1. The Respiratory System
Introduction to Gas Exchange Systems

2. Why is it
great to be a jellyfish?

3. Simple body plans save cnidarians from the “gas exchange problem”

4. Carbon Dioxide
Oxygen
Since they do not have to worry about hiving thick tissues, cnidarians can rely of diffusion for gas exchange.

5. The polychaetes

6. Respiratory Membranes in Multicellular Organisms
Must be near an oxygen source.
Must be thin.
Must be near a circulatory system.
Cannot be dry.
Must have a large surface area for diffusion.

7. Parapodia

8. Parapodia
The aquatic polychaetes have solved the gas exchangeproblem with structures called parapodia. The parapodia, illustrated on the left, increase the surface area of the respiratory membrane, allowing for the diffusion of greater concentrations of gases into the organism. The parapodia also contain a body fluid with proteins to bind to gases, thus speeding up the rate of diffusion. Other aquatic organisms such as fishes have evolved gills to increase the surface area of oxygen uptake.

13. As with all living creatures, aquatic animals respire by taking in oxygen and excreting carbon dioxide. We say that they carry out aquatic respiration, as the gas exchanges are made with the gases dissolved in water. This is the case for fish, mollusks (or molluscs UK) and crustaceans. Among the fish, the renewal of respiratory gases is assured by the regular movement of the mouth and the operculum of the gill openings.  A constant flow of water thereby passes through the respiratory organs of fish: these are the gills, which are located behind the opercula in the gill cavities. That which we call the gill opening, or sometimes gill slit, is the opening that connects the gill cavity to the external medium.  There is one of these openings on each side of the fish’s head.
Respiratory movements take place in three steps.  In step one, the fish opens its mouth and water enters the buccal cavity. Then the mouth closes and the water is pushed into the gill cavities. Finally, the opercula are lifted, and water is expelled to the exterior via the gill openings.
Each gill opening leads to four gills. Each gill is divided into two plates which are themselves subdivided into numerous gill filaments bearing numerous small and very fine gill lamellae.The surface area that is in contact with the water is thus quite large.
Gills are very rich in blood vessels. Thus, the gas exchanges take place between the water at  the gill surfaces and the blood in the interior of the gill, by passing through a very thin wall. The oxygen in the water  enters the bloodstream at the level of the gills and is then carried  by the blood circulation to all of the fish’s organs.  In a reverse process, the carbon dioxide excreted by the fish’s organs is carried by the blood  to the gills, where it is expelled into the water outside of  the body.

14. O2 in H2O O2 Used by Fish 10 20 30 40 Water Temp (°C)
Temp. Limit of Inactive Fish (25°C)
O2 in H2O
O2 Used by Fish
There are limits to gas exchange in water. Less oxygen is available at warmer temperatures. An active fish will have a higher body temperature and require more oxygen for metabolic processes. Paradoxically, less oxygen is available at these warmer temperatures. 10 20 30 40
Water Temp (°C)

15. 1L 1L Oxygen content in water 10mL/L Oxygen content in air 200mL/L
Living on land versus in the water has a selective advantage in terms of oxygen availability, since air has more oxygen than water. For example, one liter of water contains 1% oxygen, whereas the same volume of air contains 20% oxygen. One problem, however, is that the respiratory membranes of land-dwelling organisms still require moisture to enable the diffusion of gases. In order to survive on land, they required adaptations that prevent the respiratory membrane from drying out.
1% Oxygen
20% Oxygen

17. The circulation of air in the tracheal tubes enables the transport of respiratory gases to the cells of the organs without having the gases pass through the intermediary of blood, as is the case for animals using pulmonary respiration, or those using aquatic respiration with gills.
The oxygen in the air that enters the respiratory orifices is carried to the organs, where it is used. The carbon dioxide excreted by the organs takes the reverse path.
The circulation of air in the tracheal tubes (ventilation) is carried out in different ways by different insects. For example, among small insects, or those having low levels of activity, the circulation of the air is carried out in a passive fashion, via diffusion. In large insects, or those that are very active, the circulation of air is amplified by rhythmic movements of the body muscles (primarily the muscles of the abdomen and thorax): contraction of the muscles provokes expulsion of air from the tracheal tubes  to the exterior (expiration), while inspiration occurs passively when the muscles are relaxed.
To find out more:
The tracheal tubes are elastic tubes whose interior walls are covered by cuticle. They are reinforced by spiral filaments that keep them open.
Among certain insects, particularly flying insects, the tracheal tubes are dilated in places in order to form air sacs. These are compressed by the muscles during expiration.
Insects have a pair of stigmas on the sides of each segment of the abdomen and thorax. Depending on the species, these can open and close in order to regulate gas exchanges and limit water loss; they can also possess filtering threads to block the entry of foreign bodies, like dust or parasites, into the insect’s body.