According to Austin Community College, third spacing is the build-up of fluids in parts of the body that typically have little or no fluid. This is contrasted with second spacing, where excess fluids accumulate in the tissue.
The college describes first spacing as fluids being in their normal locations inside and outside cells. Lippincott's Nursing Center reports that third spacing is the result of too much fluid flowing from blood vessels into the open spaces between cells. This sometimes reduces how much blood the heart pumps and lowers blood pressure. The Center identifies several conditions that contribute to third spacing, including kidney problems, low sodium levels and loss of proteins in blood plasma.