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 -15 o C 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 23 o C 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 o C. 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 > 37 o C 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 37 o C (set point).

Thermoregulation Temperature < 37 o C 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 37 o C (set point).

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Control center 37°C set point Sensor stimulus 37°C set point negative feedback and return to normal Sensor Normal body temperature negative feedback and return to normal temperature Blood vessels constrict; sweat glands are inactive. Blood vessels dilate; sweat glands secrete. Effect above normal below normal directs response to stimulus sends data to control center change of internal conditions directs response to stimulus sends data to control center change of internal 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.