缓冲大，不是应该PH变化小吗？

Dissociation Curves of Carbon Dioxide

Figure 9.2 shows the classic form of the dissociation curve of carbon dioxide relating blood content to partial pressure.For decades there has been great interest in curves that re­ late any pair of the following: (1) plasma bicarbonate con­ centration, (2) Pco2 and (3) pH. These three quantities are related by the Henderson–Hasselbalch equation (Equation 7), and therefore the third variable can always be derived from the other two. The most famous is the Siggaard– Andersen plot, which relates Pco2 on a logarithmic scale to pH (Fig. 9.5). These graphs can be used to explore the effects of changes in respiratory and metabolic acid–base balance, but care must be taken in using these in vitro data in intact subjects. For example, if the Pco2 of an entire pa­ tient is altered, the pH changes are not the same as those of a blood sample of which the Pco2 is altered in vitro. This is because the blood of a patient is in continuity with the  extracellular fluid (of very low buffering capacity) and also with intracellular fluid (of high buffering capacity). Bicar­ bonate ions pass rapidly and freely across the various inter­ faces. As a result, the in vivo change in pH is normally greater than the in vitro change in the patient’s blood when subjected to the same change in Pco2.

二氧化碳解离曲线

图9.2示经典二氧化碳解离曲线，该曲线说明了血液CO2含量与PCO2的关系。几十年来，人们对下述任何一对相关的曲线都给予了较大的关注：（1）血浆碳酸氢盐浓度、（2）Pco2、（3）pH。这三个变量的关联可用汉-哈二氏方程（方程7）表示，因此第三个变量始终可以从另外两个变量推导出来。最著名的是CO2平衡曲线图（Siggaard – Andersen plot），它将Pco2的对数值与pH关联（图9.5）。这些图可用于探索呼吸和代谢性酸碱平衡，但在完整受试者中使用这些体外数据时需谨慎。例如，若整个患者的Pco2改变，则其体内pH变化不同于体外Pco2改变时的血样pH变化。这是因为患者血液不仅受细胞外液（缓冲力极低）还受细胞内液（缓冲力高）缓冲。碳酸氢根离子可快速自由地在体内移动。因此，相同的Pco2变化时，相比体外，患者体内血液pH值通常变化大。

大家帮我看看，理解的对吗?为什么“体内缓冲能力大”，受到“相同的Pco2变化时”，pH值通常变化反而更大呢？缓冲大，不是应该PH变化小吗？

谢谢