Let us assume that you have stepped outside where it is much colder than inside. There is an initial change in your internal body temberture. The normal body temperature is called the set point and your body temperature drops below the set point. The immediate increases of heat loss from your warm skin upsets the cynamic balance between heat gain and heat loss. Your internal body temperature falls.

The first homeostatic respons is that ablod vessels to the skin narrow, reducing the amount of warm blood flowing through the skin and therefore reducing heat loss.

Given that there is no way to stay worm, e.g., putting on a jacket, it is necessary for the body to produce more heat to stay warm. Involutary, shivering, or voluntary, exercise, muscle activith may be used to generate heat.

The chemical reactions of muscle activy generates heat which raises the body temperature. The internal body temperature rises towards the set point.

As long as the internal temperature remains below the set point, there will be a tendency to remain active and generate more heat. If excess exercise is done, the temperature will rise above the set point and sweating may occur. As the temperture increases above the set point the desire for exercise will decrease.

A reflex is a stimulus-response event. kSpecifically, this is a an involuntary, unpremeditated, unlearned "built-in" response to a particular stimulus. An example is jerking your hand away from a hot object. There are also learned or acquired reflexes. An example is swerving the car to avoid an animal in the road. In general, most reflexes, no matter how basic, are subject to alteration by learning.

The pathway mediating a reflex is known as the reflex arc. A stimulus is defined as a detectable change in the internal or external environment, such as a change in temperature, plasma potassium concentration, or blood pressure. A receptor detects the environmental chage; we may refer to the receptor as a detector.

A stimulus acts upon a receptor to produce a signal that is relayed to an integrating center. The pathway traveled by the signal between the receptor and the integrating center is known as the afferent pathway.

An integrating center often receives signals from many receptors. Some of which may be responding to quite different types of stimuli. therefore, the output of an integrating center reflects the net effects of the total afferent input, i.e., it represents an integration of numerous bits of information.

The output of an integrating center is sent to the last component of the system, a device whose change in activity constitutes the overall response of the system. this component is known as an effector. Information going from an integrating center to an effector is like a command directing the effector to alter its activity. The pathway along which this information travels is knwon as the efferent pathway.

Reflexs are used to minimize the decrease in body temperature that occur as a result to exposure to a cold environment. this diagrqm looks at a pair of variables or system responses.

The stimulus is the same for each case. The decrease in temberature stimulates temperature sensitive nerve endings and let you and your integrating center that it is cold out. One set of responses is to constrict the smooth muscle around the afferent arterioles which reduces blood flow the the skin. This decreases the amount of heat radiated from the skin and preserves your core temperature.

The second set of responses deal with the skeletal muscle. Involuntary contractions, shivering, cause the production of heat. This heat production raises your core body temperature.

Both these sets of events serve to increase your body temperature towards its set point. This is a negative feedback pathway.

Homeostasis is about maintaining a steady state or balance. For your body to maintain in steady state, the net fluxes in must equal the net fluxes out. That is the food, liquids, air, and body products created must equal the net losses of the body, excretion plus metabolism.

The inputs to the body, the fluxes in, are the substances that you eat, drink, and breath. Additionally, a variety of substances are synthesised in your body. These are all considered inputs.

Within your body substances are stored and transformed. There are numerous storage depots. We store fats, water, carbohydrates, and any of a large variety of substances. Are body draws from these storage depots to meet its metabolic and catabolic needs. What is not needed is waste.

Wastes are excreted. Are wastes are exhaled air, sweat, feces, urine, and menstrual flow. We also produce products such as milk, saliva, and seamen. Are metabolic processes tend to produce heat, heat is thus a waste product we radiate.

For any chemical in our body, three states of total-body balance are possible:
1. Loss exceeds gain, so that the total amount of the substance in the body is decreasing, and the person is said to be in negative balance;
2. Gain exceeds loss, so that the total amount of the substance in the body in increasing, and the person is ssaid to be in positive balance;
3. Gain equals loss, and the person is in stable balance.

Let us consider sodium balance. The control systems for sodium balance have the kidneys as their targets. The systems operate by inducing the kidneys to excrete into the urine an amount of sodium approximately equal to the amount ingested on a daily basis.

Let us start with a daily intake and excretion of 7 g of sodium and a stable amount of sodium in the body. In an experiment, the daily sodium consuptions rises to 15 g, and remains there indefinitely. On this same day, the kidneys excrete into the urine somewhat more than 7 g of sodium, but not all of the ngested 15 g. The result is that some excess sodium is retained in the body on that day. The person is now in a positive sodium balance.

The kidneys do somewhat better on day 3, but it is probably not until day 4 or 5 that they are excreting 15 g. From this time on, output from the body onece again equals input, and sodium balance is onece again stable.

An important point is that, although again in stabvle balance, the subject has perhaps 2 percent more sodium in their body than was there at the initial stable balance state with an ingestion of 7 g/day. It is this 2 percent extra body sodium that constitutes the continuous error signal to the control system driving the kidneys to excrete 15 g/day rather than 7 g/day.

Homeostatic control systems cannot maintain complete constancy of the internal environment in the face of continued change in the perturbing event since some change in the regulated variable must persit to serve as a signal to maintain the compensating responses. Although an increase of 2 percent does not seem large, it has been hypothexized that this small gain might facilitate the development of high blood pressure, hypertension.