When a type of substance called a Bronsted acid dissolves in water, it releases hydrogen ions, increasing the hydrogen ion concentration in the liquid. Chemists classify the measure of hydrogen ion concentration as pH: the lower the pH, the higher the concentration of hydrogen ions. Hydrogen ion concentration, or pH, plays a variety of important roles in human physiology.
TL;DR (Too Long; Didn't Read)
Chemists classify the measure of hydrogen ion concentration as pH. The pH scale goes from 0, highly acidic, to 14, highly basic. The pH level plays a variety of important roles in human physiology. Whenever hydrogen ions are in liquid that contains water, hydrogen ions quickly combine with H2O to form hydronium ions, or H3O+.
Proteins are essential to the body’s functioning, and rely on hydrogen bonds to maintain their shapes. The body has to keep the pH at a stable level to make sure the proteins keep their shapes and do their jobs. Hydrogen ions also contribute to the formation of hydrochloric acid in the stomach to digest food, and to form a molecule called pepsin, which helps break down food proteins.
The pH in your blood is tightly controlled to stay in a narrow range, from about 7.2 to 7.4, using carbon dioxide, a cellular waste product of energy metabolism, and inhaled oxygen.
The pH Scale
Hydrogen ions do not actually float around independently. Whenever they are in liquid that contains water, hydrogen ions quickly combine with H2O to form hydronium ions, or H3O+. The hydrogen ion concentration in water, then, is really the hydronium ion concentration; chemists use the two terms almost interchangeably. At room temperature, a pH measurement of 7 is neutral, meaning that there is an equal concentration of hydrogen and hydroxide (OH-) ions. The pH scale goes from 0, highly acidic, to 14, highly basic. A 14 means there is a very low concentration of hydrogen ions, while 1 means there is a very high concentration of hydrogen ions.
Proteins are large molecules that carry out many of the most important tasks in the human body. Their structure is shaped partly by special bonds called hydrogen bonds that can form between different amino acids in the protein molecule. Changing the hydrogen ion concentration in the body can change the shape or configuration of proteins in the body, so your body has a variety of mechanisms to keep the pH at a constant level. Some organelles inside your cells maintain a different pH level, however, in order to help them do their job. Lysosomes, for example, are cell organelles that maintain a low pH, which helps them break down worn-out cell components.
In the lining of your stomach, cells called parietal cells secrete hydrogen and chloride ions, which combine to form hydrochloric acid. This strong acid dramatically reduces the pH of the contents of your stomach, which helps to kill bacteria and break down molecules in your food. The hydrogen ions also affect digestion by ensuring that an enzyme called pepsin assumes the proper configuration it needs to do its job. Pepsin breaks up proteins in the food you eat for better digestion. When the contents of your stomach pass into your small intestine, your pancreas secretes bicarbonate to neutralize the acidic contents so they don't cause any ill effects.
Blood and Lungs
The pH in your blood is tightly controlled to stay in a narrow range, from about 7.2 to 7.4. When your cells break down sugars to get energy, they end up producing carbon dioxide, which diffuses back into the bloodstream. Carbon dioxide reacts with water to form carbonic acid, increasing the blood's pH. This slightly elevated hydrogen ion concentration affects hemoglobin, a protein carrying oxygen inside your red blood cells, causing it to release some of its oxygen for the cells to use. In this process, the hemoglobin then picks up some of the extra hydrogen ions and carbon dioxide and transports these back to the lungs. Carbon dioxide concentration in your lungs is lower than in the bloodstream, so the carbon dioxide diffuses out of your blood and into your lungs. The higher pH here increases hemoglobin's affinity for oxygen now, so it can take up oxygen again.
About the Author
Based in San Diego, John Brennan has been writing about science and the environment since 2006. His articles have appeared in "Plenty," "San Diego Reader," "Santa Barbara Independent" and "East Bay Monthly." Brennan holds a Bachelor of Science in biology from the University of California, San Diego.