The primary function of the respiratory system is to deliver oxygen to the cells of the body’s tissues and remove carbon dioxide, a cell waste product. The main structures of the human respiratory system are the nasal cavity, the trachea, and lungs.
All aerobic organisms require oxygen to carry out their metabolic functions. Along the evolutionary tree, different organisms have devised different means of obtaining oxygen from the surrounding atmosphere. The environment in which the animal lives greatly determines how an animal respires. The complexity of the respiratory system is correlated with the size of the organism. As animal size increases, diffusion distances increase and the ratio of surface area to volume drops. In unicellular organisms, diffusion across the cell membrane is sufficient for supplying oxygen to the cell (Figure). Diffusion is a slow, passive transport process. In order for diffusion to be a feasible means of providing oxygen to the cell, the rate of oxygen uptake must match the rate of diffusion across the membrane. In other words, if the cell were very large or thick, diffusion would not be able to provide oxygen quickly enough to the inside of the cell. Therefore, dependence on diffusion as a means of obtaining oxygen and removing carbon dioxide remains feasible only for small organisms or those with highly-flattened bodies, such as many flatworms (Platyhelminthes). Larger organisms had to evolve specialized respiratory tissues, such as gills, lungs, and respiratory passages accompanied by complex circulatory systems, to transport oxygen throughout their entire body.
The cell of the unicellular algae Ventricaria ventricosa is one of the largest known, reaching one to five centimeters in diameter. Like all single-celled organisms, V. ventricosa exchanges gases across the cell membrane.
Complete the tasks on the list below:
- Read the following two pages in the OpenStax textbook
- Systems of Gas Exchange
- On this page you can skim through Direct Diffusion, Skin and Gills, and Tracheal Systems
- Read the Mammalian Systems section
- Gas Exchange across Respiratory Surfaces
- On this page you can skim down to the Art Connection section, study the image here, and questions that follow
- The Performing spirometry video is interesting, and just 4 minutes long
- Use the Glossary at the bottom of the page to help with the vocabulary for this week
- Use the Review Questions and Free Response questions to check for understanding
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