Final answer:
The hematocrit (HCT) level increases when a patient is dehydrated due to a reduction in plasma volume. This results in a higher concentration of red blood cells, which raises the hematocrit. A patient with 42 percent hematocrit has approximately 58 percent of blood volume as plasma.
Step-by-step explanation:
When a patient is dehydrated, the hematocrit (HCT) level typically increases. Dehydration leads to a reduction in plasma volume due to inadequate water intake or excessive water losses. As plasma volume falls, the concentration of red blood cells (RBCs) in the blood rises. This elevation in RBC concentration causes the hematocrit, which measures the proportion of blood volume occupied by RBCs, to rise as well. It's useful to note that if a patient's hematocrit is 42 percent, this indicates that RBCs constitute 42 percent of their total blood volume, and therefore, approximately 58 percent of the blood is plasma.
A transient increase in hematocrit can also be induced through acclimatization to high altitudes where oxygen levels are lower, leading to an adaptive response known as mild polycythemia. However, a significant and pathological increase in hematocrit can occur due to a bone marrow disease called polycythemia vera, which can lead to increased blood viscosity and health complications.