Breathing and Exchange of Gases


The process of exchange of O2 from the atmosphere with CO2 produced by the cell is called breathing (commonly known as respiration).
External respiration (Breathing)
Site of occurrence
At the respiratory surface
At the cellular level
Exchange of Gases
O2 of air or water & CO2 of blood.
O2 of blood & CO2 of cells.
Physical process
Physiochemical process
Food is not oxidized, so no energy is produced.
Food is oxidized in mitochondria & energy is produced.
Enzymes are not involved.
A large no. of enzymes are involved.

Characteristics of respiratory surface

  1. It must be thin.
  2. It must be moist either with water or mucus.
  3. Permeable to the respiratory gases.
  4. Should have a large surface area.
  5. Rich in blood supply.

Organs of Respiration

  • Amoeba- cell membrane
  • Lower Invertebrates- body surface
  • Earthworm- moist skin
  • Insects- spiracles/ Trachea
  • Fish – Gills
  • Mammals – Lungs


  • Humans have a pair of external nostrils opening out above the upper lip. It leads to the nasal chamber through nasal passage. Nasal chamber opens into the pharynx which opens through the larynx (sound box) region into the trachea (WindPipe).
  • Trachea is supported by cartilage so it doesn’t collapse when air is not present in it.
  • Trachea further divides into two bronchi which are further divided into bronchioles. Each bronchiole gives rise to a number of very thin & vascularised bag-like structures called alveoli. The branching network of bronchi, bronchioles & alveoli comprise the lungs.


  • Lungs are present in an air- tight thoracic cavity, one on either side of the heart.
  • Lungs are covered by a thin transparent double layered, pleura membrane.
  • Between these layers, pleural fluid is present which allows free- frictionless movement of lungs & protects the lungs from mechanical shock.


Respiration involves the following steps:-
  1. Breathing or pulmonary ventilation by which atmospheric air is drawn in and CO2 rich alveolar air is released out ( Inspiration and Expiration).
  2. Diffusion of gases (O2 & CO2) across alveolar membrane
  3. Transport of gases by the blood
  4. Diffusion of O2 & CO2 between blood & tissues (internal respiration)
  5. Utilization of O2 by the cell for catabolic reactions & resultant release of CO2


Respiratory movement involves two phase:-
  1. Inspiration
  2. Expiration
  • It involves taking in fresh air in the alveoli of the lungs. It includes muscle contraction. So it is an active process.
  • Inspiration is initiated by the contraction of diaphragm which increases the volume of the thoracic chamber which causes an increase in pulmonary volume.
  • The muscles involved in these are phrenic muscles and  intercostals muscles.
  • An increase in the pulmonary volume decreases the intra- pulmonary pressure to less than the atmospheric pressure, which forces the air from outside to move into lungs i.e. inspiration
  • It involves the expelling of foul air out of the body.
  • The expiration is a passive process & simply involves the relaxation of the inspiratory muscles i.e. phrenic & external intercostal muscles.
  • Relaxation of the diaphragm reduces the thoracic volume & thereby the pulmonary volume.
  • This leads to an increase in intra- pulmonary pressure to slightly above the atmospheric pressure causing the expulsion of air from the lungs. i.e. expiration.

Breathing rate of a healthy person is 12-16 times per minute.
Instrument used to measure the volume of air involved in breathing is the Spirometer.

Respiratory Volumes and Capacity

Tidal volume (TV)
Volume of air inspired or expired during normal respiration. It is approx 500ml i.e., a healthy man can inspire or expire approx 6000 to 8000 ml of air per minute.
Inspiratory reserve volume (IRV)
Additional volume of air a person can be inspired by a forcible inspiration. This average is 2500ml to 3000ml.
Expiratory reserve volume (ERV)
Additional volume of air a person can expire by a forcible expiration. This average is 1000ml to 1100ml.
Residual volume (RV)
Volume of air remaining in the lungs even after a forcible expiration. This average is 1100 ml 1200 ml.
Inspiratory capacity (IC)
Total volume of air a person can inspire after a normal expiration. This includes tidal volume and inspiratory reserve volume (TV+IRV).
Expiratory capacity (EC)
Total volume of air a person can expire after a normal inspiration. This includes tidal volume and expiratory reserve volume (TV+ERV).
Functional residual capacity (FRC)
Volume of air that will remain in the lungs after a normal expiration. This includes ERV and RV.
Vital capacity (VC)
The maximum volume of air a person can breathe in after a forced expiration. This includes ERV, TV and IRV or the maximum volume of air a person can breathe out after a forcible inspiration.
Total lung capacity (TLC)
Total volume of air accommodated in the lungs at the end of forced inspiration. This includes RV, ERV, TV and IRV or (VC+RV).


( i)Pulmonary gas exchange (External Respiration)

  • It involves the exchange of Oxygen from alveoli to blood and Carbon dioxide from blood to alveoli.
  • The exchange of oxygen of fresh air and carbon dioxide of blood stream between alveoli and blood, by the method of simple diffusion which is based on pressure/concentration gradient.
  • Pressure contributed by an individual gas in a mixture of gases is called partial pressure and is represented as 'p'.
For example,
pO2 (partial pressure) for oxygen
pCO2 for carbon dioxide.

Uptake of oxygen (by pulmonary blood)
  • The pO2 of gaseous oxygen in the alveoli air is 104mmHg, while that of venous blood in the pulmonary artery on average is 40mmHg.
  • So the initial pressure difference allows the diffusion of oxygen into pulmonary capillary is 64mmHg. This increases blood pO2 as high as 95mmHg which passes through the capillaries.
Uptake of CO2 (by the alveolar air)
  • The pO2 of venous blood (deoxygenated blood) entering the pulmonary capillaries is 45mmHg while that of alveoli air is 40mmHg.
  • So, the initial pressure difference, which allows the diffusion of CO2 into alveolar air is only 5mmHg.
  • The decrease the pCO2 of pulmonary capillaries to about 40mm Hg.
Transport of respiratory gases in blood
Blood is the medium of transport for O2 and CO2
Transport of O2
  • About 97% of O2 is transported by RBCs in the blood. The remaining 3% of O2 is carried in a dissolved state through the plasma.
  • O2 can bind with hemoglobin in a reversible manner to form oxyhemoglobin.
  • Binding of O2 with hemoglobin depends upon following factors:-
    1. Increases with increase in pO2 of alveoli air.
    2. Increases with decrease with pCO2 of blood.
    3. Hb4 + 4O2 = Hb4O8
    4. Low hydrogen ion concentration and low temperature favors formation of oxyhemoglobin.
Oxygen dissociation Curve

O2 dissociation curve is useful in studying the effect of factors like :- pCO2, H+ ion concentration etc on binding of  O2 with hemoglobin.
  • In the alveoli where there is high pO2, low pCO2 loss H+ ion concentration and temperature favors the formation of oxyhemoglobin whereas in the tissue low pO2, high pCO2, high/more H+ ion concentration and high temperature favors for dissociation of oxygen from hemoglobin, so it is clear that O2 can bind to hemoglobin in the lung surface and get dissociation at the tissue.
Transport of CO2
  • Nearly 20 to 25% of CO2 is transported by RBCs whereas 75% of it is carried as bi- carbonate and about 7% of it is carried out by plasma in the form of carbonic acid.
  • CO2 is carried by hemoglobin as carbamino- hemoglobin.
  • The factors which affect the binding are:- pCO2 & po2.
  • When pCO2 is high & pO2 is low in the tissue more binding of CO2 occurs where as when pCO2 is low & pO2 is high in alveoli, dissociation of CO2 from carbmino- hemoglobin takes place.
  • RBCs contain high concentration of carbonic anhydrase enzyme which facilitates the following reactions in both> CO2 + H2O----H2CO3----HCO3- + H+
  • At the tissue site pCO2 is high, so CO2 diffuses into blood & forms HCO3- & H+.
  • At the alveolar side pCO2 is low which leads to the formation of CO2 & H2O. Thus CO2 trapped as bi-carbonate at the tissue level & transported to alveoli is released out as CO2.


  • In internal respiration, the exchange occurs between the oxygen of blood and the CO2 of the body cells. At the level of body tissue low pO2 favors the dissociation of oxy-hemoglobin to free O2 for internal respiration.
  • When the arterial blood reaches the peripheral tissue, it pO2 is still 95mm Hg while that of interstitial fluid is only 20mm Hg. This high initial pressure difference causes the rapid diffusion of O2 from blood into tissue.
  • At the cellular level, the pCO2 is about 45mm Hg while that of arterial 40mm Hg, so CO2 diffuses rapidly from body cell to blood capillaries through tissue fluid.

Regulation of Respiration

  • Basic rhythm of respiration is regulated by the respiratory rhythm centre present in the medulla region of the brain.
  • Another centre present in the pons region of the brain called pneumotaxic centre can moderate the function of respiratory rhythm centre.

Disorders Of Respiratory System

It is a difficulty in breathing causing wheezing due to inflammation of bronchi & bronchioles.

It is a chronic disorder in which alveolar walls are damaged due to which respiratory surface is decreased. One of the major causes of this is cigarette smoking.

Occupational Respiratory Disorder:-
In certain industries, especially those involving grinding or stone breaking, so much dust is produced. Long exposure of this can give rise to inflammation leading to fibrosis (proliferation of fibrous tissues), thus causing serious lung damage. Workers in such industries should wear protective masks.

It is the deficiency of oxygen in the body fluid.

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