BREATHING AND EXCHANGE OF GASES

Respiration and breathing:

Oxygen (O2) is utilised by the organisms to indirectly break down nutrient molecules like glucose and to derive energy for performing various activities. Carbon dioxide [CO2] which is harmful is also released during the above catabolic reactions. It is, therefore, evident that O2 has to be continuously provided to the cells and CO2 produced by the cells have to be released out. This process of exchange of O2 from the atmosphere with CO2 produced by the cells is called breathing, commonly known as respiration.

Respiratory organs: 

• Lower invertebrates like sponges, coelenterates, flatworms, etc.. exchange O2 with CO2 by simple diffusion over their entire body surface.
•  Earthwonns use their moist cuticle and insects have a network of tubes (tracheal tubes) to transport atmospheric air within the body. 
• Special vascularised structures called gills are used by most of the aquatic arthropods and molluscs whereas vascularised bags called lungs are used by the terrestrial forms for the exchange of gases.
• Among vertebrates, fishes use gills whereas reptiles, birds and mammals respire through lungs.
• Amphibians like frogs can respire through their moist skin also.
• Mammals have a well developed respiratory system.

Human Respiratory system:

• Humans  have a pair of external nostrils opening out above the upper lips. 
• It leads to a nasal chamber through the nasal passage. 
• The nasal chamber opens into nasopharynx. which is a portion of pharynx, the common passage for food and air.
•  Nasopharynx opens through glottis of the larynx region into the trachea.
•  Larynx is a cartilaginous box which helps in sound production and hence called the sound box.
•  During swallowing glottis can be covered by a thin elastic cartilaginous flap called epiglottis to prevent the entry of food into the larynx. 
• Trachea is a straight tube extending up to the mid—thoracic cavity, which divides at the level of 5th thoracic vertebra into a right and left primary bronchi. Each bronchi undergoes repeated divisions to form the secondary and tertiary bronchi and bronchioles ending up in very thin terminal bronchioles. The tracheae, primary, secondary and tertiary bronchi, and initial bronchioles are supported by incomplete cartilaginous rings. 
• Each terminal bronchiole gives rise to a number of very thin, irregular- walled and vascularised bag-like structures called alveoli. The branching network of bronchi. bronchioles and alveoli comprise the lungs
  Humans have two lungs which are covered by a double layered pleura, with pleural fluid between them. It reduces friction on the lung- surface. The outer pleural membrane is in close contact with the thoracic lining whereas the inner pleural membrane is in contact with the lung surface. 
• The part starting with the external nostrils up to the terminal bronchioles constitute the conducting part whereas the alveoli and their ducts form the respiratory or exchange part of the respiratory system.
  The conducting part transports the atmospheric air to the alveoli, clears it from foreign particles. humidifies and also brings the air to body temperature. 
• Exchange part is the site of actual diffusion of O2 and CO2 between blood and atmospheric air.
  The lungs are situated in the thoracic chamber which is anatomically an air—tight chamber. 
• The thoracic chamber is formed dorsally by the vertebral column, ventrally by the sternum, laterally by the ribs and on the lower side by the dome-shaped diaphragm.
•  The anatomical setup of lungs in thorax is such that any change in the volume of the thoracic cavity will be reflected in the lung [pulmonary] cavity. Such an arrangement is essential for breathing. as we cannot directly alter the pulmonary volume.

Respiration involves the following steps:

[i] Breathing or pulmonary ventilation by which atmospheric air is drawn in and CO2 rich alveolar air is released out.
[ii] Diffusion of gases [O2 and CO2] across alveolar membrane.
iii] Transport of gases by the blood.
(iv] Diffusion of O2 and CO2 between blood and tissues.
[v] Utilisation of O2 by the cells for catabolic reactions and resultant release of CO2 (cellular respiration as dealt in the Chapter 14).

 MECHANISM or BREATHING

 Breathing involves two stages 2 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. The contraction of external inter—costal muscles lifts up the ribs and the sternum causing an increase in the volume of the thoracic chamber in the dorso—ventral axis. The overall increase in the thoracic volume causes a similar increase in pulmonary volume. An increase in pulmonary volume decreases the intra-pulmonary pressure to less than the atmospheric pressure which forces the air from outside to move into the lungs, i.e., inspiration
Relaxation of the diaphragm and the inter-costal muscles returns the diaphragm and sternum to their normal positions and reduce the thoracic volume and 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 .Humans have the ability to increase the strength of inspiration and expiration with the help of additional muscles in the abdomen. 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.

 Respiratory Volumes and Capacities

• 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 1 100 mL.
• Residual Volume [RV]: Volume of air remaining in the lungs even after a forcible expiration. This averages l 100 mL to 1200 mL.
• lnspiratory 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+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 forced inspiration.
• Total Lung Capacity: Total volume of air accommodated in the lungs at the end of a forced inspiration. This includes RV, ERV, TV and IRV or vital capacity + residual volume.

REGULATION Of  RESPIRATION

• 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 specialised centre present in the medulla region of the brain called respiratory rhythm centre is primarily responsible for this regulation. 
• Another centre present in the pons region of the brain called pneumotaxic centre can moderate the functions of the respiratory rhythm centre. 
• Neural signal from this centre can reduce the duration of inspiration and thereby alter the respiratory rate.
•  A chemosensitive area is situated adjacent to the rhythm centre which is highly sensitive to CO2 and hydrogen ions. Increase in these substances can activate this centre, which in turn can signal the rhythm centre to make necessaiy adjustments in the respiratory process by which these substances can be eliminated.
•  Receptors associated with aortic arch and carotid artery also can recognise changes in CO2 and Ht concentration and send necessary signals to the rhythm centre for remedial actions. The role of oxygen in the regulation of respiratory rhythm is quite insignificant.

DISORDERS Of  RESPIRATORY SYSTEM

• 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.

Transport of Oxygen

Haemoglobin is a red coloured iron containing pigment present in the RBCs. O2 can bind With haemoglobin in a reversible manner to form oxyhaemoglobin. Each haemoglobin molecule can carry a maximum of four molecules of O2. Binding of oxygen with haemoglobin is primarily related to partial pressure of O2. Partial pressure of CO2, hydrogen ion concentration and temperature are the other factors which can interfere with this binding.
Every 100 ml of oxygenated blood can deliver around 5 ml of O2 to the tissues under normal physiological conditions.

Transport of Carbon dioxide

CO2 is carried by haemoglobin as carbamino-haemoglobin (about 20-25 per cent]. This binding is related to the partial pressure of CO2. p02 is a major factor which could affect this binding. When pCO2 is high and p02 is low as in the tissues. more binding of carbon dioxide occurs whereas, when the pCO2 is low and p02 is high as in the alveoli, dissociationof CO2 from carbamino—haemoglobin takes place, i.e., CO2 which is bound to haemoglobin from the tissues is delivered at the alveoli. RBCs contain a very high concentration of the enzyme, carbonic anhydrase and minute quantities of the same is present in the plasma too. This enzyme facilitates the following reaction in both directions. At the tissue site Where partial pressure of CO2 is high due to catabolism. CO2 diffuses into blood [RBCs and plasma] and forms HCO3and H+. At the alveolar site Where pCO2 is low, the reaction proceeds in the opposite direction leading to the formation of CO2 and H20. Thus, CO2 trapped as bicarbonate at the tissue level and transported to the alveoli is released out as CO2 . Every 100 ml of deoxygenated blood delivers approximately 4 ml of CO2 to the alveoli.

 Oxygen dissociation curve:

A sigmoid curve is obtained when percentage saturation  of haemoglobin with O2 is plotted against the p02. This curve is called the Oxygen dissociation curve and is highly useful in studying the effect of factors like pCO2. H‘ concentration, etc., on binding of O2  is high p02, low PCO2, lesser Ht concentration and lower temperature, the factors are all favourable for the formation of oxyhaemoglobin.

whereas in the tissues. where low p02, high pCO2, high H’ concentration and higher temperature exist. the conditions are favourable for dissociation of oxygen from the oxyhaemoglobin. This clearly indicates that O2 gets bound to haemoglobin in the lung surface and gets dissociated at the tissues.