The respiratory center is gray matter in the pons and the upper Medulla, which is responsible for rythemic respiration. This center can be divided into an inspiratory center and an expiratory center in the Medulla, an apneustic center in the lower and midpons and a pneumotaxic center in the rostral-most part of the pons. This respiratory center is very sensitive to the pCO2 in the arteries and to the pH level of the blood. The CO2 can be brought back to the lungs in three different ways; dissolved in plasma, as carboxyhaemoglobin, or as carbonic acid. That particular form of acid is almost broken down immediately by carbonic hydrase into bicarbonate and hydrogen ions. This process is then reversed in the lungs so that water and carbon dioxide are exhaled. The Medulla Oblongata reacts to both CO2 and pH levels which triggers the breathing process so that more oygen can enter the body to replace the oxygen that has been utilized. The Medulla Oblongata sends nural impulses down through the spinal chord and into the diaphragm. The impulse contracts down to the floor of the chest cavity, and at the same time there is a message sent to the chest muscles to expand causing a partial vacuum to be formed in the lungs. The partial vacuum will draw air into the lungs.
There are two other ways the Medulla Oblongata can be stimulated. The first type is when there is an oxygen debt (lack of oxygen reaching the muscles), and this produces lactic acid which lowers the pH level. The Medulla Oblongata is then stimulated. If the pH rises it begins a process known as the Bohr shift. The Bohr shift is affected when there are extremely high oxygen and carbon dioxide pressures present in the human body. This factor causes difficulty for the oxygen and carbon dioxide to attach to hemoglobin. When the body is exposed to higher altitudes the oxygen will not attach to the hemoglobin properly, causing the oxygen level to drop and the person will black out. This theory also applies to divers who go to great depths, and the pressure of the oxygen becomes poisonous. These pressures are known as pO2 and pCO2, or partial pressures. The second type occurs when the major arteries in the body called the aortic and carotid bodies, sense a lack of oxygen within the blood and they send messages to the Medulla Oblongata.
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