- Breathing: (External respiration) the process of exchange of O2 from the atmosphere with CO2 produced by the cells.
RESPIRATORY ORGANS :
- Direct respiration by diffusion from the environment – sponges, coelenterates, flat worms etc.
- Cutaneous or by skin – earthworm.
- Tracheal system – insects.
- Gills – aquatic arthropods mollusks
- Lungs – terrestrial forms.
HUMAN RESPIRATORY SYSTEM:
- External nostril opens into the nasal chamber through nasal passage.
- The nasal chamber opens into the nasopharynx.
- Nasopharynx opens through glottis of the larynx into the trachea.
- Larynx is a cartilaginous box which produce sound hence called sound box.
- Cartilaginous epiglottis covers the glottis during swallowing to prevent entry of food into trachea.
- Trachea is a straight tube extending up to themed-thoracic cavity, which divides into right and left primary bronchi at the level of 5th thoracic vertebra.
- Each bronchus undergoes repeated divisions to form the secondary and tertiary bronchi and bronchioles ending up in very thin terminal bronchioles.
- Trachea, primary, secondary and tertiary bronchi and initial bronchioles are supported by cartilaginous rings.
- Each terminal bronchiole gives rise to a number of very thin, irregular-walled and vascularised bags like structures called alveoli.
- The branching network of bronchi, bronchioles and alveoli comprises the lungs.
- There are two lungs which are covered by a double layered pleura, with pleural fluid in them.
- Lungs are situated in the thoracic chamber which is anatomically a air tight chamber.
- The thoracic chamber is formed –
- Dorsally by vertebral column.
- Ventrally by sternum.
- Laterally by ribs.
- On the lower side by dome shaped diaphragm.
- Respiration involves in following steps –
- Breathing or pulmonary ventilation by which atmospheric air is drawn in and CO2 rich alveolar air is released out.
- Diffusion of gases (O2 and CO2) across alveolar membrane.
- Transport of respiratory gases by blood.
- Diffusion of O2 and CO2 between blood and tissues.
- Utilization of O2 by the cells for catabolic reactions and resultant release of CO2.
MECHANISM OF BREATHING :
Inspiration :
- Intake of atmospheric air into the lungs.
- It occurs if the pressure within the lungs (intra-pulmonary pressure) is lower than the atmospheric pressure.
- Contraction of diaphragm which increases the volume of thoracic chamber in the anterior posterior axis.
- The contraction of external intercostals muscles lifts up the ribs and the sternum causing an increase in the volume of thoracic chamber in the dorso ventral axis.
- It causes an increase in pulmonary volume decrease the intra-pulmonary pressure to less than the atmospheric pressure.
- It forces the air out side to move in to the lungs, i.e, inspiration.
Expiration :
- Relaxation of diaphragm and inter-costal muscles returns the diaphragm and sternum to their normal positions and reduce the thoracic and pulmonary volume.
- It increases in intrapulmonary pressure slightly above the atmospheric pressure.
- It causes the expulsion of air from the lungs, i.e, expiration.
- A healthy man breathes 12-16 times/minutes.
- The volume of air involved in breathing is estimated by spirometer.
Respiratory Volumes and Capacities :
- Tidal volume: volume of air inspired or expired during a normal breathing. It is about 500 ml.
- Inspiratory reserve volume: Additional volume of air, a person inspire by a forceful inspiration. It is about 2500-3000 ml.
- Expiratory reserve volume: Additional volume of air, a person expires by a forceful expiration. It is about 1000-1100 ml.
- Residual volume: Volume of air remaining in the lungs even after a forceful expiration. It is about 1200 ml.
- Inspiratory capacity: it includes tidal volume and Inspiratory reserve volume.
- Expiratory capacity: it includes tidal volume and expiratory reserve volume.
- Functional residual capacity: This includes ERV+RV.
- Vital capacity: IRV + TV + ERV.
- Total lung capacity: RV + IRV + TV + ERV
