CIRCULATION OF BLOOD IN HUMAN BODY

The circulatory system comprises of four-chambered heart, arteries, veins and capillaries. As we ,humans, have a closed blood circulation system, the blood flows from the heart through blood vessels to various organs and back to the heart.

THE HEART :

• The heart in the human is about the size of our closed fist.
• It is protected by a thin double walled membranous covering called pericardium. It contains a lubricating pericardial fluid which protect the heart from mechanical injuries.
• The heart consists of four chambers - two upper atria and two lower ventricles.
• As compared to ventricles, the auricles have thinner muscular wall as they have to pump the blood into the very next ventricles whereas ventricles have to pump blood to long distance.

  Blood vessels entering and leaving the heart:

 

    Blood vessels entering the heart:

• The upper two chambers of the heart i.e. right atrium and left atrium receives the blood.
• The right atrium receives two large vessels- anterior vena cava (also known as superior vena cava) and posterior vena cava (also known as inferior vena cava) .Anterior vena cava receives deoxygenated blood from the upper part of the body including head , chest and arms whereas Posterior vena cava receives blood from lower region of the body including legs and abdomen.
• The left atrium receives four pulmonary veins (two from each lungs) bringing oxygenated blood.(It is the only vein in the human body that carry oxygenated blood.)   

    Blood vessels leaving the heart:

• The lower two chambers of the heart i.e. right ventricle and left ventricle pump the blood to the long distance and hence, two large blood vessels arise from here. 
• The Pulmonary artery arises from right ventricle carry deoxygenated blood to the lungs for oxygenation.(It is the only artery in the human body that carry deoxygenated blood.)
• The aorta arises from left ventricle carries oxygenated blood and supply it to all parts of the body.  

    Valves:

• Valves regulate the flow of blood in a single direction i.e. it avoid the back flow of the blood back to vessel carrying it.
• There are four valves present in heart.

1. Tricuspid valve (also known as right atrio-ventricular valve) is located between right auricle and right ventricle to avoid the back flow of the deoxygenated blood back to right auricle. Chordae tendinae hold the valves in the position preventing upturning due to pressure.
2. Biscuspid valve (also known as left atrio-ventricular valve) is present between left auricle and left ventricle.
3. Pulmonary semilunar valves are located at the opening of the right auricle into the pulmonary artery.
4. Aortic semilunar valves are located at the opening of the left ventricle into aorta.

SYSTOLE AND DIASTOLE PHASES OF THE HEART