Which of the following is an example of buffer system in blood?
a)	Haemoglobin and oxyhaemoglobin	b)	Oxygen and carbon dioxide
c)	Albumin and globulin	d)	Sodium bicarbonate and carbonic acid
In an open circulatory system,

(d)

A buffer is a chemical or combination of chemicals that can both take up and release hydrogen ions. Carbonic acid  and sodium bicarbonate  help buffering human blood because  is a weak acid that does not totally dissociate, when excess hydrogen ions are present in blood, the reaction goes to the left and carbonic acid forms to maintain the pH.

Carbonic acid            Hydrogen ion        Bicarbonate ion