**Core Concept**
The normal pH of human blood is tightly regulated by the body's homeostatic mechanisms to ensure proper functioning of enzymes and cellular processes. The pH of blood is measured on a scale from 0 to 14, with a pH of 7 being neutral, below 7 being acidic, and above 7 being alkaline.

**Why the Correct Answer is Right**
The normal pH of human blood is maintained within a narrow range of 7.35 to 7.45 by the buffer system, primarily composed of bicarbonate (HCO3-) and carbonic acid (H2CO3). The bicarbonate buffer system works in conjunction with the respiratory system to regulate pH levels. When blood pH becomes acidic, the lungs increase ventilation to expel more CO2, which reacts with water to form carbonic acid, thereby reducing pH. Conversely, when blood pH becomes alkaline, the lungs decrease ventilation to retain more CO2.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect as the normal pH of human blood is not in the acidic range of 6.0 to 6.9.
* **Option B:** This option is incorrect as the normal pH of human blood is not in the alkaline range of 7.8 to 8.5.
* **Option C:** This option is incorrect as the normal pH of human blood is not in the acidic range of 6.5 to 7.0.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that even small changes in blood pH can have significant effects on cellular function, making acid-base balance a critical aspect of medical management in various clinical scenarios.

**Correct Answer: D. 7.4**