Gaseous Exchange In Human Beings

1.0Respiratory System

A pair of external nostrils open into a pair of nasal passages. The nasal passages are highly vascularized, have fine hairs and mucus secreting cells, so that the inspired air gets warmed, moistened and becomes dust free (filter). The dust particles are entrapped in the mucus secreted by mucus cells. The nasal cavities are separated from the oral cavity by a bony palate. It is due to this reason we can breathe while we eat. Epiglottis prevents the entry of food into the respiratory tract. The nasal chambers open into the pharynx. The pharynx provides passage into the trachea or wind pipe via the larynx.

The wall of trachea has incomplete cartilaginous rings (C-shaped), which prevent the trachea from collapsing even if there is not much air in it. Trachea runs down the neck and extends into the thoracic cavity. Trachea divides into bronchi (singular : bronchus). Each bronchus divides and redivides to form finer branches called bronchioles. After repeated divisions each bronchiole ends into a cluster of tiny air chambers called air sacs or alveoli.

Route of Inhaled Air 

External nostrils → Nasal cavity → Pharynx → Larynx → Trachea → Bronchi → Bronchioles → Alveoli.

Lungs

The lungs are a pair of spongy, bag-like organs, which is enclosed by a double layered membrane or covering called pleura. 

Alveoli are structural and functional units of lungs as these are the actual site of gaseous exchange.

There are very large number of alveoli present in lungs which increase the total surface area.  Alveoli are covered with a network of capillaries. 

2.0Exchange of Gases in Alveoli

Exchange of gases takes place in the alveoli. Inhaled air in the alveoli has higher oxygen concentration than that in the blood. Hence, the oxygen diffuses from the alveoli into the blood. It passes first into the blood plasma and then combines with the haemoglobin in the red blood corpuscles to form oxyhaemoglobin. Haemoglobin is the respiratory pigment which is present in the red blood corpuscles (RBC). Haemoglobin has a very high affinity to oxygen.

Carbon dioxide in the blood capillaries is in higher concentration than that in the alveolar air. Therefore, carbon dioxide diffuses from the blood into the alveoli. CO₂ is more soluble in water than oxygen hence CO₂ is mostly transported in the dissolved form in our blood.

3.0Also Read