Breathing and Exchange of Gases

Respiration is the biochemical process in which organisms take in air (rich in O2) from the surrounding medium and give out air (rich in CO2) into the same medium.

Different organisms have different mechanisms of breathing.
•    Earthworms use their moist cuticle.
•    Insects respire through a network of tubes called tracheal system.
•    Fish respire through gills.
•    Reptiles, birds and mammalsrespire through the lungs.
•    Amphibians respire through both lungs as well as their moist skin.

Respiration in humans is achieved with the help of a well-developed respiratory system consisting of organs like nose, nasal cavity, nasopharynx, larynx, trachea, bronchi, bronchioles, alveoliand lungs.

Breathing involves two processes: inspiration and expiration.

They occur due to changes in the volume of thoracic and abdominal cavities caused by movement of diaphragm, ribs and intercostal muscles.
Inspiration and expiration are carried out by creating pressure gradientsbetween the atmosphere and the alveoli with the help of specialised muscles.

Volumes of air that are exchanged between lungs and atmosphere are called respiratory volumes.

Some of them are: Tidal Volume (TV), Inspiratory Reserve Volume (IRV), Expiratory Reserve Volume (ERV) Residual Volume (RV), Inspiratory Capacity (IC), Expiratory Capacity (EC), Functional Residual Capacity (FRC), Vital Capacity (VC) and Total Lung Capacity.

During respiration, oxygen and carbon dioxide are exchanged by simple diffusion between the blood and alveolar air, mainly based on pressure gradient.Blood serves as a medium for the transport of gases.
Rate of diffusion of gases is dependent on the partial pressure gradients of oxygen(pO2) and carbon dioxide (pCO2), their solubility as well as the thickness of the diffusion surface.

A sigmoid curve, called oxygen dissociation curve, is obtained when the percentage saturation of haemoglobin with O2 is plotted against the pO2.

Nearly 20-25% of the CO2 is transported by RBCs, whereas 70% of it is carried in the form of bicarbonates.
•    In the RBCs, enzyme carbonic anhydrase catalyses the formation of bicarbonates from carbon dioxide and water. Most of the bicarbonates leak out into plasma in exchange of Cl- ions.
•    In the plasma, CO2 is carried in the form of sodium bicarbonate, while in the erythrocytes as potassium bicarbonates.
•    CO2 is carried by haemoglobin as carbamino-haemoglobin.

The ability to maintain and moderate the respiratory rhythm according to the demands of the body tissues is due to the neural control.
Respiratory rhythm is maintained by the respiratory centre in the medulla region of brain. A pneumotaxic centre in the pons region of the brain and a chemosensitive area in the medulla can alter respiratory mechanism.

Some respiratory disorders are: Asthma, emphysema, occupational respiratory disordersetc.

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