This thesis focuses on measuring blood haemoglobin content and its relationship with physiological performance in human subjects. Haemoglobin concentration [Hb] is commonly measured in clinical practice. In contrast, total haemoglobin mass (tHb-mass) is rarely used since direct measurement methods have been expensive, inaccurate, or technically challenging. However, the modified carbon monoxide rebreathing method (oCOR) has the potential to measure tHb-mass rapidly and simply. The clinically important condition of anaemia is described and the different methods used to measure blood volume and tHb-mass preceding the oCOR technique’s development are discussed. Subsequently, the oCOR technique is described and used to measure and characterise anaemia in different patient groups. Finally, the impact of intravenous iron on the correction of anaemia and its effect on physiological reserve is explored.It was hypothesised that patients with greater physiological reserve, defined by exercise capacity measured using cardiopulmonary exercise testing (CPET), would be better able to withstand the physiological challenge of surgery. And secondly, that measurement and manipulation of tHb-mass would improve physiological reserve through improvement in measured exercise capacity. To test these, experiments were performed to refine the oCOR technique in different patient populations. Adaptations to the oCOR in conditions simulating mechanical ventilation during surgery or critical illness were explored.Finally, a perioperative interventional pilot study testing whether intravenous iron could improve haemoglobin mass and CPET derived markers of cardiorespiratory fitness was undertaken.It was demonstrated that the oCOR method was safe for measuring tHb-mass in patients with chronic liver disease and ascites without adjusting blood sample time points. In comparison with oxygen plus exercise, it was shown that oxygen alone could effectively clear carbon monoxide between tests to allow duplicate measurements within a short time frame. Finally, in a pilot interventional study, anaemic perioperative patients given intravenous iron had significant improvements in [Hb], tHb-mass and CPET derived markers of physical fitness.The modified oCOR could illustrate the aetiology of anaemia in critically ill patients, accurately measure surgical blood loss and distinguish between dilutional and true anaemia in patients with heart failure. Further research should define normative data for tHb-mass for healthy subjects across different populations. Research into patient outcomes following oCOR directed perioperative optimisation is needed to support wider adoption. Pilot data from this thesis would ensure such perioperative studies were adequately powered.