slightly underestimates the acid/base added to the system怎么理解？

Base excess (BE) was introduced by Siggaard-Andersen in 1960 as an answer to the forty-year-long quest for a reliable, stand-alone marker of metabolic acidosis/alkalosis, independent from co-existing respiratory derangements, and able to quantify the severity of the disorder [1]. Previously, several parameters had been examined.The first was actual bicarbonate (HCO3−) [2], which was quickly discarded due to its known dependency on the partial pressure of CO2 (PCO2). To eliminate the respiratory component, standard bicarbonate (HCO3−(st)) was introduced, representing the plasma bicarbonate concentration after equilibration at a PCO2 of 40 mmHg [3].Although this was certainly a step forward, HCO3−(st) does not take into account the buffer effect of weak non-carbonic acids, i.e. proteins, which normally contribute to buffering with 14–16 negative charges (A−) per liter. Indeed, when a strong acid is added to blood,both HCO3− and A− concentrations will be reduced.In an open system, which is the case of a subject properly regulating PCO2 through breathing, carbonic buffers have a predominant role (about 75–80%), however, non-carbonic buffers cannot be disregarded completely.Therefore, the difference between HCO3−(st) and the ideal, “normal” bicarbonate value, slightly underestimates the acid/base added to the system (e.g. the addition of 10 mmol/L of a strong acid to blood with HCO3− of 24 mmol/L could result in an HCO3−(st) of 16 mmol/L, instead of 24–10 = 14 mmol/L). To overcome this problem, Singer and Hastings introduced the buffer base (BB), the sum of all buffer anions

为寻找可靠的、独立的（不受呼吸紊乱干扰）并能够量化疾病严重程度的代谢性酸/碱中毒标志物，Siggaard-Andersen于1960年引入了碱剩余（BE）。之前，已尝试了几个参数。第一个是实际碳酸氢盐(HCO3-)，由于已知其受CO2分压(PCO2)影响，因此很快废弃。为消除呼吸紊乱的干扰，又引入标准碳酸氢盐(HCO3-(st))，代表PCO2为40 mmHg时平衡后的血浆碳酸氢盐浓度。尽管引入HCO3-(st)肯定是向前迈出了一步，但HCO3-(st)没有考虑弱非碳酸（即蛋白质）的缓冲效应，弱非碳酸通常有助于缓冲，每升含14-16个负电荷(A-)。的确，当血液中加入强酸时，HCO3-和A-浓度都会降低。在开放系统中，即受试者通过呼吸适当调节PCO2的情况下，碳酸缓冲液具有主要作用（约75-80%），然而，不能完全忽略非碳酸缓冲液。因此，HCO3-(st)与理想的“正常”碳酸氢盐值之间的微小差异来源于而被忽略的非碳酸缓冲液（例如，向HCO3-为24 mmol/L的血液中添加10 mmol/L强酸可导致HCO3-(st)为16 mmol/L，而不是24-10 = 14 mmol/L）。为了克服这个问题，Singer和Hastings引入了缓冲碱(BB)，即所有缓冲阴离子的总和。

slightly underestimates the acid/base added to the system 微小差异来源于而被忽略的非碳酸缓冲液？