Figure 1http://johnnydissidence.wordpress.com/tag/lung-capillaries/During internal respiration, energy, known as ATP, is also released. This is used in many reactions within cells. The energy needed by the body is stored in food, and the process of tissue respiration releases this energy. Oxygen oxidizes glucose and produces energy. This is known as aerobic respiration. The equation for glucose oxidation is; (http://jeshvin.wikispaces.com/Aerobic+and+Aearobic+Respiration) The main adaptation of the lungs is their extremely large surface area. This is created by millions of tiny alveolar sacs and the ciliated epithelial cells that line them. This increases the contact surface area of ​​oxygen entering the lungs, allowing many more molecules to diffuse simultaneously. The capillaries are also divided into a large surface network which also creates a greater exchange surface for carbon dioxide and oxygen. The second adaptation is that both the alveolar membrane and the capillary wall are surrounded by a membrane only one cell thick. This is important for the diffusion of oxygen and carbon dioxide gases as it means that the distance for the molecules to cross is much smaller. The third way the lungs adapt to increase the rate of diffusion is that both the capillaries and alveoli are made of a semipermeable membrane that allows smaller molecules such as oxygen and carbon dioxide to pass through easily. Fick's law is used to measure the rate of diffusion. He explains that for diffusion to occur successfully, the surface area must be as large as possible and there must be a significant difference between the concentration in each organ structure. The thickness... half the paper... acids and glucose produced by digestion in the hepatic portal vein and liver. The lacteals are small lymphatic vessels and are present in the villi. They absorb fatty acids and glycerol, the products of fat digestion. Layers of circular and longitudinal smooth muscle allow chyme to be pushed along the ileum by waves of muscle contractions called peristalsis. The remaining chyme is passed to the colon (Christopher A Becker). In conclusion, the body has many physiological adaptations to increase the rate of diffusion/absorption. This is specified using Fick's law. The main features are the adaptation of a large exchange surface within each organ system and the close proximity of the surrounding capillary networks. Other adaptations include membranes a single cell thick and the evolution of cells within membranes to create a moist layer to aid diffusion.