THE HUMAN RESPIRATORY SYSTEM

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.