Henderson-Hasselbalch Equation

The Henderson-Haselbalch (HH) equation in the most important equation in learning about acid-base homeostasis. Why is the HH equation important? It is important because it is useful. What is the HH useful for? It is useful for many things.


USES OF THE HENDERSON-HASSELBALCH EQUATION

1. It is a formula for linking the blood pH to the respiratory component (PCO2) and the renal component (HCO3-)
2. It has three terms which are interrelated and therefore inter-dependent: pH, PCO2 and HCO3-
3. Knowing the values of two components makes it easy to calculate the value of the third component
4. The plasma pH is affected by HCO3- and PCO2
5. The plasma PCO2 is affected by pH and bicarbonate
6. Plasma HCO3- is affected by pH and PCO2
7. HCO3- and PCO2 are regulated separately by different organs in the body - kidneys and lungs
8. Kidney function affects the amount of bicarbonate in blood, ie [HCO3-]
9. Lung function affects the amount of CO2 in blood, ie PCO2
10. Buffer ratio is the ratio of HCO3- to PCO2 in the HH equation; normal buffer ratio is 20:1. 
11. From buffer ratio, it also tells whether there is excess or lack of base component (ie Base Excess, B.E. is + or -); normal range is from -2 to +2; B.E. is +ve if there is excess of base. B.E. is -ve if there is a lack of base. Values exceeding the limits indicate there is excessive or severe lack of base.
12. The buffer ratio components will change in the same direction as the primary lesion in order for pH to return to normal. If the buffer ratio is 20:1, then pH is 7.4. 
13. If the buffer ratio is greater (eg 22:1), then there is excess base and the blood pH will be higher than pH 7.4. So the respiratory component will need to be increased in order for pH to decrease towards normal.
14. Similarly, if the buffer ratio is less than 20:1 (eg 18:1), then there is lack of base and the blood pH will be less than pH 7.4. So the renal component will need to be increased if pH is to increase towards normal.