all living cells have to be provided with nutrients, O2 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.
composition and properties of
blood and lymph (tissue fluid) and the mechanism of circulation of blood
is also explained herein.
BLOOD : Blood is a special connective tissue consisting of a fluid matrix, plasma,
and formed elements.
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.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++, HCO3–, 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.
FORMED ELEMENTS :
Erythrocytes, leucocytes and platelets are collectively called formed
elements 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–3 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 thesecells. 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–3 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 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–3. 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.
BLOOD GROUPS :
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
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).
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’.
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.
COAGULATION OF BLOOD :
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. Certainfactors released by the tissues at the site of injury also can initiate coagulation. Calcium ions play a very important role in clotting.
LYMPH (TISSUE FLUID ):
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.
CIRCULATORY PATHWAYS :
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 system.
HUMAN CIRCULATORY SYSTEM :
Human circulatory system, also called the blood vascular system consists
of a muscular chambered heart, a network of closed branching blood
vessels and blood, the fluid which is circulated.
Heart, the mesodermally derived organ, is situated in the thoracic
cavity, in between the two lungs, slightly tilted to the left. It has the size of
a clenched fist. It is protected by a double walled membranous bag,
pericardium, enclosing the pericardial fluid. Our heart has four chambers, two relatively small upper chambers called atria and two larger
lower chambers called ventricles. A thin, muscular wall called the inter-atrial septum separates the right and the left atria, whereas a thick-walled,
the inter-ventricular septum, separates the left and the right ventricles. The atrium and the ventricle of the same side are also
separated by a thick fibrous tissue called the atrio-ventricular septum.
However, each of these septa are provided with an opening through which
the two chambers of the same side are connected. The opening between
the right atrium and the right ventricle is guarded by a valve formed of
three muscular flaps or cusps, the tricuspid valve, whereas a bicuspid
or mitral valve guards the opening between the left atrium and the left ventricle. The openings of the right and the left ventricles into the pulmonary artery and the aorta respectively are provided with the
semilunar valves. The valves in the heart allows the flow of blood only in
one direction, i.e., from the atria to the ventricles and from the ventricles
to the pulmonary artery or aorta. These valves prevent any backward flow.
The entire heart is made of cardiac muscles. The walls of ventricles
are much thicker than that of the atria. A specialised cardiac musculature
called the nodal tissue is also distributed in the heart . A patch of this tissue is present in the right upper corner of the right atrium
called the sino-atrial node (SAN). Another mass of this tissue is seen in
the lower left corner of the right atrium close to the atrio-ventricular septum
called the atrio-ventricular node (AVN). A bundle of nodal fibres, atrio-
ventricular bundle (AV bundle) continues from the AVN which passes
through the atrio-ventricular septa to emerge on the top of the inter-
ventricular septum and immediately divides into a right and left bundle.
These branches give rise to minute fibres throughout the ventricular
musculature of the respective sides and are called purkinje fibres. These
fibres alongwith right and left bundles are known as bundle of His. The
nodal musculature has the ability to generate action potentials without
any external stimuli, i.e., it is autoexcitable. However, the number of action
potentials that could be generated in a minute vary at different parts of
the nodal system. The SAN can generate the maximum number of action
potentials, i.e., 70-75 min–1
, and is responsible for initiating and
maintaining the rhythmic contractile activity of the heart. Therefore, it is called the pacemaker. Our heart normally beats 70-75 times in a minute (average 72 beats min–1).
CARDIAC CYCLE :
How does the heart function? Let us take a look. To begin with, all the
four chambers of heart are in a relaxed state, i.e., they are in joint diastole.
As the tricuspid and bicuspid valves are open, blood from the pulmonary
veins and vena cava flows into the left and the right ventricle respectively
through the left and right atria. The semilunar valves are closed at this
stage. The SAN now generates an action potential which stimulates both
the atria to undergo a simultaneous contraction – the atrial systole. This
increases the flow of blood into the ventricles by about 30 per cent. The
action potential is conducted to the ventricular side by the AVN and AV
bundle from where the bundle of His transmits it through the entire
ventricular musculature. This causes the ventricular muscles to contract,
(ventricular systole), the atria undergoes relaxation (diastole), coinciding
with the ventricular systole. Ventricular systole increases the ventricular pressure causing the closure of tricuspid and bicuspid valves due to
attempted backflow of blood into the atria. As the ventricular pressure
increases further, the semilunar valves guarding the pulmonary artery
(right side) and the aorta (left side) are forced open, allowing the blood in
the ventricles to flow through these vessels into the circulatory pathways.
The ventricles now relax (ventricular diastole) and the ventricular pressure
falls causing the closure of semilunar valves which prevents the backflow
of blood into the ventricles. As the ventricular pressure declines further,
the tricuspid and bicuspid valves are pushed open by the pressure in the
atria exerted by the blood which was being emptied into them by the
veins. The blood now once again moves freely to the ventricles. The
ventricles and atria are now again in a relaxed (joint diastole) state, as
earlier. Soon the SAN generates a new action potential and the events
described above are repeated in that sequence and the process continues.
This sequential event in the heart which is cyclically repeated is called
the cardiac cycle and it consists of systole and diastole of both the atria
and ventricles. As mentioned earlier, the heart beats 72 times per minute,
i.e., that many cardiac cycles are performed per minute. From this it could
be deduced that the duration of a cardiac cycle is 0.8 seconds. During a
cardiac cycle, each ventricle pumps out approximately 70 mL of blood
which is called the stroke volume. The stroke volume multiplied by the
heart rate (no. of beats per min.) gives the cardiac output. Therefore, the
cardiac output can be defined as the volume of blood pumped out by each
ventricle per minute and averages 5000 mL or 5 litres in a healthy individual.
The body has the ability to alter the stroke volume as well as the heart rate
and thereby the cardiac output. For example, the cardiac output of an
athlete will be much higher than that of an ordinary man.
During each cardiac cycle two prominent sounds are produced which
can be easily heard through a stethoscope. The first heart sound (lub) is associated with the closure of the tricuspid and bicuspid valves whereas the second heart sound (dub) is associated with the closure of thesemilunar valves. These sounds are of clinical diagnostic significance.
ELECTROCARDIOGRAPH (ECG) :
You are probably familiar with this scene from a typical hospital television
show: A patient is hooked up to a monitoring machine that shows voltage
traces on a screen and makes the sound “… pip… pip… pip…..
peeeeeeeeeeeeeeeeeeeeee” as the patient goes into cardiac arrest. This type
of machine (electro-cardiograph) is used to obtain an electrocardiogram
(ECG). ECG is a graphical representation of the electrical activity of the
heart during a cardiac cycle. To obtain a standard ECG .a patient is connected to the
machine with three electrical leads (one to each
wrist and to the left ankle) that continuously
monitor the heart activity. For a detailed
evaluation of the heart’s function, multiple
leads are attached to the chest region. Here,
we will talk only about a standard ECG.
Each peak in the ECG is identified with a
letter from P to T that corresponds to a specific
electrical activity of the heart.
The P-wave represents the electrical
excitation (or depolarisation) of the atria,
which leads to the contraction of both the atria.
The QRS complex represents the depolarisation of the ventricles,
which initiates the ventricular contraction. The contraction starts shortly
after Q and marks the beginning of the systole.
The T-wave represents the return of the ventricles from excited to
normal state (repolarisation). The end of the T-wave marks the end of systole.
Obviously, by counting the number of QRS complexes that occur in a
given time period, one can determine the heart beat rate of an individual.
Since the ECGs obtained from different individuals have roughly the same
shape for a given lead configuration, any deviation from this shape
indicates a possible abnormality or disease. Hence, it is of a great clinical
DOUBLE CIRCULATION :
As mentioned earlier, the blood pumped by the right ventricle enters the
pulmonary artery, whereas the left ventricle pumps blood into the aorta.
The deoxygenated blood pumped into the pulmonary artery is passed on
to the lungs from where the oxygenated blood is carried by the pulmonary
veins into the left atrium. This pathway constitutes the pulmonary
circulation. The oxygenated blood entering the aorta is carried by a
network of arteries, arterioles and capillaries to the tissues from where
the deoxygenated blood is collected by a system of venules, veins and
vena cava and emptied into the right atrium. This is the systemic circulation . The systemic circulation provides nutrients, O2
and other essential substances to the tissues and takes CO2 and other harmful substances away for elimination. A unique vascular connection exists between the digestive tract and liver called hepatic portal system.
The hepatic portal vein carries blood from intestine to the liver before it is
delivered to the systemic circulation. A special coronary system of blood vessels is present in our body exclusively for the circulation of blood to and from the cardiac musculature.
REGULATION OF CARDIAC ACTIVITY :
Normal activities of the heart are regulated intrinsically, i.e., auto regulated
by specialised muscles (nodal tissue), hence the heart is called myogenic.
A special neural centre in the medulla oblangata can moderate the cardiac
function through autonomic nervous system (ANS). Neural signals through
the sympathetic nerves (part of ANS) can increase the rate of heart beat,
the strength of ventricular contraction and thereby the cardiac output.
On the other hand, parasympathetic neural signals (another component
of ANS) decrease the rate of heart beat, speed of conduction of action potential and thereby the cardiac output. Adrenal medullary hormones can also increase the cardiac output.
DISORDERS OF CIRCULATORY SYSTEM :
High Blood Pressure (Hypertension): Hypertension is the term for blood
pressure that is higher than normal (120/80). In this measurement 120 mm Hg (millimetres of mercury pressure) is the systolic, or pumping,
pressure and 80 mm Hg is the diastolic, or resting, pressure. If repeated checks of blood pressure of an individual is 140/90 (140 over 90) or higher, it shows hypertension. High blood pressure leads to heart diseases
and also affects vital organs like brain and kidney.
Coronary Artery Disease (CAD): Coronary Artery Disease, often referred
to as atherosclerosis, affects the vessels that supply blood to the heart
muscle. It is caused by deposits of calcium, fat, cholesterol and fibrous
tissues, which makes the lumen of arteries narrower.
Angina: It is also called ‘angina pectoris’. A symptom of acute chest pain
appears when no enough oxygen is reaching the heart muscle. Angina
can occur in men and women of any age but it is more common among
the middle-aged and elderly. It occurs due to conditions that affect the
Heart Failure: Heart failure means the state of heart when it is not pumping
blood effectively enough to meet the needs of the body. It is sometimes
called congestive heart failure because congestion of the lungs is one of
the main symptoms of this disease. Heart failure is not the same as cardiac
arrest (when the heart stops beating) or a heart attack (when the heart
muscle is suddenly damaged by an inadequate blood supply).