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Respiratory System in Simple and Complex Animals

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The process of diffusion is the exchange of nutrients and wastes between a cell and its environment. For instance, amoeba, a single-celled microorganism can get all the nutrients it needs and get rid of its wastes through diffusion. However, diffusion is more effective in specific distance, hence it limits the size that an individual cell can attain. This means that the larger the size of the animal, the less surface area for diffusion it has (OpenStax Biology, 2015).

Larger animals are multicellular, and they have various systems that allow them to get nutrients and remove wastes, for instance, the circulation brings nutrients and removes wastes, while the respiratory system brings O2 and removes CO2 from cells (OpenStax Biology, 2015).

An example of small animals can be sponges. Sponges have many limitations as they are fixed in one place and cannot go looking for food. Another limitation, which I will be focusing on, is that it has no respiratory organs or system; in fact, the sponge is so simple that it does not have a special area for gas exchanges, therefore they have developed other ways of exchanging gases with the environment (Moore, 2017). Sponges have small pores, called ostia which is where oxygen-containing water is drowned into them, then the water circulates throughout the sponges’ body through an action of cells called phagocyte. The phagocyte cells contain flagella, a structure that moves the water around and through the sponge. Through this process food and oxygen are brought to the sponge and wastes and CO2 are removed. Specifically, the exchange of gas in Sponges occurs through simple diffusion across each cell membrane (Moore, 2017).

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An example of complex animals can be humans. As we all know human and sponges have different ways of breathing and exchanging gas with the environment (Moore, 2017). Humans have the respiratory system which is a network of organs and tissues including lungs and blood vessels. All these parts work together to remove CO2 and to get O2 (Moore, 2017). The respiratory system has various function other than exchanging gases with the environment in fact it allows us to talk and smell, it brings air to the body temperatures and protects the airways from harmful substances (Cleveland Clinic, n.d.). The respiratory system is composed of many parts that work together like mouth and nose, trachea, bronchiole tubes and lungs. From the lungs the oxygen goes to the bloodstream and delivers oxygen to all the organs and tissues. When a person breathes out the blood carries the CO2 and other wastes of the body. The human respiratory system is connected to the circulatory system as O2 needs to be distributed through the bloodstreams (Cleveland Clinic, n.d.).

If we were to compare the sponge and human circulatory system, we can see that the sponge has a less complicated process that works through diffusion, while the human circulatory system is far more complicated as it includes more organs and tissues. Human beings cannot breathe or exchange gases through diffusion as it is too slow for the needs of the human body (Moore, 2017).

The increased size in animals is an adaption, in fact bigger body sizes increase tolerance to extreme environments, reduce mortality, and it reduces the change of the animal to become prey. For instance, throughout the observation of mammalian fossil records, we can see that mammals have evolved to bigger a size which is most likely because of selection pressure.


  1. OpenStax, Biology. OpenStax CNX. May 13, 2015. Chapter 33. The Animal Body: Basic Form and Function. Retrieved from:
  2. Moore, 2017. How Do Sponge Breath? Staining. Retrieved from:
  3. Cleveland Clinic, n.d. Respiratory System. Retrieved from:
  4. Baker, J. , Meade, A., Pagel, M. and Venditti, C. (2015). Adaptive Evolution Toward Larger Size in Mammals. Proceedings of the National Academy of Sciences of the United States of America, 112(16).
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Respiratory System in Simple and Complex Animals. (2022, December 15). Edubirdie. Retrieved December 6, 2023, from
“Respiratory System in Simple and Complex Animals.” Edubirdie, 15 Dec. 2022,
Respiratory System in Simple and Complex Animals. [online]. Available at: <> [Accessed 6 Dec. 2023].
Respiratory System in Simple and Complex Animals [Internet]. Edubirdie. 2022 Dec 15 [cited 2023 Dec 6]. Available from:
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