All living cells have to be provided with nutrients, Oxygen and other essential substances and waste or harmful substances produced, have to be removed continuously for healthy functioning of tissues.
Organisms like sponges and coelenterates circulate water from their surroundings through their body cavities to facilitate the cells to exchange these substances.
Blood is the most commonly used body fluid by most of the higher organisms including humans for this purpose.
Lymph (tissue fluid), also helps in the transport of certain substances.

Blood

Blood is a special connective tissue consisting of a fluid matrix, plasma, and formed elements. (NEET 2015)

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.
Major proteins :-
Fibrinogens - needed for clotting or coagulation of blood. (NEET 2011,2018)
Globulins - primarly are involved in defence mechanisms of the body.
Albumins - help in osmotic balance. (NEET 2009,2018)
Plasma also contains small amounts of minerals like Na+, Ca++ , Mg++, HCO3- , Cl–,
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. (NEET 2016)

Formed Elements

Erythrocytes, leucocytes and platelets are collectively called formed elements and they constitute nearly 45 per cent of the blood.

1. Erythrocytes or red blood cells (RBC)

Most abundant of all the cells in blood 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. (NEET 2015)
RBCs are devoid of nucleus in most of the mammals and are biconcave in shape.
Absence of nucleus in RBC is an adaptation that allows it. to contain more haemoglobin and carry more oxygen by providing empty space. This adaptation also aids in effective diffusionofoxygen.
RBCs are initially produced in bone marrow with a nucleus, they, then undergo enucleation at maturity, in which their nucleus is removed. (NEET 2017)
red coloured, iron containing complex protein called haemoglobin, hence the colour and name of these cells.
12-16 gms of haemoglobin in every 100 ml of blood.
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). (NEET 2012)

2. Leucocytes or white blood cells (WBC)

Colourless due to the lack of haemoglobin.
Nucleated and are relatively lesser in number which averages 6000-8000 mm–3 of blood.
Leucocytes are generally short lived.
Two main categories of WBCs – (a) Granulocytes (b) Agranulocytes

(a) Granulocytes :- three types

i. Neutrophils ii. eosinophils iii. Basophils

Neutrophils are the most abundant cells (60-65 per cent) of the total WBCs . Phagocytic cells which destroy foreign organisms entering the body.
Eosinophils (2-3 per cent) resist infections and are also associated with allergic reactions.
basophils are the least (0.5-1 per cent) secrete histamine, serotonin, heparin, etc., and are involved in inflammatory reactions. (NEET 2008,2013)

(b) Agranulocytes :- two types

i. lymphocytes ii. Monocytes

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.
Monocytes (6-8 per cent) are phagocytic cells which destroy foreign organisms entering the body.

3. Platelets or thrombocytes

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.
Reduction in their number can lead to clotting disorders which will lead to excessive loss of blood from the body. (NEET 2016)

Blood Groups

Various types of grouping of blood has been done.
Two such groupings – 1. ABO 2. Rh – are widely used all over the world.

1. ABO grouping

Based on the presence or absence of two surface antigens (chemicals that can induce immune response) on the RBCs namely A and B.
Plasma of different individuals contain two natural antibodies (proteins produced in response to antigens).
Distribution of antigens and antibodies in the four groups of blood, A, B, AB and O.
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).
Group ‘O’ blood can be donated to persons with any other blood group and hence ‘O’ group individuals are called ‘universal donors’. (NEET 2011,2012)
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’. (NEET 2014)

2. Rh grouping

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).
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.
Rh group should also be matched before transfusions.
A special case of Rh incompatibility (mismatching)
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.
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.
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.
Could be fatal to the foetus or could cause severe anaemia and jaundice to the baby and this condition is called erythroblastosis foetalis.
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.
A dark reddish brown scum formed at the site of a cut or an injury over a period of time.
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.
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 (TISSUE FLUID)

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.
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.
Elaborate network of vessels called the lymphatic system collects this fluid and drains it back to the major veins.
Fluid present in the lymphatic system is called the lymph.
Colourless fluid containing specialised lymphocytes which are responsible for the immune responses of the body. (Aipmt 2009)
Important carrier for nutrients, hormones, etc.
Fats are absorbed through lymph in the lacteals present in the intestinal villi.

CIRCULATORY PATHWAYS

Two types – (i) Open (ii) Closed.

Open circulatory system - Present in arthropods and molluscs in which blood pumped by the heart passes through large vessels into open spaces or body cavities called sinuses.
Closed circulatory system - Annelids and chordates have in which the blood pumped by the heart is always circulated through a closed network of blood vessels.

More advantageous as the flow of fluid can be more precisely regulated.

All vertebrates possess a muscular chambered heart.

i. 2-chambered heart - Fishes have with an atrium and a ventricle.

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).

ii. 3-chambered heart - Amphibians and the reptiles (except crocodiles) with two atria and a single ventricle.

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.
mixed up in the single ventricle which pumps out mixed blood (incomplete double circulation)

iii. 4-chambered heart - crocodiles, birds and mammals possess with two atria and two ventricles.

Birds and mammals, oxygenated and deoxygenated blood received by the left and right atria respectively passes on to the ventricles of the same sides.
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. (NEET 2015)

Human Circulatory System

Blood vascular system consists of a muscular chambered heart, a network of closed branching blood vessels and blood, the fluid which is circulated.
Heart - 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.
entire heart is made of cardiac muscles.
Protected by a double walled membranous bag, pericardium, enclosing the pericardial fluid.
Heart has four chambers, two relatively small upper chambers called atria and two larger lower chambers called ventricles.
Walls of ventricles are much thicker than that of the atria.
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.

Atrium and the ventricle of the same side are also separated by a thick fibrous tissue called the atrio-ventricular septum.
Each of these septa are provided with an opening through which the two chambers of the same side are connected.
Opening between the right atrium and the right ventricle is guarded by a valve formed of three muscular flaps or cusps, the tricuspid valve.
Bicuspid or mitral valve guards the opening between the left atrium and the left ventricle.
openings of the right and the left ventricles into the pulmonary artery and the aorta respectively are provided with the semilunar valves. (NEET 2018)
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 and prevent any backward flow.
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).
Same tissue 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, atrioventricular bundle (AV bundle) continues from the AVN which passes through the atrio-ventricular septa to emerge on the top of the interventricular septum and immediately divides into a right and left bundle.
Branches give rise to minute fibres throughout the ventricular musculature of the respective sides and are called purkinje fibres.
Nodal musculature has the ability to generate action potentials without any external stimuli, i.e., it is autoexcitable.
Number of action potentials that could be generated in a minute vary at different parts of the nodal system.
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.
SAN is called the pacemaker.
Our heart normally beats 70-75 times in a minute (average 72 beats min–1).

Cardiac Cycle

Cardiac Cycle - sequential event in the heart which is cyclically repeated consists of systole and diastole of both the atria and ventricles.
begin with, all the four chambers of heart are in a relaxed state, i.e., they are in joint diastole.
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.
Semilunar valves are closed at this stage.
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.
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. (NEET 2012)
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.
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.
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.
blood now once again moves freely to the ventricles.
Ventricles and atria are now again in a relaxed (joint diastole) state, as earlier.
SAN generates a new action potential and the events described above are repeated in that sequence and the process continues.

Duration of a cardiac cycle is 0.8 seconds.

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.

(NEET 2019)

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.
first heart sound (lub) is associated with the closure of the tricuspid and bicuspid valves.
second heart sound (dub) is associated with the closure of the semilunar valves.

(NEET 2015)

Electrocardiograph (ECG)

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.
Machine with three electrical leads (one to each wrist and to the left ankle) that continuously monitor the heart activity.
Detailed evaluation of the heart’s function, multiple leads are attached to the chest region.
Each peak in the ECG is identified with a letter from P to T that corresponds to a specific electrical activity of the heart.
P-wave represents the electrical excitation (or depolarisation) of the atria, which leads to the contraction of both the atria.
QRS complex represents the depolarisation of the ventricles, which initiates the ventricular contraction.
Contraction starts shortly after Q and marks the beginning of the systole.
By counting the number of QRS complexes that occur in a given time period, one can determine the heart beat rate of an individual.

(iii) T-wave represents the return of the ventricles from excited to normal state (repolarisation).

end of the T-wave marks the end of systole.
Reduction in its size signifies coronary ischemic, i.e. when the heart muscles receive insufficient oxygen as in arteriosclerotic heart disease. (NEET 2009,11,13,2019) also diagram.
Any deviation from this shape indicates a possible abnormality or disease. Hence, it is of a great clinical significance.

Double Circulation

The blood flows strictly by a fixed route through Blood Vessels—the arteries and veins.
Each artery and vein consists of three layers:
an inner lining of squamous endothelium, the tunica intima,
a middle layer of smooth muscle and elastic fibres, the tunica media.
- tunica media is comparatively thin in the veins.
an external layer of fibrous connective tissue with collagen fibres, the tunica externa.
Blood pumped by the right ventricle enters the pulmonary artery, whereas the left ventricle pumps blood into the aorta.
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.
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 and provides nutrients, oxygen, carbon dioxide and other
harmful substances away for elimination.
Unique vascular connection exists between the digestive tract and liver called hepatic portal system.
Hepatic portal vein carries blood from intestine to the liver before it is delivered to the systemic circulation. (NEET 2017)
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.
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. (NEET 2014)
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. (NEET 2011) - 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 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)
heart attack (when the heart muscle is suddenly damaged by an inadequate blood supply).