What proportion of the blood's total buffering power is accounted for by protein buffers?

1. Multiple buffer systems operate in blood: proteins, phosphate, and bicarbonate-carbonic acid. 2. Proteins are made of amino acids with both positive and negative charged groups that can bind hydrogen or hydroxyl ions. 3. Protein buffering accounts for two-thirds of the blood's buffering power, and most buffering inside cells. 4. One-quarter and One-half are both different figures a reader might guess from memory, but neither matches the specific figure given here. 5. Nearly all of it is also incorrect for the same reason: it does not match the specific figure given here. 6. So two-thirds is correct, not the smaller fractions or the overstated 'nearly all' option. _Source: OpenStax Anatomy and Physiology (CC BY 4.0), Ch 26 "Fluid, Electrolyte, and Acid-Base Balance", p.1265 §Protein Buffers in Blood Plasma and Cells_