Page 4 - 84 human physiolofy part-1

Chapter 1

1 HOMEOSTASIS

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Overview

T he human body 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. 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.

A metaphor to help us understand this process is the operation of a thermostat. The thermostat
monitors and controls room temperature. The thermostat is set at a certain temperature that is
considered ideal, the set point. The function of the thermostat is to keep the temperature in the room
within a few degrees of the set point. If the room is colder than the set point, the thermostat receives
information from the thermometer (the receptor) that it is too cold. The effectors within the thermostat
then will turn on the heat to warm up the room. When the room temperature reaches the set point, the
receptor receives the information, and the thermostat "tells" the heater to turn off. This also works
when it is too hot in the room. The thermostat receives the information and turns on the air conditioner.
When the set point temperature is reached, the thermostat turns off the air conditioner.

Our bodies control body temperature in a similar way. The brain is the control center, the receptor
is our body's temperature sensors, and the effector is our blood vessels and sweat glands in our skin.
When we feel heat, the temperature sensors in our skin send the message to our brain. Our brain then
sends the message to the sweat glands to increase sweating and increase blood flow to our skin. When
we feel cold, the opposite happens. Our brain sends a message to our sweat glands to decrease
sweating, decrease blood flow, and begin shivering. This is an ongoing process that continually works
to restore and maintain homeostasis.

Because the internal and external environment 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 dynamic equilibrium.

4 | Human Physiology