**Core Concept**
The hematocrit is the proportion of blood volume occupied by red blood cells. In the context of venous blood, the hematocrit increases due to changes in the distribution of blood volume between the vascular and interstitial compartments.

**Why the Correct Answer is Right**
When a muscle is contracted, it compresses the veins and capillaries, leading to a decrease in venous blood volume. As a result, the hematocrit increases in the remaining venous blood due to the relative decrease in plasma volume. This effect is more pronounced in the venous system because the capillary bed is already relatively small compared to the arterial system.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because hematocrit does not increase due to changes in respiratory rate or tidal volume.
**Option B:** This option is incorrect because hematocrit does not increase due to changes in cardiac output or stroke volume.
**Option C:** This option is incorrect because hematocrit does not increase due to changes in blood pH or oxygenation.

**Clinical Pearl / High-Yield Fact**
The hematocrit can also increase in conditions where there is a decrease in plasma volume, such as dehydration or hemorrhage. In these cases, the hematocrit can be used as a useful marker to estimate the severity of blood loss.

**Correct Answer: D. Muscle contraction**