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Circulation in Animals
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Introduction
Materials formed in one part of the body have to be taken up to other parts where they are
needed or to be got rid of. This is an essential requirement of most animals. This function is
performed by the body fluids.
The movement of the body fluids i.e. blood and lymph from one part of the body to the other
parts is called circulation. The circulation of blood was first discovered by William Harvey in
1628.
In lower organisms like the one celled protozoans e.g., amoeba and paramecium, the cell is in
direct contact with the surrounding water and there is direct exchange of materials between the
cell and the water. So there is no need for a circulatory system. In multicellular higher organisms
there is no direct supply of the essential materials to all parts of the body and removal of wastes
from the body cells. So there is a need for circulatory system.
Function
The functions performed by the circulatory system in all animals are, Transport of nutrients like
glucose, amino acids, vitamins, minerals.
The functions performed by the circulatory system in all animals are
1) Transport of nutrients like glucose, amino acids, vitamins, minerals
2) Transport of oxygen from the organs of respiration to the body cells.
3) Transport of carbon dioxide from the body cells to the exterior.
4) Transportation of nitrogenous wastes from the body cells to the organs of excretion.
5) Transportation of hormones from the endocrine glands to their specific target organs.
6) Transportation of water, hydrogen ions and chemical substances all over the body for their
uniform distribution.
7) Transportation of heat from one region to another for homoeothermy.
8) Transportation of intermediate metabolites from one part of the body to another.
In the closed type of circulatory system, the blood remains inside the blood vessels and does not come
out. The blood flows from arteries to veins through small blood vessels called capillaries. The closed type
of circulatory system occurs in most of the Annelids, Cephalopods and Vertebrates including man.
Animals possess two type of circulatory system. They are
a) Open type and
b) Closed type
Open Type
In this type of circulatory system the blood may be present in the blood vessels for some time but
finally it comes out of the blood vessels. The internal organs are directly bathed in blood. The
blood flows from the heart into the arteries. The artery open into large spaces called sinuses.
From the sinus the blood is carried by the veins to the heart. There are no inter connecting
vessels or capillaries between the arteries and the veins, as the blood comes out of blood vessels.
This type of circulatory system is called open type. It occurs in annelids like leeches, arthropods,
most of the molluscs and ascidians.
Closed Type
In the closed type of circulatory system, the blood remains inside the blood vessels and does not
come out. The blood flows from arteries to veins through small blood vessels called capillaries.
The closed type of circulatory system occurs in most of the Annelids, Cephalopods and
Vertebrates including man.
Characteristics of Open System
1) The blood flows at a very low velocity and at low pressure due to the absence of smooth
muscles.
2) There is direct exchange of materials between the cells and the blood because of the direct
contact between them.
3) The respiratory pigment, when present, is dissolved in the plasma of the blood and there are
no red corpuscles.
Characteristics of Closed System
1) The speed of circulation is more rapid due to the presence of muscular and contractile blood
vessels.
2) The supply and removal of materials to and from the tissues by the blood is enhanced, thereby
increasing the efficiency of circulation.
3) The volume of blood flowing through a tissue or organ is regulated by the contraction and
relaxation of the muscles of the blood vessels.
Open Circulation System in Cockroach
The heart of the cockroach is elongated, thick, muscular, tubular and 13-chambered. It lies in
the pericardial sinus of the haemocoel. Each chamber of the heart receives oxygenated blood
from the dorsal sinus through one pair of slit like openings called ostia. The heart contracts in a
postero anterior direction and the blood also flows posteroanteriorly. The alary muscles are
responsible for the circulation of blood. The first chamber leads into an aorta, which opens in the
head sinuses which are connected to the pericardial sinus through perineural and perivisceral
sinuses.
Closed Circulatory System in Humans
The organisms have a thick body wall to prevent the evaporation of water, so exchange of
materials between the body cells and the environment by diffusion is not possible.
The closed circulatory system can be represented as follows.
Diagram showing inter-relationship between artery, vein and capillary
The closed blood vascular system is a very well developed system
This is so because,
a) The organisms have a thick body wall to prevent the evaporation of water, so exchange of
materials between the body cells and the environment by diffusion is not possible.
b) The organisms have higher metabolic rate and need a greater supply of nutrients and oxygen.
They also need a more quick removal of wastes and carbon dioxide.
c) External changes of temperature.
Components of Circulatory System
These are hollow, tubular vessels which conduct the blood from the heart to the tissues and
from the tissues to the heart. There are 3 type of blood vessels, arteries, capillaries and veins.
The circulatory system consists of 3 components:
1. Blood vessels
2. Heart
3. Blood
Blood Vessels
These are hollow, tubular vessels which conduct the blood from the heart to the tissues and from
the tissues to the heart. There are 3 type of blood vessels, arteries, capillaries and veins.
Arteries
Arteries are vessels which carry blood away from the heart. They are thick walled vessels. They
are elastic in nature, have a narrow lumen, are deep seated in the body parts and have no valves
in them. The blood flowing through the arteries carry oxygenated blood in them, except the
pulmonary artery which carries deoxygenated blood to the lungs. The average diameter of a an
artery is 500 mm.
The arterial wall has three coats namely tunica interna, tunica media and tunica adventitia. The
tunica interna is composed of endothelial cells. The tunica media is the middle layer and is
compound of elastic and simple muscle fibers. The tunica adventitia is composed of white
connective tissue and elastic fibers. The smallest branches of an artery are called arterioles. The
walls of the arteries are supplied with a set of blood vessels called vaso vasonum.
Capillaries
The arterioles further divide into smaller vessels called meta arterioles which have a diameter of
70 mm which in turn divide into capillaries. They are the thinnest blood vessels and their walls
are formed of a single layer of endothelial cells. These form a connective link between the
arterioles and the veins. They were discovered by Marcello Malpighi in 1661. The endothelium
allows the exchange of materials like the nutrients, CO2, O2, hormones and waste products
between the blood and the surrounding tissue cells through the tissue fluid.
T. S. of artery and vein
Capillary bed showing arterioles and capillaries
Veins
The arteriole capillaries join to form venous capillaries which then join to form the venules and
veins. The veins are thin walled vessels. The walls of the veins have all the three layers as in an
artery, but they are comparatively thinner, less elastic and less muscular. The pressure of the
blood in a vein is low and the speed is slow. It has a wide lumen and is superficial.
Action of Semi-lunar Valve in a Vein
Valves are present in most of the veins to prevent the backward flow of blood. Veins are also
supplied with vasa vasorum, the nutrient blood vessels.
Blood
The blood is a fluid connective tissue. It is opaque and somewhat sticky. It is more viscous than
distilled water with a viscosity of 4.7 and slightly alkaline in nature, Oxygenated blood is bright
red while the deoxygenated blood is purple coloured.
Blood flows in the blood vessels due to the pumping action of the heart. It forms about 6-10% of
the body weight and about 30-35% of the ECF. Adult humans contain about 5 litres of blood.
The blood is formed of 2 parts,
a) Plasma
b) Blood corpuscles
Plasma
It forms about 55-60% of the blood volume. It is the liquid portion of blood. It contains 90%
water and the remaining 10% is formed of organic and inorganic substances. These materials
include proteins, glucose, nitrogenous wastes, enzymes, hormones and minerals.
About 200-300 gm of plasma proteins are present in the total volume of blood. They are serum
albumin, serum globulin, fibrinogen and prothrombin.
Blood Corpuscles
Red and white cells from the human blood
The blood has 3 types of blood corpuscles.
a) Erythrocytes or Red blood corpuscles or R.B.Cs.
b) Leucocytes or White blood corpuscles or W.B.Cs.
c) Blood platelets or thrombocytes.
Erythrocytes
(erythros = red; cyton = cell) the erythrocytes are oval, biconvex and nucleated in all vertebrates
except mammals. In mammals the erythrocytes are circular, biconcave and non-nucleated. The
total number of RBCs per microlitre is called RBC count. Normal RBC count in an adult human
male is 5 - 5.5 million per cubic millimeter of blood while in an adult female, it is 4.5 - 5 million
per cubic millimetre. The instrument used to determine RBC count is called haemocytometer.
Each RBC is composed of an envelope and a spongy elastic substance called stroma. Inside the
meshes of the stroma is present the iron pigment haemoglobin. Haemoglobin is a conjugated
protein composed of a protein part globin and a non protein pigment haem. Haem is an iron
containing porphyrin.
4 molecules of haem + globin
Haemoglobin.
On combining with oxygen, haemoglobin is converted to oxyhaemoglobin (H6O2).
The erythrocytes develop from the bone marrow. The life span of RBC is around 120 days. As
the erythrocyte becomes older and older, the cell membrane becomes fragile and the old RBCs
are disintegrated in the spleen which is said to be the graveyard for red cells.
The haemoglobin is broken down into 2 important components, denatured globulin and iron. The
iron is stored in the liver and is used to make new haemoglobin. The globin part is converted into
bilirubin a yellow pigment and biliverdin a green pigment used in colouring the bile.
A scanning electron micrograph of red blood cells of a mammal
Components of Circulatory System (Contd.)
The WBCs are larger in size than the RBCs but much lesser in number. They are different
from the erythrocytes in the following aspects.
The WBCs are larger in size than the RBCs but much lesser in number. They are different from
the erythrocytes in the following aspects.
a) They have no haemoglobin
b) They are bigger in size
c) They are nucleated and amoeboid
d) They have a longer life span
e) There are several varieties with different functions.
A blood smear showing red blood cells and three types of white cells also called as leucocytes
(leucos = colourless, cyton = cell)
The total number of leucocytes per microlitre is called the total leucocyte count (TLC) and is of
diagnostic value in many diseases. The number of WBCs in a healthy person ranges from 5,000
to 10,000 per cubic millimetre. The rise in WBC count is called leucocytosis while a fall in WBC
count is called leucopoenia, Leukemia is a pathological condition (blood cancer). The formation
of WBC occurs in bone marrow, lymph nodes, thymus, spleen and is called leucopoiesis.
There are several varieties of leucocytes. They are of 2 main types.
a) granulocytes
b) agranulocytes
Granulocytes
These have a granular cytoplasm and lobed nucleus. These are of 3 types on the basis of the
shapes of their nuclei and the staining reactions of their granules.
Neutrophils
These are also called as scavenger cells. They form about 79% of the total leucocyte count. It has
granular cytoplasm and a multilobed nucleus (2-7 lobed). They show amoeboid movement Their
life span is about 10-12 hours. They engulf pathogens by phagocytosis. When neutrophils die it
is known as pus. A localized accumulation of pus is called an abscess.
Basophils
They have a s shape or 2-3 lobed nucleus and a coarse cytoplasmic granules which take up basic
stains like methylene blue. They secrete heparin and histamine and so play an important role in
anticoagulation and formation of ground substance.
Eosinophils
These are slightly larger in size. The cytoplasm contains coarse granules which stain with acid
dyes like eosin. The nucleus is 2 or 3 lobed. They increase in number during allergic reactions.
They bring about distraction and detoxification of toxins.
Agranulocytes
These are non-granular white blood cells that contain non - lobulated nuclei. They are produced
in the lymph nodes and spleen and are of 2 types lymphocytes and monocytes.
Lymphocytes
These form about 25-30% of the total leucocyte count. They have a large nucleus. The cytoplasm
is basophilic without any granules. They are divided into 2 groups namely small lymphocytes
and large lymphocytes. These are formed in the thymus and lymphoid tissues like spleen, tonsils,
lymph nodes from the precursor cells called lymphoblasts.
Monocytes
These are the largest sized leucocytes forming about 5% of the total leucocyte count. They have
a oval, kidney or horse shaped nucleus. They are motile and have the power of engulfing
bacteria. These also differentiate into macrophages or scavenger cells which remove damaged
and dead cells to clean the body. They are usually formed in the lymph nodes and the spleen
from precursor cells called monoblasts.
Different Blood Corpuscles
Blood Platelets
These are colourless, oval shaped and discoidal cytoplasmic fragments formed from the giant
cells called megakaryocytes of bone marrow. The average life span of platelets is about 5-9
days. These play a major role in the process of blood clotting. At the site of injury, the platelets
release a number of platelet factors and an enzyme thromboplastin which cause the coagulation
of blood and clot formation to prevent excessive bleeding. Blood platelets also secrete a
vasoconstrictor called serotonin which causes constriction of blood vessels to reduce the blood
loss.
Heart - Shape and Position
It is a thick muscular, reddish brown, conical organ present in the mediastinal space of
thoracic cavity between 2 pleura enclosing the lungs. Its broader side is called base and it is
forward and upward while the pointed side called apex is backward and downward. It is 9 cm
broad and 12 cm long and about 300 gms in weight.
Protective covering
The heart is enclosed in a tough sac called pericardium. It is made up of an inner pericardium
and an outer fibrous pericardium. The 2 layers have between them a very narrow space, the
pericardial cavity filled with a watery coelmic fluid called pericardial fluid.
The pericardial fluid keeps the heart moist and prevents friction between the heart wall and the
surrounding tissues during the heart beat. The fibrous pericardium is formed of tough, white
fibrous tissue which protects the heart from mechanical injury and checks its over stretching or
overfilling with blood.
Structure of the Heart
External view of a Mammalian Heart
The heart is a 4 chambered muscular pump located inside the chest or the thoracic cavity. It is a
mesodermal derivative and is a myogenic heart. The heart is surrounded externally by a thin,
transparent 2 layered serous sac called pericardium. The narrow cavity between the 2 layers is
called the pericardial cavity which is filled with a watery fluid called the pericardial fluid. The
fluid performs 2 functions
a) it allows frictionless movements of the heart and
b) it protects the heart from mechanical shocks. The wall of the heart is primarily made up of
cardiac muscles called myocardium.
The heart is formed of 4 chambers, namely 2 auricles and 2 ventricles. The auricles are named as
right and left auricles and the ventricles are right and left ventricles.
A groove is present externally between the auricles and the ventricles called the coronary
sulcus.
The ventricles also home two grooves present on them called anterior interventricular sulcus
and posterior interventricular sulcus. These sulci receive coronary arteries through which the
heart receives the blood.
Auricles
The right and the left auricles are separated by a fibrous partition called interauricular septum.
Both the auricles have very thin walls because they have to push the blood only into the
ventricles. The inner surface of the auricles is very smooth except for a network of low ridges
called the musculi pectinati
Diagram of Heart cut open
The right auricle receives deoxygenated blood through the 2 major veins, namely the inferior
vena cava, superior vena cava and the coronary vein. The left auricle receives oxygenated blood
from the lungs through the pulmonary veins.
An oval depression is present on the interatrial septum called the fossa ovalis. It is actually the
remnant of foramen ovale, which is an opening in the interatrial septum of the foetal heart, which
closes at the time of birth.
Ventricles
The auricles are separated from the ventricles by an auriculo ventricular septum. The right
auricle opens into the right ventricle by a right auriculo ventricular aperture. The left auricle
opens into the left ventricle by a left auriculo ventricular aperture. The left auriculo ventricular
aperture is guarded by a valve called bicuspid valve. It has two cusps (flaps) and hence the name
bicuspid. The right auriculo ventricular aperture is guarded by a tricuspid valve containing 3
flaps. Both the tricuspid and the bicuspid valves are fastened to small conical muscles called the
papillary muscles on the ventricular wall through several tendinous strands called the chordae
tendinae.
Section through Mammalian Heart
The ventricles have thicker walls than the auricles. The walls of the left ventricle is about 3 times
as thick as that of the right ventricle. This is because the left ventricle has to pump the blood to
the farthest end of the body, while the right ventricle has to send the blood only to the nearby
lungs.
Two main blood vessels carry blood from the ventricles. One large aortic arch (aorta) carries
blood from the left ventricle to the various parts of the body except the lungs. The pulmonary
artery takes blood from the right ventricle to the lungs.
The base of the aortic arch and the pulmonary artery are guarded by semi lunar valves. Each
semi lunar valve is made up of 3 half moon cusps (flaps), attached to the wall of the aorta by one
border with the curved edge free inside the lumen of the aorta. They open during ventricular
systole and close during ventricular diastole. They allow the blood to flow into the aorta from the
ventricles and the reverse flow is prevented.
On the right wall of the right auricle is the sino-auricular node or SA node. It represents the sinus
venosus which has completely merged into the wall of the right auricle. It is called as the
pacemaker as the cardiac impulse originates from here and it determines the rate of heart beat.
Heart - Cardiac Cycle
This phase involves the contraction of the 2 auricles, pushing the blood into the respective
ventricles. There is no back flow of blood due to the presence of the bicuspid and the tricuspid
valves. The atrial systole takes 0.1 second. This is followed by the atrial diastole when both the
auricles relax simultaneously. This is about 0.7 seconds.
Circulation of Blood Through the Mammalian Heart
The aorta or the great artery arising from the left ventricle takes the oxygenated blood to the
body organs through a number of arteries. From these organs, the deoxygenated blood is
collected by the superior and inferior vena cava and brought back to the right auricle. The right
auricle pumps the deoxygenated blood into the right ventricle.
Circulation of Blood Through the Mammalian Heart
The mammalian heart is 4 chambered and shows double circulation. This means that the blood
passes through the heart twice for the body to be supplied once.
The 2 circulations are systemic circulation and pulmonary circulation.
Systemic Circulation
The aorta or the great artery arising from the left ventricle takes the oxygenated blood to the
body organs through a number of arteries. From these organs, the deoxygenated blood is
collected by the superior and inferior vena cava and brought back to the right auricle. The right
auricle pumps the deoxygenated blood into the right ventricle.
Pulmonary Circulation
The deoxygenated blood from the right ventricle is taken by the pulmonary artery to the lungs for
oxygenation. The blood after oxygenation is returned to the left auricle of the heart by the
pulmonary vein. From the left auricle the blood flows to the left ventricle.
Diagram of human circulation
Origin and Conduction of Heart Beat
The cardiac muscles of the heart have the property of excitability and conductivity. When these
muscles are stimulated, they get excited and initiate waves of electric potential called cardiac
impulses, which are conducted along the special cardiac muscle fibres on the wall of the heart
chambers.
Origin and Conduction of Heart Beat
The impulse originates from the SA node which lies on the right wall of the right auricle below
the opening of the vena cava. It is also called pacemaker as it determines the rate of heart beat.
The impulse originated from the sinu-auricular node is picked up and propagated by a special
system of tissues present in the heart. The conducting system includes the following components.
a) Auriculo ventricular node
b) Bundle of His (AV Bundle)
c) Purkinje fibres
The impulses arising from the sino auricular node is picked up by the auriculo ventricular node
(AV node) located at the posterior right border of the inter auricular septum. It functions as a
relay station and it transmits the impulses to other parts of the heart through the bundle of His.
When the SA node fails to function, it acts as a resume pace maker as it can also initiate the
cardiac impulse.
The bundle of His originates from the AV node as a bundle of tissue. Immediately after its origin
it divides into 2 branches. These branches run along the inner border of each ventricle and reach
the tip of the ventricle and then runs upwards along the outer margin of the ventricle. The bundle
of His and its branches produce minute branches called Purkinje fibres on the wall of the
ventricles.
During a heart beat, the auricles contract first and the ventricles contract later. This is because
there is no muscular continuity between the auricles and the ventricles. The auricles receive the
impulses directly from the SA node. The impulses reach the AV node about 0.03 seconds after
their origin from the SA node. So the ventricles always contract after the auricles.
Regulation of Heart Beat
Neurons connecting the heart to the cardiovascular system
The heart beat is controlled by the nervous system, hormones, temperature and pH.
Nervous Regulation
The heart is regulated to a large extent through the central nervous system. The heart nerve
branches from the vagus nerve from the medulla oblongata and the sympathetic nerve fibres
from the spinal cord.
The heart is regulated to a large extent through the central nervous system. The heart nerve
branches from the vagus nerve from the medulla oblongata and the sympathetic nerve fibres
from the spinal cord.
The vagus nerves are cardiac inhibitory nerves. When the vagus nerve is stimulated, the activity
of the heart is inhibited. The inhibitory action of the vagus nerve is brought about as follows.
a) The heart rate is slowed down
b) The conductivity of the bundle of His is reduced.
c) The force of contraction is diminished.
d) The duration of systole is diminished but that of the diastole is increased.
e) Excitability of the heart is also reduced.
The sympathetic nerves are accelerator nerves. These fibres stimulate the SA node, AV node and
also the muscles of the auricles and the ventricles Stimulation of the sympathetic nerves causes
a) Increase in frequency of heart rate
b) Increase in force of contraction
c) Increases excitability and irritability of the heart
d) Increases the conductivity of the cardiac muscle and bundle of His
thermal regulation
Adrenaline secreted by the adrenal medulla of adrenal gland accelerates the rate of heart beat
during emergency conditions. Noradrenaline increases the heart beat during normal condition.
Thyroxine and sex hormones also influence heart rate.
Thermal Regulation
Temperature
The rate of heart beat is affected by variation in temperature.
pH
During increased body activity the CO2 content of blood raises. It combines with water to form
carbonic acid in the blood. This lowers the pH, which increases the heart beat.
Rate of Heart Beat
Pulse is defined as the wave of distension and recoiling felt in the radial artery due to the
contraction of the left ventricle which force about 70-90 ml of blood into the already full aorta.
The rhythmic contraction and relaxation of the heart to pump out and receive blood is called the
heart beat. The rate of the heart beat varies from species to species. The smaller the animal, the
higher is the metabolic rate and so greater is its heart rate. The heart rate is about 25 times per
minute in a elephant and about 300 times per minute in a rat.
In an healthy adult, the normal heart beat at rest is about 70-72 times per minute. It increase
during fever, exercise, anger and pain.
Cardiac Output
It is the volume of blood ejected from the heart in to the aorta in one minute.
Pulse
Pulse is defined as the wave of distension and recoiling felt in the radial artery due to the
contraction of the left ventricle which force about 70-90 ml of blood into the already full aorta.
It can be felt in the superficial arteries such as those in the wrist, neck, temples and ankles. It is
generally felt by placing the fingers over the radial artery near the wrist. It can also be felt in the
temporal artery over the temporal bone, or the dorsalis pedis artery at the bend of the ankle.
Since each heart beat generates one pulse in the arteries, the pulse rate per minute indicates the
heart rate and is 72 per minute. It is affected by various factors such as posture of the body, age,
sex, exercise and emotions.
Lymphatic System
The lymphatic system is an accessory circulatory system which transports lymph, a fluid similar to
plasma from the intercellular spaces of tissues to the blood. It is a one way route for the movement of
interstitial fluid to blood. The lymphatic system can carry proteins and large particle matter from the
tissue spaces into the blood. Lymphatic System
Lymphatic System of a Human
The lymphatic system is an accessory circulatory system which transports lymph, a fluid similar
to plasma from the intercellular spaces of tissues to the blood. It is a one way route for the
movement of interstitial fluid to blood. The lymphatic system can carry proteins and large
particle matter from the tissue spaces into the blood.
The lymphatic system is formed of the following components
a) Lymph
b) Lymph vessels
c) Lymph nodes.
Lymph
Lymph is the interstitial fluid that flows into the lymphatic system. It is formed from blood by
the passage of substances through the wall of the blood capillaries into the inter cellular tissue
spaces. It is formed by the process of diffusion and filtration from the blood.
It consists of two parts namely fluid matrix, the plasma, in which float amoeboid cells, the white
blood corpuscles or leucocytes. It differs from blood in lacking red corpuscles, platelets and
some plasma proteins and having less calcium and phosphorus than the blood.
About 120ml of lymph flows through the lymphatic system per hour into the blood.
Lymph Vessles
Lymphatic System of Right arm and Breast
Lymphatic System of Right Leg
Lymphatic capillaries
The lymphatic system is formed of a tree like branching vessels. It starts among the tissue cells
as blind end microscopic vessels called lymphatic capillaries which are similar to veins. Their
walls are made up of endothelial cells supported by fibrous connective tissue.
The lymphatic capillaries repeatedly join together to form bigger vessels which pass through
lymph nodes, receive more tributaries and gradually increase in size. All the vessels finally join
together to form two large vessels called thoracic duct or left lymphatic duct and right lymphatic
duct.
Lymph capillaries collecting Lymph among the Tissues
Thoracic Duct
The thoracic duct is about 45 cm long and about 6 mm in diameter. It emerges from the
receptaculum chyli situated near the second lumbar vertebra. It collects all the lymph from the
hind limbs and alimentary canal. The thoracic duct also receives the left cervical duct which
collects lymph from the left forelimb, left side of the neck and chest. The thoracic duct opens
into the venous system at the junction of the left internal jugular vein and sub clavianvein.
Lacteals
The lymph vessels that remain in the center of the villi of alimentary canal are called lacteals.
They collect milk white fluid called chyle from the intestine after digestion. The lacteals absorb
fats from the intestine.
Main drainage routes of Lymphatic System
Right Lymphatic Duct
The right lymphatic duct is a smaller vessel of about 1.25 cm length. It collects lymph from the
right side of the head, the right neck, the right upper limb and the right side of the chest. It
empties into the venous system at the right subclavian vein and internal jugular vein.
LS through Lymph Vessel showing an internal Valve
Valves
The lymphatic vessels are provided with valves. They help the lymph to flow in the direction of
the chest. They prevent the reverse flow of lymph.
Section through a Lymph Gland
Lymph Nodes
Lymph nodes are oval or bean shaped bodies placed in the lymphatic vessels. They act as filters
and are sites where lymphocytes are formed. The main groups of lymph nodes lie in the neck,
axilla, thorax, abdomen and groin.
They help in the production of antibodies and in the development of immunity, Lymph nodes
produce gamma globulin. They serve a defensive role against bacterial infection.
A model showing the circulation of lymph
Lymph Movement
The lymph flow in lymphatic vessels very slowly. Forcing out fluid from the blood capillaries set
up some pressure in the tissue fluid. This pressure gradient causes the lymph to flow in the
lymphatic vessel. Movements of visverand contractions of the body muscles help by squeezing
the lymph along. The valves prevent the backward flow. The villi by their movement help the
lymph to flow in the lacteals.
Functions of Lymph
a) The lymph returns fluid and protein from the tissues to the circulation.
b) It transports lymphocytes from the lymphatic glands to the blood.
c) The lacteals transport emulsified fats from the intestine to the blood
d) It destroys the invading microorganism and foreign particle in the lymph nodes.
e) Following an infection the lymph nodes produce antibodies to protect the body against
subsequent infection.
Immunity and Immune System
Animals encounter many potentially dangerous microbes in air, water and food. So they have
involved defense mechanisms against the disease causing germs Immunity is a defense
mechanism without which we will fall a prey to all parasitic microorganisms.
Animals encounter many potentially dangerous microbes in air, water and food. So they have
involved defense mechanisms against the disease causing germs Immunity is a defense
mechanism without which we will fall a prey to all parasitic microorganisms.
Definition
Immunity is defined as the ability to eliminate complexes of protein molecules differing in
structure from those present in the healthy animals.
There are two kinds of defence mechanism against microbes
a) nonspecific defence mechanism
b) specific defence mechanism.
Non-specific defence mechanism
This is common for most type of infection. It controls infection by blocking the entry of
pathogens into the body or by destroying the microbes through means other than antibodies.
The non-specific defence mechanism is further of two types
a) External defence or first line of defence
b) Internal defence or second line of defence
External Defence
The external defence comprises physical and chemical barriers to the entry of pathogens into the
body.
Physical barriers
Skin
The outer layer of the skin is fully keratinized and is waterproof and germproof. It successfully
prevents the entry of viruses and bacteria.
Mucuous Membranes
The mucuous membrane lining the digestive, respiratory and urinogenitaltracts secrete mucus
which traps the micro organisms and immobilises them. This is then eliminated with the sputum
or removed with the faeces
Secretions of tears and movements of eyelids flush out the microorganisms settling on the eyes.
Chemical Barriers
The skin and mucuous membranes secrete certain chemical which dispose of the pathogens.
Skin Secretions
The oil and sweat secreted by the sebaceous and the sweat glands contain some fatty acids and
lactic acid which make the surface of the skin acidic with a pH of 3-5. This prevents micro
organisms from growing on the skin. The skin harbours some friendly bacteria also which
release acids and other metabolic wastes that check the growth of microbes. Lysozyme, an
enzyme present in sweat, kills many bacteria by destroying their cell wall.
Saliva
Saliva also contains lysozyme which kills micro organisms that are not the normal inhabitants of
the buccal cavity.
Bacteria which escape the action of saliva, when they reach the stomach, are killed by the
hydrochloric acid and the proteolytic enzymes of the gastric juice.
Tear
Tears, a slightly saline fluid secreted by the lachrymal gland over the eyes, also contain
lysozyme which prevents eye infections. Frequent washing of eyes reduces the disinfecting
power of the lachrymal secretions. Tears also wash off the chemical irritants of polluted air, from
the eyes.
Skin and mucous membranes may fail to keep out the invaders. Some parasites enter through the
skin e.g., hook worm, or through wounds and swellings of sweat glands and hair follicles. It is
therefore very essential to have a second line of defence for controlling the parasites that have
entered the body by breaking through the first line of defence.
Internal Defence
Second line of defence or body's internal defence is carried out by leucocytes, macrophages,
inflammatory reactions, fever, interferons and natural killer cells. These operate together to
check the damage to the body by the pathogens.
Second line of defence or body's internal defence is carried out by leucocytes, macrophages,
inflammatory reactions, fever, interferons and natural killer cells. These operate together to
check the damage to the body by the pathogens.
Leucocytes
The number of leucocytes increases due to an infection. They creep out of the capillaries by
amoeboid movement into the intercellular spaces if there is an infection. This process is called
diapedesis. The neutrophils engulf and digest the microorganisms infecting the body tissues.
They are called phagocytes and the process is known as phagocytosis. The basophils release
histamine that plays a role in inflammatory reaction. A macrophage is an irregular cell which can
engulf about 100 bacteria and dispose them.
Inflammatory Response
It is initiated by chemical signals. The invading microbes release certain chemicals. Tissue injury
causes the most cells to release histamine. Chemicals from microbes and histamines together
cause dilution of capillaries and increases the permeability of capillary wall. As a result, more
blood flows to that area, making it red and warm. Fluid leaks out into the tissue spaces causing
its swelling. This reaction of the body is called inflammatory response and is a part of the
internal defence. The plasma that accumulates at the injured site dilutes the toxins secreted by the
bacteria and lessens their effect. The neutrophils move in and eat up the invading
microorganisms. Macrophages not only phagocytise microbes but also clean up damaged cells
and remains of neutrophils destroyed in phagocytic action.
The dead phagocytes and bacteria damaged tissue cells, enzymes and the fluid and proteins
leaked from the capillaries leave the body in the form of pus. Formation of pus is a sure sign of
infection. Usually, the pus is absorbed by the body within a few days.
Fever
The inflammatory response may be localized or systemic. The localised response is confined to
the site of injury only. The systemic response affects the entire body in case of a severe infection
or a serious injury. In this case, the WBC count of the blood increases. Body temperature rises
causing fever. This may be brought about by toxins produced by the pathogen or by a protein
called pyrogen or interleukin released by macrophages.
When the pyrogens reach the brain, the body's thermostat is set at a higher temperature, allowing
the temperature of the entire body to rise. Mild fever strengthens the defence mechanism by
activating the phagocytes and by inhibiting the growth of microbes. Higher body temperature
reduces blood iron level. Bacteria need more iron as temperature increases. Therefore, the
bacterial growth is reduced in a fever ridden body.
Interferon production and activity
A very high temperature may prove dangerous. It must be brought down by giving antipyretics
and by applying cold pack.
Interferons
This works specifically against viral infections. Cells invaded by a virus produce an antiviral
protein called interferon, which is released from the infected and dying host cell. On reaching
the nearby uninfected cell, it makes them resistant to the virus infection. Interferon induces the
formation of certain proteins which prevent multiplication of the virus. Fever increases the
production of interferons.
Interferons have proved very effective in treating cold, influenza and hepatitis but have not
succeeded against cancer.
Natural Killer Cells
These cells are a kind of lymphocytes. They do not attack micro organisms directly, but lyse the
viral infected body cells and abnormal cells which could form tumours.
Specific Defence Mechanism
This mechanism provides protection against specific foreign materials and is often called the
immune system. This system forms the third line of body's defence against microbes and harmful
molecules. The most important characteristic of the immune system is that its cells have an
ability to recognise body's own cells and macromolecules, from those which are foreign invaders
or nonself.
This mechanism provides protection against specific foreign materials and is often called the
immune system. This system forms the third line of body's defence against microbes and harmful
molecules. The most important characteristic of the immune system is that its cells have an
ability to recognise body's own cells and macromolecules, from those which are foreign invaders
or nonself.
Antigens
The foreign matter that enters the body and elicits a specific immune response by lymphocytes is
an antigen (anti - antibody gen - generating)
It stimulates the immune system to produce protective chemicals or special cells to destroy the
antigens. These protective chemicals are called as antibodies. The protein molecules present on
the surface of the foreign material act as antigens. Each antigen causes the formation of a
specific antibody. Antibodies react with antigens and make them inactive or harmless.
Cells of the Immune System
Lymphocytes are the main cells of the immune system. They arise from stem cells present in
the liver in the foetus and in the bone marrow in the adult.
Lymphocytes are the main cells of the immune system. They arise from stem cells present in the
liver in the foetus and in the bone marrow in the adult.
Comparison of cell-mediated and humoral immunity
Some of them continue their maturation in the bone marrow and develop into b- lymphocytes or
B - cells. Others migrate to the thymus, a gland located in the chest above the heart and mature
there. They develop into T - cells or T - lymphocytes.
The B - cells form the humoral immune system and the T - cells form the cell mediated immune
system.
Humoral Immune System
The plasma membrane of each B cell should be sensitised by contact with a specific antigen
for the release of antibodies. The plasma cells do not migrate to the site of infection but act
through the lymph. So they form the humoral immune system. The B lymphocytes are short
lived and are replaced every few days from the bone marrow.
The plasma b - cells produce a lot of anti-bodies, which circulate in the lymph. Each person can
make 107 to 108 different kinds of antibody molecules so that there is an antibody for any
antigen.
The plasma membrane of each B cell should be sensitised by contact with a specific antigen for
the release of antibodies. The plasma cells do not migrate to the site of infection but act through
the lymph. So they form the humoral immune system. The B lymphocytes are short lived and are
replaced every few days from the bone marrow.
Cell Mediated Immune System
The cellular immune response is given by T - cells. There are separate T - cells for each type
of antigen that invades the body. The life span of the T - cells is 4-5 years or even longer. There
are four types of T - cells.
The cellular immune response is given by T - cells. There are separate T - cells for each type of
antigen that invades the body. The life span of the T - cells is 4-5 years or even longer. There are
four types of T - cells.
T - Cell difference and activity
Killer T - cells
The killer T - cells destroy the foreign antigens. They also destroy cancer cells. They are
stimulated to destroy the non - self cells with the help of the helper T - cells.
Helper T - Cells
These stimulate the B - cells. These stimulate the B-cells to produce antibodies and also
stimulate killer T - cells to destroy the nonself cells.
Suppressor T - Cells
These cells inhibit the immune response of both T and B - lymphocytes to foreign antigens when
infection has been controlled. They also prevent the immune system from attacking the body's
own cells.
Memory T- Cells
These cells are always ready to attack a pathogen if it attacks the body again.
Immunity
Immunity is the ability of an organism to recognise the foreign material or chemicals that
enter the body and to mobilise the cells and cell products to quickly remove the foreign material.
Immunity is the ability of an organism to recognise the foreign material or chemicals that enter
the body and to mobilise the cells and cell products to quickly remove the foreign material.
Development of Immunity
A person may develop immunity in three different ways.
Immunity through diseases
The first time a person gets an infection, he is likely to develop the disease because the antibody
production is slow and the virus has time to multiply and spread throughout the body.
The cells capable of producing antibodies persist in the body as memory cells for a long time,
sometimes even for life after the first infection. They quickly become active on further infection
and produce antibodies of the same type. These antibodies quickly overcome the infection, so the
disease does not appear again.
Vaccination
This is a technique where immunity is developed without infection. Weakened or dead pathogens
are injected into a person. The pathogen given is unable to cause the disease, but are sufficient to
stimulate the formation of antibodies by the hosts immune system that recognises the pathogen,
if it enters the body later.
Example: Small pox, polio, measles and rabies.
Antitoxins
Antibodies that neutralize toxins produced in the body like bacterial toxins or introduced from
outside like snake venom are called antitoxins. They are produced artificially these days.
The antigen for e.g., snake venom is injected in low doses into an animal like horse or rabbit.
After the animal develops antibodies for the antigen, its blood is drawn and allowed to clot. The
serum left behind contains antibodies. It is injected into a person bittenby a snake to neutralize
the snake venom as he would not be able to produce antibodies in sufficient quantity quickly. It
gives a temporary immunity against the antigen for a few hours or a few months.
Types of Immunity
The type of immunity inherited by the organism from the parents and protects it from birth
throughout life is known as innate or inborn immunity.
The type of immunity inherited by the organism from the parents and protects it from birth
throughout life is known as innate or inborn immunity.
Example: Human beings have innate immunity against distemper a fatal disease of dogs.
Acquired or Adaptive Immunity
This is an immunity developed by an animal in response to a disease caused by infection of
pathogens. It is very specific and prevents further attacks. It lasts for the whole life of the
organism in certain cases and for a few years in others.
Acquired immunity is further of two types - natural or active and artificial or passive
Active Immunity
Immunity is said to be active when an organisms own cells produce antibodies. It develops as a
result of contact with pathogenic organisms or their products. It may be acquired naturally or
artificially. Active immunity is produced naturally by the attack of the disease like small pox or
produced artificially by injections and vaccinations.
Passive Immunity
Immunity is said to be passive when antibodies produced in another organism are injected into a
person to induce protection against diseases. Passive immunity is developed for rabies, tetanus
toxin, or salmonella infection. It has the advantage of providing immediate relief.
But it has some problems. It is not long lasting and antibodies may cause reactions.