Buffers

A buffer is a solution of a weak acid such that the addition of [H ] to the solution will result in less of a change in the pH than if there was no buffer present.

$$R-H \leftrightarrow R^- + H^+$$ (2)

In equation 2 the weak acid, $R-H$ is on the left and the weak base, $R^-$ on the right. Addition of protons, $H^+$ to the solution will result in some combining with the base to form more of the acid. There will be less free protons in the solution and a smaller change in pH .

The $pK_a$ of a weak acid is the pH at which an equal number of molecules of $R-H$ and $R^-$ exist.

The Henderson-Hasselbach equation,see equation 3 relates the pH , $pK$ and the log of the concentrations of salt and the acid.

$$pH = pK_a + log\frac{[R^-]}{[R-H]}$$ (3)

An increase in hydrogen ion concentration in the extracellular fluid will result in movement of hydrogen ion into the cell to balance the concentration gradient. As hydrogen ions move into the cells some postassium will move outwards to balance the electric charge (Another example of the Gibbs-Donnan equillibrium which balances electric charge changes with concentration change). Thus one cause of hyper-kalemia is acidosis.

Alot of the buffering in the blood stream is done by proteins in the red blood cells especially haemoglobin. The bicarbonate buffer system is also important and permits excretion and preservation of acid by changes in CO which in turn may be eliminated from the lungs.

Table 9: Buffers

Speed	Name
Fast	HCO
	Inorganic phosphate
	Plasma Proteins
	Redblood cells
Slow	Tissue proteins
	Intracellular organic phosphates
	Bone phosphates