A. General Respiratory Organs (across animal groups)
- Body surface – Site of simple diffusion of O₂ and CO₂ in lower invertebrates (sponges, coelenterates, flatworms).
- Moist cuticle – Used by earthworms for the exchange of O₂ with CO₂.
- Tracheal tubes – A network of tubes in insects that transport atmospheric air directly within the body.
- Gills (branchial respiration) – Special vascularised structures used by most aquatic arthropods, mollusks, and fishes for the exchange of gases.
- Lungs (pulmonary respiration) – Vascularised bags used by terrestrial vertebrates (amphibians, reptiles, birds, mammals) for the exchange of gases.
- Moist skin (cutaneous respiration) – Additional respiratory surface in amphibians (e.g., frogs).
B. Human Respiratory System (14.1.1)
- External nostrils – Openings above the upper lip that lead to the nasal chamber through the nasal passage.
- Nasal chamber / nasal passage – Conducts air from the external nostrils to the pharynx.
- Pharynx – Common passage for food and air; connects the nasal chamber to the larynx.
- Larynx – Cartilaginous box (sound box) that helps in sound production; it opens into the trachea.
- Epiglottis – Thin elastic cartilaginous flap that covers the glottis during swallowing to prevent entry of food into the larynx.
- Trachea – Straight tube extending to the mid-thoracic cavity; divides into right and left primary bronchi; supported by incomplete cartilaginous rings.
- Primary bronchi (right and left) – Arise from the division of the trachea at the level of the 5th thoracic vertebra.
- Secondary and tertiary bronchi – Repeated divisions of the primary bronchi.
- Bronchioles (including terminal bronchioles) – Further divisions of the bronchi; initial bronchioles are supported by cartilage; end in alveoli.
- Alveoli – Very thin, irregular-walled, highly vascularised bag-like structures; primary sites of exchange of gases (O₂ and CO₂) between air and blood.
- Lungs (pair) – Contain the branching network of bronchi, bronchioles, and alveoli; situated in the thoracic chamber; covered by double-layered pleura.
- Pleura (double-layered pleural membranes) – Cover the lungs; pleural fluid between the layers reduces friction on the lung surface.
- Conducting part (external nostrils to terminal bronchioles) – Transports atmospheric air to the alveoli, clears foreign particles, humidifies, and brings air to body temperature.
- Respiratory/exchange part (alveoli and their ducts) – Site of actual diffusion of O₂ and CO₂ between blood and atmospheric air.
C. Structures of the Thoracic Chamber
- Thoracic chamber (thoracic cavity) – Air-tight chamber formed dorsally by the vertebral column, ventrally by the sternum, laterally by the ribs, and inferiorly by the diaphragm; any change in its volume is reflected in lung (pulmonary) volume, enabling breathing.
- Vertebral column – Forms the dorsal boundary of the thoracic chamber.
- Sternum – Forms the ventral boundary of the thoracic chamber; lifted during inspiration.
- Ribs/rib cage – Form the lateral boundaries of the thoracic chamber; raised by external intercostal muscles during inspiration.
- Diaphragm – Dome-shaped muscle forming the lower boundary of the thoracic chamber; contracts to increase thoracic volume in the anteroposterior axis during inspiration; relaxes and arches upwards during expiration.
D. Muscles Involved in the Breathing Mechanism
- External intercostal muscles – Contract to lift the ribs and sternum, increasing thoracic volume in the dorso-ventral axis during inspiration.
- Internal intercostal muscles – Assist in expiration (contraction reduces thoracic volume).
E. Neural Regulatory Centers (14.5 Regulation of Respiration)
- Respiratory rhythm center (in medulla region of the brain) – Primarily responsible for maintaining and moderating the basic respiratory rhythm.
- Pneumotaxic center (in the pons region of the brain) – Moderates the functions of the respiratory rhythm center; neural signals from it reduce the duration of inspiration and alter the respiratory rate.
- Chemosensitive area (adjacent to the respiratory rhythm center in the medulla) – Highly sensitive to increases in CO₂ and H⁺ concentration; activates the rhythm center to adjust respiration and eliminate these substances.
- Receptors associated with the aortic arch and carotid artery – Detect changes in CO₂ and H⁺ concentration and send signals to the respiratory rhythm center for corrective actions.