The condition in which the fluid portion of the blood filters into the tissues, decreasing the fluid portion of the blood in the blood vessels, is called:

Hemoconcentration

Explanation:

Hemoconcentration refers to the condition when blood plasma volume is reduced accompanied by increased concentration of erythrocytes in blood. The blood plasma volume is reduced due to its increased filtration into the tissues.

Some specific conditions that lead to hemoconcentration are diarrhea, vomiting, intense sweating, etc. The solute level of blood is increased and the specific gravity of blood is also altered.