Homeostasis

Keeping the Body

in Balance

Homeostasis in the Body• Previously, we’ve talked about a few methods cells use to

keep their internal environments constant.• Now, we will turn our attention to the human body as a

whole.

• For a complex organism, homeostasis is: “the relative constancy of the body’s internal environment”.

• When the external environment changes, the inside of the body should stay the same.

Homeostasis in the Body• In order to ensure a constant internal environment, the

body must monitor a wide variety of “variables”.• These include:

• Temperature• Water levels• Amount of waste• Blood pH (acidity) and sugar levels

Dynamic Equilibrium• The internal conditions in the body are NOT absolutely

constant – they can and do change.• The internal environment is in a dynamic equilibrium –

any conditions not at the “normal” value will be corrected by the body.

• If there is a very large change in conditions, this is called illness.

Homeostatic Control• The body controls homeostasis through a mechanism called

negative feedback.• Before we define this, let’s use an analogy (you don’t need to write this down )

• It’s the end of a long school day in January. You come home only to find that it’s oppressively cold – you can see your breath. You clamor over to the thermostat to find that it’s -15oC in the house – the thermostat somehow got set way too low. Reaching out with your quivering hand, barely able to manipulate the inconveniently-small buttons on the thermostat, you manage to set a new temperature at 23oC before finally collapsing to the floor, gasping as the burning sensation of frostbite overtakes your hand. As you hear the furnace in the basement coming to life, you cry tears of joy that promptly freeze to your face. You awaken, hours later, to find that the house is now comfortably warm. Your family, oblivious to the near-disaster, stands over you, confused. The furnace has, since, turned off.

Homeostatic Control• In homeostatic control and negative feedback, there is

always a cause-and-effect relationship that is monitored by sensors and control centers in the body.

• In our example, the cause or stimulus was the low temperature.

• The sensor was the temperature sensor in the thermostat.

• The control center, which set the desired temperature, was the thermostat, itself.

• The effector, which brought about a corrective change, was the furnace.

Homeostatic Control• Note that once the desired

temperature was reached and began to increase beyond that point, the sensor detected the increased temperature, causing the control center (thermostat) to turn off the furnace.• This prevents the furnace from

overheating the house.

• Homeostasis in the body is controlled in much the same way.

Negative Feedback• Now, we can define negative feedback.• Negative feedback is a mechanism of homeostasis in

which a body system acts to reverse a change in the body’s internal environment.• Once the change is reversed, the stimulus that activated the body

system is removed, and the body system stops whatever it was doing to reverse the change.

Negative Feedback• Negative feedback has at least three components:

1. A sensor that detects a change in the internal conditions.

2. A control center that directs a response to bring conditions back to their normal value (the set point).

3. An effector which is signalled by the control center, and whose actions cause conditions to return to normal.

Thermoregulation• Thermoregulation refers to the control of the body’s

internal temperature. The average set point for body temperature (at the body’s core) is 37.0 oC.

• Major changes from this value do happen. They can be fatal.• Can you think of any instances where major changes can be

helpful?

Thermoregulation

Mechanism:

Temperature > 37oC• Temperature sensors in the brain (sensor) send signals to

the hypothalamus (control centre). • The hypothalamus causes an increase in blood flow

(dilates blood vessels) to the skin. Sweat glands in the skin are activated. The blood vessels and sweat glands are effectors.

• Body temperature falls back to 37oC (set point).

Thermoregulation

Temperature < 37oC• Temperature sensors in the brain, again, send signals to

the hypothalamus (control centre).• The hypothalamus sends signals to decrease blood flow

to the skin (constricts blood vessels). Again, the blood vessels are effectors.

• Very low temperatures cause signals to also be sent to muscles (also effectors), causing them to shiver.

• Body temperature rises to 37oC (set point).

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Control center

Control center

37°C set pointSensor

stimulus

stimulus

37°C set point

negative feedbackand return to normal

Sensor

Normal body temperature

negative feedbackand return to normaltemperature

Blood vessels constrict;sweat glands are inactive.

Blood vessels dilate;sweat glands secrete.

Effect

Effect

above normal below normal

directs responseto stimulus

sends data tocontrol center

change ofinternal conditions

directs responseto stimulus

sends data tocontrol center

change ofinternal conditions

Osmoregulation• The body must also maintain a constant water balance.

This is called osmoregulation. Small drops in fluid concentration can cause major effects:• A drop of 5% causes extreme pain and collapse.• A drop of 10% typically results in death.

• Smaller drops (as little as 1%) cause the body to increase fluid levels.

• How might this be accomplished?• What would sensors detect?• What effects would the effectors cause?

Osmoregulation

Mechanism:• Sensors in the hypothalamus monitor how concentrated

the blood is as it circulates.• When there’s less water in the blood, the hypothalamus

(control center) causes a hormone to be released.• This hormone causes the kidneys (an effector) to retain

more water.• The nervous system (another effector) triggers the

sensation of feeling thirsty.