}]]}]}]}] 30:T2794,

### Introduction to Respiration Respiration is the process of exchange of gases ($O_2$ from the atmosphere with $CO_2$ produced by the cells) at various levels. Organisms have evolved different mechanisms for gas exchange depending on their habitat and level of organization. ### Respiratory Organs Different animals use different organs for respiration: - **Sponges, Coelenterates, Flatworms:** Exchange $O_2$ and $CO_2$ by simple diffusion across their entire body surface. - **Earthworms:** Use their moist cuticle. - **Insects:** Have a network of tubes (tracheal tubes) to transport atmospheric air directly to cells. - **Aquatic Arthropods, Molluscs:** Use gills (branchial respiration). - **Reptiles, Birds, Mammals:** Use lungs (pulmonary respiration). - **Amphibians (e.g., frogs):** Can respire through moist skin (cutaneous respiration) and lungs. Humans have a well-developed respiratory system. ### Human Respiratory System The human respiratory system consists of: - **External nostrils:** Open into a nasal chamber above the palate. - **Nasal chamber:** Opens into the pharynx. - **Pharynx:** A common passage for food and air. - **Larynx (sound box):** Cartilaginous box that helps in sound production. During swallowing, the epiglottis (a thin elastic cartilaginous flap) covers the glottis (opening of the larynx) to prevent food entry into the larynx. - **Trachea (windpipe):** A straight tube extending up to the mid-thoracic cavity, dividing at the 5th thoracic vertebra into primary bronchi. - **Bronchi:** Each primary bronchus divides repeatedly into secondary and tertiary bronchi and then into fine terminal bronchioles. - **Bronchioles:** End up in very thin, irregular-walled vascularised bag-like structures called alveoli. - **Lungs:** There are two lungs, enclosed by a double-layered pleura. Pleural fluid between the layers reduces friction. The conducting part (nostrils to terminal bronchioles) transports air, humidifies it, and brings it to body temperature. The respiratory or exchange part (alveoli and their ducts) is the actual site of gas exchange. ### Mechanism of Breathing Breathing involves two stages: inspiration and expiration. 1. **Inspiration:** Intake of atmospheric air. - Diaphragm contracts (flattens). - External intercostal muscles contract, pulling ribs and sternum upwards and outwards. - Increases thoracic volume (antero-posterior and dorso-ventral axes). - Pulmonary volume increases, decreasing intra-pulmonary pressure below atmospheric pressure. - Air rushes into the lungs. 2. **Expiration:** Release of alveolar air. - Diaphragm relaxes (domes upwards). - External intercostal muscles relax, allowing ribs and sternum to return to original position. - Decreases thoracic volume. - Pulmonary volume decreases, increasing intra-pulmonary pressure above atmospheric pressure. - Air is expelled from the lungs. **Forceful breathing:** Involves additional muscles (internal intercostals and abdominal muscles). ### Respiratory Volumes and Capacities - **Tidal Volume (TV):** Volume of air inspired or expired during a normal respiration (approx. 500 mL). - **Inspiratory Reserve Volume (IRV):** Additional volume of air a person can inspire by a forceful inspiration (2500-3000 mL). - **Expiratory Reserve Volume (ERV):** Additional volume of air a person can expire by a forceful expiration (1000-1100 mL). - **Residual Volume (RV):** Volume of air remaining in t