THE CIRCULATORY SYSTEM

The circulatory system or the transport system consists of heart, blood and blood vessels. 

The heart is the main pumping organ of a body which functions continously without stopping for a second. It is made of cardiac muscle whose contraction and relaxation makes a  pulse rate or a heart beat . 

Blood is a red colour fluid freely floating in our body to supply food, nutrients and oxygen to each and every cell and also collecting waste materials from them .

Blood vessels are the closed tubes through which the blood is circulated in our body.

 

Structure of heart 

The heart is a conical structure which lies slightly left in the chest or thoracic cavity. It's anterior end is broader while posterior end is narrower in shape . It is the size of one's fist. It is covered by two membranes called pericardial membranes. The space between these pericardial membranes is filled with a fluid called pericardial fluid which protects the heart from shocks.

Internally heart is divided into four chambers by deep grooves. The upper two chambers are called auricles or atria( atrium - singular) and the lower two chambers are called ventricles.

The muscular walls which divides the heart internally into chambers is called septa ( singular -septum) . The septum that separates  the auricles into two is called interauricular septum . The septum that separates the ventricles into two is called inter ventricular septum .  The auricles and ventricles are separated from each other byva septum called as auriculo- ventricular septum.  There are holes or apertures present in this septum for the flow of blood from  auricles into ventricles .

These apertures are guarded by the muscular flaps called valves . The valve present in the right auriculo-ventricular aperture is called tricuspid valve and the valve present in the left auriculo-ventricular aperture is called bicuspid or mitral valve . These valves facilitates the blood to flow only in one direction and prevents the backward flow of blood . From the left ventricle, arises a biggest artery called systemic aorta  which is guarded by systemic valves  and from the right ventricle arises a pmonary artery guarded by pulmonary valves .

 BLOOD VESSELS ASSOCIATED WITH HEART 

There are three blood vessels which circulates the blood in our body : the arteries,the veins and the capillaries.

The arteries

The arteries originates or arises from the heart and carry the oxygenated blood away from the heart towards the different body organs. They are made of thick walls and hence they do not collapse when they are not filled with blood . They do not have valves in them . The blood flows with a large pressure in the arteries . The biggest artery is an aorta which carry the blood to each and every part of our body. An exception is pulmonary artery that carry the deoxygenated blood  from the heart to lungs.

The veins 

The veins are the blood vessels which originates from the different organs and opens up in heart. They carry the deoxygenated blood from the body organs towards the heart . They have valves in them to prevent backward flow of blood. They have thin walls compared to arteries and the blood pressure is less in them . An exception is pulmonary vein that carry the oxygenated blood from lungs to the heart. 

The veins that carry the blood from the body organs are called caval veins . Those veins which brings the blood from anterior side ( upper) parts of the body such as head, shoulders, etc are called superior vena cava or precaval veins .

Those veins which brings the blood from posterior side( lower parts) of the body such as legs , stomach,kidneys, liver,etc are called inferior vena cave or post caval veins. 

The capillaries

Capillaries are thin blood vessels which supply the blood to each and every cell of our body. They were first observed by Marcello Malphigi in the wings of a bat . The word capillary in Latin means ' the hair' .