Homeostasis is the activity of cells throughout the body to maintain the physiological state within a narrow range that is compatible with life. Homeostasis is regulated by negative feedback loops and, much less frequently, by positive feedback loops. Both have the same components of a stimulus, sensor, control center, and effector; however, negative feedback loops work to prevent an excessive response to the stimulus, whereas positive feedback loops intensify the response until an end point is reached.
control center
compares values to their normal range; deviations cause the activation of an effector
effector
organ that can cause a change in a value
negative feedback
homeostatic mechanism that tends to stabilize an upset in the body’s physiological condition by preventing an excessive response to a stimulus, typically as the stimulus is removed
normal range
range of values around the set point that do not cause a reaction by the control center
positive feedback
mechanism that intensifies a change in the body’s physiological condition in response to a stimulus
sensor
(also, receptor) reports a monitored physiological value to the control center
set point
ideal value for a physiological parameter; the level or small range within which a physiological parameter such as blood pressure is stable and optimally healthful, that is, within its parameters of homeostasis
Water concentration in the body is critical for proper functioning. A person’s body retains very tight control on water levels without conscious control by the person. Watch this video to learn more about water concentration in the body. Which organ has primary control over the amount of water in the body?
The kidneys.
1. After you eat lunch, nerve cells in your stomach respond to the distension (the stimulus) resulting from the food. They relay this information to ________.
A) a control center
B) a set point
C) effectors
D) sensors
A
2. Stimulation of the heat-loss center causes ________.
A) blood vessels in the skin to constrict
B) breathing to become slow and shallow
C) sweat glands to increase their output
D) All of the above
C
3. Which of the following is an example of a normal physiologic process that uses a positive feedback loop?
A) blood pressure regulation
B) childbirth
C) regulation of fluid balance
D) temperature regulation
B
1. Identify the four components of a negative feedback loop and explain what would happen if secretion of a body chemical controlled by a negative feedback system became too great.
The four components of a negative feedback loop are: stimulus, sensor, control center, and effector. If too great a quantity of the chemical were excreted, sensors would activate a control center, which would in turn activate an effector. In this case, the effector (the secreting cells) would be adjusted downward.
2. What regulatory processes would your body use if you were trapped by a blizzard in an unheated, uninsulated cabin in the woods?
Any prolonged exposure to extreme cold would activate the brain’s heat-gain center. This would reduce blood flow to your skin, and shunt blood returning from your limbs away from the digits and into a network of deep veins. Your brain’s heat-gain center would also increase your muscle contraction, causing you to shiver. This increases the energy consumption of skeletal muscle and generates more heat. Your body would also produce thyroid hormone and epinephrine, chemicals that promote increased metabolism and heat production.