Acid-base balance

Acid-base balance is very important for the homeostasis of the body and almost all the physiological activities depend upon the acid-base status of the body. Acids are constantly produced in the body. However, the acid production is balanced by the production of bases so that the acid-base status of the body is maintained. An acid is the proton donor (the substance that liberates hydrogen ion). A base is the proton acceptor (the substance that accepts hydrogen ion). In spite of continuous production of acids in the body, the concentration of free hydrogen ion is kept almost constant at a pH of 7.4 with slight variations.


Hydrogen ion (H+) contains only a single proton (positively charged particle), which is not orbited by any electron. Therefore, it is the smallest ionic particle. However, it is highly reactive. Because of this, the H+ shows severe effects on the physiological activities of the body even at low concentrations. The normal H+ concentration in the extracellular fluid (ECF) is 38 to 42 nM/L. The pH is another term for H+ concentration that is generally used nowadays instead of ‘hydrogen ion concentration’. The pH scale was introduced in order to simplify the mathematical handling of large numbers. Negative logarithm of H+ concentration is taken for calculating the pH as given below. An increase in H+ ion concentration decreases the pH (acidosis) and a reduction in H+ concentration increases the pH (alkalosis). An increase in pH by onefold requires a tenfold decrease in H+ concentration. In a healthy person, the pH of the ECF is 7.40 and it varies between 7.38 and 7.42. The maintenance of acidbase status is very important for homeostasis, because even a slight change in pH below 7.38 or above 7.42 will cause serious threats to many physiological functions.


It is difficult to determine the acid-base status in the ECF by direct methods. So, an indirect method is

follow ed by using Henderson-Hasselbalch equation. In this, to determine the pH of a fluid, the concentration of bicarbonate ions (HCO3 –) and the CO2 dissolved in the fluid are measured. The pH is calculated as follows: Where, pK is constant with pH of 6.1 Thus, In addition to this, the pH of plasma is also determined by using an instrument called pH meter. Normal acid-base ratio is 1:20, i.e. the ratio of 1 part of CO2 (derived from H2CO3) and 20 parts of HCO3–. If this ratio is altered, the pH also is altered leading to either acidosis or alkalosis. Thus, the pH of arterial blood is an indirect measurement of H+ concentration and it reflects the balance of CO2 and HCO3 –.


Body is under constant threat of acidosis because of the production of large amount of acids. Generally, two types of acids are produced in the body:

1. Volatile acids

2. Non-volatile acids.

1. Volatile Acids

Volatile acids are derived from CO2. Large quantity of CO2 is produced during the metabolism of carbohydrates and lipids. This CO2 is not a threat because it is almost totally removed through expired air by lungs.

2. Non-volatile Acids

Non-volatile acids are produced during the metabolism of other nutritive substances such as proteins. These acids are real threat to the acid-base status of the body. For example, sulfuric acid is produced during the metabolism of sulfur containing amino acids such as cysteine and metheonine; hydrochloric acid is produced during the metabolism of lysine, arginine and histidine. Fortunately, body is provided with the best regulatory mechanisms to prevent the hazards of acid production.

Compensatory Mechanism

Whenever there is a change in pH beyond the normal range, some compensatory changes occur in the body to bring the pH back to normal level. The body has three different mechanisms to regulate acid-base status:

1. Acid-base buffer system, which binds free H+

2. Respiratory mechanism, which eliminates CO2

3. Renal mechanism, which excretes H+ and conserves the bases (HCO3–).

Among the three mechanisms, the acid-base buffer system is the fastest one and it readjusts the pH within seconds. The respiratory mechanism does it in minutes. Whereas, the renal mechanism is slower and it takes few hours to few days to bring the pH back to normal. However, the renal mechanism is the most powerful mechanism than the other two in maintaining the acidbase balance of the body fluids.

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