Parietal Cells and Protons
Stomach acid is primarily made up of chloride and hydrogen ions (the latter of which are often referred to as protons). It is the protons that make the contents of the stomach acidic. Because there is a higher concentration of protons in the stomach acid than there are in the cells surrounding the stomach, these hydrogen ions have to be pumped into the stomach. The cells in charge of this process are called parietal cells and they use a special molecular pump called the hydrogen/potassium ATPase. The parietal cells have a protein that pumps hydrogen ions out of the cell and potassium ions into the cell. Both ions are positively charged and this two-directional transportation is used to keep the parietal cells from developing an imbalance of positive and negative charges within the cell. This protein is powered by a molecule called adenosine triphosphate, also known as ATP, which is used to store and deliver energy throughout the cell. By using energy to pump the protons, the parietal cells are able to concentrate them into the stomach.
Ion Flow and Parietal Cells
In order for the parietal cells to be able to pump protons into the stomach, many different ions have to be transported around to keep the parietal cells from getting electrically charged or becoming alkaline or basic (the opposite of acidic). First, the parietal cell has to make hydrogen ions by breaking down a molecule of water, which turns into a proton and bicarbonate (which is basic). The bicarbonate (which is negatively charged) has to be removed from the cell so it is transferred into the blood in exchange for a chloride ion (which is also negatively charged, keeping the cell from developing an electrical charge). The cell then uses ATP to pump hydrogen ions out and potassium ions in. Because potassium is positively charged and chloride is negatively charged, the cell can then allow the two ions to escape the cell through special channels without altering its electrical balance.
Acid Secretion Control
Stomach acid is produced only when the parietal cells are instructed to pump protons into the stomach. The parietal cells are primarily controlled by three chemical signals: acetylcholine, gastrin and histamine. Acetylcholine is a chemical that is produced by nerves connected to the stomach and it allows the nervous system to increase stomach acid production during eating. Gastrin is a chemical that is made by cells in the stomach when food is present. Finally, histamine is made by cells called enterochromaffin-like cells, which are located outside of the stomach. Although each signal by itself can cause some parietal cell activation, when all three signals are present, the parietal cells begin vigorously pumping protons into the stomach, generating the acid necessary for proper digestion.
