Respiration through lungs is called pulmonary respiration.
The mechanism of pulmonary respiration: It includes breathing movement, exchange of gases, in lungs, transport of gases by blood and exchange of gase by tissue.
The thoracic cavity is a air tight chamber which is enclosed dorsally by vertebral column, ventrally by sternum, laterally by ribs, anteriorly by neck and posteriorly by diaphragm. Diaphragm is a dome-shaped partition of skeletal muscle in between thoracic cavity and abdominal cavity.
Breathing includes two process- inspiration and expiration. Inspiration is a process of intake of air into lungs. It is a active process. When the external intercostals muscles contract the diaphragam becomes flat and space inside thoracic cavity increases. Simultaneously the high pressure air from outside rushes into lungs. Expiration is a process of expulsion of air from the lung.
In this process the internal intercostals muscles contract and diphragam becomes original dome-shaped and the space inside thoracic cavity decreases, lungs are compressed and air is expelled out. In female diaphragm does not take part an important role in inspiration to prevent injury to foetus in uterus? Therefore, ribs play important role in female whereas diaphragm in male.
The recoding of volume of movement of air into and out of lungs is called spirometry and it is measured with the help of spirometer. The volume of air in animal inhales and exhales with each breath is called tidal volume. It averages about 500ml in humans. The maximum volume of air that can be inhaled and exhaled during forced breathing is called vital capacity. It is about 3.5-4.5 litres.
Vital capacity is higher in athletes, mountain dwellers and lower women, old-age, cigarette smoking persons. The volume of air is left in lungs after forceful expiration called residual volume. It is about 1200ml. the volume of air that can not be inspired over and above the normal tidal volume is called inspiratory reserve volume or IRS (3000ml). The amount of air that can not be expired forcefully beyond the normal tidal expiration is called expiratory reserve volume. It is approximately 1100ml. Dead space is the volume of air (150 ml) in nasopharynx, trachea, bronchi which does not available for gas exchange.
Rate of Respiration:
The rate of respiration in humans is 15-25 per minute. In babies, it is 35 per minute.
The Control of Breathing:
Our breathing centers are located in two regions of the brain- the medulla oblongata (with inspiratory centre and expiratory centre) and pons varolli (with pneumotoxic centre). Inspiratory centre stimulates inspiratory muscle (diaphragm) to control the rhythm of respiration. Expiratory center remainsinactive during normal breathing but it controls both expiration and inspiration during exercise.Pneumotoxic centre controls the switch off point of the breathing sending signal to inspiratory centre. The medulla’s control centre detects low pH of tissue fluid during formation of more carbonic acids, the centre increases the rate of breathing to eleiminate the excess CO2. The concentration of O2 has little effect on breathing centers. However, when O2 level is severely depressed, O2 sensors in the aorta and carotid arteries send signals to breathing control center to increase the breathing rate.
Pulmonary Exchange of Gases (External Respiration):
External respiration takes place between blood in alveolar capillaries and alveolar air. The partial pressure of O2 (Po2) in the alveolar air is 100 mm Hg and in venous blood is 40mm Hg. So oxygen from alveolar air diffuses into venous blood in lungs.on the other hand, the partial pressure of CO2 or Pco2 in venous blood is 46mm Hg and in alveolar air is 40 mm Hg. Therefore, CO2 diffuses from venous blood to alveolar air in lungs. However, N2 is physiologically inert with regard to respiration.