How can anemia negatively influence gas exchange?

1. Disciplina de Pneumologia, Universidade Federal de São Paulo, São Paulo (SP) Brasil. Tissues obtain energy primarily from oxygen. Adequate oxygen levels in the inhaled air and a preserved ventilation/ perfusion ratio (V • /Q) are the major determinants of oxygen supply to the blood. However, peripheral supply depends on effective oxygen transport to the tissue mitochondria, a critical task that is performed by hemoglobin. Without hemoglobin, cardiac output would have to increase up to 20 times in order to meet resting metabolic demands, and this would certainly be incompatible with life.(1) Arterial oxyhemoglobin saturation (SaO2) represents the percentage of occupied oxygen binding sites on hemoglobin. In patients with chronic anemia, SaO2 is usually within the reference range, as is the amount of oxygen dissolved in the blood. However, the arterial oxygen content (which essentially depends on oxygen saturation levels and hemoglobin concentration) is reduced. This results in inadequate tissue supply, particularly when oxygen demand is increased, as occurs during exercise. Compensatory mechanisms include a hyperkinetic cardiovascular response and increased peripheral oxygen extraction.(2) A reduction in mixed venous oxygen tension might contribute to arterial oxyhemoglobin desaturation during exercise, especially in patients with chronic cardiopulmonary disease.