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Modeling and control in physiology 27

Fig. 23 Physiological positive loops: childbirth.

Fig. 24 Positive and negative feedback loops of the menstrual cycle. (The figure has
been adopted from Carroll, R.G., 2007. Female reproductive system. In: Elsevier’s
Integrated Physiology. Elsevier, pp. 177–187.)

set point. One of the most typical examples of positive loops is childbirth,
shown in Fig. 23, where contractions enhance the change until the baby
comes out.
The most classic example including at the same time positive and neg-
ative loops is the menstrual cycle (Carroll, 2007). This is shown in Fig. 24.
Here, note that there are three different feedback loops that are occurring.
The first feedback loop, FSH and LH were secreted to start the development
of the follicle. These two hormones work together to start estrogen secre-
tion. The estrogen has a negative feedback on FSH, which causes it to stop
being secreted. The second feedback loop occurs with the increased amount
of estrogen that is experienced at the midpoint in the cycle. This increased
amount of estrogen causes a positive feedback to occur on the LH cells in the
pituitary. LH secretion rises, and ovulation occurs. The final feedback loop is
a negative feedback. After ovulation, the follicle cells are transformed into