 In order to access the energy from their food, animals need to do cell respiration. In order for them to do cell respiration, and not just the less efficient fermentation, every animal cell needs oxygen. Cell respiration also creates carbon dioxide as a waste product, and this needs to be removed or would become dangerous for the cell. Gas exchange in animals is coordinated by a respiratory system, working together with a circulatory system. Note it is not the same as cellular respiration. All animals have some kind of respiratory surface to facilitate the movement of oxygen in and carbon dioxide out of the body. First, the gases are dissolved in water before they can diffuse across cell membranes. Both oxygen and carbon dioxide are only ever passively transported by diffusion, and so the system relies on concentration gradients, respiratory surfaces with very large surface areas and the presence of water. The oxygen supplying medium is the air for land animals and water for aquatic animals. Endotherms, warm-blooded animals, have a much higher rate of energy usage, and because of that, a higher requirement for gas exchange than ectotherms, which are cold-blooded animals. Depending on where they live, their size, and their energy needs, animals have differing structures to facilitate gas exchange. Some simple animals that live in water, like sponges and jellyfish, are thin enough that every cell of their bodies can facilitate diffusion of gases into their bodies from the environment. Earthworms can breathe through all of their skin and only need to circulate gases to the insides of their bodies from there. Most animals, however, have a specific respiratory organ with the very large, usually folded or branched respiratory surfaces. The three most common respiratory organs are trachea, gills, and lungs. Fish, sea stars, and crustaceans have gills. Gills are outward-facing folds of respiratory surfaces. The concentration of oxygen in salt or freshwater is much lower than that in air. It's only about 1% compared to 20% in the air around us. Marine animals have other features to push more water across their gills to increase their uptake of oxygen. Insects have tracheal systems. These are a series of tubes throughout the body that have gas intake holes along the flanks and channel oxygen into different reaches of the insect. Other animals have lungs, which are internal spongy organs filled with branches of ever-decreasing size. Let's look at the respiratory systems of mammals. The key parts at the entry to the lungs are the nasal and oral cavities where air comes in and out. The pharynx, this is the part at the back of your throat where food and air go down, and the larynx. This is the top of the trachea, which moves up when you swallow to let food go down into the stomach. It's also a voice box in most mammals. From there, air goes down the trachea or windpipe and into both lungs. The first tubular branches are the bronchi, and then with each branch further along, they're called bronchioles. At the very ends of the bronchioles are little gas sacs called alveoli. This is where the gas exchange happens. Mammals breathe by negative pressure breathing, basically by sucking air in. This is facilitated by the muscles of the thoracic cavity formed by the rib cage and the diaphragm. They move up, down and outwards to create a lower pressure in the lungs compared within the atmosphere. Gases flow from high to low pressure areas so new air is pulled into the lungs.