Breathing and Exchange of Gases
Cells use O2 to break down glucose, amino acids and fatty acids for energy, releasing harmful CO2. The exchange of O2 from the atmosphere with CO2 produced by the cells is called breathing (respiration).
Respiratory organs across animals
- Sponges, coelenterates, flatworms: simple diffusion over the body surface.
- Earthworms: moist cuticle.
- Insects: network of tracheal tubes.
- Aquatic arthropods and molluscs: gills (branchial respiration).
- Terrestrial forms / fishes use gills; amphibians, reptiles, birds, mammals use lungs (pulmonary respiration). Frogs can also use moist skin (cutaneous respiration).
Human respiratory system
External nostrils -> nasal chamber -> pharynx -> larynx (sound box; epiglottis guards entry of food) -> trachea -> at the 5th thoracic vertebra divides into right and left primary bronchi -> secondary, tertiary bronchi -> bronchioles -> terminal bronchioles -> alveoli. The conducting part (nostrils to terminal bronchioles) transports, cleans, humidifies and warms air; the exchange part (alveoli and ducts) is the site of diffusion. Lungs lie in the air-tight thoracic chamber (dorsal vertebral column, ventral sternum, lateral ribs, lower diaphragm) and are covered by double-layered pleura.Mechanism of breathing
- Inspiration: contraction of diaphragm and external intercostals increases thoracic and pulmonary volume, lowering intra-pulmonary pressure below atmospheric pressure, so air moves in.
- Expiration: relaxation of these muscles reduces volume, raising intra-pulmonary pressure above atmospheric pressure, so air moves out.
- A healthy human breathes 12-16 times/minute; a spirometer measures lung volumes.