(There is another page for Reproduction)
Hormones are chemicals – usually peptides or similar, or lipids – which are carried in the blood in minute quantities, from endocrine glands where they are secreted, to often distant and multiple target organs and cells. They usually lead to slow-acting, irreversible, long-term responses. Their involvement in control and homeostasis is thus very different from the action of the nervous system.
Link to the SAT syllabus content: endocrine syllabus
The IB syllabus 6.6
INSULIN & GLUCAGON and the homeostatic control of blood sugar levelThe control of glucose levels in the blood – reading: Diabetes (and gene technology)
LEPTIN AND APPETITE
There was a lot of speculation about how appetite was controlled. The answer seems to lie in a hormone LEPTIN which is produced around the body in fat storage cells (adipose cells). The ‘ob’ gene responsible for synthesising this hormone is switched on or off according to the density of fat in the adipose cells. Leptin then targets the hypothalamus, which leads to secondary effects in bone tissue and has an effect upon appetite.
MELATONIN – circadian rhythms and controlling jet lag
Melatonin is a hormone produced by the pineal gland and is responsible (by interacting with other hormones and body systems) for maintaining natural circadian rhythms. Melatonin is most active at night and its production can be affected by caffeine (drinking coffee in the evening) and by crossing time zones, when daylight and darkness lose their customary synchronisation. In fact jet lag may be treatable by melatonin.
Testosterone is the hormone which in males leads to the development of both primary and secondary sexual characteristics. It is controlled by the SRY (Sex determining Region of the Y chromosome) gene on the Y chromosome.
NEGATIVE & POSITIVE FEEDBACK LOOPS IN THE MENSTRUAL CYCLE
William Harvey (1578-1657) is best known for his published ideas about the circulation of blood around the body, with the heart as its pump. He developed his ideas by dissecting deer given to him by King James from the royal parks and forests.
In addition to his research into blood and the circulation, Harvey was one of the first to study embryology (the study of reproduction in its earliest stages) by observing the development of the chick in the egg. He performed many dissections of mammal embryos at various stages of formation. From these experiments Harvey was able to formulate the first new theory of animal generation since antiquity, emphasizing the primacy of the egg, even in mammals. Prior to Harvey’s work, it was thought that the male sperm was the primary source of new life, and that the egg was simply an empty home, so to speak, for the sperm to develop.
Harvey was hampered in his research into embryology by a lack of a microscope, which was not invented until some 17 years after his death. With a microscope, he would have been able to substantiate many of his perfectly correct ideas.
Thanks to Harvey’s willingness to abandon old wisdom and observe and test for himself, we have our modern understanding of physiology.
Follow this link for more about William Harvey and embryology: https://embryo.asu.edu/pages/william-harvey-1578-1657