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 210
