Chemical Control in Mammals

Endocrine System

The endocrine system operates via secretions called hormones directly into the bloodstream.
Endocrine glands include the pituitary, thyroid, pancreas, adrenal, ovaries, and testes.
Hormones are chemical messengers, carrying information from a gland to a target cell via the bloodstream.
Hormones can be either proteins or steroids; protein hormones are not lipid-soluble and so have receptors on cell surfaces, whereas steroid hormones are lipid-soluble and enter the cell to bind with receptors.

Feedback Mechanisms

A crucial aspect of hormonal regulation is negative feedback, which reduces the deviation from an ideal normal value, and is common to all homeostatic mechanisms.
In contrast, positive feedback increases any deviation from the ideal normal value.
The hypothalamus monitors the level of hormones in the blood and can either increase or decrease their production based on negative feedback.

Key Hormones

Insulin is a hormone produced by the pancreas that controls the level of glucose in the bloodstream.
The production of insulin is an example of negative feedback; when blood glucose levels are too high, insulin is produced to reduce them.
Adrenaline is produced by the adrenal glands and prepares the body for ‘fight or flight’ in stressful situations.
Thyroxine, produced by the thyroid, controls the metabolic rate, heart rate, muscle growth and development.

Hormonal Disorders

Imbalances in the hormonal system can lead to diseases such as Diabetes which is caused by insufficient insulin production.
Hyperthyroidism and Hypothyroidism are conditions caused by over or under-production of thyroxine respectively.

The Menstrual Cycle

The menstrual cycle is an example of a system regulated by hormones, with FSH, LH, progesterone and oestrogen playing key roles.
FSH stimulates the development of a mature follicle in the ovary, which then secretes oestrogen.
High levels of oestrogen cause a surge in LH which leads to ovulation.
After ovulation, the ruptured follicle develops into the corpus luteum, which secretes progesterone to maintain the lining of the uterus.
If no fertilised egg embeds in the uterus, progesterone levels fall, triggering the shedding of the uterus lining, or menstruation.

Remember, understanding how these hormones interact with each other and their direct effects on the body is crucial for understanding chemical control in mammals.