Category : UPSC



  • Oxygen (\[{{O}_{2}}\]) is utilized by the organisms to indirectly break down nutrient molecules like glucose and to derive energy for performing various activities. Carbon dioxide (\[C{{O}_{2}}\]) which is harmful is also released during the above catabolic reactions.
  • It is, therefore, evident that \[{{O}_{2}}\] has to be continuously provided to the cells and \[C{{O}_{2}}\] produced by the cells have to be released out. This process of exchange of \[{{O}_{2}}\] from the atmosphere with \[C{{O}_{2}}\] produced by the cells is called breathing, commonly known as respiration.
  • Respiration involves the following steps :
  • Breathing or pulmonary ventilation by which atmospheric air is drawn in and \[C{{O}_{2}}\] rich alveolar air is released out.
  • Diffusion of gases (\[{{O}_{2}}\] and \[C{{O}_{2}}\]) across alveolar membrane-
  • Transport of gases by the blood.
  • Diffusion of \[{{O}_{2}}\] and \[C{{O}_{2}}\] between blood and tissues,
  • Utilisation of \[{{O}_{2}}\] by the cells for catabolic reactions and resultant release of \[C{{O}_{2}}\].
  • Breathing involves two stages: inspiration during which atmospheric air is drawn in and expiration by which the alveolar air is released out. The movement of air into and out of the lungs is carried out by creating a pressure gradient between the lungs and the atmosphere. Inspiration can occur if the pressure within the lungs (intra-pulmonary pressure) is less than the atmospheric pressure, i.e., there is a negative pressure in the lungs with respect to atmospheric pressure. Similarly, expiration takes place when the intra-pulmonary pressure is higher than the atmospheric pressure.
  • The diaphragm and a specialised set of muscles - external and internal intercostals between the ribs, help in generation of such gradients. Inspiration is initiated by the contraction of diaphragm which increases the volume of thoracic chamber in the antero-posterior axis.
  • On an average, a healthy human breathes 12-16 times/minute. The volume of air involved in breathing movements can be estimated by using a spirometer which helps in clinical assessment of pulmonary functions.
  • Tidal Volume (TV): Volume of air inspired or expired during a normal respiration. It is approx. 500 mL., i.e., a healthy man can inspire or expire approximately 6000 to 8000 mL of air per minute.
  • Inspiratory Reserve Volume (IRV): Additional volume of air, a person can inspire by a forcible inspiration. This averages 2500 mL to 3000 mL.
  • Expiratory Reserve Volume (ERV): Additional volume of air, a person can expire by a forcible expiration. This averages 1000 mL to 1100 mL.
  • Residual Volume (RV): Volume of air remaining in the lungs even after a forcible expiration. This averages 1100 mL to 1200 mL.                                
  • By adding up a few respiratory volumes described above, one can derive various pulmonary capacities, which can be used in clinical diagnosis,                                  
  • Blood is the medium of transport for \[{{O}_{2}}\] and \[C{{O}_{2}}\] About 97 per cent of \[{{O}_{2}}\] is transported by RBCs in the blood. The remaining 3 per cent of \[{{O}_{2}}\] is carried in a dissolved state through the plasma. Nearly 20-25 per cent of \[C{{O}_{2}}\] is transported by RBCs whereas 70 per cent of it is carried as bicarbonate. About 7 per cent of \[C{{O}_{2}}\], is carried in a dissolved state through plasma,                                                    
  • Haemoglobin is a red coloured iron containing pigment present in the RBCs. \[{{O}_{2}}\] can bind with haemoglobin in a reversible manner to form oxyhaemoglobin. Each haemoglobin molecule can carry a maximum of four molecules of \[{{O}_{2}}\].
  • Binding of oxygen with haemoglobin is primarily related to partial pressure of \[{{O}_{2}}\]. Partial pressure of \[C{{O}_{2}}\] hydrogen ion concentration and temperature are the other factors which can interfere with this binding.
  • \[C{{O}_{2}}\] is carried by haemoglobin as carbamino-haemoglobin (about 20-25 per cent),
  • Human beings have a significant ability to maintain and moderate the respiratory rhythm to suit the demands of the body tissues. This is done by the neural system. A specialized centre present in the medulla region of the brain called respiratory rhythm centre is primarily responsible for this regulation.
  • Asthma is a difficulty in breathing causing wheezing due to inflammation of bronchi and bronchioles.
  • Emphysema 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 Disorders: In certain industries, especially those involving grinding or stone-breaking, so much dust is produced that the defense mechanism of the body cannot fully cope with the situation. Long exposure can give rise to inflammation leading to fibrosis (proliferation of fibrous tissues) and thus causing serious lung damage. Workers in such industries should wear protective masks.
  • Cells utilise oxygen for metabolism and produce energy along with substances like carbon dioxide which is harmful.
  • Respiration are exchange of respiratory gases at the tissues and cellular respiration in which oxidation of glucose releases energy and carbon dioxide. Respiration in the presence of oxygen is termed aerobic respiration. Respiration in the absence of oxygen is called anaerobic respiration.                               
  • Our muscle cells can also respire anaerobically, but only for a short time, when there is a temporary deficiency of oxygen. During heavy exercise, fast running, cycling, walking for many hours or heavy weight lifting, the demand for energy is high. But the supply of oxygen to produce the energy is limited. Then anaerobic respiration takes places in the muscle cells to fulfil the demand of energy.
  • Breathing means taking in air rich in oxygen and giving out air rich in carbon dioxide with the help of respiratory organs. The taking in of air rich in oxygen into the body is called inhalation and giving out of air rich in carbon dioxide is known as exhalation. It is a continuous process which goes on all the time and throughout the life of an organism.
  • The number of times a person breathes in a minute is termed as the breathing rate.
  • Yeasts are single-celled organisms. They respire anaerobically and during this process yield alcohol. They are, therefore, used to make wine and beer.
  • On an average, an adult human being at rest breathes in and out \[15-18\]times in a minute. During heavy exercise, the breathing rate can increase upto 25 times per minute. While we exercise, not only do we breathe fast, we also take deep breaths and thus inhale more oxygen.
  • The percentage of oxygen and carbon dioxide in inhaled air is 21 per cent oxygen and \[0.04\]per cent carbon dioxide.
  • The percentage of oxygen and carbon dioxide in exhaled air is \[16.4\]per cent oxygen and \[4.4\]per cent carbon dioxide.
  • Animals that live in water need to use the oxygen dissolved in water. Since the amount of dissolved oxygen is fairly low compared to the amount of oxygen in the air, the rate of breathing in aquatic organisms is much faster than that seen in terrestrial organisms. Fishes take in water through their mouths and force it past the gills where the dissolved oxygen is taken up by blood.

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