The primary function of cardio vascular system is to provide
adequate supply, to all the cells of the body, the materials
needed for their proper function and that carries away the
waste products of their metabolism.

It is a well organised transport system, where blood is 
circulated in a closed system and heart acts as a pump.

The cardio cascular system includes
 
    a) Heart
    b) Arteries
    c) Capillaries
    d) Veins

These differ in their structure and functions.

The volume of blood is limited in the body approximately
5 to 6 litres. The same quantity of blood is used again and
again by circulation.

Blood is oxygenated and circulated through the body in 5
minutes. Blood does the same job repeatedly.

Blood gets reduced in the tissues and oxygenated in the lungs.

Blood has to pass alternatively through lungs and tissues.

Circulatory system is divided into 2 functionally opposite parts.

1. Systemic circulation (with high resistance path) - passing
   through the tissues.

2. Pulmonary circulation (with low resistance path) - passing
   through the lungs.

Parts of Heart

Heart has 4 chambers. Upper chambers are called as Atria -
left atrium and right atrium. Lower chambers are called as 
Ventricles - left ventricle and right ventricle.

The 2 left chambers are separated from 2 right chambers by
a partition called as Septum.

Aorta, the main artery starts from left ventricles, which
carries blood to tissues.

Pulmonary trunk starts from right ventricle and carries
de-oxygenated blood to the lungs.

The right atrium receives all the impure blood from the
2 main veins - Inferior vena cava and Superior vena cava.

The left ventricles receives the oxygenated blood from the
lungs through pulmonary veins.

4 chambers of the heart perform 4 different functions.

Circular path

The left ventricle pumps oxygenated blood to the tissues.
It gives up Oxygen and becomes reduced. The reduced blood
comes back to the heart through the veins and is received
by the right atrium.

From the right atrium it passes into the right ventricle,
which pumps it into the lungs. Here it becomes re-oxygenated
and is returned to the left atrium through the pulmonary
veins. Next, it enters the left ventricle and is pumped out
into the greater circulation again. Circulation goe on in
this way.

Systemic circulation begins in the left ventricle and ends
in right atrium.

Pulmonary circulation starts in the right ventricle and ends
in the left atrium.

Hence, right half of the heart is concerned with impure blood
and left half is concerned with pure blood.

2 technical terms are used in connection with heart i.e
Systole and Diastole.

Systole means contraction and Diastole means relaxation.

Valves of the heart

There should not be any mixture of arterial (pure) and venous
(impure) blood. In other words, circulation must be strictly
one way. This is regulated by the action of the values.
There are 4 sets of valves in the heart.

The right atrioventricular opening is gaurded by tri-cuspid
valve.

The left atrioventricular opening is gaurded by bi-cuspid
valves.

The opening of aorta and pulmonary artery are gaurded by
semi-lunar valves.

Heart is a miracle of constants. The 2 ventricles contract
simultaneously as also the 2 atria.

The same amount of blood passes out of the ventricles at the
same time during systole.

The same amount of blood enters the heart at the same time
during diastole.

Any discrepancy in the time or in the quantioty of blood
may ultimately cause heart failure.

Structure of cardio vascular system

The blood flow is as follows.

    Aorta
    Big arteties
    Small arteries
    Arterioles
    Capillaries (7-9 um diameter)
    Venules
    Veins
    Vena cava

Valves of the veins

Valves are present in most of the veins, particularly of those
of the lower limb. These valves present backflow from the
heart.

Factors that maintain circulation

a) Pumping action of the heart
b) Elastic nature of the arteries
c) Pressure gradient
 
   Blood pressure gradually falls from the left to the right
   side of the heart. In the big arteries the average pressure
   is 120 mm of Hg, in the arterioles the pressure sharply
   falls to about 50 - 60 mm of Hg, in the capillaries the
   pressure is about 15 mm of Hg. In the veins, the pressure
   falls further while near the heart the pressure is
   0 mm of Hg.

   Due to this pressure gradient, blood passes from the higher
   to the lower pressure i.e from the left to the right side
   of the heart.

d) Respiration

   During inhalation, the thorasic pressure falls and abdominal
   pressure rises. Hence, with each inhalation, venous blood is
   sucked up by the thorax and is pumped out by the abdomen.
   In this way, respiration helps venous return and acts as a
   great force in maintaining circulation.

e) Muscular exercise

   When muscles contract, they squeeze the capillaries and veins
   and thus help venous return. This is aided by the valves of
   veins which prevent the passage of blood back towards the
   capillary bed.

f) Effect of gravity

   Above the level of heart, it helps venous return. But, below
   the level of heart, it works against it.

Heart beats at the rate of 75 times per minute.