What We Should Know About the Physiology of the Pulmonary System

Gas exchange between the blood and the alveoli is brought about by different processes which include:
• Ventilation of the alveoli;
• Mixing of inspired air and alveoli air and
• Diffusion of gases across the alveolar membrane.

Under resting conditions, 5 litres of blood perfuse the pulmonary capillaries, and 6 litres of air enter and leave the lungs every minute. 250-300ml of oxygen is taken by the pulmonary capillary blood to the tissues and 200-250ml of carbondioxide is left out into the alveolar air. With exercise these values go up considerably.

The nervous control of breathing
Respiration is controlled by the respiratory centre situated in the medulla and pons. The respiratory centre consists of inspiratory center, expiratory center and pneumotaxic enter which control the rhythm, depth, and rate of respiration. Principal muscles of respiration are the diaphragm and intercostals muscles. When there is demand for increasing the ventilation, the accessory muscles of respiration come into play.

Chemical control of breathing
Rise in carbon-dioxide tension in the arterial blood is the strongest direct stimulus to the respiratory centre to increase ventilation. Next in importance is hypoxia. Hypoxia stimulates the chemoreceptors of the carotid and aortic bodies attached to external carotid artery and ascending aorta respectively. A fall in pH stimulates breathing directly by its action on the respiratory center.

Cheyne-Stokes breathing
In this type of abnormal rhythm of respiration, there is increase in the rate and depth of respiration, which reach a maximum and then they come down to reach a period of apnea. These cycles repeat. Cheyne-Stokes respiration is indicative of serious functional impairment of the respiratory centre. It is seen in Cardiac failure, metabolic acidosis, increased intracranial tension, narcotic poisoning, and sometimes even during sleep.

Biot's breathing
This is a type of irregular breathing in which 3-4 respirations occur in clusters with apneic pauses. The respiration resembles sighs. This is commonly seen in meningitis and brain damage.

Gas exchange in the alveoli

Air in the conducting airways is functionally inert and its volume is called the "anatomical dead space". Functionally effective ventilation that the alveoli receive is called "alveolar ventilation". When the alveoli are not perfused with blood, ventilation becomes ineffective. Ventilation occurring in alveoli which are not properly perfused with blood is called "dead space ventilation". Gas exchange in the alveoli is adversely affected by uneven ventilation, uneven perfusion or defective diffusion.

Causes of alveolar hypoventilation
• Obstructive airway disease which produces uneven distribution of ventilation.
• Thoracic deformities and neuromuscular disease like myasthenia and poliomyelitis.
• Conditions causing central respiratory failure
• Reduction in functional lung volume as in atelectasis, fibrosis etc.

Hyperventilation
This occurs commonly in major pulmonary embolism, anxiety, neurocirculatory asthenia, meningitis, encephalitis, therapy with drugs like epinephrine, poisoning with salicylates of aspirin, hyperthyroidism, hypoxia, and acidosis. Hyperventilation results in excessive removal of Carbondioxide and consequent alkalosis.