6.6 Hormones, homeostasis and reproduction
Lesson One: Introduction to the endocrine system, diabetes, and the hormones of weight gain.
- Hormones – a chemical messenger, carried in the blood.
- Homeostastis – maintaining a constant internal environment in the body
- Reproduction – producing new members of a population
Introductory Activity: Introduction to the endocrine system. Research, draw and label the following organs on the human body outline. They are an important part of the endocrine system:
- Pineal gland
- Thyroid gland
- Adrenal gland
- Adipose tissue (sub-cutaneous)
Endocrine system as we need to know it
Figure 1: Endrocrine system
In this unit we will exlore the relevance of the endocrine system to 5 interesting themes:
- How do people become diabetic?
- Why do people get fat, is it hormones?
- Why is jet-lag so hard to handle and can we take medicine for it?
- Can we have babies without having sex?
- What does it mean to be male or female?
Diabetes is the inability to regulate blood sugar levels. There are two kind of diabetes:
- Diabetes type 1: Called ‘early-onset diabetes’, is caused by auto-immune damage to the B-cells in the pancreas. Auto-immune means that the bodies own immune system attacks it (we don’t know why, although it could be genetic).This means that insufficient insulin is produced in the body.
- Diabetes type 2: Called ‘late-onset diabetes’ is caused by a reduced sensitivity of the body to insulin. It is still not clear what causes diabetes type 2, but it has been correlated with diets with high-sugar diets, fatty-diets, lack of exercise and prolonged obesity and genetic factors.
The Pancreas and blood sugar levels
The glucose levels in the blood are normally around 4-8mmol/litre for a non-diabetic. Regulation within these limits is achieved by two cells in the pancreas, alpha and beta cells.
These two kinds of cells are found in a part of the pancreas called the ‘islets of Langerhans’, after German physician Paul Langerhans who discovered them.
Alpha cells secrete: Glucagon
Beta cells secrete: Insulin
memory trigger – “You make glucagon when all the glucose is gone. Take sugar in, insulin”
Figure: Alpha and Beta cells in the pancreas
The regulation of blood sugar level takes place through Positive (encourages a change) and negative feedback (counteracts a change)
- For example if glucose levels rise, alpha cells in the pancreas secrete insulin, causing glucose levels to fall. This is negative feedback.
- If glucose levels fall, beta cells in the pancreas secrete glucagon, causing glucose levels to rise again. This is also negative feedback.
Think: How could you judge if you have a genetic-predisposition to diabetes? How could you reduce your chances of developing type 2 diabetes?
The glycemic index is very important for diabetics, and compares the relative effects of different foods on our blood glucose levels
- Food with a low glycemic index will be digested more slowly, and have a more gradual effect on our blood sugar levels.
- Foods with a high glycemic index will be digested and metabolised more quickly, and have a rapid effect on our blood sugar level.
Activity: Test your breakfast on the GI scale (high, medium, or low?) click http://www.glycemicindex.com
Question 2 Why do people get fat, is it hormones?
to stimulate discussion questions:
Weight gain basically happens because the energy intake of the body is less than the energy used by the bodies metabolism.
However, some people seem to gain or lose weight a lot more easily than others….do hormones have something to do with it?
The role of thyroxine
Thyroxine is a hormone made by the thyroid gland, and it controls your base metabolic rate (how fast the chemical reactions in your body take place).
The secretion of thyroxine:
- Increases metabolic rate, particularly in the more metabolically active organs (muscle, liver, brain).
- This causes the production of body heat to increase, and also the production of protein and cell division.
Symptoms of thyroid deficiency include:
- Forgetfulness and depression
- Lack of energy and continual fatigue
- feeling cold all the time
- impaired brain development in children
- weight gain despite lack of appetite (why?)
Another related hormone is leptin. Leptin is produced by adipose tissues in the body, and targets taste receptors in the mouth to depress appetite. It also affects the appetite control centre in the hypothalamus. In this way, the body has a natural feedback mechanism which prevents over-eating. However studies in mice have shown that some animals have a defective gene for leptin, so it is not produced.
Figure 2 The mouse on the right has a leptin deficient gene
Think: could the same be true of humans?
Lesson two – the sleep cycle
Objectives: understand the role of melatonin in controlling the sleep cycle
- Circadian rhythm: the body’s natural cycle of sleep and waking, a sense of timing controlled by the brain.
- Suprachiasmatic nuclei (SCN): a part of your hypothalamus that controls the circadian rhythms, it responds to stimuli of light from your retina to update the circadian rhythm.
- Pineal gland: the control gland of the endocrine system. It also secretes ‘melatonin’ which is a sleepy hormone
- Melatonin: a hormone which depresses the speed of your metabolism, causing your core body temperature to cool and stimulating sleep.
Question 3: Why is jet lag so hard to handle and can we take medicine for it?
Our body keeps track of time using circadian rhythms (body clocks). This involves a region of your hypothalamus called the SCN (suprachiasmatic nuclei) and the pineal gland.
- SCN sets a 24 hour rhythm cycle for the body of sleep/wakefullness.
- Pineal gland secretes a hormone called melatonin (I’m feeling sleepy hormone).
Melatonin causes drowsiness, and a drop in body-core temperature. Lowering body-core temperature causes metabolism to slow down. The SCN communicates with the pineal gland to ensure that Melatonin:
- rises at night time
- Falls during the day time.
Jet-lag is defined as fatigue following an international flight which has crossed timezones, and results from your circadian rhythm being out of sync with the time zone you are now in.
Q) What is the time difference between Costa Rica and:
- New Zealand
- San Francisco
If you travelled from those timezones, by how much would your body clock be out of sync with Costa Rican time?
It is possible that treatment with melatonin may help you to recover.
Lesson three: The menstrual cycle
Objectives: to understand how hormones control the menstrual cycle
- Estrogen: a sex hormone that causes the lining of the uterus to build up, in anticipation of an embryo. Also causes FSH to rise.
- Progesterone: a sex hormone that causes the lining of the uterus to be maintained, in anticipation of an embryo. Suppresses FSH and LH
- FSH (Follicle-stimulating hormone) is a hormone that stimulates the maturation of an egg follicle. Also causes levels of oestrogen to increase.
- LH (leutenising hormone) is a hormone that causes a mature egg to be releases from an egg follicle.
- Egg follicle is a spherical group of cells that contain the egg and secrete hormones, found in the ovaries
- Corpeus luteum: the left over egg follicle after the ripe egg has been released. Secretes progesterone.
- Pituitary gland: control gland of the endocrine system. Secretes FSH and LH
IVF treatment refers to In-Vitro (glass) Fertilisation. That is when fertilisation happens in a laboratory. This allows for a woman to impregnate herself using laboratory controlled sperm. The process is:
- Menstrual cycle is halted using a nasal spray containing powerful hormones
- Woman is treated with FSH and LH by injection. This causes her to super-ovulate (produce more than one egg)
- Human Chorionic Gonadotropin is added to cause final maturation of the egg follicles
- The ova are collected using a flexible syringe, inserted through the vagina.
- The ova are mixed with the desired sperm in laboratory controlled conditions
- Embryos begin to develop in the laboratory, and are selected based on there viability.
- A healthy embryo is implanted into the womb, using a flexible syringe.
- The implanted embryo continues to develop normally.
Think: What interesting ethical questions arise from the IVF procedure?
The control of hormones in the menstrual cycle
The role of the two hormones oestrogen and progesterone:
- Oestrogen causes the lining of the uterus to build up, in order to receive an embryo.
- Progesterone maintains the lining of the uterus.
- It is the drop of progesterone, failing the implantation of an embryo, which causes menstruation to occur.
- Leutenising hormone (LH) and FSH (Follicle-stimulating Hormone) are also involved, and work with oestrogen to cause ovulation.
- Ovulation – the release of an egg, from an ovary.
The other hormones….
- FSH – follicle stimulating hormone, stimulates the maturation of an immature egg (or follicle)
- LH- Leutenising hormone, causes ovulation by causing digestion of the follicle wall. Also helps turn the follicle into the corpus luteum (hormone secreting structure left over).
FSH and LH are produced by the pituitary gland (in the brain).
The subtleties of the positive and negative feedback loops involved in the menstrual cycle is difficult to track. An attempt to clarify is shown below:
Q) Those who practice the ‘rhythm’ method of contraception attempt to limit intercourse to times when an egg is not present. Which days would suit this aim, judging from the graph?
Q) Explain what would happen to the hormone levels, and the lining of the uterus if pregnancy was to occur?
Positive and negative feedback in the menstrual cycle
Positive feedback causes something to increase, negative feedback causes something to decrease
Q) Negative or positive feedback?
- a thermostat
- runaway global warming
Menstrual cycle feedback:
- Oestrogen makes the follicles more receptive to FSH, and FSH stimulates the follicles to produce more oestrogen (positive feedback)
- After ovulation, progesterone inhibits production of FSH and LH (negative feedback)
What does it mean to be male of female?
discussion: How much of maleness and femaleness is a societal construct?
real biological differences between male and female:
- reproductive system
- brain size (average)
- Chromosomes XX and XY
- muscle mass (average)
- height (average)
How many more differences are there in men and women in society, are any of these not biological?
History of discovering the importance of fertilisation
Before the use of microscopes, eggs and sperm could not be seen. Dr William Harvey tried to solve the mystery of how copulation could lead to birth, by studying deer.
Aristotle taught that the male produces a seed, which forms an egg when it mixes with menstrual blood.
Dr William Harvey analysed the uteri of female deer that had mated, for up two two months after coitus. He failed to find any evidence of Aristotles theory.
Q) Could there be other theories waiting to be proved by new evidence?
You should be familiar with the structure of the male/female reproductive system, in terms of the main organs involved.
Male and female reproductive systems