Cardiovascular System – Heart and Blood Vessels

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The Heart:


Heart Position     

  • It is a first—sized cone shaped hollow muscular organ that is situated between the two lungs and above the diaphragm and behind the sternum in the chest. The base is tilted to the left. It is guarded by rib cage. The heart contracts at regular intervals, forcing blood through the circulatory system.
  • A thin, tough membrane called the pericardium surrounds and holds the heart in position. This membrane has a slippery surface inside and allows the heart to move easily when it beats. The fluid present in between the pericardial membranes is called the pericardial fluid. It helps to reduce friction between the walls of the heart and surrounding tissues when the heart is beating.

External Structure: 

Heart Anterior View

The Heart wall:

Heart Wall

  • The heart wall has three layers.
  • a) Epicardium: (made up of single layer flat epithelial cells called mesothelium b) Myocardium: (made up of cardiac muscles responsible for beating of heart) c) Endocardium: (made up of single layer flat epithelial cells called endothelium).

Internal Structure of Heart and Working:

Heart Internal Structure

  • Heart is divided into two halves the right side and the left side by a thick, muscular wall called atrioventricular septum.
  • Each half has two chamber the upper auricles (singular atrium) and the lower ventricles. The walls of auricles are comparatively thinner, while walls of ventricles are comparatively thicker, muscular walls. Thus heart has four chambers. The right side of the heart contains de-oxygenated blood while the left side of the heart contains oxygenated blood.
  • The right ventricle pumps blood to the lungs through pulmonary arteries. The right auricle receives blood from the rest of the body through superior vena cava and inferior vena cava. The superior vena cava returns blood from the head and arms; the inferior vena cava from the rest of the body (except the lungs!). The entry of blood from vena cavae is guarded by valves. During joint diastole blood enters from vena cavae inrto right auricle.
  • The entry of blood from right auricle to right ventrical is guarded by an atrioventricular valve having three flaps and called tricuspid valve.
  • During atrial systole auricles contract, the tricuspid valve opens and the blood from right auricle is emptied in right ventricle.
  • The opening of right ventricle in pulmonary arteries is guarded by pulmonary valve. During ventricular systole the tricuspid wall is closed, ventricles contract, pulmonary valve opens. The blood from right ventricle is forced into pulmonary arteries. Pulmonary arteries are further bifurcate into two vessels carrying blood to left and right lungs.
  • This deoxygenated blood gets oxygenated in lungs and return back to left auricle of the heart through four pulmonary veins (two from each lung).
  • During joint diastole the left auricle is filled with blood coming from lungs.
  • The flow of blood from left auricle to left ventricle is guarded by a valve having two flaps called bicuspid valve. During auricular systole the left auricle contracts, the bicuspid or mitral valve opens. Blood is forced into left ventricle.

Heart Systole

  • During ventricular systole the left ventricle contracts, the blood in it is forced at very high pressure through another semi-lunar valve (the aortic valve), into the aorta. Aorta carries blood throughout the body.
  • After moving through different parts of the body, the blood return backs to right auricle of heart through vena cavae.
  • Diastole:

    When all the heart muscles relax, deoxygenated blood from the head and the body enters the right auricle through the superior vena cava and the inferior vena cava respectively. At the same time oxygenated blood from the lungs enters the left auricle through the pulmonary vein. This phase is known as diastole.

  • Systole:

    When auricles and ventricles contract the state is called systole. There are two systole. a) auricular systole and b) ventricular systole. During auricular systole, the auricles filled with blood, contract, forcing the blood into the ventricles. During ventricular systole the right ventricle contracts and forces deoxygenated blood to lungs through pulmonary arteries and left ventricle contracts and force oxygenated blood to different parts of body through aorta.

Blood Vessels:

  • When blood leaves the heart, it is transported around the body by three kinds of blood vessels—arteries, capillaries and veins.
  • Arteries carry blood away from the heart whereas veins carry blood back to the heart.
  • Capillaries are thin blood vessels. They have very thin walls, through which oxygen, food, carbon dioxide  and waste products are exchange between the blood and the body cells.

Heart Blood Vessels


  • Arteries carry oxygenated blood away from the heart to all other parts of the body. Except pulmonary arteries because pulmonary arteries carry deoxygenated blood from the heart to the lungs.
  • Their walls are thick and elastic because the blood is pumped out of the heart under high pressure.
  • The passage way is narrow due to thick walls.
  • The arteries divides to form smaller vessels called arterioles.
  • The arterioles divides many times to form a dense network of capillaries.


  • Veins carry de-oxygenated blood from other parts of the body back to the heart. Except pulmonary veins because pulmonary veins carry oxygenated blood from the lungs to the heart.
  • Their walls are thin compared to the arteries as the blood travels smoothly, at low pressure.
  • The passage way is wider. Veins have valves that prevent the blood from flowing back.
  • Capillaries come together to form venules. Venules further group together to form veins.


  • Capillaries are blood vessels that connect the arteries and the veins.
  • Their thin walls allow oxygen, nutrients, carbon dioxide and waste products to pass to and from the tissue cells.

Blood pressure:

  • Blood moves through our circulation system because it is under pressure, caused by the contraction of the heart and by the muscles that surround our blood vessels.
  • Blood  Pressure is the force of blood against the walls of the arteries. It is measured with an instrument called sphygmomanometer.
  • Usually two readings are taken to know the blood pressure.The systolic pressure as the heart beats.The diastolic pressure as the heart relaxes between beats.
  • Normal systolic pressure is about 120 mm Hg for males; 110 mm Hg for females. Average systolic pressure rises with age. Normal diastolic pressure is about 80 mm Hg for males and 70 mm Hg for females.
  • Blood pressure is lowest when you sleep and  rises when you get up. It can also change when you are excited, nervous or active or perform exercises.
  • Hypertension (high blood pressure) is diagnosed when the diastolic pressure is >10 mm Hg above the normal; the systolic pressure is of less concern.


  • Pulse is the pressure wave that travels through the arteries after each ventricular systole. Pulse can be felt in any artery that lies near the surface of the body.
  • Pulse rate is higher in children, females and in standing position. It is lower in adults, males and in lying position. It increases with disturbing emotional state.

Heart Beat:

  • Diastole and systole produce the two distinct tone sound ‘lub-dub’ of the heart which can easily be heard through an instrument called the stethoscope. These sound constitute the heartbeat.
  • The closing of bicuspid and tricuspid valves makes a sound – ‘lub’.  The closing of semilunar valve causes a sound – dub’.
Science > Biology > CirculationYou are Here
Physics Chemistry  Biology  Mathematics

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