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Human Circulatory System

The human circulatory system which is also known as blood vascular system comprises muscular chambered heart, a network of closed branching blood vessels and fluid in the form of blood.

Blood vessels – Arteries, Capillaries, and Veins. Arteries can carry blood from the heart to the of the parts of the body. Veins can carry blood from organs to the heart.

Let’s see their features in detail.


  • Have thick walls of muscles.
  • These help in carrying blood away from the heart. All arteries except pulmonary artery carry blood from the heart to other parts.
  • Pulmonary artery brings blood from the heart to the lungs and hence termed as Pulmonary artery as term pulmonary indicates its relation to lungs.
  • These have internal lumens that are small passages for blood.
  • These have relatively high pressure as the heart pushes the blood with the pumping.


  • Its function is to carry blood from organs to the heart.
  • It always carries deoxygenated blood except for the pulmonary vein which carries blood from the lungs to the heart.
  • The walls are thin.
  • The internal lumen is large.
  • These have low blood pressure.


  • Found in the lungs and muscles.
  • These are very fine and thin
  • These have low blood pressure
  • Capillaries function in exchange of gases. Through capillaries, the oxygen reaches the tissues while tissue also gives out the carbon dioxide into the capillaries.


  • It is mesodermally formed organ
  • It is located between two lungs in the thoracic cavity.
  • It is slightly tilted towards left.
  • The size of the heart is roughly the size of a clenched fist.
  • The heart is enveloped in a double walled bag of membrane called pericardium which encloses the pericardial fluid.
  • The heart has four chambers.
    • Two small upper chambers – Atria
    • Two large lower chambers – Ventricles
  • Interatrial Septum – It is a thin, muscular wall that separates the right and the left atria.
  • Atrioventricular Septum – It is a thick fibrous tissue that separates the left and the right ventricles.
  • Valves – These septa have an opening between the right atrium and right ventricle and is shielded by a valve made of three muscular cusps or flaps, the tricuspid valve, bicuspid valve or mitral valve. These two valves guard the opening between the left atrium and the left ventricle.
    Semilunar valves guard the opening of the right and left ventricles that extends to the pulmonary artery and the aorta respectively.
    The function of Valves – The valves allow the blood flow in one direction. It makes them move from atria to ventricles and then from ventricles to the pulmonary artery or aorta. Also, these valves prevent the blood from flowing backwards.
  • Cardiac Muscles – Muscles that make the heart. The walls of ventricles are thicker than the walls of atria. These muscles feature auto excitability and generate a rhythmic contractile activity of the heart.
    Normal heart beating rate is 70 to 75 times in a minute.

Double Circulation

  • The blood which is pumped by the right ventricle enters the pulmonary artery
  • The blood is pumped into the aorta by the left ventricle.
  • Pulmonary Circulation:
    • The deoxygenated blood pumped into the pulmonary artery is passed on to the lungs from where the oxygenated blood is brought by the pulmonary veins into the left atrium.
    • This pathway is called pulmonary circulation.
  • Systemic Circulation:
    • The oxygenated blood entering the aorta is carried by a network of arteries, capillaries, and arterioles to the tissues. Here the deoxygenated blood is collected by a system of veins, vena cava, and veins and is further emptied into the right atrium.
    • This is a systemic circulation.
    • It provides oxygen, nutrients and other essential substances to the tissues and collects carbon dioxide and other toxic substances away for elimination.
  • Hepatic-Portal System
    • It is a unique vascular connection that exists between the liver and the digestive tract.
    • Its function is to carry blood from the intestine to the liver through a hepatic portal vein before it is released in the systemic circulation.
  • Coronary System
    • It is a special system of blood vessels which is exclusively involved in the circulation of blood to and from the cardiac muscles.

Effect of adrenaline and exercise

During exercise, more energy is required by muscle cells than usual. Thus to fulfil this requirement the body will respire more and as a result, will require more glucose and oxygen. To fulfil the requirements, the heart starts pumping vigorously which results in the increase in heart rate and stroke volume.

The changes in the heart rate are easily shown when we are stressed, angry or frightened. It also induces an increase in heart rate and stroke volume.

Adrenaline is the hormone which is released in these stressed conditions. It prepares us for flight or fight and increases the heart rate and stroke volume.

Pulse rate can be observed to feel see the changes in the heart rate. It can be felt at the side of the neck or wrist.

Disorders of the circulatory system

High blood pressure (hypertension)

  • It is the term used for higher blood pressures than normal (120/80).
  • 120 mm Hg – systolic or pumping pressure
    80 mm Hg – diastolic or resting pressure.
  • A pressure above this standard signifies hypertension.
  • High blood pressure is one of the major cause of various heart diseases and also affect the functioning of the kidney and brain.

Coronary Artery Disease

  • It is often known as atherosclerosis.
  • It affects the vessels supplying blood to the muscles of the heart.
  • It is caused by deposition of fat, fibrous tissue, calcium, and cholesterol that makes the lumen of the arteries very narrow.


  • It is also known as ‘angina pectoris’.
  • Acute chest pain occurs when oxygen is not enough in the heart muscles.
  • It can occur in both men and women but it is common among middle-aged and elderly individuals.
  • It happens due to conditions that affect the flow of blood.

Heart Failure

  • When the heart does not pump the blood efficiently and enough to meet the requirements of the body.
  • It is sometimes referred to as congestive heart failure as it often is accompanied by the congestion of the lungs.
  • Heart failure is different from cardiac arrest (the heart stops beating) and heart attack ( the muscles of the heart get damaged by an inadequate supply of blood).