🗻Big Picture: Lungs are special vascularized bags that are used by terrestrial forms (e.g. humans, apes, etc.) for the exchange of gases.
As we breathe the air enters our body via the external nostrils opening out above the upper lips.
It leads to a nasal chamber through the nasal passage. The pharynx opens through the larynx region into the trachea (Larynx: a cartilaginous box which helps in the production of sound and hence is also known as the sound box)
Next, the air inhaled is led into the trachea which is a straight tube extending up to the mid-thoracic cavity, which divides at the level of the 5th thoracic vertebra into a right and left primary bronchi.
Each bronchus undergoes repeated divisions to form the secondary and tertiary bronchi and bronchioles, ending up in very thin terminal bronchioles.
Each terminal bronchiole gives rise to a number of very thin, irregular-walled and vascularised bag-like structures called alveoli.
🤯 Lungs comprise of the branching network of bronchi, bronchioles and alveoli.
Breathing is just the the process of taking air into and expelling it from the lungs.
Respiration is the process in living organisms involving the production of energy, typically with the intake of oxygen and the release of carbon dioxide from the oxidation of complex organic substances.
🤯 The oxygen metabolises in the mitochondria to prduce ATP- the energy currency of our cells. Meanwhile, the opposite is true for plants. While performing photosynthesis plants release oxygen to produce food for themselves.
👆 Important point to note: Such an arrangement is essential for breathing, as we cannot directly alter the pulmonary volume.
Inspiration: process by which atmospheric air is drawn into the lungs.
Expiration: process by which air in the lungs is expelled into the atmosphere.
Primary site for exchange of gases are the alveoli. Exchange of gases takes place by simple diffusion based on the concentration gradient of the gases.
Two important factors which affect the rate of diffusion of gases are:
The actual exchange of gases takes place based on the difference in the partial pressure of gases, leading to the formation of a concentration gradient i.e. gases diffuse from regions of higher concentration to regions of lower concentrations.
From lungs to blood: The concentration of oxygen in the blood is lower than that of the alveoli. Due to the concentration gradient, oxygen diffuses into the bloodstream from the alveoli.
From tissues to blood: Similar to the above case, a reverse concentration gradient is present for carbon dioxide letting the gas diffuse from tissues to blood and from the blood to the alveoli.
🤯 Solubility of carbon dioxide is much higher than that of oxygen (almost 20 times more!). As a result, the amount of carbon dioxide that can diffuse through the diffusion membrane per unit difference in partial pressure is much higher compared to that of oxygen.
Oxygen is carried by haemoglobin, a red coloured iron-containing pigment. Binding of oxygen with haemoglobin is affected, primarily, by partial pressure of oxygen. When partial pressure of oxygen is high (eg: in alveoli) binding of oxygen with haemoglobin occurs. When partial pressure of oxygen is low (eg: in tissues) oxygen disassociates.
Carbon dioxide is also carried by haemoglobin in the form of carbamino-haemoglobin. Binding of carbon dioxide is also affected by its partial pressure in a region.
🤯 Only 20-25% carbon dioxide is carried as carbamino-haemoglobin. 70% of carbon dioxide in the blood is carried as bicarbonate.
Source: Kleinman et al. N Engl J Med. 1967.
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 necessary adjustments in the respiratory process by which these substances can be eliminated.
Receptors associated with the aortic arch and the carotid artery also can recognise changes in CO2 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.
Asthma: A difficulty in breathing causing wheezing due to inflammation of the bronchi and bronchioles.
Emphysema: 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 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.
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