You have learnt that all living cells have to be provided with nutrients, O
and other essential substances. Also, the waste or harmful substances
produced, have to be removed continuously for healthy functioning of
tissues. It is therefore, essential to have efficient mechanisms for the
movement of these substances to the cells and from the cells. Different
groups of animals have evolved different methods for this transport. Simple
organisms like sponges and coelenterates circulate water from their
surroundings through their body cavities to facilitate the cells to exchange
these substances. More complex organisms use special fluids within their
bodies to transport such materials. Blood is the most commonly used body
fluid by most of the higher organisms including humans for this purpose.
Another body fluid, lymph, also helps in the transport of certain substances.
In this chapter, you will learn about the composition and properties of
blood and lymph (tissue fluid) and the mechanism of circulation of blood
is also explained herein.
18.1 BLOOD
Blood is a special connective tissue consisting of a fluid matrix, plasma,
and formed elements.
18.1.1 Plasma
Plasma is a straw coloured, viscous fluid constituting nearly 55 per cent of
the blood. 90-92 per cent of plasma is water and proteins contribute 6-8
per cent of it. Fibrinogen, globulins and albumins are the major proteins.
18.1 Blood
18.2 Lymph (Tissue
18.3 Circulatory
18.4 Double
18.5 Regulation of
Cardiac Activity
18.6 Disorders of
Fibrinogens are needed for clotting or coagulation of blood. Globulins
primarly are involved in defense mechanisms of the body and the albumins
help in osmotic balance. Plasma also contains small amounts of minerals
like Na
, Ca
, Mg
, Cl
, etc. Glucose, amino acids, lipids, etc., are
also present in the plasma as they are always in transit in the body. Factors
for coagulation or clotting of blood are also present in the plasma in an
inactive form. Plasma without the clotting factors is called serum.
18.1.2 Formed Elements
Erythrocytes, leucocytes and platelets are collectively called formed
elements (Figure 18.1) and they constitute nearly 45 per cent of the blood.
Erythrocytes or red blood cells (RBC) are the most abundant of all
the cells in blood. A healthy adult man has, on an average, 5 millions to
5.5 millions of RBCs mm
of blood. RBCs are formed in the red bone
marrow in the adults. RBCs are devoid of nucleus in most of the mammals
and are biconcave in shape. They have a red coloured, iron containing
complex protein called haemoglobin, hence the colour and name of these
cells. A healthy individual has 12-16 gms of haemoglobin in every
100 ml of blood. These molecules play a significant role in transport of
respiratory gases. RBCs have an average life span of 120 days after which
they are destroyed in the spleen (graveyard of RBCs).
Leucocytes are also known as white blood cells (WBC) as they are
colourless due to the lack of haemoglobin. They are nucleated and are
relatively lesser in number which averages 6000-8000 mm
of blood.
Leucocytes are generally short lived. We have two main categories of WBCs
– granulocytes and agranulocytes. Neutrophils, eosinophils and basophils
are different types of granulocytes, while lymphocytes and monocytes
are the agranulocytes. Neutrophils are the most abundant cells (60-65
per cent) of the total WBCs and basophils are the least (0.5-1 per cent)
among them. Neutrophils and monocytes (6-8 per cent) are phagocytic
cells which destroy foreign organisms entering the body. Basophils secrete
histamine, serotonin, heparin, etc., and are involved in inflammatory
reactions. Eosinophils (2-3 per cent) resist infections and are also
T lymphocyte
B lymphocyte
Figure 18.1 Diagrammatic representation of formed elements in blood
associated with allergic reactions. Lymphocytes (20-25 per cent) are of
two major types – ‘B’ and ‘T’ forms. Both B and T lymphocytes are
responsible for immune responses of the body.
Platelets also called thrombocytes, are cell fragments produced from
megakaryocytes (special cells in the bone marrow). Blood normally
contains 1,500,00-3,500,00 platelets mm
. Platelets can release a variety
of substances most of which are involved in the coagulation or clotting of
blood. A reduction in their number can lead to clotting disorders which
will lead to excessive loss of blood from the body.
18.1.3 Blood Groups
As you know, blood of human beings differ in certain aspects though it
appears to be similar. Various types of grouping of blood has been done.
Two such groupings – the ABO and Rh – are widely used all over the
world. ABO grouping
ABO grouping is based on the presence or absence of two surface antigens
(chemicals that can induce immune response) on the RBCs namely A
and B. Similarly, the plasma of different individuals contain two natural
antibodies (proteins produced in response to antigens). The distribution
of antigens and antibodies in the four groups of blood, A, B, AB and O
are given in Table 18.1. You probably know that during blood transfusion,
any blood cannot be used; the blood of a donor has to be carefully matched
with the blood of a recipient before any blood transfusion to avoid severe
problems of clumping (destruction of RBC). The donor’s compatibility is
also shown in the Table 18.1.
Blood Group Antigens on Antibodies Donor’s Group
RBCs in Plasma
A A anti-B A, O
B B anti-A B, O
AB A, B nil AB, A, B, O
O nil anti-A, B O
TABLE 18.1 Blood Groups and Donor Compatibility
From the above mentioned table it is evident that group ‘O’ blood can
be donated to persons with any other blood group and hence ‘O’ group
individuals are called ‘universal donors’. Persons with ‘AB’ group can
accept blood from persons with AB as well as the other groups of blood.
Therefore, such persons are called ‘universal recipients’.
281 Rh grouping
Another antigen, the Rh antigen similar to one present in Rhesus monkeys
(hence Rh), is also observed on the surface of RBCs of majority (nearly 80
per cent) of humans. Such individuals are called Rh positive (Rh+ve)
and those in whom this antigen is absent are called Rh negative (Rh-ve).
An Rh-ve person, if exposed to Rh+ve blood, will form specific antibodies
against the Rh antigens. Therefore, Rh group should also be matched
before transfusions. A special case of Rh incompatibility (mismatching)
has been observed between the Rh-ve blood of a pregnant mother with
Rh+ve blood of the foetus. Rh antigens of the foetus do not get exposed to
the Rh-ve blood of the mother in the first pregnancy as the two bloods are
well separated by the placenta. However, during the delivery of the first
child, there is a possibility of exposure of the maternal blood to small
amounts of the Rh+ve blood from the foetus. In such cases, the mother
starts preparing antibodies against Rh antigen in her blood. In case of
her subsequent pregnancies, the Rh antibodies from the mother (Rh-ve)
can leak into the blood of the foetus (Rh+ve) and destroy the foetal RBCs.
This could be fatal to the foetus or could cause severe anaemia and
jaundice to the baby. This condition is called erythroblastosis foetalis.
This can be avoided by administering anti-Rh antibodies to the mother
immediately after the delivery of the first child.
18.1.4 Coagulation of Blood
You know that when you cut your finger or hurt yourself, your wound
does not continue to bleed for a long time; usually the blood stops flowing
after sometime. Do you know why? Blood exhibits coagulation or clotting
in response to an injury or trauma. This is a mechanism to prevent
excessive loss of blood from the body. You would have observed a dark
reddish brown scum formed at the site of a cut or an injury over a period
of time. It is a clot or coagulam formed mainly of a network of threads
called fibrins in which dead and damaged formed elements of blood are
trapped. Fibrins are formed by the conversion of inactive fibrinogens in
the plasma by the enzyme thrombin. Thrombins, in turn are formed from
another inactive substance present in the plasma called prothrombin. An
enzyme complex, thrombokinase, is required for the above reaction. This
complex is formed by a series of linked enzymic reactions (cascade
process) involving a number of factors present in the plasma in an inactive
state. An injury or a trauma stimulates the platelets in the blood to release
certain factors which activate the mechanism of coagulation. Certain
factors released by the tissues at the site of injury also can initiate
coagulation. Calcium ions play a very important role in clotting.
As the blood passes through the capillaries in tissues, some water along
with many small water soluble substances move out into the spaces
between the cells of tissues leaving the larger proteins and most of the
formed elements in the blood vessels. This fluid released out is called the
interstitial fluid or tissue fluid. It has the same mineral distribution as
that in plasma. Exchange of nutrients, gases, etc., between the blood and
the cells always occur through this fluid. An elaborate network of vessels
called the lymphatic system collects this fluid and drains it back to the
major veins. The fluid present in the lymphatic system is called the lymph.
Lymph is a colourless fluid containing specialised lymphocytes which
are responsible for the immune responses of the body. Lymph is also an
important carrier for nutrients, hormones, etc. Fats are absorbed through
lymph in the lacteals present in the intestinal villi.
The circulatory patterns are of two types – open or closed. Open
circulatory system is present in arthropods and molluscs in which blood
pumped by the heart passes through large vessels into open spaces or
body cavities called sinuses. Annelids and chordates have a closed
circulatory system
in which the blood pumped by the heart is always
circulated through a closed network of blood vessels. This pattern is
considered to be more advantageous as the flow of fluid can be more
precisely regulated.
All vertebrates possess a muscular chambered heart. Fishes have a
2-chambered heart with an atrium and a ventricle. Amphibians and the
reptiles (except crocodiles) have a 3-chambered heart with two atria and a
single ventricle, whereas crocodiles, birds and mammals possess a
4-chambered heart with two atria and two ventricles. In fishes the heart
pumps out deoxygenated blood which is oxygenated by the gills and
supplied to the body parts from where deoxygenated blood is returned to
the heart (single circulation). In amphibians and reptiles, the left atrium
receives oxygenated blood from the gills/lungs/skin and the right atrium
gets the deoxygenated blood from other body parts. However, they get mixed
up in the single ventricle which pumps out mixed blood (incomplete double
circulation). In birds and mammals, oxygenated and deoxygenated blood
received by the left and right atria respectively passes on to the ventricles of
the same sides. The ventricles pump it out without any mixing up, i.e., two
separate circulatory pathways are present in these organisms, hence, these
animals have double circulation. Let us study the human circulatory