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Human Circulatory System

human circulatory system transportation

Human circulatory system:

When we talk of transportation in our day to day life, we think of only vehicles on roads, water and air. Apart from virtual transportation, our body also contains a large network of blood vessels for transportation of oxygen, nutrients as well as waste products outside the body. Transportation in human beings is carried out by circulatory system.

Circulatory System:

human circulatory system (2)

There are several components involved in the circulatory system:

  • Blood
  • Heart
  • Lungs
  • Blood vessels
  • Capillaries
  • Lymphatic vessels
  • Lymph


human blood

  • Blood is a fluid (liquid) connective tissue.

Composition of blood:

  • It is mainly composed of plasma and blood cells.
  1. Plasma:
  • Plasma, also called blood plasma, the liquid portion of blood.
  • It contains 90% water.
  • Plasma acts as a transport medium for delivering nutrients to the cells of the various organs of the body.
  • It also transports waste products derived from cellular metabolism to the kidneys, liver, and lungs for excretion.
  • It plays a critical role in maintaining normal blood pressure.
  • Plasma constitutes 55% of the total blood volume.
  1. Blood cells:
  • Blood cells constitute 45% of the total blood volume.
  • There are mainly three types of blood cells:
  • Red blood cells
  • White blood cells
  • Platelets

Difference between RBC, WBC and platelets:

Red Blood Cells
White Blood Cells
Blood Platelets
These blood cells are
circular, biconcave
and disc-shaped.
WBC’s are irregular,
colourless, larger than
RBCs. Also, they have a
large and lobed nucleus.
Blood platelets are oval or
round shaped, cells have no
RBC’s are produced in
long bones of the body.
WBC’s are produced in
the bone marrow, lymph
glands and spleen.
Platelets are formed in the
bone marrow.
The lifespan of RBCs is
120 days.
WBCs have lifespan of
13 to 20 days.
The lifespan of platelets is
8 to 9 days.
RBCs are made up of an
iron-containing respiratory
pigment called haemoglobin.
Haemoglobin transports
oxygen from the lungs to
WBCs provide immunity. Blood platelets play an
important role in blood


Function of blood:

  • The main function of blood is to transport oxygen from the lungs to the tissues and carbon dioxide from the tissues to the lungs.
  • It also carries cellular waste products from the tissues to the kidneys.
  • It supplies nutrients absorbed from the intestine to the tissues.
  • Carries hormones from the place where they are produced to the target organ.
  • The function of WBC is to provide immunity against diseases.
  • Blood platelets helps in clotting of blood thus prevents excess blood loss from the body.
  • It also helps in water balance in tissues and organs of the body.
  • It also maintains the body temperature by distributing the heat in different parts of the body.



  • The heart is a muscular organ in most animals, which pumps blood through the blood vessels of the circulatory system.
  • It is located at the left side in chest cavity between the lungs.
  • The size of heart in human being (adult) is approximately as their fist.
  • The human heart is covered by a double membrane called pericardium. It contains the lubricating pericardial fluid.
  • During the contraction and relaxation of the heart it is the pericardial fluid which provides lubrication.
  • This fluid also protects the heart from mechanical injuries.
  • The heart has 4 chambers:
    a) Two upper chambers e. left atrium and right atrium (thin-walled).
    b) Two lower chambers i.e. left ventricle and right ventricle (thick-walled).
  • The superior vena cava brings deoxygenated blood from head, chest and arms, to the right atrium.
  • The inferior vena cava brings blood from the posterior region of the body, including the abdomen and legs, to the right atrium.
  • The blood from the right atrium enters the right ventricle.
  • From the right ventricle, the blood is enters the lungs through the pulmonary artery.
  • Oxygenated blood from the lungs is enters the left atrium by four pulmonary veins.
  • From the left atrium, the blood enters the left ventricle
  • From the left ventricle, through the aorta oxygenated blood is sent to all parts of the body.
  • Heart valves prevent the backflow of blood or regulate the flow of blood in a single direction.
  • The tricuspid valve is located between the right atrium and the right ventricle has three projections or cups.
  • The bicuspid valve/mitral valve is located between the left atrium and the left ventricle has two projections or cups.
  • Semilunar valves guards the opening of the left ventricle into the aorta and the opening of the right ventricle into the pulmonary artery.

Double circulation:

human circulatory system

  • Human circulatory system consists of two loops inside the body by which blood circulates. That is why, they are said to utilize double circulatory system.
  • Deoxygenated blood from different parts of the body comes to the heart and it pumps this blood throughout the body.
  • The oxygenated blood from the lungs returns to the heart, which it pumps again into different parts of the body.
  • The blood makes one complete round the body thus passes twice through the heart. This is double circulation.
  • The circulatory system has main role in the transport of gases, nutrients, waste products etc.
  • There are two pathways in which blood flows in double circulation. They are:
    a) Pulmonary pathway: The pulmonary pathway carries the deoxygenated blood from the right side of the heart to the lungs. In the lungs, exchange of oxygen and carbon dioxide takes place and the blood is now oxygenated.
    b) Systemic pathway: Now, oxygenated blood travels from the left side of the heart to the other areas of the body. At different organ sites, exchange of gases, nutrients, and waste through lymph occurs. Veins collect the deoxygenated blood from the body parts and pour it back into the right auricle

Blood pressure:


  • It is the pressure exerted by blood on the walls of the blood vessels.
  • The maximum blood pressure during one heartbeat is systolic pressure, and the minimum blood pressure in between two heartbeat is diastolic pressure.
  • A person’s blood pressure is usually expressed in systolic pressure over diastolic pressure.
  • It is measured in millimeters of mercury (mmHg), above the atmospheric pressure in surrounding.
  • Adult human has the normal blood pressure of 120/80 mm Hg.
  • Blood pressure also varies according to the age and health of a person.
  • Low blood pressure is called hypotension, while high blood pressure is also called hypertension.
  • A sphygmomanometer is an instrument used to measure blood pressure.

Blood vessels:

blood vessel

  • The blood vessels are part of the circulatory system and function to transport blood throughout the body.
  • Blood vessels are present throughout the body.

There are mainly three types of blood vessels:

  1. Artery
  2. Vein
  3. Capillaries

Difference between artery vein and capillaries:

Artery  Vein Capillaries
Artery is a blood vessel
having a elastic and thick
Vein is a blood vessel having
thin wall.
Capillaries are very narrow
blood vessel which has very
thin walls.
Arteries carry blood from the
heart to different parts of the
It brings blood from different
parts to the heart.
It forms a network throughout
the body in all living cells
connecting arteries to veins.
On regulatory demand of the
body it can dilate or constrict.
It can’t dilate or constrict
under normal condition.
Capillaries can dilate or
construct according to the
requirement of tissue.
It doesn’t contain any valve. Vein contains valves that
allow the blood to flow in one
direction towards the heart.
It doesn’t have any valve.
All arteries carry oxygenated
blood except pulmonary
All veins carry deoxygenated
blood except pulmonary vein.
It contains mixed blood as it
connects arteries and veins
Aorta is the largest artery. N/A N/A
It has narrow cavity through
which the blood flows.
It has broad cavity through
which the blood flows.

Lymph and Lymphatic System:

lymph and lymphatic system

  • Lymph is the fluid that circulates throughout the lymphatic system.
  • It is formed when the tissue fluid (the fluid which lies in the interstices of all body tissues) is collected through lymph capillaries.
  • Then it is transported through larger lymphatic vessels to lymph nodes.
  • Lymph vessels along with small sac-like structures called lymph nodes form the lymphatic system.
  • The lymphatic system is part of the vascular system and also an important part of the immune system.
  • The lymphatic system is not a closed system as the circulatory system.

Blood clotting or coagulation:

blood coagulation

Coagulation or blood clotting is the process by which blood changes from a liquid to a gel (when lesion/wound/cut), forming a blood clot.

Blood clotting mechanism:

  • Coagulation involves a cellular (platelet) and protein (coagulation factor) component.
  • Platelets immediately form a plug and release an enzyme at the site of injury.
  • This enzyme converts fibrinogen present in the blood plasma into fibrin strands which reinforce the platelet plug.
  • Fibrin along with the trapped RBCs contracts and forms a clot, thus stopping the bleeding.
  • Proteins (coagulation factors) act in a series of chemical reactions to strengthen the plug and allow healing to begin.
Also read,
Life processes
Nutrition in plants Nutrition in animals
Respiration in plants Human respiratory system
Transportation in plants Control and coordination


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