Respiratory and cardiovascular system

Abstract

Oxidative metabolism is essential for our cellular life. Although tissues such as skeletal muscle can operate for short periods anaerobically, human life does not continue for long in the absence of a ready supply of oxygen. Adequate oxygen delivery to tissues is essential for aerobic metabolism and disorders of delivery ultimately become life-threatening. The factors contributing to oxygen delivery are summarised in the oxygen flux equation: OXYGEN FLUX = CARDIAC OUTPUT × ARTERIAL OXYGEN CONTENT The cardiac output is the product of heart rate and stroke volume and amounts to about 5 litres per minute. The arterial oxygen content is the product of the blood’s haemoglobin concentration multiplied by the haemoglobin’s % saturation. The latter is determined by the partial pressure of oxygen in the blood. This is higher in arterial than in venous blood. A small, additional amount of oxygen is carried dissolved in the blood, the amount again determined by the oxygen partial pressure. The five litres of arterial blood delivered to the tissues each minute contain about 1000ml of oxygen. Only a quarter of this (250ml) is needed to support resting metabolism. There is therefore a large safety factor in oxygen delivery. This can be utilized, in concert with adaptive changes to cardiac output, vascular resistance and pulmonary ventilation, in situations such as muscular exercise, where oxygen demand increases dramatically, or at high altitude where inspired oxygen is low. Oxygen delivery depends on the cardiovascular system, respiratory system and the blood. In the lungs, blood in the alveoli is brought into close proximity with alveolar air so that oxygen can diffuse easily into the blood and carbon dioxide, a major waste product of metabolism, can diffuse into the alveolar air. Alveolar air is kept refreshed with atmospheric air by pulmonary ventilation which keeps the partial pressures of oxygen and carbon dioxide in alveolar air and pulmonary capillary blood in a constant equilibrium. This process ensures that pulmonary venous blood and systemic arterial blood have high oxygen and low carbon dioxide partial pressures. Once in the blood, almost all of the oxygen combines with haemoglobin and is transported by the cardiovascular system to the tissues.