Which of the following is true about a dog's packed cell volume (PCV) when he is dehydrated?

When a dog is dehydrated, the packed cell volume (PCV) tends to be high. This occurs because dehydration reduces the fluid component of the blood, concentrating the red blood cells and resulting in an increased proportion of cells to plasma. As a result, the PCV, which measures the percentage of blood volume occupied by red blood cells, will reflect this concentration and therefore show a higher value.

In cases where a dog is adequately hydrated, the PCV would generally be within a normal range. If the dog experiences fluctuations in hydration status, this may lead to variations in the PCV, but fundamentally, significant dehydration results in an elevated PCV due to the relative increase in red blood cell concentration. Thus, the correct understanding aligns with the observation that dehydration leads to a high PCV.