Which chemical buffer system resists ph changes in the extracellular fluid (ecf) ?

The answer is carbonic acid-bicarbonate buffer. This buffer system is regulated by the lungs and kidneys due to the regulation of CO2 and bicarbonate ions respectively. It is formed by the dissolving of carbon dioxide from the tissue into the blood plasma. A weak acid-base conjugate is a good buffer due to its partial dissociation in water.