Regional Circulations

The major role of the circulatory system is to supply all tissue beds with appropriate oxygen and nutrients and effectively remove the end products of metabolism. During critical illness or injury, the circulatory system must adjust flow to organs and tissues to assure delivery of adequate oxygen and metabolic substrate to vital organs (i.e., the heart and the brain). The uninterrupted flow of oxygen and nutrients is necessary to sustain viability and normal function of the many specialized tissues. Since energy is needed for any function in the human body and it can be provided only by nutrients and oxygen, through billions of years of evolution, each organ system developed regulatory mechanisms that couple their function and energy consumption with substrate delivery by the circulatory system. These mechanisms are often customized for the unique functions of a given organ system. The control of blood flow to the brain, heart, lungs, kidneys, splanchnic circulation, and skin each has unique characteristics that will be reviewed in this chapter. For example, the renal circulation receives blood flow well in excess of tissue energy requirements; this excess flow facilitates the regulation of salt and water. During critical illness, these functions are less critical, and blood can be redirected from the kidney to preserve blood supply to vital organ systems. In addition to regulatory mechanisms that determine blood flow at the organ level, the control of the microcirculation can be disrupted by critical illness or its treatment. These alterations explain the apparent lack of correlation between restoration of macrohemodynamic parameters and microcirculatory flow in some patients with shock. The latter is currently difficult to measure at the bedside but may be suggested by ongoing organ dysfunction and accumulation of metabolic products such as lactic acid.