Regulation of the Acid-Base Balance
Several factors contribute to the regulation of the acid-base balance. These include the blood buffering characteristics and the internal and external cell compartments, the gaseous exchange in the lungs and the excretory functions of the kidneys. These systems are in a relationship of functional equilibrium with each other.
The blood pH regulation is carried out by different buffer systems consisting of a weak acid and its corresponding base in a particular ratio. If protons or hydroxide ions are added, the pH level will be hardly changed, although ratio of the acid to the corresponding base will change. At the same time, the blood buffer capacity is determined by the combined effect of the groups of anions working as buffers:
- Bicarbonate buffer (52%)
- Protein buffer (15%)
- Phosphate buffer (2%)
- Haemoglobin buffer (31%)
The bicarbonate buffer system, followed by the buffer characteristics of haemoglobin, plasma proteins and the phosphate buffer, is mainly responsible for a constant pH level in the blood. The particular ability of the buffer systems to react enables them to regulate the blood pH to a level within the normal range (pH 7.35 – 7.45) extraordinarily quickly.
The bicarbonate buffer interacts directly with respiration through the formation of carbon dioxide. At the same time, carbonic acid is created from hydrogen carbonate, which breaks down into water and carbon dioxide, which is breathed out. Quantitatively, bicarbonate contributes most to the blood buffer capacity. The most important intra-cellular buffer of the erythrocytes (red blood cells) is the redox system of haemoglobin. At the same time, phosphate and protein buffers (e.g. serum protein containing the amino-acid histidin) also play an important role.
Although the strain of acid can be avoided through breathing out carbon dioxide, the kidneys serve primarily to excrete protons created during the breakdown of different acids. This excretion is necessary, because the emergence of protons clearly exceeds the absorption of substances effective as bases, when a usual mixed diet is consumed.