Biological Rhythms

A striking characteristic of many body functions is the rhythmical changes they manifest. The most common type is the circadian rhythm, which cycles approximately once every 24 h. Waking and sleeping, body temperature, hormone concentrations in the blood, the excretion of ions into the urine, and many other functions undergo circadian variation (Figure 7-7). Other cycles have much longer periods, the menstrual cycle (approximately 28 days) being the most well known.

What have biological rhythms to do with homeostasis? They add yet another "anticipatory" component to homeostatic control systems, in effect a feedforward system operating without detectors. The negative-feedback homeostatic responses we described earlier in this chapter are corrective responses, in that they are initiated after the steady state of the individual has been perturbed. In contrast, biological rhythms enable homeostatic mechanisms to be utilized immediately and automatically by activating them at times when a challenge is likely to occur but before it actually does occur. For example, there is a rhythm in the urinary excretion of potassium such that excretion is high during the day and low at night. This makes

Circadian time (hours)

a

e

10

m

o

« <B

rm

5

n

or

h

0

o

«

*—)

15

F

or u

o o

10

a

ma

re

a

O IT

Circadian time (hours)

4 A.M. Noon 8 P.M. 4 A.M. Noon 8 P.M. Time of day (hour)

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

Get My Free Ebook


Post a comment