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 1. 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. Negative Feedback MechanismsĪny homeostatic process that changes the direction of the stimulus is a negative feedback loop. Homeostasis is controlled by the nervous and endocrine system of mammals. Positive feedback loops actually push the organism further out of homeostasis, but may be necessary for life to occur. Homeostatsis is maintained by negative feedback loops. The effector is a muscle (that contracts or relaxes) or a gland that secretes. The receptor senses the change in the environment, then sends a signal to the control center (in most cases, the brain) which in turn generates a response that is signaled to an effector. When a change occurs in an animal’s environment, an adjustment must be made. If the blood’s glucose rises after a meal, adjustments are made to lower the blood glucose level by getting the nutrient into tissues that need it or to store it for later use. For instance, if the body becomes too warm, adjustments are made to cool the animal. A change in the internal or external environment is called a stimulus and is detected by a receptor the response of the system is to adjust the deviation parameter toward the set point. While there are normal fluctuations from the set point, the body’s systems will usually attempt to go back to this point. The goal of homeostasis is the maintenance of equilibrium around a point or value called a set point.

Even an animal that is apparently inactive is maintaining this homeostatic equilibrium. It is equilibrium because body functions are kept within specific ranges. It is dynamic because it is constantly adjusting to the changes that the body’s systems encounter. Homeostasis means to maintain dynamic equilibrium in the body.

These changes might be in the level of glucose or calcium in blood or in external temperatures. Describe thermoregulation of endothermic and ectothermic animalsĪnimal organs and organ systems constantly adjust to internal and external changes through a process called homeostasis (“steady state”).Discuss positive and negative feedback mechanisms used in homeostasis.Describe the factors affecting homeostasis.By the end of this section, you will have completed the following objectives:

Negative feedback system