After obtaining approval from the Institutional Review Board of our institution, written informed consent is obtained from patients undergoing colon surgery are enrolled in this prospective study and randomly allocated into one of two groups: Group-C (n=15) and Group-HES (n=15).

All recruited patients will be given patient identification number (PIN) for the present study of 01-30 according to their order of interview and recruitment. Investigators will prepare 15 yellow and 15 green cards, which will be inserted in 30 thick-paper envelopes. Then, all envelopes will be sealed, mixed and randomly allocated to get numbers of 01 to 30 (Envelop number).

According to the color of the card, attending anesthesiologists will give crystalloid for yellow card or HES for green cards, respectively, to maintain stroke volume variation < 15 during volume controlled ventilation (8 ml/kg tidal volume) of O2/air mixture.

Patient data and statistical analyses:

Patient's data whole blood viscosity, PaO2, FiO2, Hemoglobin, urine amount, s-glucose will be determined from the patients' medical record after patient's discharge.

All statistical analyses will be performed after the 30th patient's discharge and data acquisition

Inclusion Criteria:

• Patients undergoing colon surgery
• Patients provided a written informed consent.
• Patients with preoperative serum hemoglobin concentration >13 g/dL (male) and >12 g/dL (female)
• Patients with PaO2/FiO2 ratio >150

Exclusion Criteria:

• Patients with history of anemia, dyspnea, active infection.
• Patients with endocrine disease
• Patients received or receiving intraoperative and preoperative blood salvaged, allogenic blood transfusion, anti-platelet drug, anti-fibrinolytic agents or recombinant human erythropoietin, or undergoing acute normovolemic hemodilution.