In order to keep at a constant point, known as the set point, homeostasis relies on negative feedback mechanisms. This means that whenever there’s a change in a system it’s reversed by a corrective mechanism so that the system can return to its original state. The corrective mechanism corresponds to the size of the change. Negative feedback can never keep completely on the set point but instead oscillates about it.

Positive feedback is where a change that occurs in a system causes more change to occur. Within bodily systems this tends to be very dangerous as it can lead to complete control breakdown.

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Control of Mammalian Oestrus

The oestrus cycle is the process in which a female mammal produces eggs. During this period most mammals then become receptive to mating. The cycle length varies from animal to animal, ranging from five days in mice to a year in red deer.

Due to the fact that it lasts a month in humans the cycle is known as the menstrual cycle (‘mens’ means month in Latin). Humans and also other primates are different from other mammals in that the female tends to be sexually receptive throughout the whole year.

In humans, two glands produce four hormones that control the menstrual cycle:

• the pituitary gland: this is located just under the hypothalamus in the brain and secretes luteinising hormone (LH) and follicle stimulating hormone (FSH)
• the ovaries: they secrete progesterone and oestrogen

Using negative and positive feedback loops, hormones affect the release of each other. This results in each hormone being secreted in sequence. Within the 28 day cycle the levels of each hormone changes.

The menstrual cycle takes place as follows:

• The pituitary glands release FSH. This travels to the ovaries where it stimulates the development of a Graafian follicle in which is contained an ovum cell.
• The developing follicle secretes oestrogen. This stimulates the reformation of the endometrium wall in the uterus which was shed in the last menstruation. Oestrogen also travels to the pituitary gland where it inhibits FSH release. As the follicle develops, however, and the blood oestrogen levels increase, this rise stimulates the pituitary gland to secrete LH and FSH.
• At around day 14 of the cycle a sudden increase in LH causes the follicle, which is now completely developed, to burst. The ovum is released into the oviduct, a process known as ovulation. The remaining follicle is stimulated further by LH to form into the corpus lutuem which then proceeds to release progesterone.
• The uterus is stimulated by the progesterone to complete the endometrium wall. This is now ready for an embryo. This hormone also travels to the pituitary gland and inhibits FSH and LH release which then inhibits the ovaries from releasing progesterone and oestrogen.
• Over the following 10 days prostaglandins secreted by the ovaries causes the corpus luteum degenerate. This means that the level of progesterone in the blood decreases. Once this hormone has reached a low enough level menstruation can start. This also allows the pituitary gland to start releasing FSH again and so the cycle begins once more.

However, if an egg is fertilised then an embryo can implant itself in the uterus. Once implanted, the embryo releases a hormone known as HCG (human chorionic gonadotrophin) which prevents the degeneration of the corpus luteum. It’s this hormone which pregnancy kits test for in urine. Progesterone continues to be secreted and menstruation isn’t triggered. Progesterone also inhibits the pituitary gland from secreting FSH which prevents any more ova from maturing.