Buffers

Buffers

The body has a very large buffer capacity.

This can be illustrated by considering an experiment where dilute hydrochloric acid was infused into a dog

(Pitts RF. Mechanisms for stabilizing the alkaline reserves of the body. Harvey Lect 1952-1953; 48 172-209. PubMed.)

In this experiment, dogs received an infusion of 14 mmols H+ per litre of body water. This caused a drop in pH from 7.44 ([H+] = 36 nmoles/l) to a pH of 7.14 ([H+] = 72 nmoles/l) That is, a rise in [H+] of only 36 nmoles/l.

SO: If you just looked at the change in [H+] then you would only notice an increase of 36 nmoles/l and you would have to wonder what had happened to the other 13,999,964 nmoles/l that were infused (14 mmolsH+/L = 14,000,000 nmoles/L.)

Where did the missing H+ go?

They were hidden on buffers and so these 13,999,964 nmoles/L of hydrogen ions were hidden from view.

Buffering hides from view the real change in H+ that occurs.

Because of the large buffering capacity, the actual change in [H+] is so small it can be ignored in any quantitative assessment, and instead, the magnitude of a disorder has to be estimated indirectly from the decrease in the total concentration of the anions involved in the buffering. The buffer anions, represented as A-, decrease because they combine stoichiometrically with H+ to produce HA. A decrease in A- by 1 mmol/l represents a 1,000,000 nano-mol/l amount of H+ that is hidden from view and this is several orders of magnitude higher than the visible few nanomoles/l change in [H+] that is visible.) - As noted above in the comments about the Swan & Pitts experiment, 13,999,994 out of 14,000,000 nano-moles/l of H+ were hidden on buffers and just to count the 36 that were on view would give a false impression of the magnitude of the disorder.

The Major Body Buffer Systems
Site	Buffer System	Comment
ISF	Bicarbonate	For metabolic acids
	Phosphate	Not important because concentration too low
	Protein	Not important because concentration too low
Blood	Bicarbonate	Important for metabolic acids
	Haemoglobin	Important for carbon dioxide
	Plasma protein	Minor buffer
	Phosphate	Concentration too low
ICF	Proteins	Important buffer
	Phosphates	Important buffer
Urine	Phosphate	Responsible for most of 'Titratable Acidity'
	Ammonia	Important - formation of NH4+
Bone	Ca carbonate	In prolonged metabolic acidosis