Respiratory Systems

is in the small intestine where the vast majority of food is absorbed, particularly the jejunum, which is the middle segment of the small intestine. The food that is not absorbed is eliminated through the large intestine and the anus.

RESPIRATORY SYSTEMS

The goal of the respiratory system is to exchange oxygen and carbon dioxide between the animal and the environment. Animals breathe both voluntarily and involuntarily. The amount of air taken in and the rate of breathing is regulated mainly by the brain’s respiratory system. There is inhalation and exhalation. Inhalation fills the lungs with oxygenated air. The oxygen reaches alveoli which are small sacs that exchange the oxygen with carbon dioxide, deep within the lungs. Outside of the lungs, cellular respiration takes place, in which oxygen participates in the breakdown of glucose into CO2, making ATP in the process.

Evolutionarily speaking, there has been a change in the way organisms allow for oxygen and carbon dioxide exchange. As the animal complexity and size has increased, the respiratory system developed structures (the alveoli) with a large surface area in order to allow for the maximal rate of diffusion. The diffusion process is completely passive and goes from an area of high concentration of oxygen and carbon dioxide to a lower concentration of these gases.

For small multicellular organisms, diffusion across an outer membrane is enough to meet their oxygen needs. This works only up to a one-millimeter distance from the cell to the exterior of the animal. This is how flatworms and cnidarians operate; they are flat or tubular so every cell participates in gradual diffusion across the cells.

Other than lungs and diffusion, animals have developed other effective ways to have gas exchange. Earthworms and amphibians respire through their skin. There is a dense interconnected capillary network just below the skin that helps participate in gas exchange. It requires that the skin be moist in order to have gases dissolve and diffuse across this surface.

Water-living organisms use oxygen dissolved in water. Fish and other aquatic organisms will use gills in order to extract oxygen from the surrounding water. Water passes over

the gills and oxygen diffuses into the fish’s circulatory system. Gills are folded and branched so that they take in a great deal of oxygen. Gills are also seen in crustaceans, mollusks, and annelids but these will have coelomic fluid rather than blood. Figure 50 shows the respiratory system of fish:

Insects use tracheal systems in order to respire. The respiratory systems and the circulatory systems in these organisms are separate. Instead, there are tubes that carry oxygen to the entire body efficiently. The tubes are made of chitin as is the rest of the exoskeleton of the insect. Insect bodies have respiratory openings called spiracles along the thorax and abdomen that allow oxygen to pass through the body, regulating the diffusion of oxygen and carbon dioxide. The air enters and exits the tracheal system through the spiracles.

Amphibians have evolved many different ways of breathing in order to survive the different aspects of their life cycle. Younger amphibians like tadpoles have gills in order to breathe; they are confined to water. The gills disappear as the animal grows so that lungs take their place (in many cases). The lungs are more primitive than higher animals with an inefficient or absent diaphragm and the need for additional respiration through the skin.

Birds have a unique respiratory system that differs from other vertebrates. They have small lungs and few air sacs. They do not have diaphragms nor do they have a pleural cavity. The gas exchange does not occur through the alveoli but happens between the air capillaries and the blood capillaries. Because flight takes a great deal of oxygen, they have evolved a relatively sophisticated way to exchange gas. Besides lungs, they have air sacs (a posterior air sac and an anterior air sac) that allows for efficient oxygen uptake, particularly during high altitude situations.

Mammals have lungs that participate in gas exchange. There is air inspired through the nose that is warmed and filtered as it enters the respiratory tract. There is a conducting zone of the upper respiratory tract and a respiratory zone that participates in gas exchange. The latter zone is primarily made up of the alveoli in the lungs. Figure 51 is an image of the human respiratory system: