Blood transports oxygen, nutrients, hormones, heat and waste products throughout the body. It is composed of blood plasma and formed elements including red blood cells, white blood cells and platelets. Red blood cells contain hemoglobin and transport oxygen, white blood cells help fight infection, and platelets help form blood clots to stop bleeding. Blood is categorized into blood groups based on antigens on red blood cells. The major blood groups are ABO and Rh, which determine blood type compatibility for transfusions. Blood transports materials between tissues and organs, helps regulate pH and temperature, and protects the body through clotting and immune functions.

1. BLOOD
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2.  Most cells of a multicellular organism cannot move around to obtain oxygen and
nutrients or eliminate carbon dioxide and other wastes. Instead, these needs are met
by two fluids: blood and interstitial fluid.
 Blood is a connective tissue composed of a liquid extracellular matrix called blood
plasma that dissolves and suspends various cells and cell fragments.
 Interstitial fluid is the fluid that bathes body cells and is constantly renewed by the
blood.
 Blood transports oxygen from the lungs and nutrients from the gastrointestinal tract,
which diffuse from the blood into the interstitial fluid and then into body cells.
 Carbon dioxide and other wastes move in the reverse direction, from body cells to
interstitial fluid to blood.
 Blood then transports the wastes to various organs—the lungs, kidneys, and skin—for
elimination from the body.
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4. FUNCTION
1. Transportation: As you just learned, blood transports oxygen from the lungs to
the cells of the body and carbon dioxide from the body cells to the lungs for
exhalation. It carries nutrients from the gastrointestinal tract to body cells and
hormones from endocrine glands to other body cells. Blood also transports
heat and waste products to various organs for elimination from the body.
2. Protection: Blood can clot, which protects against its excessive loss from the
cardiovascular system after an injury. In addition, its white blood cells protect
against disease by carrying on phagocytosis. Several types of blood proteins,
including antibodies, interferons, and complement, help protect against
disease in a variety of ways.
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5. 3. Regulation. Circulating blood helps maintain homeostasis of
all body fluids. Blood helps regulate pH through the use of
buffers. It also helps adjust body temperature through the
heat absorbing and coolant properties of the water in blood
plasma and its variable rate of flow through the skin, where
excess heat can be lost from the blood to the environment.
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6. PHYSICAL CHARACTERISTICS OF BLOOD
 Blood is denser and more viscous (thicker) than water and feels slightly
sticky.
 The temperature of blood is 38C (100.4F), about 1C higher than oral or
rectal body temperature, and it has slightly alkaline pH ranging from 7.35 to
7.45.
 The color of blood varies with its oxygen content. When it has a high oxygen
content, it is bright red. When it has a low oxygen content, it is dark red.
 The blood volume is 5 to 6 liters (1.5 gal) in an average-sized adult male and
4 to 5 liters (1.2 gal) in an average-sized adult female. The difference in
volume is due to differences in body size.
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7. COMPONENTS OF BLOOD
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1. BLOOD PLASMA
2. FORMED ELEMENTS

8. 1. BLOOD PLASMA
 When the formed elements are removed from blood, a straw-colored
liquid called blood plasma (or simply plasma) is left.
 Blood plasma is about 91.5% water and 8.5% solutes, most of which
(7% by weight) are proteins.
 Some of the proteins in blood plasma are also found elsewhere in the
body, but those confined to blood are called plasma proteins.
 Among other functions, these proteins play a role in maintaining
proper blood osmotic pressure, which is an important factor in the
exchange of fluids across capillary walls.
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2. FORMED ELEMENT

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13. Origin & Development Blood Cells
 The process by which the formed elements of blood
develop is called hemopoiesis or hematopoiesis.
 Before birth, hemopoiesis first occurs in the yolk sac of an
embryo and later in the liver, spleen, thymus, and lymph
nodes of a fetus.
 Red bone marrow becomes the primary site of
hemopoiesis in the last three months before birth, and
continues as the source of blood cells after birth and
throughout life.
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14.  Red bone marrow is a highly vascularized connective tissue
located in the microscopic spaces between trabeculae of
spongy bone tissue.
 It is present chiefly in bones of the axial skeleton, pectoral and
pelvic girdles, and the proximal epiphyses of the humerus and
femur.
 About 0.05–0.1% of red bone marrow cells are derived from
mesenchyme and are called pluripotent stem cells or
hemocytoblasts. These cells have the capacity to develop into
many different types of cells.
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17. RBC
 Red blood cells (RBCs) or erythrocytes (e-RITH-ro¯-sı¯ts; erythro- red; -cyte cell)
contain the oxygen-carrying protein hemoglobin, which is a pigment that gives whole
blood its red color.
 A healthy adult male has about 5.4 million red blood cells per microliter (L) of blood,*
and a healthy adult female has about 4.8 million.
 RBCs are biconcave discs with a diameter of 7–8 micrometer .
 Mature red blood cells have a simple structure.
 Their plasma membrane is both strong and flexible, which allows them to deform
without rupturing as they squeeze through narrow capillaries.
 RBCs lack a nucleus and other organelles and can neither reproduce nor carry on
extensive metabolic activities.
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19. RBC Life Cycle
 Red blood cells live only about 120 days because of the wear and tear their
plasma membranes undergo as they squeeze through blood capillaries.
Without a nucleus and other organelles, RBCs cannot synthesize new
components to replace damaged ones.
 The plasma membrane becomes more fragile with age, and the cells are
more likely to burst, especially as they squeeze through narrow channels in
the spleen.
 Ruptured red blood cells are removed from circulation and destroyed by fixed
phagocytic macrophages in the spleen and liver, and the breakdown products
are recycled, as follows
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23. Granular Leukocytes
 Granular Leukocytes After staining, each of the three types of granular leukocytes
displays conspicuous granules with distinctive coloration that can be recognized under
a light microscope.
 The large, uniform sized granules within an eosinophil are eosinophilic ( eosin-
loving)—they stain red-orange with acidic dyes. The granules usually do not cover or
obscure the nucleus, which most often has two lobes connected by a thick strand of
chromatin.
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24. Granular Leukocytes
 The round, variable-sized granules of a basophil are basophilic ( basic
loving)—they stain blue-purple with basic dyes. The granules commonly
obscure the nucleus, which has two lobes.
 The granules of a neutrophil are smaller, evenly distributed, and pale lilac in
color, the nucleus has two to five lobes, connected by very thin strands of
chromatin.
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25. Agranular Leukocytes
 Even though so-called agranular leukocytes possess cytoplasmic granules, the
granules are not visible under a light microscope because of their small size and poor
staining qualities.
 The nucleus of a lymphocyte is round or slightly indented and stains darkly. The
cytoplasm stains sky blue and forms a rim around the nucleus. Lymphocytes may be
as small as 6–9 m in diameter or as large as 10–14 m in diameter. As noted earlier,
there are three types of lymphocytes: T lymphocytes (T cells), B lymphocytes (B cells),
and natural killer (NK) cells.
 Monocytes are 12–20 m in diameter. The nucleus of a monocyte is usually kidney
shaped or horseshoe shaped, and the cytoplasm is blue-gray and has a foamy
appearance. The color and appearance are due to very fine azurophilic granules which
are lysosomes
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27. PLATELET
 Under the influence of the hormone thrombopoietin, myeloid stem cells
develop into megakaryocyte-colony-forming cells that in turn develop into
precursor cells called megakaryoblasts. Megakaryoblasts transform into
megakaryocytes, huge cells that splinter into 2000 to 3000 fragments.
 Each fragment, enclosed by a piece of the plasma membrane, is a platelet
(thrombocyte). Between 150,000 and 400,000 platelets are present in each
microL of blood.
 Each is disc-shaped, 2–4 m in diameter, and has many vesicles but no
nucleus. Platelets help stop blood loss from damaged blood vessels by
forming a platelet plug. Their granules also contain chemicals that, once
released, promote blood clotting. Platelets have a short life span, normally
just 5 to 9 days.
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31. HEMOSTASIS
 Hemostasis not to be confused with the very similar term
homeostasis, is a sequence of responses that stops bleeding.
 When blood vessels are damaged or ruptured, the hemostatic
response must be quick, localized to the region of damage, and
carefully controlled in order to be effective.
 Three mechanisms reduce blood loss:
1. vascular spasm,
2. platelet plug formation, and
3. blood clotting (coagulation).
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32. BLOOD CLOTTING
 Normally, blood remains in its liquid form as long as it stays within its vessels.
If it is drawn from the body, however, it thickens and forms a gel.
 Eventually, the gel separates from the liquid.
 The straw-colored liquid, called serum, is simply blood plasma minus the
clotting proteins.
 The gel is called a clot.
 It consists of a network of insoluble protein fibers called fibrin in which the
formed elements of blood are trapped.
 The process of gel formation, called clotting or coagulation, is a series of
chemical reactions that culminates in formation of fibrin threads.
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33.  Clotting involves several substances known as clotting (coagulation) factors.
 These factors include calcium ions (Ca2), several inactive enzymes that are
synthesized by hepatocytes (liver cells) and released into the bloodstream,
and various molecules associated with platelets or released by damaged
tissues.
 Most clotting factors are identified by Roman numerals that indicate the
order of their discovery (not necessarily the order of their participation in the
clotting process).
 Clotting is a complex cascade of enzymatic reactions in which each clotting
factor activates many molecules of the next one in a fixed sequence. Finally, a
large quantity of product (the insoluble protein fibrin) is formed. Clotting can
be divided into three stages
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37. BLOOD GROUPS
 The surfaces of erythrocytes contain a genetically determined assortment of
antigens composed of glycoproteins and glycolipids.
 These antigens, called agglutinogens occur in characteristic combinations.
 Based on the presence or absence of various antigens, blood is categorized
into different blood groups.
 Within a given blood group, there may be two or more different blood types.
There are at least 24 blood groups and more than 100 antigens that can be
detected on the surface of red blood cells.
 Here we discuss two major blood groups—ABO and Rh.
 Other blood groups include the Lewis, Kell, Kidd, and Duffy systems.
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38. BLOOD GROUPS
 Blood plasma usually contains antibodies called agglutinins that
react with the A or B antigens if the two are mixed.
 These are the anti-A antibody, which reacts with antigen A, and
the anti-B antibody, which reacts with antigen B.
 Although agglutinins start to appear in the blood within a few
months after birth, the reason for their presence is not clear.
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40.  Consider what happens if a person with type A blood receives a transfusion of
type B blood.
 The recipient’s blood (type A) contains A antigens on the red blood cells and
anti-B antibodies in the plasma. The donor’s blood (type B) contains B antigens
and anti-A antibodies. In this situation, two things can happen.
 First, the anti-B antibodies in the recipient’s plasma can bind to the B antigens
on the donor’s erythrocytes, causing agglutination and hemolysis of the red
blood cells.
 Second, the anti-A antibodies in the donor’s plasma can bind to the A antigens
on the recipient’s red blood cells, a less serious reaction because the donor’s
anti-A antibodies become so diluted in the recipient’s plasma that they do not
cause significant agglutination and hemolysis of the recipient’s RBCs.
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41.  People with type AB blood do not have anti-A or anti-B antibodies in their
blood plasma.
 They are sometimes called universal recipients because theoretically they can
receive blood from donors of all four blood types. They have no antibodies to
attack antigens on donated RBCs. People with type O blood have neither A nor
B antigens on their RBCs and are sometimes called universal donors because
theoretically they can donate blood to all four ABO blood types.
 Thus, blood should be carefully cross-matched or screened before transfusion.
 In about 80% of the population, soluble antigens of the ABO type appear in
saliva and other body fluids, in which case blood type can be identified from a
sample of saliva.
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43. Rh Blood Group
 The Rh blood group is so named because the antigen was discovered in the
blood of the Rhesus monkey.
 The alleles of three genes may code for the Rh antigen.
 People whose RBCs have Rh antigens are designated Rh (Rh positive); those
who lack Rh antigens are designated Rh (Rh negative).
 Normally, blood plasma does not contain anti-R antibodies. If an Rh person
receives an Rh blood transfusion, however, the immune system starts to make
anti-Rh antibodies that will remain in the blood. If a second transfusion of Rh
blood is given later, the previously formed anti-Rh antibodies will cause
agglutination and hemolysis of the RBCs in the donated blood, and a severe
reaction may occur.
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Thrombocytopenia

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