Veterinary Histology: Blood

Front 1

Blood Function

Back 1

- fluid connective tissue
- carries oxygen and nutrients
- brings hormones to cells and CO2 and waste products away for excretion
- regulates acid-base, osmotic balance and body temperature

Front 2

Blood Volume

Back 2

- expressed relative to body weight
- domestic animals have blood volumes of ~9% of their body weight except cats (~6%)
- as a single blood draw, a maximum of 1% of the animal's body weight can be removed

Front 3

Blood components

Back 3

- plasma: protein-rich fluid component
- cellular component: RBCs, WBCs, platelets

Front 4

Plasma

Back 4

- aqueous solution with 90% water and 10% other substances (proteins, lipids, glucose, etc)
- albumin is the predominant protein
- other proteins include hormones, clotting factors, immunoglobulins
- low molecular weight substance in plasma can easily go through capillary walls and are in equilibrium with interstitial fluid

Front 5

Albumin

Back 5

- maintains oncotic pressure of blood
- cannot cross the capillary walls so their effect on the osmotic pressure of the capillary interiors will balance out the tendency for fluid to leak out of the capillaries (pulls fluid in)
- if albumin is low you can get fluid loss into tissues (edema) or into the abdomen (ascites)

Front 6

Normal Appearance of Plasma

Back 6

- clear and colorless except in the horse where it is clear and light yellow (bilirubin)
- may also appear light yellow in cattle eating green plants that contain carotenoids

Front 7

Serum

Back 7

- the fluid portion of the blood after the blood cells and the fibrin clot have been removed
- it is what is left of the plasma after the clotting factors have been removed
- has a lower total protein than plasma
- serum is used to test for clinical chemistry determinations and when looking for antibodies because the anticoagulants used in plasma will interfere

Front 8

Hematopoiesis

Back 8

- the continuous replacement of cells in the blood
- in the adult this occurs in the bone marrow of long, flat bones
- pluripotent hematopoietic stem cells are the origin of all formed elements of the blood (CFU-L and M lines)

Front 9

Hematopoietic organs

Back 9

- bone marrow is the major site of hematopoiesis in the adult (especially in long, flat bones)
- lymphoid stem cells originate in the bone marrow but complete maturation in the thymus (T cells) and secondary lymphoid tissues
- spleen has potential for extramedullary hematopoiesis

Front 10

Erythrocytes

Back 10

- most common cell in the bloodstream
- enclose a large quantity of the red oxygen-binding protein hemoglobin that allows for efficient gas exchange
- hemoglobin is 95% of the total protein in a RBC
-

Front 11

Hemoglobin

Back 11

- two basic parts: iron containing pigment heme and the protein globin
- main function is oxygen uptake in the lungs, carriage in RBCs (oxyhemoglobin) and release in other tissues
- process of O2 transfer is depending on partial pressure of oxygen at these sites
- hemoglobin also carries CO2 (carboxyhemoglobin) and transports it to the lungs for excretion

Front 12

Advantages of Hemoglobin within cells

Back 12

- slower turnover time: heme has a short half life (hours) and RBCs have a lifespan of several months
- ability to maintain Hbg iron in the functional ferrous state
- prevents the osmotic effects that would be produced by levels of free Hgb consistent with the amount normally in RBCs (more water would be pulled into the capillaries)

Front 13

Histologic appearance of RBCs

Back 13

- appear homogenously orange to pink
- area of central pallor reflects the biconcavity of the cell
- Dogs have the largest RBCS (7micrometers)
- Cats, horses and cows: 5.7mm and some variation in size
- Sheep: 4.5mm and some variation in size
- Goats have very small RBCs and can have spindle shaped RBCs and they have a higher RBC count to compensate for their small size
- Camelids have oval shaped, nondeformable RBCs

Front 14

Formation of RBCs

Back 14

- nucleus and organelles are lost to make room for hemoglobin

Front 15

RBC plasma membrane

Back 15

- 50% of the plasma membrane of the RBC is made up of protein
- some of the proteins function as receptors, some are transport proteins
- shape of RBC is maintained by cytoskeletal proteins (spectrin)

Front 16

Anemia

Back 16

- number or volume or quantity of hemoglobin in the blood is below normal
- can be caused by blood loss, accelerated RBC destruction (hemolysis) and decreased RBC production by the marrow
- if there is an increase in reticulocytes then you know that anemia is not due to decreased RBC production but is due to either blood loss or hemolysis

Front 17

Polycythemia

Back 17

- increase in the number or concentration of circulating RBCs
- can be caused by dehydration, which increases the concentration of RBCs by decreasing the proportion of plasma
- can be seen in response to chronic hypoxia

Front 18

Leukocytes

Back 18

- defense against infection
- WBCs are inactive while circulating in the blood
- they cross venules and capillaries to carry out their functions in the CT
- only 1% of blood volume is leukocytes

Front 19

Classification of leukocytes

Back 19

- all WBCs have lysosomal azurophilic primary granules in the cytoplasm
- they are classified as agranulocytes or granulocytes
- they are motile cells that travel in the blood

Front 20

Agranulocytes

Back 20

- do not have secondary granules
- includes lymphocytes and monocytes

Front 21

Granulocytes

Back 21

- have secondary granules
- segmented (lobulated) nuclei

Front 22

Leukocyte Distribution

Back 22

- motile cells
- move from the blood into tissues following a sequence of steps:
1. margination: gathering and adhesion of leukocytes to the endothelial cells (mediated by adhesion molecules on WBC surface)
2. Diapedesis (emigration): the passing of leukocytes through teh intact wall of a blood vessel via temporary spaces that form between the junctions of endothelial cells
3. Migration: WBCs follow chemoattractant molecules and move through the tissues toward the site of inflammation or infection
- number of marginated WBCs equals or surpasses the number of circulating cells
- in a blood sample you only get the cells from the circulating (non-marginated) pool

Front 23

Neutrophil Appearance

Back 23

- most numerous granulocyte
- granules are neutral staining
- stay in the blood for 6-10 hours
- nucleus is lobulated
- immature neutrophils have a non-segmented nucleus that looks like a horseshoe

Front 24

Neutrophil Function

Back 24

- first type of leukocyte to reach a site of tissue damage
- most important defense against pathogenic bacteria
- their granules kill ingested bacteria
- substances released by bacteria and endothelial cells attract neutrophils that then migrate into the tissues and the neutrophils become active in phagocytosis
- operate in inflamed or necrotic cells by utilizing anaerobic glycolysis
- bacterial infections are often associated with an increased number of neutrophils in the blood
- tissues can be damaged if the content of granules are released from the cell--> pus

Front 25

Neutrophil Bacteriocidal Activity

Back 25

- primary/azurophilic granules have potent bacteriocidal action
- myeloperoxidase system: most effective microbicidal and cytotoxic mechanism of leukocytes
- during phagocytosis there is a burst of oxygen leading to formation of free radicals which kill bacteria
- low pH in phagosomes destroy bacteria
- Lactoferrin is found in secondary granules and it binds iron which is an important bacterial nutrient
- Lysozyme degrades peptidoglycans in the walls of bacteria

Front 26

Eosinophils

Back 26

- have a ribbon shaped nuclei and abundant cytoplasm
- secondary granules are large and bright orange and they contain the major basic protein that is efficient in killing parasitic worms, eosinophil peroxidase and eosinophil cationic protein to also kill parasites
- large amount of species variation in size and shape of eosinophils
- increased amount of eosinophils in the blood can be a sign of a parasitic infection or allergic disease
- eosinophils circulate in the blood for about 20 minutes and then enter the tissues where they can survive for a long time so the number of them in blood does not reflect the amount in the body

Front 27

Eosinophil Function

Back 27

- are phagocytic and can ingest some bacteria but not to the same extent as neutrophils
- granules contain major basic protein, eosinophil peroxidase and eosinophil cationic protein
- they secrete their products into the local environment in response to large particles like parasites that cannot be engulfed
- involved in regulation of allergic and inflammatory processes by production of cytokines and inflammatory mediators

Front 28

Basophils

Back 28

- least common leukocytes
- survive 10-12 days in the blood
- nucleus is a horseshoe or ribbon shaped
- granules are similar to those in mast cells

Front 29

Basophil Function

Back 29

- granules contain heparin and histamine and compounds that affect smooth muscle in some organs, other leukocytes and blood vessels
- involved in promoting the early steps of inflammation associated with allergic hypersensitivity reactions
- a massive release of granules from basophils can lead to anaphylactic shock

Front 30

Mast cells

Back 30

- related to basophils
- not considered a blood leukocyte although they arise from a bone marrow precursor
- an immature form is released into the blood and the cells mature in the tissue
- found in tissues, near blood vessels
- function similar to basophils and are important in allergic reactions

Front 31

Lymphocytes

Back 31

- most abundant agranulocyte
- important for defending the body against viral infection and fighting cancer
- lymphocyte precursors are produced in the bone marrow and then populate lymphoid tissues
- classified into T and B cells according to function (T cells are mostly circulating)
- only leukocyte that returns from the tissue back to the circulation after diapedesis

Front 32

Lymphocyte Appearance

Back 32

- spherical cells with circular nuclei
- small amount of cytoplasm
- small, medium and large lymphocytes exist with the small ones predominating

Front 33

Monocytes

Back 33

- largest leukocyte
- nucleus varies in shape
- cytoplasm is granular and may have vacuoles
- leave the bloodstream, migrate to CT and transform into macrophages
- stay in the blood for about 40 hours and CT for up to several months
- can undergo mitosis at maturity while granulocytes cannot

Front 34

Monocyte Function

Back 34

- phagocytosis
- immunity: secrete interleukins, tumor necrosis factor, interferons, growth factors

Front 35

Platelets

Back 35

- not individual cells but are small cytoplasmic fragments of megakaryocyte in bone marrow
- no nucleus (except in birds)
- has two regions:
1. periphery with a transparent zone called the hyalomere
2. central zone with granules called the granulomere
- survive for 8-10 days
- low platelet count or platelet function defects show a tendency to bleed especially via nose bleeds, gums and cutaneous capillary beds

Front 36

Platelet Structure

Back 36

- system of open canaliculi that connect with invaginations of the plasma membrane to increase surface area for secretion
- dense tubular system holds calcium
- several types of granules that contain factors taht stimulate coagulation, ADP and ATP, growth factor, etc
- microtubules and microfilaments

Front 37

Platelet Function

Back 37

- clot formation:
1. adhesion of platelets forms a temporary aggregation
2. degranulation releases clotting factors and a vasoconstrictor from the granules
3. clotting factors rom platelets and damaged tissue triggers a clotting cascade that stimulates formation of fibrin from fibrinogen and the fibrin traps RBCs and platelets to form a definitive clot
4. platelets cause clot retraction which pulls in teh fibrin in reduces the size of the clot to allow return of normal blood flow
5. clot is removed by proteolytic activity of plasmin
- also play a role in stimulation and regulation of inflammation and tissue repair