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BLOOD

The blood circulation was famously described by William Harvey in 1628.

This chapter discusses the nature of blood, the fluid component of the cardiovascular system. In

adults, circulating blood provides each of the body’s roughly 75 trillion cells a source of nutrients

and oxygen, and a way of removing wastes. The blood also transports specialized cells that defend

tissues from infection and disease.

Blood: A specialized fluid connective tissue that contains cells suspended in a fluid matrix, it

provides communication between different parts of the body and with external environment.

Properties:

It comprises about 1/11 portion or 8% of total body weight.

It is alkaline fluid i.e. pH is 7.35-7.45.

It is sticky, viscous and metallic in taste.

It is red in color due to presence of hemoglobin.

Temperature of blood is slightly higher than body temperature i.e. 370c.

Its specific gravity is 1.050-1.060.

Its density is 1060 kg/m3 very close to water’s density of 1000kg/m3.

Functions: blood performs many functions in the body. All those functions are categorized under

three groups. Those are transportation, regulation and protection functions.

Transportation functions:

Transportation of oxygen from lungs to all parts of the body.

Transportation of nutrients from gastrointestinal tract (GIT) to all parts of the body.

Transportation of hormones from endocrine glands to their target sites.

Transportation of carbon dioxide from tissues to lungs.

Transportation of metabolic waste materials to their excretory organs like kidneys,

lungs, lungs etc…

Regulation functions:

Regulation of blood pH with the help of electrolytes and buffers.

Regulation of body temperature.

Regulation of normal blood volume.

Protection functions:

Coagulation: preventing blood loss, when a blood vessel was damaged, platelets and

plasma proteins initiates clot formation, halting blood loss.

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Preventing infection: including circulation of W.B.C and detection of foreign

materials by anti-bodies.

Composition of blood:

Blood is composed of a straw colored transparent fluid called plasma, in which different

types of cells are suspended. Plasma constitutes about 55% and cells about 45% of total blood

volume.

Blood = Blood cells + Plasma.

Blood cells = Red bllod cells + White bllod cells + Platelets.

Plasma = serum + Clotting factors.

Serum is defined as plasma without clotting factors.

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Blood plasma:

Blood plasma is a straw colored sticky fluid. Althogh it is mostly water (about 90%)

plasma contains over 100 different dissolved solutes including nutrients, gases, hormones,

vitamins, wastes etc…

S.no Parameter Description

1 water It occupies 90% of plasma volume

It acts as dissolving and suspending mediumfor solutes of blood

and absorbs heat.

2 solutes These occupy 10% of plasma volume..

Proteins These occupy 8% of plasma volume.

Albumin 60% of plasma proteins.

Exerts osmatic pressure to maintain water balance between

blood and tissues.

Globulin 36% of plasma proteins.

Pesent in three forms α,β and γ-globulins.

α,β-globulins are produced by liver and these involved in

transportation. Transport globulins bind small ions,hormones,

fatty acids, thyroid hormones, some steroid hormones, and other

substances.

γ-globulins involved in release of antibodies from plasma cells

during immune response.

Antibodies, also called immunoglobulins, attack foreign

proteins and pathogens.

Clotting factors 4% of plasma proteins.

These includes fibrinogen, prothrombin.

Other protein part includes several enzymes, hormones, antibodies and complement.

Non-protein

nitrogenous

substances

These are byproducts of cellular metabolism i.e. urea, uric acid,

creatinine and ammonium salts.

Neutrients these includes glucose, amino acids, fatty acids, glycerol,

triglycerides, cholesterol and vitamins.

Electrolytes Cations: sodium, potassium, calcium, magnesium etc…

Anions: chorides, phosphates, sulphates, bicarbonates etc…

These maintain osmatic pressure and blood pH.

Respiratory

gases

Oxygen and carbon dioxide.

Pigments Bilirubin and biliverdin.

The components of plasma include plasma proteins, other solutes, and water.

Plasma Proteins: Plasma contains significant quantities of dissolved proteins, namely albumins,

globulins, and fibrinogen. These three types make up more than 99 percent of the plasma

proteins. The remainder consists of circulating enzymes, hormones, and prohormones.

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Albumins: Albumins make up the majority of the plasma proteins. Albumins are also important

for transporting fatty acids, thyroid hormones, some steroid hormones, and other substances.

Globulins: Globulins comprise the second most-abundant proteins in plasma. Transport

globulins bind small ions, hormones, and compounds that might otherwise be removed by the

kidneys or that have very low solubility in water. Important examples of transport globulins

include the following:

Hormone-binding proteins, which provide a reserve of hormones in the bloodstream.

Examples include thyroid-binding globulin and transthyretin, which transport thyroid

hormones, and transcortin, which transports ACTH.

Metalloproteins, which transport metal ions. Transferrin, for example, is a metalloprotein

that transports iron (Fe2+).

Apolipoproteins, which carry triglycerides and other lipids in blood. When bound to

lipids, an apolipoprotein becomes a lipoprotein.

Steroid-binding proteins, which transport steroid hormones in blood. For example,

testosterone-binding globulin (TeBG) binds and transports testosterone.

Fibrinogen: The third major type of plasma protein, fibrinogen, functions in clotting by the

conversion of fibrinogen (a soluble protein) to fibrin (an insoluble protein).

Other Plasma Proteins: The remaining 1 percent of plasma proteins is composed of specialized

proteins whose levels vary widely. Peptide hormones including insulin, prolactin (PRL), and the

glycoproteins thyroid-stimulating hormone (TSH), follicle-stimulating hormone (FSH), and

luteinizing hormone (LH) are normally present in circulating blood.

Origins of the Plasma Proteins: The liver synthesizes and releases more than 90 percent of the

plasma proteins, including all albumins and fibrinogen, most globulins, and various

prohormones. Because the liver is the primary source of plasma proteins, liver disorders can alter

the composition and functional properties of blood. For example, some forms of liver disease can

lead to uncontrolled bleeding due to the inadequate synthesis of fibrinogen and other proteins

involved in clotting.

BLOOD CELLS:

Red blood cells(R.B.C) erythrocytes or bags of haemoglobin

The most abundant blood cells are the red blood cells (RBCs), which account for 99.9 percent

of the blood cells. These cells give whole blood its deep red color because they contain the red

pigment hemoglobin. A single drop of whole blood contains approximately 260 million RBCs,

and the blood of an average adult has 25 trillion RBCs. RBCs account for roughly one-third of all

cells in the human body.

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Red blood cell is a circular, biconcave, disc shaped, non-nucleated cell. It contains respiratory

pigment called haemoglobin. These are 7.5µ in diameter, 2.0µ thickness at edges and 1.7µ

thickness in center. R.B.C’S does not contain nucleus, mitochondria, ribosomes, endoplasmic

reticulum and centriole. Due to its biconcave shape, it has following advantages:-

Giving Each RBC a Large Surface-Area-to-Volume Ratio: Each RBC carries oxygen

bound to intracellular proteins. That oxygen must be absorbed or released quickly as the

RBC passes through the capillaries of the lungs or peripheral tissues. The greater the

surface area per unit volume, the faster the exchange between the RBC’s interior and the

surrounding plasma. The total surface area of all the RBCs in the blood of a typical adult is

about 3800 square meters, (nearly 4600 square yards), some 2000 times the total surface

area of the body.

Enabling RBCs to Bend and Flex: When Entering Small Capillaries and Branches. Red

blood cells are very flexible. By changing shape, individual RBCs can squeeze through

capillaries as narrow as 4 µm.

Enabling RBCs to Form Stacks: These stacks, known as rouleaux, form and dissociate

repeatedly without affecting the cells involved. An entire stack can pass along a blood

vessel that is only slightly larger than the diameter of a single RBC, whereas individual

cells would bump the walls, bang together, and form logjams that could restrict or prevent

blood flow.

Normal count: Adult male - 5.0 millions/cu.mm

Female - 4.5 millions/cu.mm

Infant - 6-7 millions/cu.mm

Foetus - 8.0 millions/cu.mm

High altitude

Exercise

High temparature

Excitement

Injection of adrenaline

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Variations in R.B.C count:

Diurnal variation about 5% variation change occurs in 24 hours. During sleep lowest count,

then gradually it increases and become maximum in the evening. Other factors which increase

erythrocyte count are shown in the side diagram.

Composition of R.B.C: R.B.C’s composed of 60-70% water and 30-40% solids. In 30-40% of

solids, haemoglobin constitutes about 97% and other components like organic, inorganic

phospholipids and stroma proteins. Stroma proteins includes sprctrin, actomyocin and lecithin.

Spectrin-causes contraction of R.B.C.

Actomyocin-useful to maintain biconcave structure of R.B.C.

Lecithin it is an antioxidant, useful to protect body from H2O2.

The process of formation of R.B.C’s is known as erythropoiesis.

Life span of R.B.C’S: Because they lack nuclei and ribosomes, circulating mammalian RBCs

cannot divide or synthesize structural proteins or enzymes. As a result, the RBCs cannot perform

repairs, so their life span is relatively short normally less than 120 days.

Fate of R.B.C’S: After completing its life span R.B.C’S enzymes lose their capacity to protect

R.B.C’S against certain damages and disintegration. At last R.B.C’S become rigid, fragile and

spherical. When these R.B.C’S pass through the spleen, due its peculiar structure of splenchic

R.B.C’S gets rupture and broken down into fragments. Reticulo endothelial system acts on these

fragments and they release hemoglobin. This Hb is further broken down and converts into

stercobilinogen and urobilinogen, which are excreted out. Heme part of Hb converts into ferritin

and haemosiderin and store in liver, release into blood when body require iron as transferrin.

Functions of erythrocytes:

Carry O2 and CO2 .

Acid base balance.

Maintanence of viscosity of blood.

Provide specific gravity to the blood.

Carrier of blood group factors (antigens)on their surface membrane(hence responsible

for blood group reactions).

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White blood cells( leucocytes)

Leukocytes are the largest blood cells and they account for about 1% of the blood

volume. These cells have an important function in defending the body against microbes and

other foreign material. Depending on the presence and absence of granules in their cytoplasm

these are classified into granulocytes and agranulocytes.

Granulocytes – All granulocytes have mullobed nuclei in their cytoplasm. Their

names represent the dyes they take up on staining.

o Eosinophils take up the red acid dye, eosin.

o Basophils take up alkaline methylene blue

o Neutrophils takes up both the dyes and appear in purple color.

Agranulocytes – These are leukocytes without granules in their cytoplasm and with a

large nucleus. These are two types: Monocytes and Lymphocytes.

The neutrophils produces a trypsin-like enzyme with which they digest the bacteria

and dead tissue. Due to this, the dead tissue in an inflamatory area becomes liquefied

and the so called PUS is formed.

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Platelets:

SHAPE: platelets are small irregularly rounded oval cells or rod shaped bodies.

SIZE: 2-4µ in diameter, but becoming much larger in active phase.

NORMAL RANGE: 1,50,000-4,00,000/ cu.mm.

LIFE SPAN: 7-14 days.

FUNCTION: blood clotting to prevent blood loss from damaged blood vessels.

Haemoglobin(Hb): (mol.wt: 64,000daltons)

Haemoglobin present inside the RBCs. Haemoglobin consists of four pyrrole rings

which are linked up with methine bridge(-CH=), central ferrous ion and this heme moity

binds with polypeptide (globin).

Structure and chemistry of haemoglobin:

Chemically haemoglobin is a conjugated metalloprotein. On break down Hb produces

heme and globin.

Neutrophil Eosinophil Basophil Lymphocyte Monocyte

Size 12-14µm 16µm 14-16µm 9-16µm 16-20µm

Differential

W.B.C count

40-75% 1-6% 0.5% 20-45% 2-6%

Description Nucleus is lobed

and the lobes

vary from2-5.

Fine pink and

violet granules in

the cytoplasm

Nucleus is

usually

bilobed.

Numerous

coarse dark

pink/

reddish

granules in

the

cytoplasm.

Nucleus is bilobed,

unclear, masked

by granules.

Few or large

number of dark

blue coarse

granules present in

the cytoplasm and

on the nucleus

with unclear cell

boarder.

Single lobed

nucleus fills

the entire cell.

A thin rim of

bluish

cytoplasm

without

granules.

Single bean or

kidney shaped

eccentric nucleus

placed with

chromatin

reticulum.

Abundant grayish

white cytoplasm

without granules.

Life span 6 hours to a few

days

8-12 days few hours to few

days

Years for

memory cells,

weeks for all

else

Hours to days

Function These attack and

destroy invading

bacteria, viruses,

and other

injurious agents.

these can attack

and destroy

bacteria even in

the circulating

blood.(phagocyto

sis)

Eosinophils

are weak

phagocytes,

and they

exhibit

chemotaxis

in allergic

reactions.

Secrete histamine,

heparin and

smaller quantities

of bradykinin and

serotonin. which

prevents intra-

vascular clotting.

mount

immune

response by

releasing

antibodies

Phagocytize large

quantities of

bacteria, viruses,

necrotic tissue, or

other foreign

particles in the

tissue.phagocytosi

s

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Heme:-

It is an iron containing compound.

It belongs to the class of protoporphyrins.

The iron present in the Hb is in ferrous(Fe++) state and even after combination with oxygen

it maintains ferrous state.

On break down of old RBCs heme will form and this will store in the liver, in the form of

ferritin. When ever body requires iron content, liver releases iron in the form of

transferrin.

Globin:

It is a simple protein belongs to class globulins.

On further metabolism in liver it will convert into amino acids and useful as nutrients.

The RBCs of an embryo or a fetus contain a different form of hemoglobin, known as fetal

hemoglobin, which binds oxygen more readily than does the hemoglobin of adults. For this

reason, a developing fetus can “steal” oxygen from the maternal bloodstream at the placenta.

The conversion from fetal hemoglobin to the adult form begins shortly before birth and

continues over the next year. The production of fetal hemoglobin can be stimulated in adults

by the administration of drugs such as hydroxyurea or butyrate. This is one method of

treatment for conditions, such as sickle cell anemia or thalassemia, that result from the

production of abnormal forms of adult hemoglobin.

Normal ranges of haemoglobin:

Males : 14-17 gms/dl

Females : 12-16 gms/dl

14.5grams/100ml was considered as 100% haemoglobin.

Functions of haemoglobin:

Transportation of oxygen: it picks up oxygen in alveoli of lungs and releases in the

tissues, where ever hypoxia conditions are present.

Hb + O2 HbO2

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Heamoglobin binds oxygen and forms oxyhemoglobin (HbO2) in the lungs and this is

transported through blood to all parts of the body.

Transportation of carbon dioxide: The carbon dioxide (CO2) formed at the tissues due

to biochemical reactions binds (only 20%) with Hb and forms carboxy hemoglobin.

Hb + CO2 HbCO2

It acts as a blood buffer.

In some diseased states the iron of hemoglobin is oxidized and converted into ferric

state (Fe+++), this condition is known as Methymoglobinemia. In this state oxygen can’t be

released easily from such hemoglobin molecules. Normally the presence of enzyme

methymoglobin reductase keeps iron in ferrous state.

Erythropoiesis

Erythropoiesis is the process by which red blood cells (erythrocytes) are produced. It is

stimulated by decreased O2 delivery to the kidneys, which then secrete the

hormone erythropoietin. This activates erythropoiesis in the hemopoietic tissues of bone

marrow.

It is defined as the development of

erythrocytes from haemocytoblast (stem cell).

When there is decreased levels of RBCs in the

blood, oxygen carrying capacity of blood will

decrease.

Gross changes occurring in the erythropoiesis:

Haemocytoblast is large cell (15-20µ),

having nucleus and it lacks hemoglobin.

RBCs are small cells (when compared with

haemocytoblast) having size 7.5µ, non-

nucleated cells having hemoglobin.

So, major changes have to occur in the

erythropoiesis are:

Cell size has to reduce, from 15-20µ to

7.5µm.

Nucleus has to disappear

Staining reaction of the cytoplasm

changes from basophilic to acidophilic

(this is because of the decrease in the

amount of RNA and DNA);

Hemoglobin has to develop.

First two changes that means, decrease in cell size and disappear of nucleus is done by

the action of folic acid, Vitamin-B12 and nicotinic acid. Development of hemoglobin is done

with the help of iron, Vitamin-B12 and globin.

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The process of erythropoiesis takes place in 6-9 days. Erythropoiesis starts in third

week of intrauterine fetus. This was done in blood vessel walls. Hence it is known as

intravascular erythropoiesis. After third month and up to fifth month erythropoiesis will

occur in spleen and liver. Then this process will occur in red bone marrow of long bones and

flat bones.

Blood clotting: (Hemostasis)

Normally, blood flows smoothly over the lining of blood vessel (endothelium) walls.

But if a blood vessel wall breaks, a series of reactions is set in motion to accomplish

hemostasis or stoppage of blood flow. This response, which is fast and localized, involves

many substances normally present in plasma, as well as some are released by platelets and

injured tissue cells.

The process of hemostasis (hemo, blood _ stasis, halt), the stopping of bleeding, halts

the loss of blood through the walls of damaged vessels. At the same time, it establishes a

framework for tissue repairs. Hemostasis consists of three phases:

1. Vascular phase,

2. Platelet phase, and

3. Coagulation phase.

The Vascular Phase

Cutting the wall of a blood vessel triggers a contraction in the smooth muscle fibers of the

vessel wall. This local contraction of the vessel is a vascular spasm, which decreases the

diameter of the vessel at the site of injury. Such a constriction can slow or even stop the loss

of blood through the wall of a small vessel. The vascular spasm lasts about 30 minutes, a

period called the vascular phase of hemostasis.

During the vascular phase, changes occur in the endothelium of the vessel at the injury site:

• The Endothelial Cells Contract and Expose the Underlying Basement Membrane to the

Bloodstream.

• The Endothelial Cells Begin Releasing Chemical Factors and Local Hormones. Endothelial

cells also release endothelins, peptide hormones that (1) stimulate smooth muscle contraction

and promote vascular spasms and (2) stimulate the division of endothelial cells, smooth

muscle cells, and fibroblasts to accelerate the repair process.

The Platelet Phase

The attachment of platelets to the basement membrane, and to exposed collagen fibers marks

the start of the platelet phase of hemostasis. The attachment of platelets to exposed surfaces

is called platelet adhesion. As more and more platelets arrive, they begin sticking to one

another as well. This process, called platelet aggregation, forms a platelet plug that may

close the break in the vessel wall if the damage is not severe or the vessel is relatively small.

Platelet aggregation begins within 15 seconds after an injury occurs.

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As they arrive at the injury site, the platelets become activated. The first sign of activation is

that they become more spherical and develop cytoplasmic processes that extend toward

nearby platelets. At this time, the platelets begin releasing a wide variety of compounds,

including (1) adenosine diphosphate (ADP), which stimulates platelet aggregation and

secretion; (2) thromboxane A2 and serotonin, which stimulate vascular spasms; (3) clotting

factors, proteins that play a role in blood clotting; (4) platelet-derived growth factor (PDGF),

a peptide that promotes vessel repair; and (5) calcium ions, which are required for platelet

aggregation and in several steps in the clotting process.

The Coagulation Phase

The vascular and platelet phases begin within a few seconds after the injury. The coagulation

phase does not start until 30 seconds or more after the vessel has been damaged.

Clotting Factors

Normal blood clotting depends on the presence of clotting factors in the plasma. Important

clotting factors include Ca2+and 11 different proteins.

Clotting Factors

Factor Nature Name Pathway

I Protein Fibrinogen Common

II Protein Prothrombin Common

III Lipoprotein Tissue factor Extrinsic

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IV Ion Calcium ions Entire process

V Protein Proaccelerin Extrinsic and intrinsic

VII Protein Proconvertin Extrinsic

VIII Protein factor Antihemophilic factor Intrinsic

IX Protein factor Plasma thromboplastin Intrinsic

X Protein Stuart-Power factor Extrinsic and intrinsic

XI Protein antecedent Plasma thromboplastin Intrinsic

XII Protein Hageman factor Intrinsic

XIII Protein factor Fibrin-stabilizing factor Stabilizes fibrin

Many of the proteins are proenzymes, which, when converted to active enzymes, direct

essential reactions in the clotting response. The activation of one proenzyme commonly

creates an enzyme that activates a second proenzyme, and so on in a chain reaction, or

cascade. During the coagulation phase, enzymes and proenzymes interact.

Above figure shows the cascades involved in the extrinsic, intrinsic, and common pathways.

The extrinsic pathway begins outside the bloodstream, in the vessel wall; the intrinsic

pathway begins inside the bloodstream, with the activation of a circulating proenzyme. These

two pathways converge at the common pathway.

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The Extrinsic Pathway

The extrinsic pathway begins with the release of Factor III, also known as tissue factor

(TF), by damaged endothelial cells or peripheral tissues. The greater the damage, the more

tissue factor is released and the faster clotting occurs. Tissue factor then combines with Ca2+

and another clotting factor (Factor VII) to form an enzyme complex capable of activating

Factor X, the first step in the common pathway.

The Intrinsic Pathway

The intrinsic pathway begins with the activation of proenzymes (usually Factor XII)

exposed to collagen fibers at the injury site (or to a glass surface of a slide or collection tube).

This pathway proceeds with the assistance of PF-3, a platelet factor released by aggregating

platelets. Platelets also release a variety of other factors that accelerate the reactions of the

intrinsic pathway. After a series of linked reactions, activated Factors VIII and IX combine to

form an enzyme complex capable of activating Factor X.

The Common Pathway

The common pathway begins when enzymes from either the extrinsic or intrinsic pathway

activate Factor X, forming the enzyme prothrombinase. Prothrombinase converts the

proenzyme prothrombin into the enzyme thrombin (THROM-bin). Thrombin then completes

the clotting process by converting fibrinogen, a soluble plasma protein, to insoluble strands of

fibrin.

Interactions among the Pathways When a blood vessel is damaged, both the extrinsic and the

intrinsic pathways respond. The extrinsic pathway is shorter and faster than the intrinsic

pathway, and it is usually the first to initiate clotting. In essence, the extrinsic pathway

produces a small amount of thrombin very quickly. This quick patch is reinforced by the

intrinsic pathway, which later produces more thrombin.

The time required to complete clot formation varies with the site and the nature of the injury.

In tests of the clotting system, blood held in fine glass tubes normally clots in 8–18 minutes

(the coagulation time), and a small puncture wound typically stops bleeding in 1–4 minutes

(the bleeding time).

Calcium Ions, Vitamin K, and Blood Clotting

Calcium ions and vitamin K affect almost every aspect of the clotting process. All three

pathways (intrinsic, extrinsic, and common) require Ca2+, so any disorder that lowers plasma

Ca2+ concentrations will impair blood clotting.

Adequate amounts of vitamin K must be present for the liver to synthesize four of the clotting

factors, including prothrombin. Vitamin K is a fat-soluble vitamin, present in green

vegetables, grain, and organ meats, that is absorbed with dietary lipids. Roughly half of the

daily requirement is obtained from the diet, and the other half is manufactured by bacteria in

the large intestine. A diet inadequate in fats or in vitamin K, or a disorder that affects fat

digestion and absorption (such as problems with bile production), or prolonged use of

antibiotics that kill normal intestinal bacteria may lead to a vitamin K deficiency. This

condition will cause the eventual breakdown of the common pathway due to a lack of clotting

factors and, ultimately, deactivation of the entire clotting system.

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BLOOD GROUPING:

When blood from one person is transfused to another person, it must be ensured

that donar’s blood matches perfectly with the blood of recipient otherwise there will be

mismatching and may lead to death. Blood grouping was best demonstrated by two systems.

Those are

1. ABO system(1900)

2. Rhesus system(1940)

ABO system:

RBC plasma membranes, antigens at their external surfaces, which identify each of us as

unique from all others. One person’s RBC proteins may be recognized as foreign if

transfused into someone with a different red blood cell type, and the transfused cells may be

agglutinated (clumped together) and destroyed. Since these RBC antigens promote

agglutination, they are more specifically called agglutinogens.

The presence or absence of each antigen allows each person’s blood cells to be classified into

several different blood groups. ABO Blood Groups As shown in Table, the ABO blood

groups are based on the presence or absence of two agglutinogens, Antigen-A and Antigen-

B.

O blood group, which is not having Anti-A and Anti-B. It is the most common ABO

blood group.

AB blood group with both antigens is least prevalent.

The presence of either the A or the B agglutinogen results in group A or B, respectively.

Unique to the ABO blood groups is the presence of antibodies called agglutinins in the

plasma. The agglutinins act against RBCs carrying ABO antigens that are not present on a

person’s own red blood cells. A newborn lacks these antibodies, but they begin to appear in

the plasma within two months and reach adult levels between 8 and 10 years of age. As

indicated in Table, a person with neither the A nor the B antigen (group O) possesses both

anti-A and anti-B antibodies, also called α or anti-A and β or anti-B agglutinins respectively.

Those with group A blood have anti-B antibodies, while those with group B have anti-A

antibodies. Neither antibody is produced by AB individuals.

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Rhesus system: The Rh blood typing system is so named because one Rh antigen

(agglutinogen D) was originally identified in rhesus monkeys. Later, the same antigen was

discovered in humans. Most Americans (about 85%) are Rh+ (Rh positive), meaning that

their RBCs carry the D antigen. As a rule, a person’s ABO and Rh blood groups are reported

together, for example, O+, A–, and so on.

Unlike the ABO system antibodies, anti-Rh antibodies are not spontaneously formed in the

blood of Rh– (Rh negative) individuals. However, if an Rh– person receives Rh+ blood, the

immune system becomes sensitized and begins producing anti-Rh antibodies against the

foreign antigen soon after the transfusion. Hemolysis does not occur after the first such

transfusion because it takes time for the body to react and start making antibodies. But the

second time, and every time thereafter, a typical transfusion reaction occurs in which the

recipient’s antibodies attack and rupture the donor RBCs.