The hydrogen ion concentration is extremely important to the structural and functional integrity of living systems. Even slight changes in may cause profound changes in the large molecules and molecular complexes composing organisms. The molecular balance in a living system is usually so delicate that even slight changes may be incompatible with life.
Consequently, it is important that we understand something of the regulation of in living systems. Hydrogen ion concentration is expressed in terms of PH units, representing a measure of the acidity or the alkalinity of a solution. The scale of pH units goes from 0 to 14.
The midpoint of this scale is 7, the pH of pure water. Since this is where hydrogen and hydroxyl ions are equal in number, a pH of 7 represents neutrality, that is, a midpoint between acidity and alkalinity. Solutions with pH values of less than 7 have more hydrogen ions in the solution than hydroxyl ions, and those with pH values greater than 7 have more hydroxyl ions in solution than hydrogen ions. The pH scale is not an arbitrary set of numbers. It is based on the quantitative measurement of the number of hydrogen ions, which is one 10 millionth of a gram, present in 1 liter of pure water.
Since, it is awkward to deal with numbers or fractions involving many zeros, it is much simpler to express the number given above exponentially as 107. In turn, it is simpler to deal with whole numbers than with exponential numbers, and so the pH of water is expressed by the number 7.
Similarly, solutions which have more or fewer hydrogen ions are expressed by numbers on the pH scale, and perhaps you can see why the pH value goes down as the goes up. In summary, the pH of a solution is the negative logarithm of its hydrogen ion concentration.
The body fluids of most organisms must be maintained at a point rather close to neutrality. In view of the fact that most organisms are frequently exposed to acids and bases of sufficient strength to alter the relatively constant pH values of their body fluids, there must be a way to maintain this constancy. Organisms do indeed possess a mechanism for the maintenance of a steady pH through the presence of compounds that form a buffering system.
A buffering system usually consists or a weak acid and a salt which is chemically related to the acid. For instance, sodium bicarbonate and carbonic acid constitute a buffering system, since carbonic acid is a is a weak acid and sodium bicarbonate is its salt.
Let us suppose that a strong acid is added to this buffering system. In this case, we would expect the addition of such a flood of hydrogen ions to lower the pH drastically. Actually, it would do so, were it not for the buffering system.
It will be noted that the hydrogen ions released by the ionization of hydrochloric acid are used to form carbonic acid, which ionizes only to a slight degree. Therefore, it is changed very little and the pH remains approximately the same. In other words, a strong acid is converted by a salt into a weak acid which is chemically related to the salt. Living organisms possess buffering systems, without which pH constancy would not be possible.
By virtue of these mechanisms, organisms are protected from the acids or bases in their environment, including those acids or bases which may be produced within living systems themselves.
Of course, it is possible to add so much acid or base to a system that buffering agents are completely swamped. In this event, marked changes in pH will occur, and few organisms can withstand more than a slight deviation from their characteristic pH.