the principles of homeostasis Flashcards
receptors and effectors
It is impossible to maintain a living mammal in a completely stable state because everything causes minute changes. Instead, the body maintains a dynamic equilibrium, with small fluctuations over a narrow range of conditions. This is known as homeostasis.
Receptors and effectors are vital for the body to maintain this dynamic equilibrium. As you have seen, sensory receptors detect changes in the internal and external environment of an organism. In homeostasis, it is essential to monitor changes in the internal environment, for example, the pH of the blood, core body temperature, and concentrations of urea and sodium ions in the blood.
Information from the sensory receptors is transmitted to the brain and impulses are sent along the motor neurones to the effectors to bring about changes to restore the equilibrium in the body. Effectors are the muscles or glands that react to the motor stimulus to bring about a change in response to a stimulus. Both are vital in a homeostatic system - detecting change is no use without the means to react to that change, but effectors cause chaos unless responding to a need.
feedback systems
Homeostasis depends on sensory receptors detecting small changes in the body, and effectors working to restore the status quo. These precise control mechanisms in the body are based on feedback systems that enable the maintenance of a relatively steady state around a narrow range of conditions.
negative feedback systems
Most of the feedback systems in the body involve negative feedback.
A small change in one direction is detected by sensory receptors. As a result, effectors work to reverse the change and restore conditions to their base level. Negative feedback systems work to reverse the initial stimulus. You have seen negative feedback in action in the control of blood sugar levels by insulin and glucagon. Negative feedback systems are also important in many other aspects of homeostasis including temperature control and the water balance of the body. The general principles of negative feedback systems are shown in Figure 2.
positive feedback systems
There are relatively few positive feedback systems in the body. In a positive feedback system, a change in the internal environment of the body is detected by sensory receptors, and effectors are stimulated to reinforce that change and increase the response. One example occurs in the blood clotting cascade. When a blood vessel is damaged, platelets stick to the damaged region and they release factors that initiate clotting and attract more platelets. These platelets also add to the positive feedback cycle and it continues until a clot is formed. Another example of a positive feedback mechanism is seen during childbirth. The head of the baby presses against the cervix, stimulating the production of the hormone oxytocin. Oxytocin stimulates the uterus to contract, pushing the head of the baby even harder against the cervix and triggering the release of more oxytocin. This continues until the baby is born.
