Chapter 9

BLOOD

OBJECTIVES:

- Be able to describe various components of blood - plasma and formed elements.

- Be able to describe the morphological differences among formed elements RBC, platelets, and leukocytes.

- Be able to describe the role of RBC in respiration, platelets in blood coagulation, and WBC in immune response.

- Be able to describe the role of blood in the formation of tissue fluid, supplying nutrients to body tissues, forming waste products in the kidney, regulating body temperature, and maintaining pH and water balance.

- Be able to describe the development of RBC, WBC, platelets and the factors that regulate the formation of each cell type.

- Be able to describe the various blood tests and their significance in clinical hematology.

 

Functions of Blood:

1. Absorbs nutrients from digestive system and supply these nutrients to body tissues.

2. Carries oxygen from lungs to tissues.

3. Carries carbon dioxide from tissues to lungs.

4. Carries waste products from tissues to the kidney for excretion.

5. Plays a significant role in thermo regulation.

6. Contains cells like granulocytes and agranulocytes which protect the body against diseases.

7. Acts as a buffer in maintaining the normal pH of the body.

8. Contains proteins such as albumin, which serves as a carrier for lipids and hormones; fibrinogen, which is involved in the coagulation of blood; and gamma globulin, which functions in the immune system of the body.

9. Maintains the water balance of the body (Homeostasis).

10. Serves as a diagnostic tool in clinical tests. Knowledge of the following parameters of blood is essential in the study of clinical hematology.

a. RBC number per ml

b. Differential leucocytic count

c. Total leukocytic count

d. Changes in the morphology of blood cells

e. Ratio of one cell type to another type

f. Hb concentration

g. Blood chemistry

h. Packed cell volume

 

Components of Blood

Blood is composed of two major components:

1. Plasma - fluid portion

2. Formed elements - cellular portion

Fig. 9-1 illustrates a blood sample (containing anticoagulant) before and after centrifugation.

 

Blood Plasma

The following is a list of various plasma constituents:

A. Water

B. Gases

1. oxygen

2. carbon dioxide

3. nitrogen

C. Proteins

1. Albumin

2. Globulins

3. Fibrinogen

D. Glucose, lactate, pyruvate

E. Lipids

1. Fat

2. Lecithin

3. Cholesterol

F. Nonprotein nitrogenous substance

1. Amino acids

2. Urea

3. Uric acid

4. Creatine

5. Creatinine

6. Ammonia salts

G. Inorganic Substances

1. Sodium 7. Phosphate

2. Potassium 8. Iron

3. Calcium 9. Manganese

4. Magnesium 10. Cobalt

5. Chloride 11. Copper

6. Sulfate 12. Zinc

13. Iodine

H. Enzymes and Hormones

I. Vitamins

J. Pigments

 

Plasma Proteins

The three major types of plasma proteins are albumin, globulin and fibrinogen. In the human, sheep, goat, rabbit, dog and cat, albumin is more abundant than globulin; in the horse, pig and cow, the relative proportions of albumin and globulins are nearly equal or the globulins tend to predominate.

Albumin is the smallest of the plasma proteins with a molecular weight of approximately 59,000. It is synthesized by the liver. Albumin accounts for nearly 80% of colloid osmotic pressure of plasma. This colloid osmotic pressure is responsible for the water-holding capacity of blood and prevents excessive loss of fluid from the blood to the tissues (edema) by opposing the hydrostatic blood pressure in the capillaries.

The globulins include a group of proteins whose molecular weight varies from 80,000 to several million. The gamma globulins, one of the more important fraction, include immunoglobulins or antibodies that are involved in the immunological defense mechanism of the body. The gamma globulins are synthesized by plasma cells in the lymph node, spleen and bone marrow. In most newborn animals (except rodents and primates), plasma gamma globulins are either lacking or present in minute amounts since the placenta is impermeable to these protein molecules. The fetus is not capable of synthesizing them. To counteract this deficiency the dam concentrates large quantities of gamma globulin in her colostrum. When the newborn ingests the colostrum, the gamma globulin easily crosses the still imperfect intestinal barrier and provides the newborn with passive immunity in the form of antibodies.

Another globulin fraction, the beta globulins, function in the transport of hormones, metal ions and lipids.

Examples of these are:

1. Transferrin - mainly functions in the transport of iron.

2. Ceruloplasmin - functions in the transport of copper.

3. Chylomicrons, low density lipoproteins - lipid transport.

The third plasma protein is fibrinogen, a long asymmetrical molecule with a molecular weight of 330,000. Fibrinogen is synthesized in the liver and is essential in the clotting mechanism of blood. In the clotting mechanism, circulating prothrombin is converted to thrombin which in turn activates fibrinogen to fibrin, the foundation of the fibrous network of a clot. When fibrinogen is removed from plasma, the resulting fluid is called serum.

 

Plasma Proteins in Some Domestic Animals (gms/100ml)

Aniaml

Total Plasma Protien

Fibrinogen

Total Serum Protein

Albumin

Globulin

Horse

6.84

0.34

6.50

3.25

3.25

Cow

8.32

0.72

7.60

3.63

3.97

Sheep

5.74

0.36

5.38

3.07

2.31

Goat

7.27

0.60

6.67

3.96

2.71

Dog

6.72

0.52

6.20

3.57

2.63

Cat

---

---

7.58

4.01

3.57

Pig

---

---

6.30

2.03

3.27

______________________________________________________

(Data from P.L. Altman and D.S. Dittmer, Blood and Other Body Fluids, Fed. of Am. Soc. for Exp. Biology, 1961.)

 

Cellular Components of Blood

Contains three major cell types:

1. Erythrocytes (RBC)

2. Leukocytes (WBC)

3. Thrombocytes (Platelets)

 

Erythrocytes

- Gas transporters

- Biconcave discs. Exceptions: oval shaped in camel, sickle shaped in some species of deer. Osmolality of plasma affects the shape of RBC. In hypertonic solution, RBC will crenate and shrink; in hypotonic solution, it will swell and hemolyze.

- The cell membrane enclosing the Hb is flexible. This flexibility allows the cell to change its shape as it passes through the blood vessels.

- The size of RBC varies among domestic species.

Largest (7 micrometer) in dogs while smallest (4 micrometer) in goats. It is 5 - 6 micrometer in horses and cows.

- Mature RBC does not have nucleus, Golgi bodies, RER, ribosomes, and mitochondria. Exception: nucleated RBC in birds; RBC may show crenation in the pig; RBC of the horse has a tendency to clump (rouleaux formation).

- Normal values for total number of erythrocytes per ml of blood in different domestic species:

cattle = 5-8 x l06

horse = 7-13 x l06

dog = 6-9 x l06

pig = 5-8 x l06

goat = 8-15 x l06

- It should be noted that number of erythrocytes in a species is inversely proportional to the size of RBC.

- Physiologic alteration: The number of RBC is significantly increased when an animal living on plains is suddenly moved to mountains.

You should become familiar with the following definitions as they relate to erythrocytes:

1. Erythropenia 14. Myeloid

2. Polycythemia 15. Hemopoiesis

3. Microcytosis 16. Hemoglobin

4. Macrocytosis 17. Erythropoiesis

5. Anisocytcsis 18. Agglutination

6. Poikilocytosis 19. Rouleaux

7. Hypochromasia 20. Erythroblastosis

8. Polychromasia 21. Howell Jolly Body

9. Basophilic stippling 22. Cabots Ring

10. Hemolysis 23. Erythroblast

11. Anemia 24. Normoblast

12. Hematocrit 25. Crenation

13. Reticulocyte 26. Sedimentation rate

 

LEUKOCYTES

- Divided into two groups:

1. granulocytes 2. agranulocytes.

- Granulocytes are characterized by cytoplasmic granules and are further divided into three categories based upon the staining characteristics of granules:

l. Neutrophil 2. Eosinophil 3. Basophil.

- Agranu!ocytes do not contain granules and are divided into two categories:

1. Lymphocyte

2. Monocyte.

 

DIFFERENTIAL LEUKOCYTIC VALUES IN SOME DOMESTIC SPECIES

(PERCENTAGE %)

CELL TYPE

HORSE

COW

DOG

CAT

SHEEP

SWINE

Neutrophils

30-65

15-45

60-77

35-75

10-50

28-47

Lymphocytes

25-70

45-75

12-30

20-55

40-75

39-62

Monocytes

0-7

2-7

3-10

1-4

0-6

2-10

Eosinophils

0-11

2-20

2-10

2-12

0-10

0-11

Basophils

0-3

0-2

rare

rare

0-3

0-2

Band

0-2

0-1

0-4

0-2

0-2

0-5

TOTAL LEUCOCYTIC VALUES IN SOME DOMESTIC SPECIES

SPECIES

TOTAL NUMBER OF WBC PER ML OF BLOOD

Horse

5.5-12.5 x103

Cow

4.0-12.0 x 103

Dog

6.0-17.0x 103

Cat

5.5-19.5 x 103

Sheep

4.0-12.0 x 103

Swine

11.0-22.0 x 103

 

NEUTROPHILS

- Also termed polymorphonuclear leukocytes. Poly means many, morph means shape, i.e. many shapes of the nuclei.

- 10-14 micrometer in diameter.

- Nucleus is multilobed (2-4). The lobes are connected with each other by thin chromatin strands.

- The numbers of nuclear lobes vary depending upon the age of neutrophils. In young neutrophils, the nucleus has a single elongated shape giving it the name band cell. As the cell ages, the nucleus becomes more and more lobulated.

- Cytoplasm is usually pale, grayish-blue and contains fine dust-like granules.

- First line of defense.

- Neutrophils as well as band cells markedly increase in number in response to infectious diseases.

 

EOSINOPHILS

- 10-15 micrometer in diameter.

- The nucleus is bilobed, darkly stained and may not show any nucleoli.

- The cytoplasm is characterized by distinct eosinophilic granules which vary in size, shape, number, and staining reaction depending upon the species.

Horse: Granules are extremely large, brick-red in color, and give the cell a mulberry-like appearance.

Cattle: Granules are small, round, bright orange, and completely fill the cell.

Dog: Granules may vary from 2-3 to many in number, but rarely fill the cell. Granules are usually the same color as erythrocytes.

Cat: Granules are rod shaped, non-refractile, and stain grayish-orange.

- Eosinophilic granules are lysosomal in nature.

- Eosinophils are frequently found in gut connective tissue, especially in horse.

- Their numbers are significantly increased in allergic reactions and parasitic diseases. During these conditions, eosinophils are believed to phagocytose and destroy antigen-antibody complexes.

 

BASOPHILS

- Rarely seen in peripheral blood (0.5 -1%)

- Nucleus is elongated, often bent, and is usually obscured by basophilic granules.

- Basophilic granules resemble granules of mast cells in that they are metachromatic and secrete heparin and histamine.

- Their functicn is not clear. Mast cells of the connective tissue may have their origin from basophils.

LYMPHOCYTES

- Morphologically, lymphocytes are two types:

1. Small: 8 micrometer in diameter; cytoplasmic nuclear ratio, 1:9.

Nucleus is round, heterochromatic, and surrounded by a thin rim of cytoplasm at one end.

2. Large: 12 micrometer in diameter; cytoplasmic nuclear ratio, 1:1.

Nucleus is indented, heterochromatic and surrounded by cytoplasm.

- Functionally, lymphocytes are two types:

1. B Lymphocyte - responsible for humoral response.

2. T Lymphocyte - responsible for cell-mediated immune response.

- Lymphocytes continue to circulate back and forth between the blood vessels and other body tissues.

(Note, except lymphocytes, all other leukocytes, once leaving the blood vessels, do not re-enter the circulation.)

MONOCYTES

- Largest of the circulating blood cells; 15-20 micrometer in diameter.

- Nucleus may be oval, bean or horse-shoe shaped. The nuclear chromatin is lightly stained and finely granular.

- The cytoplasmic nuclear ratio is 6:4; the cytoplasm contains vacuoles in the dog and ox, but homogenous granules in the horse. Both vacuoles and granules contain lysosomal enzymes.

- The cells are motile, they leave the vessels to become phagocytic macrophages/histiocytes of the C.T.

- Osteoclasts arise from monocytes, and so are the giant cells and

macrophages, regardless of their location in the body.

 

THROMBOCYTES (Blood Platelets)

- Nucleated cells in avian species; however, in mammals, these are small, non-nucleated, basophilic bodies, 2-4 micrometer in size.

- Consist of cytoplasm, organelles and inclusions; bounded by a cell membrane.

- The central portion is referred to as "granulomere" that has mitochondria, dense bodies and alpha granules; the peripheral cytoplasm is termed "hyalomere" that is devoid of organelles except for microfilaments and a ring of microtubules.

- Platelets are derived from cytoplasmic processes of the largest cell of the bone marrow, the megakaryocytes.

- Platelets in the event of an endothelial injury adhere to the subendothelial tissue; this adherence is used to seal defects in the wall of blood-vessels.

- Apart from adhesion, platelets contain many factors that activate conversion of blood fibrinogen to fibrin at the clotting site.

 

HEMOPOIESIS

Hemopoiesis is a continuous replacement of blood cells (RBC, Leukocytes,

Blood Platelets) as they wear out or lost, to keep their concentration

in blood constant and proportional.

A. Divisions of Hemopoiesis

1. Myelopoiesis - Formation of granulocytes (granulopoiesis),

monocyte (monopoiesis), erythrocytes (erythropoiesis) and platelets (thrombopoiesis).

2. Lymphopoiesis - Formation of lymphocytes and plasma cells (Note, plasma cells are not normally seen in blood).

B. Sites of Myelopoiesis

1. Embryonic

a. Blood islands in the yolk sac.

b. Fetal liver hemopoietic tissue.

c. Hemopoietic tissue in the spleen and lymph nodes.

d. Bone marrow in the developing bones.

2. Adult

a. Red/myeloid marrow mostly in the axial skeleton, skull, ribs, sternum, vertebrae and pelvis.

b. Extramedullar myelopoiesis may be seen in the liver and spleen, especially when bone marrow is diseased.

C. Theories of Hemopoiesis

1. Uniphylectic: Each blood cell type differentiates from the same

stem cell i.e., reticular cell.

2. Diphylectic: This proposes two stem cells; the lymphoblast for lymphopoiesis and hemocytoblast (pleuripotent stem cell, reticular cell) for myelopoiesis.

3. Polyphylectic: Separate stem cells for each blood cell type.

 

NOTE: The widely recognized theory is Unitarian. However, it should be noted that precursors of lymphocytes (lymphoblasts) following their differentiation in the bone marrow migrate to Bursa Fabricius (or its homologue in mammals) and thymus.

Development of RBC

Fig. 9-2 illustrates different stages in the development of RBC.

Summary of Changes in the Development of RBC

1. Cell size decreases and organelles are lost.

2. Nucleus, initially large and pale with nucleoli, gets smaller and stains more darkly due to the condensation of chromatin.

3. Cytoplasm forms hemoglobin, thus its staining affinity changes from basophilic to acidophilic.

4. Small cell with orange cytoplasm and a dark nucleus is the metarubricyte which loses its mitotic activity.

5. Reticulocyte contains rlbosomal granules in the cytoplasm referred to as basophilic stippling, which can be demonstrated in blood smears stained with supravital dye such as methylene blue.

NOTE: Reticulocytes are not found in the blood of healthy horses, sheep, goats or cows. However, in the dog and cat, 0.5 - 1 % reticulated cells may appear in the peripheral blood. In the pig, there mav be as many as 2%. Their number is increased in acute hemorrhage and in hemlolytic anemias.

Development of Granulocytes

Development of granulocytes is illustrated in Fig. 9-3.

Summary of Changes in the Development of Granulocytes

1. Myeloblast develops into progranulocyte which begins to acquire non-specific azurophilic granules in the cytoplasm.

2. Myelocyte starts to have granules specific for one of the three kinds of granulocytes in their staining affinity.

3. The differentiation of a myelocyte into a granulocyte includes

i. decrease in the cell size

ii. increase in the darkening and lobation of the nucleus

iii. increase in the number of specific granules.

 

Development of Blood Platelets

1. Reticular cell enlarges to become a megakaryoblast.

2. The nucleus of a megakaryoblast divides and divides without cytoplasmic division. The resulting nuclei rejoin, taking on a lumpy polyploid form.

3. Fine cytoplasmic azurophil granules accumulate as the cell becomes a large granular megakaryocyte.

4. Megakaryocytes send cytoplasmic pseudopodia into the lumen of sinusoids where they are cast off into the blood as platelets.

Development of Lymphocytes

1. Reticular cells develop into lymphoblasts.

2. Lymphoblasts leave the bone marrow and enter the lymphoid organs where they will differentiate into lymphocytes. Whereas the differentialion of lymphocytes into B-lymphocytes occurs in the bone marrow, their differentiation into T-lymphocytes occurs in the thymus.

Development of Monocytes

Development of monocytes is controversial. Most recent studies indicate that they develop from monoblasts within the bone marrow.

Factors Affecting Blood Cell Formation

1. Bacterial infection increases the level of circulating granulocytes and their rate of production.

2. Erythropoietin, a hormone released by the kidney, in response to decreased oxygen level, stimulates erythropoiesis.

3. Similar but not yet identified mediators are assumed to be responsible for increased production of granulocytes.

4. Steroid hormones depress the formation of eosinophils and other cells.

5. Erythropoiesis requires adequate dietary components, e.g., iron, folic acid and vitamin Bl2.