, the spleen will swell up and store it until it's needed, like a sponge that can soak up a lot ~.I/..'. of water. When the animal is physically active, the spleen contracts and puts the stored blood back into circulation, so there is more oxygen going to the muscles (this is like squeez- ing the sponge). More about the spleen later, because it has other important functions.

ere's the question. How do you know if you can draw would result in a blood loss of 16%. This is still not a problem, H200 ml of blood from an animal without causing serious but we're getting closer to trouble. problems? Our limit will be 25% of the total blood volume, which is more blood than you would routinely draw from an A Pint's a Pound the World Around animal, since an animal that loses 25% of its total blood Another way to determine the total blood volume is to figure volume has about a 50:50 chance of survival, but let us that blood makes up an average of 6% to 8% (average 7%) examine a worst case scenario. of an animal's lean body weight. Of course, that will give you First, you need to know how much blood an animal the weight of total blood volume, so you need to convert that has. The total blood volume for any animal can be estimated to a liquid measure. A pint of water weighs a pound. So using the animal's lean body weight. Lean is the operative because blood is mostly water, we'll use that conversion rate. word here. A 13.5 kg (30-pound) house cat is not lean. So From here we need to convert it into metric measure because if you want to figure the total blood volume on this cat, that's how syringes are calibrated. There are 2 pints in a quart, think of it as a 3.5- to 4.5-kg (8- to 12-pound) cat. As a and a quart is approximately equivalent to a liter. There are broad rule of thumb, figure 50 to 100 ml (average 75) of 1000 ml in a liter. So now it becomes a simple math problem. blood/kg lean body weight. High-strung animals tend to Using these guidelines, a dog weighing 75 pounds (34 kg) have more volume because they are always active-pacing, will have a total blood volume of approximately 5.25 pounds (75 bouncing, running-so they need more oxygen in their pounds x 7%). That's 5.25 pints of blood. If 2 pints are 1 quart, muscles. 5.25 pints are 2.625 quarts, or approximately 2.625 liters. Move Using these guidelines, a 454-kg (1OOO-pound) horse will the decimal point to the right three places and the total blood have a total blood volume of about 34,000 ml or 34 liters volume of this dog is 2625 ml. A 200 ml blood loss for this (454 kg x 75 ml of blood/kg = 34,050 total blood volume). animal would amount to about a 10% loss of total blood Taking 200 ml of blood from this horse would result in a volume. This would not be a significant loss for the dog. blood loss of 0.5% of the total blood volume (200 ml divided On the other hand, a cat weighing 10 pounds (4.5 kg) will by 34,000 ml and multiplied by 100 to get a percentage). Not have a blood volume of 0.7 pound, or 0.7 pint. This converts a problem. to 350 ml. A blood loss of 200 ml would result in a loss of Now let's consider a 16-kg (35-pound) dog with a total 57% of its total blood volume. The cat would probably die of blood volume of 1193 ml. Drawing 200 ml from this dog shock before you could get that much blood out of it.