Acid-base balance

Acidosis and alkalosis

The acid or alkali status of a solution is determined by the pH. In chemistry a neutral pH is 7, solutions with a pH less than 7 are acids whilst those with a pH above 7 are alkali.


It is useful to have an idea of what pH is when considering acid/base problems. pH is determined by hydrogen ion [H⁺] concentration and specifically by the equation:


pH uses an inverse logarithmic scale so when hydrogen ion concentration goes up the pH value goes down. As its logarithmic when the pH value changes by 1 the hydrogen ion concentration has changed by a factor of 10.


Metabolism produces approximately 60 mmol of H⁺ ions each day and the majority is combined with bicarbonate (HCO₃-) to form CO₂ and water which is mostly excreted through the lungs.

The bicarbonate used up for handling these hydrogen ions is replaced by the kidneys.

Controlling pH

The body tries to maintain a a pH between 7.35 and 7.45. pH homeostasis is achieved via the following:

• 1. Respiratory control: adjusting ventilation to control CO₂ levels
• 2. Metabolic buffers: several buffers help control pH levels by 'mopping up' excess acid or alkali. This includes the bicarbonate buffer system, heamaglobin system (chloride shift), plasma protein buffer system and phosphate buffer system.
• 3. Renal pH control: The kidneys adjust excretion of H⁺ ions into the urine and secretion of bicarbonate into the blood by the renal tubular cells of the kidneys.
• 4. Potassium exchange: The body can exchange potassium (K⁺) and hydrogen (H⁺) ions between the intracellular and extracellular spaces. If the blood becomes too acidic excess H⁺ can be pumped from the blood into the cells with potassium moving in the opposite direction. Vice versa If the blood is too basic (low H⁺ / alkali) the body can pump hydrogen ions out of cells into the blood with potassium moving in the opposite direction in exchange.

Buffers

Buffers are weak acids (HA) or weak bases (A^-) which are able to donate or accept H⁺ ions. Buffers are often referred to as buffer pairs as they may exist in two forms depending on whether they have accepted or donated a H⁺ ion e.g. H₂CO₃ and HCO₃-.

The ratio between the pair is determined by hydrogen ion concentration [H⁺] and the dissociation constant (K) for that buffer pair. This relationship is described by the Henderson-Hasselbalch equation:


Changes in [A-] or [HA] will have a predictable effect on pH as shown in the table below:


Buffers work best when pH is close to their pK value.