Homeostatic systems, such as the processes regulating thermoregulation (body temperature ), work to maintain a constant internal environment. Like other homeostatic systems, thermoregulation employs negative feedback control: the resulting heat inhibits the system from calling for more. Review Figure 9.1, Animation 9.2, Animation 9.3, and Animation 9.4
Homeostatic systems rely on specialized behaviors to help regulate physiological parameters. For example, most species have specialized behaviors to help warm or cool the body. Review Figure 9.2
Our cells function properly only when the concentration of salt in the intracellular compartment of the body is within a critical range. The extracellular compartment is a source of replacement water for osmosis and a buffer between the intracellular compartment and the outside world. Review Figure 9.5
Thirst is a powerful motivator, triggered either by hypovolemic thirst (decreased volume of the extracellular fluid) or by osmotic thirst (increased extracellular saltiness). Because of the importance of solute concentration, we must regulate salt intake in order to regulate water balance effectively. Review Figure 9.6
Specialized osmosensory neurons detect the concentration of extracellular fluid. Baroreceptors in the major blood vessels monitor blood pressure and volume. Review Figure 9.7
Hypovolemic and osmotic thirsts are triggered by different mechanisms and differ in their immediate effects, but both forms of thirst ultimately trigger a complex shared thirst network. Review Figure 9.8
Our digestive system breaks down food and uses most of it for energy. Insulin helps most body cells to use glucose for fuel (the brain can use glucose directly) and promotes the storage of excess food energy. Another pancreatic hormone, glucagon, helps liberate glucose from storage. Review Figure 9.9
To the dismay of dieters, the body responds to caloric restriction by reducing metabolism, thus limiting weight loss. Review Figure 9.10
In endotherms, most food energy is used for basal metabolism. Metabolism readily shifts to compensate for changes in the availability of food. Review Figure 9.12
An appetite controller located in the arcuate nucleus of the hypothalamus responds to levels of several peptide gut hormones. When activated, arcuate POMC neurons act to decrease appetite, and arcuate NPY neurons act to stimulate appetite. Leptin and insulin provide important hormonal signals about longer-term energy storage. Ghrelin and PYY3-36 provide more-acute signals from the gut. Ghrelin stimulates and PYY3-36 inhibits the arcuate appetite control system. Review Figure 9.15, Activity 9.1
Obesity is a pervasive problem that is difficult to treat through diet, drugs, or surgery. The only long-lasting medical intervention for obesity is bariatric surgery, but several drug strategies based on a new understanding of appetite control offer promise. Review Figure 9.17, Video 9.5