Structure
The main
respiratory structures in man are paired, lobed, highly spongy and elastic
organs - the lungs. These are located in the chest cavity. Each lung in
turn is made up of millions of tiny, balloon or grape like blind sacs, alveoli.
These alveoli are very thin walled and richly supplied with blood vessels.
These are the actual site of gas exchange. But air or oxygen from the
atmosphere cannot reach these alveoli directly. It has to pass through the
conducting system of air-tubes (Nostrils > Nasal > cavity > larynx
> trachea (wind-pipe)> bronchi > bronchioles). These air-tubes are
highly divided and branched into finer tubes, which carry the oxygen to the
alveoli of lungs and carbon -dioxide in the reverse direction.
Mechanism
Breathing is brought about by alternate
contraction and relaxation of done-shaped structure, known as diaphram and
external costal muscles inserted on ribs. These two together expand the
thoracic cavity by flattening the diaphragm and moving the sterna and rib cage
outwards and upwards, during inspiration. Since, this causes an increase in the
volume of thoracic cavity, the atmospheric air rushes in, due to difference in
pressure between outside and inside. the process is reversed during expiration.
Once the atmospheric air reaches the alveoli,
the exchange of respiratory gases (O₂ & CO₂) occurs by simple diffusion.
The membrane separating the alveoli from blood capillaries is extremely
thin (0.1u) and the inspired air rich in oxygen. The partial pressure of O₂
(PO₂) in inspired air is 100 mm Hg; therefore, oxygen diffuses from alveoli
(high PO₂) to blood vessels with low partial pressure (40mmHg). The blood has
low PO₂) to blood vessels with low PO₂, because it is returning from
the cells or tissues undergoing metabolism. Hence, it's rich in carbon dioxide
and low in oxygen.
Transport
On reaching the blood vessels, major amount of
respiratory gases, particularly oxygen, is transported in combination with the
iron-rich respiratory pigment hemoglobin. Some amount however, is also
transported in the soluble form carbon dioxide is transported in the
form of bicarbonate ions.
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