Homeostatic Mechanisms

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The human organism consists of trillions of cells all working together for the maintenance of the entire organism. While cells may perform very different functions, all the cells are quite similar in their metabolic requirements. Maintaining a constant internal environment with all that the cells need to survive (oxygen, glucose, mineral ions, waste removal, and so forth) is necessary for the well-being of individual cells and the well-being of the entire body. The varied processes by which the body regulates its internal environment are collectively referred to as homeostasis.

What is homeostasis?

Homeostasis in a general sense refers to stability, balance or equilibrium. It is the body's attempt to maintain a constant internal environment. Maintaining a stable internal environment requires constant monitoring and adjustments as conditions change. This adjusting of physiological systems within the body is called homeostatic regulation. Homeostatic regulation involves three parts or mechanisms: 1) the receptor, 2) the control center and 3) the effector. The receptor receives information that something in the environment is changing. The control center or integration center receives and processes information from the receptor. And lastly, the effector responds to the commands of the control center by either opposing or enhancing the stimulus. This is an ongoing process that continually works to restore and maintain homeostasis. For example, in regulating body temperature there are temperature receptors in the skin, which communicate information to the brain, which is the control center, and the effector is our blood vessels and sweat glands in our skin. Because the internal and external environments of the body are constantly changing and adjustments must be made continuously to stay at or near the set point, homeostasis can be thought of as a synthetic equilibrium. Since homeostasis is an attempt to maintain the internal conditions of an environment by limiting fluctuations, it must involve a series of negative feedback loops.

Set point: the desired value in a closed-loop feedback system, as in regulation of temperature or pressure. 2. the point at which a thermostat has been set, as for optimum efficiency. 3. Physiology . an internal regulatory system for maintaining a relatively stable physiological condition in the face of changing external circumstances, as body temperature in a varying climate. Compare homeostasis. 4. a hypothetical regulator in the body that maintains a relatively stable body weight. 5. the weight range thus theoretically maintained

Positive and Negative Feedback

When a change of variable occurs, there are two main types of feedback to which the system reacts:

1. Negative feedback: a reaction in which the system responds in such a way as to reverse the direction of change or chaos. Since this tends to keep things constant, it allows the maintenance of homeostasis and stability. Most systems in the body, and in nature tend to be negative feedback systems. For instance, when the concentration of carbon dioxide in the human body increases, the lungs are signaled to increase their activity and expel more carbon dioxide. Thermoregulation is also an example of negative feedback. When body temperature rises (or falls), receptors in the skin and the hypothalamus sense a change, and will trigger a command from the brain. This command, in turn, affects the correct response, in this case a decrease in body temperature.

Example: Home Heating System vs. Negative Feedback When you are at home, you set your thermostat to a desired temperature. Let's say today you set it at 70 degrees. The thermometer in the thermostat waits to sense a temperature change either too high above or too far below the 70 degree set point. When this change happens the thermometer will send a message to the "Control Center", or thermostat, which in turn will then send a message to the furnace to either shut off if the temperature is too high or kick back on if the temperature is too low. In the home-heating example the air temperature is the "NEGATIVE FEEDBACK." When the Control Center receives negative feedback it triggers a chain reaction in order to maintain room temperature.

2. Positive feedback: a response is to amplify the change in the variable or a move towards chaos. This has a destabilizing effect, which does not result in homeostasis. Positive feedback is less common in naturally occurring systems than negative feedback, but it has its applications. For example, in nerves, a threshold electric potential triggers the generation of a much larger action potential. Blood clotting in which the platelets process mechanisms to transform blood liquid to solidify is an example of positive feedback loop. Another example is the secretion of oxytocin which provides a pathway for the uterus to contract, leading to child birth.

Harmful Positive Feedback:  Although Positive Feedback is needed within homeostasis it also can be harmful at times. When you have a high fever it causes a metabolic change that can push the fever higher and higher. In rare occurrences the body temperature reaches 113°F / 45°C and the cellular proteins stop working and the metabolism stops, resulting in death.

Summary: Sustainable systems require combinations of both kinds of feedback. Generally with the recognition of divergence from the homeostatic condition, positive feedbacks are called into play, whereas once the homeostatic condition is approached, negative feedback is used for "fine tuning" responses. This creates a situation of "metastability," in which homeostatic conditions are maintained within fixed limits, but once these limits are exceeded, the system can shift wildly to a wholly new (and possibly less desirable or chaotic) situation of homeostasis.

Cruise Control on a car as a simple metaphor for homeostasis

When a car is put on cruise control it has a set speed limit that it will travel. At times this speed may vary by a few miles per hour but in general the system will maintain the set speed. If the car starts to go up a hill, the systems will automatically increase the amount of fuel given to maintain the set speed. If the car starts to come down a hill, the car will automatically decrease the amount of fuel given in order to maintain the set speed. It is the same with homeostasis- the body has a set limit on each environment. If one of these limits increases or decreases, the body will sense and automatically try to fix the problem in order to maintain the pre-set limits. This is a simple metaphor of how the body operates—constant monitoring of levels, and automatic small adjustments when those levels fall below (or rise above) a set point.

Homeostatic imbalance

Many diseases involve a disturbance of homeostasis. As an organism ages, the efficiency in its control systems becomes reduced. The inefficiencies gradually result in an unstable internal environment that increases the risk of illness, and leads to the physical changes associated with aging. Certain homeostatic imbalances, such as high core temperature, a high concentration of salt in the blood, or low concentration of oxygen, can generate homeostatic emotions (such as warmth, thirst, or breathlessness), which motivate behavior aimed at restoring homeostasis (such as removing a sweater, drinking or slowing down).

Ecological Application

The concept of homeostasis is central to the topic of Ecological Stoichiometry. There, it refers to the relationship between the chemical composition of an organism and the chemical composition of the nutrients it consumes. Stoichiometric homeostasis helps explain nutrient recycling and population dynamics. Throughout history, ecological succession was seen as having a stable end-stage called the climax , sometimes referred to as the 'potential biodiversity' of a site, shaped primarily by the local climate. This idea has been largely abandoned by modern ecologists in favor of nonequilibrium ideas of how ecosystems function, as most natural ecosystems experience disturbance at a rate that makes a "climax" community unattainable.

The Biosphere

In the Gaia hypothesis, James Lovelock stated that the entire mass of living matter on Earth (or any planet with life) functions as a vast homeostatic superorganism that actively modifies its planetary environment to produce the environmental conditions necessary for its own survival. In this view, the entire planet maintains homeostasis. Whether this sort of system is present on Earth is still open to debate. However, some relatively simple homeostatic mechanisms are generally accepted. For example, it is sometimes claimed that when atmospheric carbon dioxide levels rise, certain plants are able to grow better and thus act to remove more carbon dioxide from the atmosphere. However, warming has exacerbated droughts, making water the actual limiting factor on land.