ABSTRACT
I. Introduction Mammalian species evolved with a four-chambered heart and two lungs. The evolutionary advantages of these structures are that they prevent mixing of fully oxygenated and deoxygenated blood in the gas-exchange units and allow low pressures in the delicate alveolar capillaries. However, there is a price to pay. The passage of flow between the two halves of the heart becomes subject to the pressure and volume swings associated with the generation of airflow in the lungs. When lung mechanics are optimal and ventilatory demands are small, the changes in pleural pressure required for airflow are small and thus their effect on cardiac chambers is also small. However, when ventilatory demands increase or the mechanics of the ventilatory system are altered by disease, the effects can become large and significantly impact on circulatory flow. This chapter will review the basics of the interaction between the respiratory and cardiovascular systems. There are numerous components to circulatory-ventilatory interactions, which can make the analysis very complex. However, a few components quantitatively dominate the interactions, and these will be emphasized in this review. I will also discuss only mechanical factors and not the neural-humeral responses to lung inflation that can impact on the circulation (1-7). Before discussing circulatoryventilatory interactions, it is necessary to review some of the basics of the determinants of cardiac output.
