CHAPTER 18

BODY FLUIDS AND CIRCULATION

Functions of circulatory system
• To provide nutrients to all living cells in the body
• To transport O2 and CO2 in the body
• To transport the harmful substances produced in the cells to the organs for excretion
• To the transport of hormones from the glands to the target places
Methods of transport
• 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.
Parts of Human Circulatory System
• Heart – pumping organ
• Blood vessels– the path through which blood flows
• Blood and lymph – circulating fluids
BLOOD
Blood is a special connective tissue consisting of a fluid matrix, plasma, and formed elements.
Plasma:-
• Plasma constitute about 55% of the blood
• Important constituents of plasma are
 water forming 90– 92%
 proteins forming 6– 8%
 small amount of minerals like Na+ ,Ca++, Mg++,HCO3,Cl- and substances like glucose,
amino acids lipids etc
Proteins are of 3 types fibrinogen, albumin and globulin
 Fibrinogens are needed for clotting or coagulation of blood.
 Globulins are involved in defense mechanisms of the body
 Albumins help in osmotic balance.
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% of the blood.
Erythrocytes (Red Blood Cells /RBCs):-

 They are the most abundant of all the cells in blood. A healthy adult man has, on an
average, 5millions to 5.5millions 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 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(White Blood Cells/ WBCs):-
• Leucocytes are also known as white blood cells (WBC) as they are due to the lack of
haemoglobin.
• They are nucleated and are 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 (60-65%), eosinophils (2-3%) and basophils (0.5-1%) are different types of
granulocytes, while lymphocytes(20-25%) and monocytes (6-8%)are the agranulocytes.

• Neutrophils are the most abundant cells of the total WBCs and basophils are the least
among them
• Neutrophils and monocytes are phagocytic cells which destroy foreign organisms
entering the body
• Basophils secrete histamine, serotonin, heparin, etc., and are involved in inflammatory
reactions
• Eosinophils resist infections and are also associated with allergic reactions
• Lymphocytes are of 2 types B lymphocytes and T lymphocytes. These are responsible
for immune responses of the body.
Blood Platelets (Thrombocytes)
 They 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 GROUPING
Types: ABO grouping and Rh grouping
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)
• 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 distribution of antigens and antibodies in the four groups of blood, A, B, AB and O are

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 and , such persons are called ‘universal recipients’.
Rh Grouping:-

 Rh grouping is based on the presence or absence of Rh antigen on the surface of RBCs.

 Rh antigen is similar to the one present in Rhesus monkeys (hence Rh)
 Rh antigen is 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.
Rh incompatibility

 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
• 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
• A dark reddish brown scum formed at the site of a cut or an injury over a period of time
is the clot or coagulum
• It is 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
• Thrombin, 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

LYMPH OR 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.
BLOOD VESSELS
• Arteries and veins are the main blood vessels.
• The branches of arteries are called arterioles , which further branches into capillaries in
the tissues.
• The capillaries in the tissues join to form venules which further join to form the veins.

TYPES OF CIRCULATORY PATHWAYS

The circulatory patterns are of two types –Open and Closed
Open circulatory system
 It is present in arthropods and molluscs
 blood pumped by the heart passes through large vessels into open spaces or body
cavities called sinuses.
Closed circulatory system
 It is present in annelids and chordates
 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.
HEARTS OF VERTEBRATES
• All vertebrates possess a muscular chambered heart.
• Fishes have a 2-chambered heart an atrium and a ventricle.
• Amphibians and the reptiles (except crocodiles) have a 3-chambered heart
two atria and a single ventricle
• Birds and mammals possess a 4-chambered heart two atria and two ventricles.
TYPES OF CIRCULATION IN VERTEBRATES
1.Single circulation in Fishes
• In fishes the heart pumps out deoxygenated blood. This is oxygenated by the gills
• Oxygenated blood Is supplied to the body parts
• Deoxygenated blood from body parts is returned to the heart
• As the blood passes through the heart only once it is called single circulation
(As the heart receive only deoxygenated blood fish heart is referred as venous heart)
2. Incomplete double circulation (Mixed 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

3. 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

HEART
• Heart, the mesodermally derived organ
• It 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.

Structure of Heart -Chambers

• Our heart has four chambers- two upper chambers called atria and two lower
chambers called ventricles
• A thin, muscular wall called the interatrial septum separates the right and the left atria,
• 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.
• These septa are provided with an opening through which the two chambers of the same
side are connected
• Right side of the heart carry deoxygenated and left side carry oxygenated blood
Blood Vessels
• Arteries – blood vessel which start from heart to the different parts of the body
They start from ventricles
• Veins --blood vessels taking blood from the different parts of the body to the heart
They open into atria
• Major blood vessels are
Aorta – start from left ventricle
Pulmonary artery – start from right ventricle
Vena cava – open into right atrium
Pulmonary veins open into left atrium

Valves
Tricuspid valve
• guards the opening between the right atrium and the right ventricle
• it is formed of three muscular flaps or cusps
Bicuspid or mitral valve
• guards opening between the left atrium and the left ventricle.
Semilunar valves
• Guards the opening of the right ventricle to the pulmonary artery and left ventricle to
the aorta
(Chordae tendineae are thread like bands of fibrous tissue which attach the tricuspid and
bicuspid valves with the papillary muscles within the heart)
Function of valves
• Valves in the heart allows the flow of blood in one direction, from atria to ventricle, from
ventricles to arteries
• Back flow is prevented by valves

Cardiac Musculature
 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 distributed in the heart

Distribution of Nodal Tissue

Sino atrial node (SAN)
• A patch of tissue is present in the right upper corner of the right atrium
Atrio ventricular node (AVN)
• Mass of tissue is seen in the lower left corner of the right atrium close to the atrio-
ventricular septum
AV Bundle
• A bundle of nodal fibres, continues from the AVN which passes through the atrio-
ventricular septa to emerge on the top of the interventricular septum
Right and left AV Bundle
• Branches of AV Bundle.
Purkinje fibres
 • Minute fibres arising from right and left AV bundle , branches throughout the
ventricular musculature of the respective sides
These fibres along with 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
 The sequential events 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.
systole -- contraction diastole – relaxation
• The important stages of cardiac cycle are
 Joint diastole
 Atrial systole
 Ventricular systole & atrial diastole
 Ventricular diastole

Joint Diastole
• The tricuspid and bicuspid valves are open
• Passive movement of blood occurs from veins to the atria and then to the ventricles
• Blood from the pulmonary veins flows into the left atrium and then into the left
ventricle through the bicuspid valve
• Blood from the vena cava flows into the right atrium and then to right ventricle through
tricuspid valve
• The semilunar valves are closed at this stage.

Atrial Systole
 The SAN 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.

Ventricular Systole & Atrial Diastole

• 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
• The atria undergoes relaxation (diastole), coinciding with the ventricular systole.
• Ventricular systole increases the ventricular pressure causing the closure of tricuspid
and bicuspid valves
• Semilunar valves guarding the pulmonary artery (right side) and aorta (left side) will
open and blood flows through these vessels into the circulatory pathways.
 Ventricular Diastole
 The ventricles relax 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.

STROKE VOLUME & CARDIAC OUTPUT

• The heart beats 72 times per minute
• 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
• This averages 5000 mL or 5 litres in a healthy individual.

Electrocardiograph (ECG)
• The graphical representation of the electrical activity of heart during cardiac cycle is
called electrocardiogram (ECG)
• The machine (electro-cardiograph) is used to obtain an electrocardiogram
• To obtain a standard ECG the patient is connected to the machine with three electrical
leads(one to each wrist and to the left ankle)
• For a detailed evaluation of the heart function multiple leads are attached to chest
region
• The various events in cardiac cycle is represented by the peaks named as p –wave, QRS
complex, and T - wave
• 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 T-wave represents the return of the ventricles from excited to normal state
(repolarisation of ventricles).
Clinical significance of ECG
• 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 significance.

CIRCULATORY PATHWWAYS IN DOUBLE CIRCULATION

PULMONARY CIRCULATION
• The deoxygenated blood from the right ventricle is pumped into the pulmonary artery is
passed on to the lungs
• from the lungs, the oxygenated blood is carried by the pulmonary veins into the left
atrium
• This pathway constitutes the pulmonary circulation.
SYSTEMIC CIRCULATION
• The oxygenated blood from the left ventricle entering the aorta is carried by a network
of arteries, arterioles and capillaries to the tissues
• From the tissues 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, O2and other essential substances to the
tissues and takes CO2 and other harmful substances away for elimination.

HEPATIC PORTAL CIRCULATION

• 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.
CORONARY 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
• This is 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.
• 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 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 blood flow.

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)