6.6 Hormones, Homeostasis, Reproduction
Essential idea: Hormones are used when signals need to be widely distributed.
Thyroxine is a hormone produced by the thyroid gland. It's key role is in controlling the metabolism of cells. If affects almost every physiological process in the body including growth and development. Most hormones affect more than one target tissue in more than one way. |
Understandings
6.6.U1 Insulin and glucagon are secreted by β and α cells of the pancreas respectively to control blood glucose concentration.
6.6.U2 Thyroxine is secreted by the thyroid gland to regulate the metabolic rate and help control body temperature.
6.6.U3 Leptin is secreted by cells in adipose tissue and acts on the hypothalamus of the brain to inhibit appetite.
6.6.U4 Melatonin is secreted by the pineal gland to control circadian rhythms.
6.6.U5 A gene on the Y chromosome causes embryonic gonads to develop as testes and secrete testosterone.
6.6.U6 Testosterone causes pre-natal development of male genitalia and both sperm production and development of male secondary sexual characteristics during puberty.
6.6.U7 Estrogen and progesterone cause pre-natal development of female reproductive organs and female secondary sexual characteristics during puberty.
6.6.U8 The menstrual cycle is controlled by negative and positive feedback mechanisms involving ovarian and pituitary hormones. [The roles of FSH, LH, estrogen and progesterone in the menstrual cycle are expected.]
Applications
6.6.A1 Causes and treatment of Type I and Type II diabetes.
6.6.A2 Testing of leptin on patients with clinical obesity and reasons for the failure to control the disease.
6.6.A3 Causes of jet lag and use of melatonin to alleviate it.
6.6.A4 The use in IVF of drugs to suspend the normal secretion of hormones, followed by the use of artificial doses of hormones to induce superovulation and establish a pregnancy.
6.6.A5 William Harvey’s investigation of sexual reproduction in deer. [William Harvey failed to solve the mystery of sexual reproduction because effective microscopes were not available when he was working, so fusion of gametes and subsequent embryo development remained undiscovered.]
Skills
6.6.S1 Annotate diagrams of the male and female reproductive system to show names of structures and their functions.
6.6.U1 Insulin and glucagon are secreted by β and α cells of the pancreas respectively to control blood glucose concentration.
6.6.U2 Thyroxine is secreted by the thyroid gland to regulate the metabolic rate and help control body temperature.
6.6.U3 Leptin is secreted by cells in adipose tissue and acts on the hypothalamus of the brain to inhibit appetite.
6.6.U4 Melatonin is secreted by the pineal gland to control circadian rhythms.
6.6.U5 A gene on the Y chromosome causes embryonic gonads to develop as testes and secrete testosterone.
6.6.U6 Testosterone causes pre-natal development of male genitalia and both sperm production and development of male secondary sexual characteristics during puberty.
6.6.U7 Estrogen and progesterone cause pre-natal development of female reproductive organs and female secondary sexual characteristics during puberty.
6.6.U8 The menstrual cycle is controlled by negative and positive feedback mechanisms involving ovarian and pituitary hormones. [The roles of FSH, LH, estrogen and progesterone in the menstrual cycle are expected.]
Applications
6.6.A1 Causes and treatment of Type I and Type II diabetes.
6.6.A2 Testing of leptin on patients with clinical obesity and reasons for the failure to control the disease.
6.6.A3 Causes of jet lag and use of melatonin to alleviate it.
6.6.A4 The use in IVF of drugs to suspend the normal secretion of hormones, followed by the use of artificial doses of hormones to induce superovulation and establish a pregnancy.
6.6.A5 William Harvey’s investigation of sexual reproduction in deer. [William Harvey failed to solve the mystery of sexual reproduction because effective microscopes were not available when he was working, so fusion of gametes and subsequent embryo development remained undiscovered.]
Skills
6.6.S1 Annotate diagrams of the male and female reproductive system to show names of structures and their functions.
Homeostasis
Homeostasis is the tendency for an organism or cell to maintain a constant internal environment within tolerance limits.
Internal equilibrium is maintained by adjusting physiological processes, including:
Homeostasis is the tendency for an organism or cell to maintain a constant internal environment within tolerance limits.
Internal equilibrium is maintained by adjusting physiological processes, including:
- Body temperature (normally between 36 – 38ºC)
- Carbon dioxide concentration (normally 35 – 45 mmHg)
- Blood pH (normally between 7.35 – 7.45)
- Blood glucose levels (normally 75 – 95 mg/dL)
- Water balance (varies depending on individual body size)
- When specialised receptors detect an internal change to conditions, a response is generated to correct the change
- Most homeostatic responses involve an effect that is antagonistic to the detected stimulus (negative feedback)
- When levels return to equilibrium, the effector ceases to generate a response and an internal balance is therefore maintained
- If a physiological condition moves outside of tolerance limits, disease will occur as a consequence
Endocrine System
The endocrine system is a system of ductless glands that release chemicals (hormones) into the blood to regulate body functions
Endocrine Glands
Endocrine glands secrete their product (hormones) directly into the bloodstream, rather than through a duct (e.g. exocrine gland)
The endocrine system is a system of ductless glands that release chemicals (hormones) into the blood to regulate body functions
- A hormone is a chemical messenger that is transported via the bloodstream to act on distant target cells
- Hormones are specific and will only activate cells or tissues that possess the appropriate target receptor
- The endocrine system is slower to initiate, but has a more prolonged response when compared to the nervous system
Endocrine Glands
Endocrine glands secrete their product (hormones) directly into the bloodstream, rather than through a duct (e.g. exocrine gland)
- Major endocrine glands include the pancreas, adrenal gland, thyroid gland, pineal gland and the gonads (ovaries and testes)
- The hypothalamus and pituitary gland are neuroendocrine glands and function to link the nervous and endocrine systems
- Some organs may also secrete hormones despite not being endocrine glands (e.g. adipose tissue secretes leptin)
Understandings:
6.6.U1 Insulin and glucagon are secreted by β and α cells of the pancreas respectively to control blood glucose concentration.
The body needs glucose to make ATP (via cell respiration), however the amount required will fluctuate according to demand
Regulating Blood Glucose Concentrations
6.6.U1 Insulin and glucagon are secreted by β and α cells of the pancreas respectively to control blood glucose concentration.
The body needs glucose to make ATP (via cell respiration), however the amount required will fluctuate according to demand
- High levels of glucose in the blood can damage cells (creates hypertonicity) and hence glucose levels must be regulated
- These hormones are released from pancreatic pits (called the islets of Langerhans) and act principally on the liver
- Insulin is released from beta (β) cells of the pancreas and cause a decrease in blood glucose concentration
- This may involve stimulating glycogen synthesis in the liver (glycogenesis), promoting glucose uptake by the liver and adipose tissue, or increasing the rate of glucose breakdown (by increasing cell respiration rates)
- Glucagon is released from alpha (α) cells of the pancreas and cause an increase in blood glucose concentration
- This may involve stimulating glycogen breakdown in the liver (glycogenolysis), promoting glucose release by the liver and adipose tissue, or decreasing the rate of glucose breakdown (by reducing cell respiration rates)
Regulating Blood Glucose Concentrations
6.6.A1 Causes and treatment of Type I and Type II diabetes.
Diabetes mellitus is a metabolic disorder that results from a high blood glucose concentration over a prolonged period
Diabetes mellitus is a metabolic disorder that results from a high blood glucose concentration over a prolonged period
- It is caused by the body either not producing insulin (Type I) or failing to respond to insulin production (Type II)
- It is treated with either insulin injections (Type I only) or by carefully monitoring and controlling dietary intake (Type II)
6.6.U2 Thyroxin is secreted by the thyroid gland to regulate the metabolic rate and help control body temperature.
Thyroxin is a hormone secreted by the thyroid gland in response to signals initially derived from the hypothalamus
The Role of Thyroxin in Body Temperature Regulation
Thyroxin is a hormone secreted by the thyroid gland in response to signals initially derived from the hypothalamus
- Thyroxin acts on nearly every tissue in the body and is essential to the proper development and differentiation of cells
- This can be achieved by stimulating carbohydrate and lipid metabolism via the oxidation of glucose and fatty acids
- Thyroxin is released in response to a decrease in body temperature in order to stimulate heat production
- Iodine deficiency will cause the thyroid gland to become enlarged, resulting in a disease known as goitre
The Role of Thyroxin in Body Temperature Regulation
6.6.U3 Leptin is secreted by cells in adipose tissue and acts on the hypothalamus of the brain to inhibit appetite.
Leptin is a hormone produced by adipose cells that regulates fat stores within the body by suppressing appetite
The Role of Leptin in Hunger and Satiety
Leptin is a hormone produced by adipose cells that regulates fat stores within the body by suppressing appetite
- Leptin binds to receptors located within the hypothalamus to inhibit appetite and thereby reduce food intake
- Conversely, periods of starvation lead to a reduction in adipose tissue and hence less leptin is released, triggering hunger
- This means they are more likely to feel hungry, less likely to recognise when they are full and are hence more likely to overeat
- Leptin resistance also develops with age, increasing the potential for weight gain later in life (e.g. the ‘middle-age spread’)
The Role of Leptin in Hunger and Satiety
6.6.A2 Testing of leptin on patients with clinical obesity and reasons for the failure to control the disease.
Because leptin suppresses appetite, it was considered as a form of treatment for individuals with clinical obesity
Leptin trials were initially conducted by surgically fusing the blood circulation of obese and healthy mice (parabiosis)
Because leptin suppresses appetite, it was considered as a form of treatment for individuals with clinical obesity
- Theoretically, leptin injections would reduce hunger and limit food intake in obese individuals, leading to weight loss
Leptin trials were initially conducted by surgically fusing the blood circulation of obese and healthy mice (parabiosis)
- This experiment was conducted using mice that were either obese due to a leptin gene mutation or a defective leptin receptor
- Leptin in the blood of the healthy mouse was transferred to the obese mouse
- The obese mouse responded to the leptin and began to lose weight, demonstrating the potential viability of leptin treatment
- Leptin was transferred to the healthy mouse (the obese mouse overproduced leptin to compensate for low receptor sensitivity)
- The obese mouse remained obese as its body could not respond to leptin
- The healthy mouse became emaciated due to the abnormally high levels of leptin transferred into its bloodstream
Human Experiments
Most humans have naturally high levels of leptin in the bloodstream
Most humans have naturally high levels of leptin in the bloodstream
- When linked to leptin activity, most cases of obesity are caused by an unresponsiveness to leptin and not a leptin deficiency
- Hence, in clinical trials, very few participants experienced significant weight loss in response to leptin injections
- However, many patients did experience adverse side effects from leptin injections, including skin irritations
- For these reasons, leptin treatments are not considered to be an effective way of controlling obesity
6.6.U4 Melatonin is secreted by the pineal gland to control circadian rhythms.
Melatonin is a hormone produced by the pineal gland within the brain in response to changes in light
Melatonin Secretion by the Pineal Gland
Melatonin is a hormone produced by the pineal gland within the brain in response to changes in light
- Light exposure to the retina is relayed via the suprachiasmatic nucleus (in the hypothalamus) and inhibits melatonin secretion
- Melatonin is therefore secreted in response to periods of darkness, resulting in higher concentrations at night
Melatonin Secretion by the Pineal Gland
Circadian Rhythms
Melatonin secretion by the pineal gland of the brain plays a pivotal role in the control of circadian rhythms
Melatonin is the hormone responsible for synchronising circadian rhythms and regulates the body's sleep schedule
Melatonin Secretion Over a Day-Night Cycle
Melatonin secretion by the pineal gland of the brain plays a pivotal role in the control of circadian rhythms
- Circadian rhythms are the body’s physiological responses to the 24 hour day-night cycle
- Circadian rhythms are driven by an internal (endogenous) circadian clock, although they can be modulated by external factors
Melatonin is the hormone responsible for synchronising circadian rhythms and regulates the body's sleep schedule
- Melatonin helps control your sleep and wake cycles (circadian rhythms).
- Very small amounts of melatonin are found in foods such as meats, grains, fruits, and vegetables.
- An internal 24- hour clock controls your natural cycle of sleeping and waking hours.
- Melatonin levels generally begin to rise in the mid to late evening, remaining high for most of the night, and then drop in the early morning hours.
- Light from the sun can also affect how much melatonin your body produces. During the shorter days of the winter months, your body may produce melatonin either earlier or later in the day than usual. This change can lead to symptoms of seasonal affective disorder (SAD), or winter depression.
- Natural melatonin levels slowly drop with age. Some older adults make very small amounts of it or none at all.
Melatonin Secretion Over a Day-Night Cycle
6.6.A3 Causes of jet lag and use of melatonin to alleviate it.
6.6.U6 Testosterone causes pre-natal development of male genitalia and both sperm production and development of male secondary sexual characteristics during puberty.
The main male reproductive hormone is testosterone, which is secreted by the testes and serves a number of roles:
The main male reproductive hormone is testosterone, which is secreted by the testes and serves a number of roles:
- It is responsible for the pre-natal development of male genitalia
- It is involved in sperm production following the onset of puberty
- It aids in the development of secondary sex characteristics (including body hair, muscle mass, deepening of voice, etc.)
- It helps to maintain the male sex drive (libido)
6.6.U7 Estrogen and progesterone cause pre-natal development of female reproductive organs and female secondary sexual characteristics during puberty.The main female reproductive hormones (secreted by the ovaries) are estrogen and progesterone, which serve several roles:
- If the SRY gene on the Y chromosome is not present in the embryo, the gonads develop into ovaries
- Estrogen and progesterone which are secreted by the mother’s ovaries and then by the placenta, will cause the female reproductive organs to develop in the absence of testosterone
- During puberty, estrogen and progesterone cause the development of secondary sexual characteristics in females, including breast development, menstrual cycle and pubic and armpit hair
6.6.U8 The menstrual cycle is controlled by negative and positive feedback mechanisms involving ovarian and pituitary hormones [The roles of FSH, LH, estrogen and progesterone in the menstrual cycle are expected.]
The menstrual cycle describes recurring changes that occur within the female reproductive system to make pregnancy possible
- Each menstrual cycle lasts roughly one month (~28 days) and begins at puberty (menarche) before ending with menopause
There are two key groups of hormones which control and coordinate the menstrual cycle:
- Pituitary hormones (FSH and LH) are released from the anterior pituitary gland and act on the ovaries to develop follicles
- Ovarian hormones (estrogen and progesterone) are released from the ovaries and act on the uterus to prepare for pregnancy
Flowchart of Hormonal Actions During Menstrual Cycle
6.6.A4 The use in IVF of drugs to suspend the normal secretion of hormones, followed by the use of artificial doses of hormones to induce superovulation and establish a pregnancy.
In vitro fertilisation (IVF) refers to fertilisation that occurs outside of the body (in vitro = 'in glass’)
In Vitro Fertilisation Procedure
In vitro fertilisation (IVF) refers to fertilisation that occurs outside of the body (in vitro = 'in glass’)
- It involves using drugs to suspend normal ovulation (down regulation), before using hormone treatments to collect multiple eggs (superovulation)
- Drugs are used to halt the regular secretion of FSH and LH – this in turn stops the secretion of estrogen and progesterone
- By arresting the hormonal cycle, doctors can take control of the timing and quantity of egg production by the ovaries
- The drug treatment usually takes about two weeks and is typically delivered in the form of a nasal spray
- Superovulation involves using artificial doses of hormones to develop and collect multiple eggs from the woman
- The patient is firstly injected with large amounts of FSH to stimulate the development of many follicles
- The follicles are then treated with human chorionic gonadotrophin (hCG) – a hormone usually produced by a developing embryo
- hCG stimulates the follicles to mature and the egg is then collected (via aspiration with a needle) prior to the follicles rupturing
- The extracted eggs are then incubated in the presence of a sperm sample from the male donor
- The eggs are then analysed under a microscope for successful fertilisation
- Approximately two weeks prior to implantation, the woman begins to take progesterone treatments to develop the endometrium
- Healthy embryos are selected and transferred into the female uterus (or the uterus of a surrogate)
- Multiple embryos are transferred to improve chances of successful implantation (hence multiple births are a possible outcome)
- Roughly two weeks after the procedure, a pregnancy test is taken to determine if the process has been successful
In Vitro Fertilisation Procedure
Summary of the Key Stages of IVF
- Stop normal menstrual cycle (with drugs)
- Hormone treatments to promote super ovulation
- Extract multiple eggs from the ovaries
- Sperm collected, then prepared (via capacitation) and injected into egg
- Fertilisation occurs externally under controlled conditions (in vitro)
- Implantation of multiple embryos into uterus (either patient or surrogate)
- Test for pregnancy after ~ two weeks
6.6.A5 William Harvey’s investigation of sexual reproduction in deer.
Historical Perspective of Sexual Reproduction
One of the earliest theories as to how animals reproduce sexually was the 'soil and seed' theory proposed by Aristotle
The ‘soil and seed’ theory was a popular doctrine for hundreds of years before it was eventually debunked by William Harvey
Historical Perspective of Sexual Reproduction
One of the earliest theories as to how animals reproduce sexually was the 'soil and seed' theory proposed by Aristotle
- According to this theory, the male produces a ‘seed' which forms an ‘egg' when mixed with menstrual blood (the ‘soil’)
- The ‘egg’ then develops into a fetus inside the mother according to the information contained within the male 'seed’ alone
The ‘soil and seed’ theory was a popular doctrine for hundreds of years before it was eventually debunked by William Harvey
- William Harvey studied the sexual organs of female deer after mating in an effort to identify the developing embryo
- He was unable to detect a growing embryo until approximately 6 – 7 weeks after mating had occurred
- He concluded that Aristotle’s theory was incorrect and that menstrual blood did not contribute to the development of a fetus
- Harvey was unable to identify the correct mechanism of sexual reproduction and incorrectly asserted that the fetus did not develop from a mixture of male and female ‘seeds’
Modern Theory of Sexual Reproduction
Our current understanding of the mechanism of sexual reproduction is based on evidence discovered using light microscopes
Modern Understanding of Gametogenesis
Our current understanding of the mechanism of sexual reproduction is based on evidence discovered using light microscopes
- Viable microscopes for such investigations were not invented until 17 years after the death of William Harvey
Modern Understanding of Gametogenesis
6.6.S1 Annotate diagrams of the male and female reproductive system to show names of structures and their functions.