Overview
This lecture explains how different organisms exchange gases with their environment, focusing on respiratory system structures, the path of air into the mammalian lung, and protective mechanisms.
Gas Exchange in Organisms
- The respiratory system delivers oxygen to cells and removes carbon dioxide waste.
- Unicellular organisms use direct diffusion across their cell membrane for gas exchange.
- Larger animals evolved specialized tissues (gills, lungs) and circulatory systems for efficient gas transport.
Direct Diffusion and Skin Respiration
- Small multicellular organisms (<1 mm) use direct diffusion across outer membranes for gas exchange.
- Flatworms and cnidarians have flat shapes to increase surface area for diffusion.
- Earthworms and amphibians use moist skin (integument) and dense capillaries for gas exchange.
Gills in Aquatic Organisms
- Aquatic animals use gills—highly folded and branched tissues—for gas exchange in water.
- Oxygen diffuses from water (high concentration) into blood (low concentration) in gills.
- Mollusks, annelids, crustaceans, and fish have gills.
Tracheal Systems in Insects
- Insects use a tracheal system—networks of tubes—to transport oxygen directly to tissues.
- Air enters through body openings called spiracles and diffuses through chitin-lined tubes.
Mammalian Respiratory System
- Air enters through the nasal cavity, where it is warmed, humidified, and filtered by mucus and cilia.
- The path of air: nasal cavity → pharynx → larynx → trachea → bronchi → bronchioles → alveoli.
- The right lung has three lobes; the left lung has two (to accommodate the heart).
- Bronchioles branch into alveolar ducts leading to alveolar sacs, where gas exchange occurs.
Gas Exchange in the Lungs
- Alveoli are thin-walled sacs surrounded by capillaries, optimizing gas diffusion.
- Oxygen diffuses from alveoli into blood; carbon dioxide diffuses from blood into alveoli.
- The lungs contain ~300 million alveoli, providing a large surface area (~75 m²) for gas exchange.
Protective Mechanisms of the Respiratory System
- Nasal hairs and mucus trap dust, microbes, and particulate matter.
- Cilia in the bronchi and bronchioles move mucus and trapped particles upwards to be swallowed.
- Smoking damages cilia, resulting in excess mucus and higher risk of respiratory issues.
Key Terms & Definitions
- Diffusion — Passive movement of molecules from higher to lower concentration.
- Gills — Respiratory organs in aquatic animals for gas exchange with water.
- Tracheal system — Network of tubes in insects for gas distribution.
- Spiracles — Openings in insect bodies for air entry into the tracheal system.
- Alveoli — Tiny air sacs in the lungs where gas exchange occurs.
- Cilia — Hair-like projections that move mucus and debris out of airways.
Action Items / Next Steps
- Review the respiratory system video linked in the section.
- Be able to trace the path of air from the external environment to the alveoli.
- Study protective mechanisms and be prepared to explain how the respiratory system defends itself.