Integrated Physiological Responses
At the turn of the twenty-first century, all things physiology were about integration. This means understanding mechanisms from molecular to whole animal levels. Such knowledge allows science to be quickly transferred from the laboratory bench to the hospital bed (translational physiology). Regulation of circulating blood volume is one such topic.
Hemorrhagic shock such as occurs in combat or in automobile accidents reduces the circulating volume of blood and causes hypovolemic hypotension. This has far-reaching consequences for the body and survival. It also exemplifies how multiple organs and systems respond in a coordinated fashion when the physiological homeostasis of the cardiovascular system is upset. In the experimental physiology laboratory, hemorrhagic shock and hypovolemic hypotension have been among the time-tested classic experiments used to teach advanced students about the integration of organ systems physiology.
Imagine an adult whose organ systems are in the physiological steady state under resting conditions. Assume his heart rate, cardiac output, and mean systemic arterial blood pressure are 75 beats per minute, 5 liters per minute, and 100 mmHg. All is well because his cardiovascular system is in a state of homeostasis. Now consider the same person after an emergency crisis brought on by the precipitous loss of 30 to 40 percent of his circulating blood volume. There are two phases of response to the crisis. The first could be characterized in the early seconds to minutes after the onset of hypovolemia. The second, a considerably different picture, would take place hours to days later and only if the victim was still alive.