Heat can be exchanged by four mechanisms: (a) radiation, (b) evaporation, (c) convection, or (d) conduction. Heat will be conducted from one surface to another during direct contact with the surfaces, such as an animal resting on a warm rock. Convection currents of air remove heat from the surface of dry skin as the air passes over it. Heat can be removed with liquid from a surface during evaporation. Heat comes from the sun in this manner and radiates from dry skin the same way. Radiation is the emission of electromagnetic “heat” waves. Heat can be exchanged between an animal and its environment through four mechanisms: radiation, evaporation, convection, and conduction ( Figure 14.22). These animals are able to maintain a level of activity at cooler temperature, which an ectotherm cannot due to differing enzyme levels of activity. Endotherms are animals that rely on internal sources for body temperature but which can exhibit extremes in temperature. An animal that maintains a constant body temperature in the face of environmental changes is called a homeotherm. In contrast to ectotherms, which rely on external temperatures to set their body temperatures, poikilotherms are animals with constantly varying internal temperatures. This group has been called cold-blooded, but the term may not apply to an animal in the desert with a very warm body temperature. Animals that do not control their body temperature are ectotherms. Another example of acclimatization is animals that have seasonal changes in their coats: a heavier coat in the winter ensures adequate heat retention, and a light coat in summer assists in keeping body temperature from rising to harmful levels.Īnimals can be divided into two groups: some maintain a constant body temperature in the face of differing environmental temperatures, while others have a body temperature that is the same as their environment and thus varies with the environment. In order to adjust to the lower oxygen levels at the new altitude, the body increases the number of red blood cells circulating in the blood to ensure adequate oxygen delivery to the tissues. This occurs, for instance, when an animal migrates to a higher altitude than it is accustomed to. This is called a process of alteration of the set point in a feedback loop.Ĭhanges can be made in a group of body organ systems in order to maintain a set point in another system. Medication can lower blood pressure and lower the set point in the system to a more healthy level. The result is the maintenance of an elevated blood pressure that can have harmful effects on the body.

The body no longer recognizes the elevation as abnormal and no attempt is made to return to the lower set point. An example of this is blood pressure: over time, the normal or set point for blood pressure can increase as a result of continued increases in blood pressure. When this happens, the feedback loop works to maintain the new setting. It is possible to adjust a system’s set point. (credit: modification of work by Jon Sullivan) Blood sugar levels are controlled by a negative feedback loop. Negative feedback loops are the predominant mechanism used in homeostasis. The effects of PTH are to raise blood levels of the element. If calcium levels decrease, specialized cells in the parathyroid gland sense this and release parathyroid hormone (PTH), causing an increased absorption of calcium through the intestines and kidneys and, possibly, the breakdown of bone in order to liberate calcium. This is still a negative feedback loop, but not in the direction expected by the use of the term “negative.” Another example of an increase as a result of the feedback loop is the control of blood calcium. However, if an animal has not eaten and blood glucose levels decrease, this is sensed in another group of cells in the pancreas, and the hormone glucagon is released causing glucose levels to increase. Insulin causes blood glucose levels to decrease, as would be expected in a negative feedback system, as illustrated in Figure 14.20. Specialized cells in the pancreas sense this, and the hormone insulin is released by the endocrine system. When an animal has eaten, blood glucose levels rise. An example is animal maintenance of blood glucose levels. In other words, if a level is too high, the body does something to bring it down, and conversely, if a level is too low, the body does something to make it go up. It may either increase or decrease the stimulus, but the stimulus is not allowed to continue as it did before the receptor sensed it. Any homeostatic process that changes the direction of the stimulus is a negative feedback loop.

Negative feedback examples pancreas