We compared a house, with its roof and walls, to a human body, with a skeletal structure and internal organs.

But the similarities go beyond the physical structure. Water circulates through a house, just as blood and other fluids circulate within an animal's body. Wires in a house transmit electricity; nerves transmit sensory signals in an animal.

In addition to these systems is a systems that involves control and regulation.

On a winter’s day, you don’t have to physically turn on the furnace to warm your house – a thermostat senses a drop in temperature and activates the furnace for you. When the temperature inside the house reaches a comfortable level, the thermostat turns off the furnace again.

The body works in a similar way.

If you wear light summer clothes in cold weather, nerve endings in your skin sense a temperature drop and this information is transmitted to the brain.

Temperature sensors in the brain then transmit signals to the major muscles. Your body will start to shiver. The energy produced by the shivering process produces heat, which helps prevent temperature loss. Blood vessels near the skin are also constricted to aid the conservation of heat.

The shivering process is equivalent to turning on the furnace in the house, and the skin’s temperature sensors are the body’s equivalent of the thermostat.

Maintaining the body’s internal steady state – for example the maintenance of temperature and water balance – is called homeostasis

Once the weather warms up again, shivering stops. This isn’t a conscious process. Your body ‘knows’ when to stop shivering.

This type of control system is called negative feedback, because a change in the variable being monitored – in this case heat - triggers the control mechanism to stop the action. Most animal control systems work in this way.

Each system has receptors that detect change, in this case skin receptors that signal a temperature drop.

Also present is a control center that processes the receptor’s information. In most of the body’s homeostatic mechanisms the control center is a region of the brain called the hypothalamus.

Finally, an effector carries out the correct response. In this case the effector is the body’s skeletal muscles.

Homeostasis can be seen in another area of regulation in the body, the maintenance of salt balance.

If there is less water in your body fluids and the concentration of salts in the blood increases, you become dehydrated. Specialized neurons in the hypothalamus sense the salt concentration and do two things.

First, they invoke brain interactions that make you thirsty!

Second, they trigger the pituitary gland to produce a substance called ADH – antidiuretic hormone.

ADH travels in the bloodstream to the kidneys, which filter waste products from fluids in the body and excrete excess fluids as part of the urine. ADH makes the kidneys excrete less water from the fluids in the body.

Once the concentration of salts in the blood has decreased to an acceptable level, the sensation of thirst and the production of ADH diminish. The negative feedback process is complete.

Organ systems have to work together to maintain homeostasis.

Temperature control involves thermostats in the brain, muscles to carry out the shivering reaction, and interaction with the circulatory system to minimize the blood supply to the skin.

The regulation of salt balance also involves a complex set of interactions – in this case the brain, and the circulatory system are also involved, with the addition of the kidneys as the effector.

Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education