Lesson 1, Topic 1
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08-Balance of Body Functions

April 12, 2021

Balance of body functions

Homeostasis
Although structurally different from one another, all living organisms maintain mechanisms that ensure survival of the body and success in propagating its genes through its offspring.

Survival depends on maintaining relatively constant conditions within the body. Homeostasis is what physiologists call the relative constancy of the internal environment. The cells of the body live in an internal environment made up mostly of water combined with salts and other dissolved substances.

Like fish in a fishbowl, the cells are able to survive only if the conditions of their watery environment remain relatively stable—that is, only if conditions stay within a narrow range. The temperature, salt content, acid level (pH), fluid volume and pressure, oxygen concentration, and other vital conditions must remain within acceptable limits. To maintain a narrow range of water conditions in a fishbowl, one may add a heater, an air pump, and filters.

Likewise, the body has mechanisms that act as heaters, air pumps, and the like to maintain the relatively stable conditions of its internal fluid environment (Figure 1-10).

Because external disturbances and the activities of cells themselves cause frequent fluctuations inside the body, conditions are continuously drifting away from homeostasis. Therefore, the body must constantly work to maintain or restore stability, or homeostasis. For example, the heat generated by muscle activity during exercise may cause the body’s temperature to rise above normal. The body must then release sweat, which evaporates and cools the body back to a normal temperature.

Feedback control
To accomplish such self-regulation, a highly complex and integrated communication control system is required. The basic type of control system in the body is called a feedback loop.

The idea of a feedback loop is borrowed from engineering. Figure 1-11, A, shows how an engineer would describe the feedback loop that maintains stability of temperature in a building. Cold winds outside a building may cause the building temperature to drop below normal. A sensor, in this case a thermometer, detects the change in temperature. Information from the sensor feeds back to a control center—a thermostat in this example—that compares the actual temperature to the normal temperature and responds by activating the building’s furnace. The furnace is called an effector because it has an effect on the controlled condition (temperature). Because the sensor continually feeds information back to the control center, the furnace will be automatically shut off when the temperature has returned to normal.

As you can see in Figure 1-11, B, the body uses a similar feedback loop to restore body temperature when we become chilled. Nerve endings that act as temperature sensors feed information to a control center in the brain that compares actual body temperature to normal body temperature. In response to a chill, the brain sends nerve signals to muscles that cause rapidly repeated contractions. This shivering produces heat that increases our body temperature. We stop shivering when feedback information tells the brain that body temperature has increased to normal.

Negative feedback
Feedback loops such as those shown in Figure 1-11 are called negative feedback loops because they oppose, or negate, a change in a controlled condition. Most homeostatic control loops in the body involve negative feedback because reversing changes back toward a normal value tends to stabilize conditions—exactly what homeostasis is all about.

Think about the opposite circumstance of that shown in Figure 1-11, as when we become overheated during hot weather. Temperature receptors detect a body temperature higher than normal, and the brain sends signals to the sweat glands to cool us down through evaporation. Thus the conditions are reversed and balance is restored.

Another example of a negative feedback loop occurs during exercise. As muscles contract, they produce additional CO2 that is transported by blood. This increase in blood CO2 levels is detected by sensory receptors that transmit the information to respiratory control centers in the brain. This triggers an increase in breathing rate that brings the blood CO2 level back down toward normal.

An additional example is the excretion of larger than usual volumes of urine when the volume of fluid in the body is greater than the normal, ideal amount.

Positive feedback
Although not common, positive feedback loops do exist in the body and are sometimes also involved in normal function. Positive feedback control loops are stimulatory. Instead of opposing a change in the internal environment and causing a “return to normal,” positive feedback loops temporarily amplify or reinforce the change that is occurring. This type of feedback loop causes an ever-increasing rate of events to occur until something stops the process. An example of a positive feedback loop includes the events that cause rapid increases in uterine contractions before the birth of a baby (Figure 1-12).

Another example of normal positive feedback regulation in the body is the activity of blood cells called platelets, which become increasingly sticky in response to damage to a blood vessel. Circulating platelets rapidly cling to the damaged area and release chemicals that attract additional platelets that accumulate at the site of damage to form a blood clot. The blood clot forms to control bleeding.

In each of these cases, the process rapidly increases until the positive feedback loop is stopped suddenly by the birth of a baby or the formation of a clot. In the long run, such normal positive feedback events also help maintain constancy of the internal environment.
However, negative feedback can abnormally turn into positive feedback, possibly causing a deadly shift in body function.

For example, consider the role of blood pressure and the effect that severe bleeding may have on blood pressure. A normal blood pressure is necessary to ensure that blood flows through blood vessels at an appropriate rate. When blood is lost, as occurs with severe bleeding, blood pressure drops. To compensate, the heart beats faster to try to restore normal pressure. Unfortunately, this increases the loss of blood, which causes a further drop in blood pressure and an even faster heart rate. The response is accelerated, and the amplification of blood loss caused by this positive feedback loop can rapidly turn deadly. Applying pressure to the wound can stop or slow the loss of blood and stop the positive feedback loop.

Normal fluctuations
It is important to realize that normal homeostatic control mechanisms can maintain only a relative constancy. All homeostatically controlled conditions in the body do not remain absolutely constant. Rather, conditions normally fluctuate near a normal, ideal value. Thus body temperature, for example, rarely remains exactly the same for very long—instead it fluctuates up and down near a person’s normal body temperature.

Take a moment to scan Appendix C at evolve​.elsevier.com. It lists some of the normal ranges of physiological variables often measured when assessing a patient’s health. Notice that nearly every “normal value” listed is shown as a range instead of a single number. Ranges are used because different people may have slightly different set points, some set points change under different circumstances, and the values normally fluctuate close to (but not exactly at) the set point value.

Because all organs function to help maintain homeostatic balance, we discuss negative and positive feedback mechanisms often throughout the remaining chapters of this book.
Before leaving this brief introduction to physiology, we must pause to state an important principle: the ability to maintain the balance of body functions is related to age. During childhood, homeostatic functions gradually become more and more efficient and effective. They operate with maximum efficiency and effectiveness during young adulthood. During late adulthood and old age, they gradually become less and less efficient and effective.

Changes and functions occurring during the early years are called developmental processes. Changes occurring after young adulthood are called aging processes. In general, developmental processes improve efficiency of functions. Aging processes, on the other hand, often diminish efficiency of body functions.