General Principle of Buffer Action
Maintaining the constancy of the internal environment requires immediate reactions. Buffer systems act as chemical "shock absorbers." Structurally, each buffer is formed by a weak acid and its conjugate base in a strictly defined ratio.
When a small amount of an exogenous acid or base is added to the solution, the buffer system reacts with it. Hydrogen ($H^+$) or hydroxyl ($OH^-$) ions are bound by the buffer components, converting into weak, minimally dissociating compounds or water. Thus, free aggressive ions are neutralized, and the pH level remains stable.
Bicarbonate Buffer System
This buffer consists of weak carbonic acid ($H_2CO_3$) and the bicarbonate ion ($HCO_3^-$) in a 1:20 ratio (acid to base).
The mechanism of action depends on the type of shift:
- In acidosis: Excess $H^+$ ions are bound by the base ($HCO_3^-$) to form carbonic acid ($H_2CO_3$). Then, catalyzed by the enzyme carbonic anhydrase, it breaks down into water and carbon dioxide. Because excess $CO_2$ is continuously eliminated by the lungs, the bicarbonate system is referred to as an "open system".
- In alkalosis: $OH^-$ ions interact with the hydrogen ions within carbonic acid to form water molecules ($H_2O$).
Although this buffer accounts for only 7–9% of the total buffering capacity of whole blood, it is critically important for extracellular fluids. In plasma, lymph, and cerebrospinal fluid, it makes up an impressive 97–98% of the total buffer capacity.
Phosphate and Protein Systems
Two other systems play auxiliary yet essential roles in various fluid compartments.
The phosphate buffer consists of the dihydrogen phosphate ion ($H_2PO_4^-$) and the monohydrogen phosphate ion ($HPO_4^{2-}$). Their normal ratio is 1:4. The main task of this system in the blood is to maintain an optimal balance of bicarbonate buffer components. This is described by the reaction equation: $H_2CO_3 + HPO_4^{2-} \leftrightarrow H_2PO_4^- + HCO_3^-$
The protein buffer system functions due to the amphoteric nature of proteins. Depending on the environment, proteins behave differently:
- In an alkaline environment, they dissociate to release $H^+$ ions, acting as acids.
- In an acidic environment, they bind or release $OH^-$ ions.
When $H^+$ concentration rises, proteins form weakly dissociating acids, and in the presence of excess $OH^-$, they promote water formation. The capacity of plasma proteins is relatively modest, accounting for only about 2% of the extracellular fluid's buffering capacity.
Hemoglobin Buffer System
This is the most powerful buffer system in the body, accounting for 75% (3/4) of the blood's total buffering capacity. Its incredible efficacy is due to two factors: the amphoteric properties of the protein moiety (globin) and the influence of oxygenation on the acid-base characteristics of hemoglobin.
Key properties of different hemoglobin forms:
- Deoxyhemoglobin (reduced hemoglobin, $Hb$) — is a stronger base, meaning it readily binds free hydrogen ions ($H^+$).
- Oxyhemoglobin ($HbO_2$) — behaves as a stronger acid compared to its reduced form.
Physiological mechanism in tissues: When blood passes through tissue capillaries, it releases oxygen and takes up carbon dioxide. Oxyhemoglobin turns into deoxyhemoglobin. Carbon dioxide entering the erythrocyte (acting as an acid component) is almost completely and instantaneously neutralized, because the newly formed deoxyhemoglobin acts as a strong base and absorbs the hydrogen ions.