Biology 12

Chapter 4 Homeostatic Mechanisms • MHR 111

stability. For cells, tissues, and organisms that
require constant conditions to function normally,
positive feedback is usually disastrous, causing
serious health issues. An exception occurs in the
process of childbirth, as shown in Figure 4.7. Near
the end of a pregnancy, the baby’s head pushes
against the opening of the uterus. The pressure on
the uterine muscles triggers the contraction of these
muscles and sends messages to the hypothalamus,
which releases a hormone (oxytocin) that increases
the strength of the next contraction. Each

subsequent contraction pushes the baby’s head

farther into the uterine opening, which triggers

successively stronger contractions until the baby

is born. (You will learn more about the role of

the hypothalamus and hormones in Chapter 6.)

In this section, you have looked at how the

body’s negative feedback systems affect its ability

to maintain a stable state. In the next section, you

will learn about the kidney’s role in homeostasis.

SECTION REVIEW

1. Explain how the brain helps to regulate body
   temperature.


2. Provide one example of dynamic equilibrium in
   the body.


3. Describe the role of the skin in maintaining
   homeostasis in the body.


4. Explain the difference between hypothermia
   and hyperthermia.


5. Examine the graph on the right showing the
   relationship between body temperature and ambient
   (environmental) temperature in a cat and a lizard.
   How is the cat able to maintain a more constant
   body temperature than the lizard?


6. Explain the difference between a negative
   feedback loop and a positive feedback loop.


7. Explain how illness can upset the dynamic
   equilibrium of the body. Use a flowchart to show how
   feedback mechanisms help or hinder the body to
   recover from illness.


8. Some people experience symptoms of mountain
   sickness when they travel to high altitude locations.
   Symptoms can range from mild headaches and

decreased appetite to more serious symptoms,

including confusion, hallucinations, shortness of

breath, and (in extreme cases) coma. In most cases,

people eventually adjust to the change in altitude

without lingering side effects. Do some research to

determine the homeostatic mechanisms involved in

adjusting to high altitude conditions. If you were

planning a mountain climbing expedition, what

precautions would you recommend to help you

and the other travellers avoid developing

mountain sickness?

(^01020)

10

20

30

40

30 40

Rectal temperature (

°C)

Ambient temperature (°C)

lizard

cat

MC

C

K/U

I

K/U

K/U

K/U

K/U

Oxytocin stimulates uterine

contractions and pushes

fetus toward cervix

5

Figure 4.7In childbirth, positive
feedback helps the baby emerge
from the womb.

Head of fetus pushes

against cervix

1

Nerve impulses

from cervix are

transmitted to brain

2

Brain stimulates

pituitary gland to

secrete oxytocin

3

Oxytocin carried

in bloodstream

to uterus

4