This chapter discusses applied respiratory physiology, where respiratory diseases cannot be managed satisfactorily without a proper understanding of respiratory physiology. Hypoxia is a state in which tissues suffer from the effects of oxygen deficiency. Depending on the cause, hypoxia is generally divided into four categories such as hypoxic, anaemic, stagnant and histotoxic hypoxia. Stagnant hypoxia occurs due to a reduction in cardiac output and blood flow, as in cardiac failure, haemorrhage and circulatory shock. Excess of deoxygenated haemoglobin in small vessels may be there because all arterial blood in the body is poorly oxygenated. Dyspnoea means difficulty in breathing. During voluntary breath holding, tissues continue to use oxygen and produce carbon dioxide. Cheyne-Stokes breathing may be seen physiologically in sleep, particularly in infants and at high altitude. Oxygen therapy may be required for respiratory failure due to lung disease or poisoning, such as with hypnotics or tranquilizers.