Explain the Functions of the Human Circulatory System

Functions of the Human Circulatory System


Human circulatory system consists of following components:

  1. i) Heart
  1. ii) Blood Vessels (Arteries, Veins and Capillaries)

iii) Blood


Structure of Heart:

  1. i) Human heart is muscular contractile organ.
  1. ii) Human heart is located in between the two lungs, beneath the chest bone.

iii) Heart is protected by ribs.

  1. iv) Moreover heart is enclosed in tough, fibrous and extensible membrane called pericardium. Its functions are as under:
  2. It acts as shock absorber.
  3. It protects the heart from friction.
  4. It prevents the over flow of blood.
  5. It keeps the heart moist.
  1. v) Heart muscles are called cardiac muscles.
  1. vi) Heart consists of four chambers.
  2. Two upper atria (Single Atrium).
  3. Two lower ventricles.

Function of Heart:

  1. i) Atria are thin walled independent chambers. They receive blood.
  1. ii) Right atrium receives deoxygenated blood from all parts of the body and left atrium receives oxygenated blood from lungs.

iii) Ventricles are thick walled chambers. They pump blood.

  1. iv) Right ventricle pump deoxygenated blood to lungs and left ventricle pump blood to all parts of body through aorta.
  1. v) Valves are present between atria and ventricles which prevents the backward flow of blood.
  1. vi) The right atrio-ventricular valve consists of three tissues and called tricuspid valve.

vii) The left atrio-ventricular valve consists of two tissues and called bicuspid valve.

viii) The alternating contraction and relaxation of the heart chambers is called the cardiac cycle.

  1. ix) The period of ventricular contraction is called systole and their relaxation is called diastole.

Pulmonary circulation:

In this process right atrium receives deoxygenated blood from all parts of body and send it right ventricle. Right ventricle sends this blood to lungs. This circulation is called pulmonary circulation.

Systemic circulation:

In this process left ventricle receives oxygenated blood from lungs and send it to left atrium which sends this blood to all parts of the body. This circulation is called systemic circulation.

The oxygenated blood is supplied to heart itself through a pair of coronary arteries. They arise from aorta.

2-Blood Vessels:

Blood vessels consists of Arteries, Capillaries and Veins.


  1. i) The vessels that carry blood from heart to all parts of the body are called arteries.
  1. ii) They are made up of three layers.
  2. Innermost layer is called endothelium.
  3. Middle layer is made up of smooth muscles and elastic tissues.
  4. Outer layer is made up of collagen fibers and other supporting fissues.

iii) Because of elastic walls, the arteries stretched when blood enters and then recoil slowly. It is called pulse.

iii) They withstand the high blood pressure and maintain the flow of blood.


  1. i) Arteries on reaching to different body parts divides into very small vessels called capillaries.
  1. ii) They are very thin walled. They consist of single celled layer.

iii) They are so narrow that only one RBC can move in line.

  1. iv) Gases, hormones, and other wastes are exchanged by simple diffusion.
  1. v) They join to form veins.


  1. i) The vessels that bring blood from all parts of body to heart carrying blood back to the heart from all parts of body are called veins.
  1. ii) They are less elastic.

iii) Valves are present in veins which prevent the backflow of blood.


Higher multicellular organisms have a reddish fluid for transport of materials through circulatory system called blood. In a healthy person blood is approximately 8% of body weight. A normal adult has 6-7 liters of blood.

Functions of blood:

  1. i) It helps to transport materials in all the body.
  1. ii) It transports oxygen from lungs to all body cells.

iii) It bring back Co2 from all body cells to lungs.

  1. iv) It brings all wastes from body to kidney for their removal.
  1. v) Blood carries hormones from glands to all body parts.
  1. vi) Beside transport blood also has protective function e.g. blood clotting and phagocytosis of germs.

Composition of blood:

Blood is composed of:


Plasma is complex fluid. It is 55% of whole blood. It consists of water, soluble proteins, glucose, amino acids, lipids and enzymes etc.

Blood Cells:

Blood cells are of three types

  1. i) Erythrocytes (Red blood corpuscles)
  1. ii) Leucocytes (White blood corpuscles)

iii) Thrombocytes (Platelets)

Properties of Erythrocytes or RBC:

  1. i) There are 5 million RBC in 1mm3
  1. ii) They are produced in long bones, ribs and sternum.

iii) In mature mammals RBC loose their nucleus.

  1. iv) They are biconcave and disc shaped.
  1. v) They carry oxygen and other material. They also bring Co2 from all parts of the body back to lungs.
  1. vi) They live for 90-120 days.

Properties of Leucocytes or WBC:

  1. i) There are 5000-7000 WBC in 1mm3.
  1. ii) They are produced in bone marrow, thymus gland and lymph nodes.

iii) They have nucleus.

  1. iv) They are irregular in shape and colourless. They are of several types.
  1. a) They defend the body by destroying bacteria and producing antibodies.
  2. b) They live for 6-7 days.

Properties of Thrombocfles or Platelets:

  1. i) There are 15,00,000 to 35.00.000 Platelets in 1cm3.
  1. ii) In mammals they are not complete but in other vertebrates they are complete cell.

iii) They have not nucleus.

  1. a) They help to clot blood at the wounds and stop bleeding.

Blood pressure:

It is pressure per unit area exerted by blood.

It is measured by a device called sphygmomanometer which has a column of mercury. It is measured from the artery of upper arm. The measurement of blood pressure indicate systolic and diastolic blood pressure. A normal systolic blood pressure should be 12omm of Hg and diastolic blood pressure should be 8omm of Hg. Period of ventricular contraction is called systole and their relaxation is called diastole.

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