Coupon Accepted Successfully!



The primary function of the respiratory system is to supply the blood with oxygen in order to deliver oxygen to all parts of the body. The respiratory system does this through breathing. When we breathe, we inhale oxygen and exhale carbon dioxide. This exchange of gases takes place in this system - means of getting oxygen to the blood.
In the human body the respiratory organs consist of
  1. Nasal passage
  2. Trachea and larynx
  3. Bronchi
  4. Lungs
  5. Diaphragm
  6. Rib muscles
The organs of respiration are shown in the figure.

Parts of Human Respiratory System


Breathing involves inspiration and expiration. During inspiration the lungs get filled with air due to inflation of the lungs. To facilitate this, the ribcage moves outward and forward. The diaphragm which is normally dome-like, flattens. Expiration is brought about by deflation of the lungs which is facilitated by the ribcage and diaphragm returning to their former position. 


The Lungs

The lungs are a pair of conical hollow organs, the right one being larger than the left one. The lower surfaces of the lungs are concave to accommodate the diaphragm which divides the body cavity into the thoracic and abdominal cavities. The lungs are enclosed in a double layered membrane called pleura. The lungs are well protected in the body case consisting of the breast bone on the front (ventral side) and the ribs on the sides (lateral sides) and the vertebral column on the back (dorsal side).
Structure of Alveoli


The respiratory tract is lined internally with a mucous membrane. There are numerous ciliated cells lining the passage of the trachea and bronchi. The mucous membrane which is kept moist always traps any small particles of dust or microbes in the air so that these do not enter the lung alveoli. The ciliated cells in the trachea and bronchi push up these particles.
The epithelium in the conducting bronchioles (up to the terminal bronchioles) is pseudo-stratified and columnar. The epithelium decreases in height as the tubules become smaller in diameter. In the terminal bronchioles the epithelium is a single layer of columnar or cuboidal cells. These possess cilia. The mucous cells are few in number and muscles are absent in the terminal bronchioles.  Elastic and reticular fibers and muscles are present but cartilage is totally absent.
In the alveoli, the epithelium is extremely thin and made up of a simple squamous non-ciliated epithelium.
The cartilaginous rings in the walls of the trachea and bronchi make them elastic and prevent them from collapsing. The air is allowed to pass freely.

Mechanism of Respiration

The process of respiration involves the breathing-in of the oxygen-rich air from outside (inspiration) and breathing-out of the air loaded with carbon dioxide (expiration).
Mechanism of Inspiration
During inspiration the intercostal muscles in between the ribs pull them upwards and outward. The muscular diaphragm contracts and flattens. Because of these movements of the ribs and diaphragm the chest expands and the volume of the thoracic cavity is increased. The simultaneous expansion of the pleural cavities surrounding the lungs creates a partial vacuum. The result is the rushing in of external air through the trachea into the lungs. The process of inspiration is also called inhalation.
Mechanism of Expiration
The expelling of air or exhalation from the lungs takes place when the size and pressure of the thoracic cavity are reduced. During this process the ribs are lowered by the intercostal muscles and the diaphragm becomes raised to resume its original position. As a result of these movements the size of the thoracic cavity is reduced and the lungs become compressed. The pressure in the lungs is higher than the atmospheric pressure and air rushes out of the lungs through the trachea and nose.
Even after maximum forced exhalation, some air always remains in the lungs. This is called residual volume.
The maximum volume of inspirable air is called inspiratory capacity. ital capacity is the amount of air that can be forcibly exhaled after a maximum inspiration.
Vital capacity is the amount of air that can be forcibly exhaled after a maximum inspiration. Vital capacity is taken as the measurement of pulmonary (lung) function.
The volume of air normally inhaled which is equal to the volume of exhaled air, is called tidal volume.
Gaseous exchange in the Lungs
The movement of air in and out of the lungs and the distribution of air within the lungs are referred to as ventilation of lungs are referred to as ventilation of lungs. The exchange of oxygen (O2) and carbon dioxide (CO2) between alveolar air and lung capillaries, takes place by simple diffusion. The diffusion of gases is caused due to the differential partial pressure of the respiratory gases. Gases move from a high pressure area to a low partial pressure area. Venous blood enters the lung capillaries carrying oxygen at a pressure of 40 mm (at rest) and carbon dioxide at a pressure of 46 mm. The oxygen pressure of the alveolar air is 100 mm and the carbon dioxide pressure is 40 mm. Therefore oxygen passes from the alveolar air into blood and carbon dioxide from blood to air sacs. Arterial blood leaves the lungs, carrying oxygen at a pressure of 100mm and carbon dioxide at a pressure of 40mm. The average tissue fluid pressure of oxygen is 40 mm and carbon dioxide 46 mm. Therefore, oxygen passes from the blood to tissue fluid and carbon dioxide from tissue fluid into blood.


Mechanism of Exchange of Oxygen and Carbon dioxide in the Lungs

Transport of Respiratory Gases

Oxygen Transport

In mammals, the respiratory gases (O2 and CO2) are transported through blood. Blood itself is not the carrier of the respiratory gases. It contains an iron-containing respiratory pigment called haemoglobin which is confined to the red blood cells. The haemoglobin has a special affinity for the respiratory gases. When the blood which is low in O2 reaches the lungs, the haemoglobin of the red blood corpuscles becomes loaded with O2 to form oxyhaemoglobin.
Oxyhaemoglobin on reaching the cellular level, where partial pressure of oxygen is very low, dissociates into free oxygen and reduced haemoglobin. The O2 on reaching the cell oxidises the food and liberates energy and this energy is stored in the form of a energy rich compound called ATP. A small amount of oxygen also gets dissolved in the blood plasma to form a physical solution.
Carbondioxide Transport
The production of CO2 due to the oxidation of digested food by the cells results in high tissue carbon dioxide partial pressure. This causes the diffusion of CO2 from the cells into the blood through the capillary walls. The CO2 in the blood is transported in two ways, through the plasma and through the blood corpuscles. A small amount of CO2 combines with haemoglobin to form carboxy-haemoglobin inside the red blood corpuscles and is transported to the lungs in this form. A greater portion of CO2 reacts with water to form carbonic acid. Bicarbonates are formed from this carbonic acid and these are transported both in the plasma and red blood corpuscles. From these complexes, CO2 is released when blood is oxygenated in the lungs. Nearly 80% of the CO2 in the plasma is transported in the form of bicarbonates of sodium and potassium and the remaining 20% is transported in the form of carboxy-haemoglobin in the red blood cells.
Besides haemoglobin there are other pigments for the transportation of respiratory gases in animals. All vertebrates have haemoglobin. However, other animals possess pigments like haemocyanin and haemerythrin.
Regulation of Respiration
The regulatory centre for respiration is situated in the medulla of the brain. This respiratory centre is extremely sensitive to the concentration of CO2 in the arterial blood. A decrease or increase in the concentration of CO2 in the arterial blood results in slowing or in the acceleration of respiration respectively. This is reflected in changes in the rate and depth of breathing. In case the breath is held by closing the nose with fingers, CO2 builds up in the blood. When the nose opens again, the breathing becomes accelerated.


Hypoxia is a condition of oxygen shortage in the body. This condition can be caused by a variety of reasons. Hypoxia could be due to lack of oxygen in the inspired air. This type of hypoxia is known as arterial hypoxia. The mountain sickness in climbing to high altitudes without the aid of oxygen cylinders is due to arterial hypoxia. The person suffering from mountain sickness experiences breathlessness and headache. The skin and mucous membranes acquire a bluish hue and moderate to severe nervous symptoms may also be seen.
Anaemia literally means ‘no blood’. The word is applied to any condition in which the red blood cell count is significantly reduced. In females RBC count below 4 million/mm3 and in males a count below 4.5 million/mm3 causes anaemia. This condition can occur due to a variety of reasons. Any condition in which the rate of destruction of RBC is excessive and greater than the ability of the bone marrow to cope with, results in anaemia. In case of carbon monoxide (CO) poisoning, some of the haemoglobin gets irreversibly bound up with CO. The carbon monoxide has a higher affinity for haemoglobin than for oxygen. The result is the reduction of the amount of haemoglobin available for carrying oxygen in the blood. This in turn results in a case of hypoxia called anaemia hypoxia which is due to the reduced oxygen carrying capacity of the blood.
Hypoxia may also result when the capacity of the tissues to use the oxygen supplied to them is lost. This type of hypoxia is produced by cyanide poisoning. Cyanides, by blocking the electron transport chain in tissue cells reduce the capacity of the tissue to utilize oxygen.
Excess of CO2 accumulates when a person is suffocated. The simultaneous lack of oxygen and the increased CO2 together cause what is known as asphyxia. This condition is the cause of respiratory and circulatory failures and death in case of suffocation.
Some Common Respiratory Disorders:-
Pneumonia:- Some bacterial infections cause lymph and mucus to accumulate in the alveoli and bronchioles. Pneumonia is a disease caused by bacteria or virus and seriously affects breathing. Failure to provide proper medical attention in case of the disease may prove fatal.
Emphysema:- When the gas exchange area of the lungs is reduced by breaking up of the walls separating the alveoli, it is called emphysema.
Tuberculosis:- Tuberculosis is a disease of the lungs caused by the bacterium Mycobacterium tuberculosis. The bacterium can spread and cause infection in another person through the sputum of the infected person. The mucus expelled from the trachea via the mouth is called the sputum. It contains the trapped dust particles which were not filtered through the nose. Vaccination in the form of treatment is usually followed for the disease. The BCG (Bacille Calmette- Guerin) vaccine effectively controls tuberculosis.


Any irritation of the mucous membrane of the larynx, pharynx or bronchi by particles of dust, food, smoke or inflammation of these parts due to infection usually lead to what is known as cough. It is a response by which the irritants are expelled from the respiratory tract along with air. The forceful passage of the air through the vocal cords in the larynx produces the characteristic sound of coughing. Heavy smoking is known to produce not only persistent coughing but has been proved to be a major cause for lung cancer.
Influenza or Flu
Fever, pain all over the body along with discharge through the nose are usually symptoms of influenza or flu which is caused by a viral infection of the mucous membrane of nose, throat and respiratory tract.
It is caused by the inflammation of the pleural membranes covering the lungs.

Test Your Skills Now!
Take a Quiz now
Reviewer Name