Respiration is the process by which oxygen is delivered to cells and body systems for use. It consists of levels, several of which are breathing, internal respiration, and external respiration. Breathing is the process by which air is inhaled and exhaled through the mouth. External respiration is the exchange of oxygen and carbon dioxide between the air and blood in the lungs and internal respiration is the delivery of oxygen to cells by hemoglobin in the blood. All three of these levels of respiration involve chemistry, making respiration an essentially chemical process.
             Breathing, the inhaling and exhaling of air, occurs due to the principle of diffusion. Diffusion is the flow of particles from a high concentration to a lower concentration. Diffusion is one of the body's ways of maintaining homeostasis. During inspiration, the pulling in of air, the diaphragm moves downwards, which increases the volume of the thoracic cavity. This increase in volume lowers the air pressure in the alveoli to below atmospheric pressure. The pressure is lowered because the same amount of gas is distributed over a larger area. Because air always flows from a region of high pressure to a region of lower pressure, air rushes in through the respiratory tract. This is diffusion. Expiration, the blowing out of air, works in a similar way. The diaphragm relaxes, which decreases the volume of the thoracic cavity. When the volume decreases, air pressure increases, and air is pushed out through diffusion (Gas Exchange in Humans 1). Neural messages called action potentials are actually what make breathing possible. Motor neurons throughout the body carry an action potential using the neurotransmitter acetylcholine (ACh), a chemical messenger. This action potential tells the diaphragm to contract and relax. An action potential occurs when positive ions, or cations, outside the cell are pumped inside and negative ions, or anions, are pumped to the outside. ...