Our Marvelous Bodies: An Introduction to the Physiology of Human Health

11. Integrated Physiological Responses

162 11 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. Hypovolemic Hypotension 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  beats per minute,  liters per minute, and 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  to  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. Integrated Physiological Responses Among other changes, the physiological responses to the early stages of hypovolemic shock involve cardiovascular reflexes that make emergency adjustments to the crisis. The second phase involves delayed, more long-term adaptations by the kidneys, the adrenal glands, and higher brain structures such as the hypothalamus. During these two periods, this person’s body will have gone from a state of physiological harmony—one where vitals signs are at steady state and equilibrium, gradients and feedback control are in balance, and homeostasis is maintained—to a near-death state of disharmony and commotion. Baroreceptor Reflexes and Cardiac Output In the above example, hypovolemia occurs after blood volume has declined from approximately five to about three liters. Correspondingly, blood pressure will plummet, from mmHg to perhaps as low as  or mmHg. This will cause havoc for blood perfusion of most organs and tissues. That is, the homeostasis of oxygen and nutrient supply and demand will be upset, and the survival of organs—the victim’s life—put in jeopardy. The volume of blood in the vascular compartment is one of the physical determinants of blood pressure. When blood volume declines so does blood pressure, and when blood volume increases blood pressure follows. A reduction in blood volume coupled with hypotension are sensory signals that activate several cardiovascular reflexes. One of the most important of these is the baroreceptor reflex. The sensory receptors of this five-component reflex arc are located, bilaterally, in the carotid sinuses near the bifurcation of the common carotid arteries into their internal and external carotid branches. Carotid sinus baroreceptors—also called pressure receptors, pressoreceptors, or mechanoreceptors —are designed to detect both mean systemic arterial blood pressure and beat-to-beat pulsatile blood pressure. Deviations in either or both of these from their physiological values either activate or inactivate the receptors. In our subject, as mean and pulsatile blood pressures decline, stretch or deformation of the walls of the carotid sinuses is reduced. This results in a reduced frequency of afferent sensory signals being transmitted to the brain stem each second. The action potentials are conducted via the sinus or Hering nerves to the pons and the medulla oblongata. These two brain stem structures and adjacent tissues constitute the location of many of the body’s important regulatory control centers, such as the cardiovascular, respiratory, thermoregulatory, and satiety centers. The cardiovascular control centers of the medulla subserve both cardiac and vascular functions. When activated, one set of neurons within the cardiac center stimulates an increase in heart rate and is called the cardioacceleratory center. Other neurons cause a decrease in heart rate when stimulated and are referred to as the cardioinhibitory center. Similar classifications apply to the INTEGRATED PHYSIOLOGICAL RESPONSES 163 vasomotor centers that are divided into vasopressor (which constricts blood vessels) and vasodepressor (dilates vessels) regions. The efferent motor action potentials coming out of the...