Chapter 49 – Maintaining the Internal Environment –
Much of this chapter has been covered in the other chapters to one degree or another.
However, several topics
remain: disposal of nitrogen, balancing body heat, evolution of the vertebrate kidney, and osmoregulation.
these topics are important to your education as young science students.
As always, the Concept Outline and the
Chapter Review Outline provide excellent places to begin your preparation in understanding this material.
is defined on P. 1040.
Basically, the body operates at an optimal level within very narrow ranges of
There is an optimal “set point” for each physiological function.
For example, there is an
optimal body temperature, blood pH, blood protein concentration, blood nitrogen level, blood glucose level, blood
oxygen level, hydration, and so forth.
The body cannot maintain these set points in absolute terms, i.e. the
temperature of the body is going to change with activity, exposure to sun (or lack of sun), changes in air or water
When the physiological state of any condition begins to change, the body begins to initiate activity to
bring the physiological state back to “normal.”
, as I understand it.
Negative Feedback Loops
– Fig. 49.2; 49.3.
Stimulus moves physiological condition from normal ‘set point.’
Sensor picks up the change, measures the deviation.
CNS communicates with effector to moderate condition so that
physiological state returns to normal.
Figs.49.5 and 47.19 are good exampled.
Blood glucose levels go up, set point
is violated, insulin is secreted, blood glucose levels go down, original set point achieved, no more insulin is secreted.
Positive Feedback Loops
– Positive feedback loops are uncommon because the feedback system tends to move the
set point farther from the normal state.
Two examples are often given:
childbirth (Fig. 49.7, and orgasm in the sex
– Fig. 49.6 describes a nonbiological example of antagonistic reactions.
example can be seen in Fig. 47.19.
In this case, consider glucagon.
Glucose levels rise, the set point is violated,
insulin is secreted, glucose levels fall, no more insulin; BUT if glucose is need, glucagon is secreted, glucose levels
rise – maybe too high, glucagon shuts off, but insulin is secreted.
So one molecule is antagonistic to the other.
– this term is not in your textbook, but it describes the body’s anticipation to a certain condition and the
physiological adjustments to deal with the condition.
Most of this chapter deals with water balance between the intracellular and extracellular environments (Fig. 49.8).
There is a difference between