**Core Concept**
The partial pressure of oxygen (PO2) is a critical parameter in respiratory physiology, indicating the amount of oxygen dissolved in the blood. It plays a vital role in maintaining adequate oxygenation of tissues and organs.

**Why the Correct Answer is Right**
The partial pressure of a gas in a mixture is determined by its concentration and the total pressure of the mixture. In the context of oxygen, the PO2 is typically measured in arterial blood, where it is approximately 75-100 mmHg at sea level. This value is crucial for assessing the adequacy of oxygenation and detecting potential respiratory or cardiac problems. The partial pressure of oxygen is influenced by factors such as altitude, respiratory rate, and the efficiency of gas exchange in the lungs.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not specify a valid partial pressure value for oxygen. The partial pressure of oxygen at sea level is generally higher than what is typically seen in clinical practice.
* **Option B:** This option is incorrect because it does not accurately represent the partial pressure of oxygen in arterial blood. The PO2 in venous blood is generally lower than in arterial blood due to the extraction of oxygen by tissues.
* **Option C:** This option is incorrect because it does not correspond to a valid partial pressure value for oxygen. The partial pressure of oxygen in the atmosphere at sea level is approximately 159 mmHg, but this value is not typically measured or reported in clinical practice.

**Clinical Pearl / High-Yield Fact**
A PO2 of less than 60 mmHg in arterial blood is generally considered indicative of hypoxemia, which may require supplemental oxygen therapy to prevent tissue damage.

**Correct Answer:** D. 80