3
Stimulus
4
Response
2
HOMEOSTASIS DISTURBED:
Blood pH increases.
5
HOMEOSTASIS RESTORED:
Blood pH decreases.
1
HOMEOSTASIS DISTURBED:
Blood pH decreases.
HOMEOSTASIS RESTORED:
Blood pH increases.
Stimulus
Response
HOMEOSTASIS FIGURE 20.24
Summary of the Chemoreceptor Reflex
The chemoreceptor reflex maintains homeostasis in response to changes in blood CO
2
and H
+
concentrations (pH). (1) Blood pH is within its normal range.(2) Blood pH increases outside the normal range. (3) Chemoreceptors in the medulla oblongata detect increased blood pH. Control centers in the braindecrease sympathetic stimulation of the heart and adrenal medulla. (4) Heart rate and stroke volume decrease, reducing blood flow to the lungs. (5) Thesechanges cause blood pH to decrease (as a result of increase in blood CO
2
). (6) Blood pH returns to its normal range; homeostasis is restored. Observe theresponses to a decrease in blood pH outside its normal range by following the
red arrows.
707