oxygen is utilised by the organisms to indirectly break down simple molecules like glucose, amino acids, fatty acids, etc., to derive energy to perform various activities and release Carbon dioxide (harmful) in above catabolic reactions.
Process of exchange of Oxygen from the atmosphere with carbon dioxide by the cells is called breathing, commonly known as respiration.

Respiratory Organs

Lower invertebrates like sponges, coelenterates, flatworms, etc., exchange Oxygen with carbon dioxide by simple diffusion over their entire body surface.
Earthworms use their moist cuticle.
Insects have a network of tubes (tracheal tubes) to transport atmospheric air within the body.
Special vascularised structures called gills (branchial respiration) are used by most of the aquatic arthropods and molluscs.
Vascularised bags called lungs (pulmonary respiration) are used by the terrestrial forms for the exchange of gases.
In vertebrates, fishes use gills whereas amphibians, reptiles, birds and mammals respire through lungs.
Amphibians like frogs can respire through their moist skin (cutaneous respiration) also.

Human Respiratory System

External nostrils opening out above the upper lips leads to a nasal chamber through the nasal passage.
Nasal chamber opens into the pharynx, a portion of which is the common passage for food and air.
Pharynx opens through 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.

Bronchi undergoes repeated divisions to form the secondary and tertiary bronchi and bronchioles ending up in very thin terminal bronchioles.
Tracheae, primary, secondary and tertiary bronchi, and initial bronchioles are supported by incomplete cartilaginous rings.
Terminal bronchiole gives rise to a number of very thin, irregular-walled and vascularised bag-like structures called alveoli.
Branching network of bronchi, bronchioles and alveoli comprise the lungs.
Two lungs which are covered by a double layered pleura, with pleural fluid between them which reduces friction on the lung-surface.
Outer pleural membrane is in close contact with the thoracic cavity lining.
Inner pleural membrane is in contact with the lung surface.
Starting with the external nostrils up to the terminal bronchioles constitute the conducting part.
conducting part transports the atmospheric air to the alveoli, clears it from foreign particles, humidifies and also brings the air to body temperature.

Alveoli and their ducts form the respiratory or exchange part of the respiratory system.

Exchange part is the site of actual diffusion of Oxygen between blood and atmospheric air.

Lungs are situated in the thoracic chamber which is anatomically an air-tight chamber.

thoracic chamber is formed dorsally by the vertebral column,
ventrally by the sternum,
laterally by the ribs
lower side by the dome-shaped diaphragm.
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 essential for breathing, as we cannot directly alter the pulmonary volume.

Respiration involves the following steps:

Breathing or pulmonary ventilation by which atmospheric air is drawn in and carbon dioxide rich alveolar air is released out.
Diffusion of oxygen and carbon dioxide across alveolar membrane.
Transport of gases by the blood.
Diffusion of Oxygen and carbon dioxide between blood and tissues.
Utilisation of Oxygen by the cells for catabolic reactions and resultant release of carbon dioxide.

Mechanism of Breathing

- Breathing involves two stages :

Inspiration during which atmospheric air is drawn in
Expiration by which the alveolar air is released out.
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.
Expiration takes place when the intra-pulmonary pressure is higher than the atmospheric pressure.
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.
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.
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.

Expiration - elaxation 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.
We have the ability to increase the strength of inspiration and expiration with the help of additional muscles in the abdomen.
A healthy human breathes 12-16 times/minute.
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 1100 mL. (NEET 2013,2017,2018)
Residual Volume (RV): Volume of air remaining in the lungs even after a forcible expiration. This averages 1100 mL to 1200 mL for this alveoli are not collapse.
Various pulmonary capacities, which can be used in clinical diagnosis.
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). (NEET 2019)
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. (AIPMT 2008)
Total Lung Capacity (TLC): 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.

Exchange of Gases

Alveoli are the primary sites of exchange of gases.
Exchange of gases also occur between blood and tissues.
Oxygen and Carbon dioxide are exchanged in these sites by simple diffusion.
Factors that affects rate of diffusion
on pressure/concentration gradient.
Solubility of the gases
thickness of the membranes involved in diffusion.
Pressure contributed by an individual gas in a mixture of gases is called partial pressure and is represented as pO2 for oxygen and pCO2 for carbon dioxide.
As the solubility of Carbon dioxide 20-25 times higher then that of Oxygen that can diffuse through the diffusion membrane per unit difference in partial pressure is much higher compared to oxygen.
The diffusion membrane is made up of three major layers (NEET 2011, Fig. 17.4)
the thin squamous epithelium of alveoli,
the endothelium of alveolar capillaries
the basement substance (composed of a thin basement membrane supporting the squamous epithelium and the basement membrane surrounding the single layer endothelial cells of capillaries) in between them.
Total thickness is much less than a millimetre. Therefore, all the factors in our body are favourable for diffusion of Oxygen from alveoli to tissue and carbon dioxide from tissue to alveoli.

Transport of Gases

Blood is the medium of transport for oxygen and carbon dioxide.

• Oxygen - about 97% of oxygen is transported by red blood cells RBC in blood. 3% of oxygen is carried in dissolve state through the plasma.

• Carbon dioxide - nearly 22-25 % of carbon dioxide transported by red blood cells.

70 % carbon dioxide is carried as bicarbonate. (NEET 2013,2015)
7 percent of carbon dioxide is carried in dissolved state through plasma.

Transport of Oxygen

Haemoglobin is a red coloured iron containing pigment present in the RBCs.
Oxygen 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 oxygen
partial pressure of carbon dioxide
Hydrogen ion concentration
temperature

Oxygen- Dissociation curve - A sigmoid curve is obtained when percentage saturation of haemoglobin with Oxygen plotted against the partial pressure of oxygen.

Useful in studying the factors like Hydrogen ion concentration or pCO2 etc.

In Alveoli - where high partial pressure of oxygen, low partial pressure of carbon dioxide, low hydrogen ion concentration(high pH) and low temperature.

all are favorable for formation Oxyhaemoglobin.

In tissue - where low partial pressure of oxygen, high partial pressure of carbon dioxide, high hydrogen ion concentration(low pH) and high temperature.

all condition are favourable for dissociation of oxygen from the Oxyhaemoglobin.(NEET 2016)

Every 100 ml of oxygenated blood can deliver around 5 ml of Oxygen to the tissues under normal physiological conditions.

Transport of Carbon dioxide

Carbon dioxide is carried by haemoglobin as Carbamino-haemoglobin. (20-25 %)

Major factors to affect this binding

partial pressure of Oxygen
partial pressure of carbon dioxide

• In tissue - Partial pressure of carbon dioxide is high and partial pressure of Oxygen is low. binding of carbon dioxide occur.

• In Alveoli - Partial pressure of carbon dioxide is low and partial pressure of Oxygen is high.

Dissociation of carbamino-haemoglobin.

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 following reaction :-

Carbon dioxide 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 4ml of carbon dioxide to alveoli.

Regulation of Respiration

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 Carbon dioxide and hydrogen ions.
Increase in these substances can activate this centre, which in turn can signal the rhythm centre to make necessary 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 Carbon dioxide and H+ 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. (NEET 2018,2019)
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. (NEET 2013,2015,2016,2018)
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. Another example is silicosis. (NEET 2013,2017)
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.