Final answer:
Increased erythropoietin levels in the blood would cause an increase in an individual's hematocrit as EPO stimulates the production of red blood cells in the bone marrow and is secreted in response to low oxygen levels.
Step-by-step explanation:
Hematocrit refers to the proportion of red blood cells in the blood, and several factors can affect its level.
Erythropoietin (EPO) is a hormone that is crucial for the production of red blood cells (erythrocytes). When the body senses low oxygen levels, EPO is released, stimulating the bone marrow to produce more erythrocytes. One scenario in which EPO levels would increase is when the body experiences low atmospheric pressure, for example, at high altitudes, leading to increased production of red blood cells. This response is a part of the body's attempt to increase its oxygen-carrying capacity under oxygen-scarce conditions.
Out of the options provided, increased erythropoietin levels in the blood would cause an increase in an individual's hematocrit. Activation of aortic baroreceptors, decreased expression of the globin genes, and other listed factors would not directly result in an increased hematocrit.