Oxygen delivery and demand

The most important function of the cardiovascular and respiratory systems is to transport a continuous supply of oxygen to the tissues. Oxygen is needed for the efficient generation of adenosine triphosphate (ATP) from substrates such as glucose. Tissues vary in their tolerance of hypoxia. The brain is most susceptible; cerebral function is lost within seconds of losing its oxygen supply and irreparable damage occurs after about five minutes, but all tissues are eventually damaged by prolonged hypoxia. Tissue hypoxia occurs if oxygen delivery is too low relative to the oxygen needs of the tissue. For any given metabolic rate, there is a critical oxygen delivery below which oxygen consumption and ATP production fall, and anaerobic metabolism gives rise to increasingly severe lactic acidosis. Oxygen delivery depends on blood flow and arterial oxygen content, which in turn depends on arterial partial pressure of oxygen, the affinity of haemoglobin for oxygen, and the haemoglobin concentration. ATP production can also fall if the ability of the tissue to extract oxygen or mitochondrial utilization of oxygen is impaired. Tissue damage may result if one or more of these factors are abnormal. Improving the match between oxygen supply and oxygen need is an important goal in many seriously ill patients and can be achieved by lowering oxygen requirements and/or increasing the oxygen supply. The inter-relationships between the factors that contribute to oxygen supply must be considered because improving one of these factors may impair another.