Clinical assessment of bleeding and calculation of perioperative blood loss do not provide an accurate estimate of blood volume. This makes rational fluid management difficult, which is of high importance for patient-related outcomes. The availability of carbon monoxide(CO)-rebreathing allows for simple, non-invasive and precise measurement of changes in blood volume. The purpose of the study is to investigate and describe changes in blood volume and fluid distribution perioperative in patients undergoing total hip arthroplasty. Furthermore, to investigate the association between changes in blood volume and incidence of orthostatic insufficiency.