Environmental Physiology 4437 Midterm
Rather than referring to everything dealing with responses to environmental and psychosocial situations as "stress," scientists have provided us with a relatively new term called allostasis. Allostasis literally means "maintaining stability or homeostasis through change". This term was introduced by Sterling and Eyer in the late 1980’s to describe how the cardiovascular system adjusts to resting and active states of the body. This concept, however, can be applied to other physiological mediators, such as the secretion of cortisol into the blood stream and so on. Rheostasis is a condition or state in which, at any one instant, homeostatic defenses are still present but over a span of time there is a change in the regulated level. This idea that nothing is truly “constant” has been around since the 1800s but it was really nailed down by a scientist named Nicholas Mrosovsky in 1990. There are two parts of rheostasis, programmed and reactive. Programmed rheostasis are changes that occur at certain times or phases in the life cycle while reactive rheoostasis are changes that occur in response to a stimulus that may or may not be encountered. Some common examples of rheostasis would be fever or a circadian rhythm such as body temperature regulation. Homeorhesis has been used in modern science to replace “homeostasis”. Homeostasis referred to a static system rather than a fluid state in the internal environment, while homeorhesis takes into account the fluidity of change and trajectory within a specific time frame. In other words homeorhesis more accurately describes the adaptations and constant interactions necessary to one's well-being in an ever-changing environment. A common example of homeorhesis would be pregnancy. The body is regulating the various systems to maintain allostasis by adapting to the demands of the pregnancy but after the child is born the body returns back to its original state or previous trajectory. These systems of regulation systems are an integral part of animal agriculture because to fully understand how a particular species or production system will react to an environmental or psychophysiological stressor we must first understand how the body will perceive the imbalance. Pregnancy has a dramatically different effect on the body compared to chronic heat stress and for this reason they fall into different categories of regulation and must be approached medically from different angles.
"Allostatic Load and Allostasis." MacArthur SES & Health Network. University of California: San Francisco, 2008. Web. 16 Oct. 2013. . "Homeorhesis." Encyclopedia Britannica Online. Encyclopedia Britannica, n.d. Web. 17 Oct. 2013. .
A mechanism that selectively cools the brain during hyperthermia is a well-accepted fact in animals. Selective brain cooling (SBC) during hyperthermia in humans however is of considerable debate. Several authors have rejected the idea of human SBC for the following reasons: SBC is not logical because this mechanism removes the error signal activating the defense against hyperthermia. Unlike other animals, humans do not pant and thus do not possess a powerful heat sink at a short distance from the brain. Humans do not have a carotid rete, which acts as the site of countercurrent heat exchange between the arterial and venous blood flowing in and out of the brain. Also the high and constant arterial blood flow of the brain is sufficient to cool the brain under all conditions and finally the relatively low tympanic temperature (Tv) recorded in hyperthermic humans is not a sign of SBC, but rather is the sign of contamination of data because of humans low head skin temperature. In contradiction of these points however literature and careful experimentation has shown us that these points against selective brain cooling in humans are somewhat false. In the argument involving not having a carotid rete; many...
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