Chapter 21: Gas Exchange and Excretory Systems .
Section: Gas Exchange in Multicellular Organisms .
Question: Where are the structures involved in gas exchange located?.
Answer: Organs for gas exchange have evolved differently in various organisms, yet they all have a common function: exchanging gas molecules between cells and the external environment.
Support: 21.1 Variation in Gas Exchange Systems .
Organs for gas exchange have evolved differently in various organisms, yet they all have a common function: exchanging gas molecules between cells and the external environment.
Small multicellular animals, such as planarians, obtain oxygen and give off carbon dioxide directly through their body surfaces.
The respiratory organs found in many animals that live in water are called gills.
Gills provide these water dwellers with a large respiratory surface.
Water contains dissolved oxygen, but only in small amounts.
To satisfy the need for oxygen in a fish, for example, water must constantly pass over the gill surfaces.
Gills are highly efficient gas exchange organs.
The filaments of gills are made up of thin disklike structures lined up parallel with the direction of water movement.
21.2 Gas Exchange in Terrestrial Organisms .
When organisms moved onto land, air became the source of their ocygen.
Water is relatively poor in oxygen.
It contains only about 5 to 10 mL of oxygen per liter of water.
Air , in contrast, is rich in oxygen, containing about 210mL of oxygen per liter of air.
As organisms became fully adapted to land, an entirely new gas exchange apparatus evolved.
Two principal reasons may account for the loss of gills.
First, air is less buoyant than water.
Second, water is lost to the air through evaporation.
In insects , a system of small, branched air ducts, called tracheae carry oxygen throughout the body.
In many vertebrates, lungs are the organs of gas exchange.
Among mammals, including humans, air is taken into and forced out of the lungs through the actions of two groups of muscles, the diaphragm and the rib muscles.
