The proposed project, entitled "HemOx Transfusion Study", investigates the functionality of red blood cells (erythrocytes) in the context of oxygen transport in the blood. Erythrocytes contain haemoglobin, a protein responsible for oxygen transport in the bloodstream. Haemoglobin can bind up to four oxygen molecules. This haemoglobin-oxygen binding is particular and efficient, allowing oxygen to be taken up easily in the lungs and released just as readily in oxygen-consuming tissues. In this way, large amounts of oxygen can be transported rapidly and efficiently despite its low solubility in blood. If a patient suffers significant blood loss, for example during surgery, the amount of oxygen-transporting haemoglobin molecules may become so low that a transfusion (administration) of packed red blood cells from a blood donor becomes necessary. In major surgical procedures, which are more frequently associated with blood transfusions, special devices are used to process blood lost from the surgical site and reinfuse it into the same patient. During the storage of packed red blood cells, storage-related damage to the erythrocytes occurs, affecting their shape and functionality. In particular, the haemoglobin-oxygen binding is impaired; the binding affinity increases markedly during storage as a result of metabolic changes within the erythrocytes. This study therefore aims to investigate whether these storage-related changes in packed red blood cells, compared with processed autologous blood from the patient, normalise again after administration in the body, and over what period of time. The rejuvenation of haemoglobin-oxygen binding functionality and oxygen transport capacity of transfused blood is critical for the patient. Understanding the dynamics of this process is intended to inform clinicians about the efficiency of their interventions and to stimulate further research. The intraoperative processing of autologous blood will be examined and analysed in comparison with the administration of packed red blood cells. In this way, the clinical value of processed autologous blood will be evaluated in comparison with packed red blood cells. In the longer term, the processing and/or optimisation of packed red blood cells prior to administration could improve patient care.

Inclusion Criteria:

1. Understands and agrees to comply with the study procedures and provides written informed consent as documented by signature.

2. Female and male subjects ≥ 18 years of age at time of consent.

3. Patient undergoes elective surgery according to Surgery List in Protocol Appendix and blood transfusion is likely to happen at the discretion of the investigator.

   Inclusion criterion 4 only refers to WOCBP

4. Negative urine pregnancy test, not breastfeeding

Exclusion Criteria:

1. Known hemoglobinopathy
2. Indication for irradiation of blood products
3. preoperative Anemia (Hb <11 mg/dl for women and Hb <12 mg/dl for men)
4. Contraindication for intraoperative cell salvage (septic surgery, tumor surgery)
5. Contraindication for transesophageal Doppler ultrasound (craniofacial dysplasia or fracture, history of esophageal perforation or rupture, esophageal stricture, esophageal varices)

Data underpinning research papers will be made available for reuse in repository "ZENODO" at the time of the publication of the article with CC-BY 4.0 licence.