6.6 Hormones, homeostasis and reproduction Flashcards
What do cells in the pancreas respond to?
Changes in blood glucose levels
What is initiated when glucose concentration deviates substantially from a set point of aobut 5mmolL-1?
Homoestatic mechanisms mediated by the pancreatic hormones insulin and glucagon are initiated
How many glands in the pancreas?
The pancreas is effectively two glands in one organ
What tissue is most of the pancreas?
Exocrine glandular tissue
What do exocrine glandular tissue from the pancreas do?
They secrete digestive enzymes into ducts leading to the small intestine
What is the small region of tissue that makes up the pancreas alongside exocrine glandular tissue?
There are small regions of endocrine tissue called islets of Langerhans dotted through the pancreas
What do islets of Langerhans do?
They secrete hormones directly into the blood stream
What are the two cell types in the islets of Langerhans that secrete different hormones?
Alpha cells and Beta cells
What do alpha cells in the islets of Langerhans synthesis and secrete?
They synthesize and secrete glucagon if the blood glucose level falls below the set point
What does glucagon stimulate?
This hormone stimulates breakdown of glycogen into glucose in liver cells and its release into the blood, increasing the concentration
What do Beta cells in the pancreas synthesize and secrete?
They synthesize insulin and secret it when the blood glucose concentration rises above the set point
What does insulin stimulate?
- uptake of glucose by various tissues, particularly skeletal muscle and liver
- conversion of glucose to glycogen
What does insulin do?
Reduces blood glucose concentratioin
How are most hormones broken down?
Broken down by the cells it acts upon, so its secretion must be ongoing
What is diabetes?
Is the condition where a person has consistently elevated blood glucose levels, leading to the presence of glucose in the urine
What are the consequences of diabetes?
- Continuously elevatedglucose damages tissues, particularly their proteins
- Impairs water reabsorption from urine while it is forming in the kidney
-> resulting in an increase in the volume of urine and body dehydration
When should a person get tesed for diabetes?
If they need to urinate more frequently, is constantly thirsty, feels tired and craves sugary drinks
What are the two types of diabetes?
Type I and Type II
What is type I diabetes?
aka early-onset diabetes
* inability to produce sufficient quantities of insulin
* It is an autoimmune disease arising from the destruction of beta cells in the islets of Langerhans by the body’s own immune system
* causes are still being researched
Why is type I diabetes an autoimmune disease?
the body’s own immune system is detroying the beta cells in the islets of Langerhans
Why is type I diabetes an autoimmune disease?
the body’s own immune system is detroying the beta cells in the islets of Langerhans
What is type II diabetes?
aka late-onset diabetes
* inability to process or respond to insulin because of a deficiency of insulin receptors or glucose transporters on target cells
* Onset is slow
* Disease may go unnotices for many years
* Causes are not well understood
* Main risk factors are sugary, fatty diets, prolonged obesity due to habitual overeating and lack of exercise, together with genetic factors that affect energy metabolism
What is the current treatment of type I diabetes?
- regularly testing the blood glucose concentration and injecting insulin when it is too hight or likely to become too high
- Injections are often done before a meal to prevent a peak of blood glucose as the food is digested and absorbed
Why is timing of the insulin injection important?
Insulin molecules do not last long in the blood
What are some better treatments being developed for type I diabetes?
- Using implanted devices that can release exogenous insulin into the blood as and when it is necesary
- A permanent cure may be achievable by coaxing stem cells to become fully functional replacement beta cells
How is type II diabetes treated?
- adjusting the diet to reduce the peaks and troughs of blood glucose
- Small amounts of food should be eaten frequently rather than infrequent large meals
- foods with high sugar content should be avoided
- Starchy food should only be eaten if it has a low glycemic index, indicating that it is digested slowly
- High fibre foods should be included to slow the digestion of other foods
- Strenous exercise and weight loss are beneficial as they improve insulin uptake and action
What is the hormone thyroxin secreted by?
the thyroid gland in the neck
Why is the chemical structure of thyroxin unusual?
The thyroxin molecule contains four atoms of iodine
A deficiency of what prevents the synthesis of thyroxin?
Prolonged deficiency of iodine as the thyroxin molecule contains four atoms of iodine
What are the targets of the hormone thyroxin?
All cells in the body are targets.
Thyroxin regulates the body’s metabolic rate, so all cells need to respond but the most metabolically active such as liver, muscle and brain are the main targets
What does higher metabolic rate support?
More protein synthesis and growth and it increases the generation of body heat
What does thyroxin regulate?
metabolic rate and help control body temperature
In a person with normal physiology, how does thyroxin work?
Cooling triggers increased thyroxin secretion by the thyroid gland which stimulates heat production so body temperature rises
What is it called when there is a thyroxin deficiency?
Hypothyroidism
What are the symptoms of hypothyroidism?
- lack of energy and feeling tired all the time
- forgetfulness and depression
- weight gain despite loss of appetite as less glucose and gat are being broken down to release energy by cell respiration
- feeling cold all the time because less heat is being generated
- constipation because contractions of muscle in the wall of the guy slow down
- impaired brain development in children
What is leptin?
A protein hormone
What is leptin secreted by?
Adipose cells (fat storage cells)
What is the concentration of leptin the blood controlled by?
food intake and the amount of adipose tissue in the body
What is the target of leptin?
group of cells in the hypothalamus of the brain that contribute to the control of appetite
What does leptin bind to?
Binds to receptors in the membrane of the group of cells in the hypothalamus
What happens when adipose tissue increase?
blood leptin concentration rise, causing long-term appetite inhibition and reduced food intake
What would you expect of leptin concentration of an obese person?
Most obese humans have exceptionally high blood leptin concentrations
Why may an obese person not respond to leptin?
The target cells in the hypothalamus may have become resistant to lepin so fail to respond to it, even at high concentrations
Whats the consequence of an obese person not responding to leptin?
- Appetite is not inhibited and food intake is excessive
- More adipose tissue develops, causing a rise in blood leptin concentration but the leptin resistance prevents inhibition of appetite
- Injection of extra leptin inevitably fails to control obesity if the cause is leptin resistance (just as insulin injections alone are effective with early-stage type II diabetes)
What is a very small proportion of cases of obesity in humans due to?
Mutations in the genes for leptin synthesis or its various receptors on target cells
How do people with obesity due to mutations respond to leptin injections?
Significant weight loss
Why is leptin not suitable in children and young adults?
It has shown to affect the development and functioning of the reproductive system
What are circadian rhythms?
rhythms in behaviour that fits into a 24-hour cycle
Humans can continue in continious light or darkness because an internal system is used to control the rhythm
What do circadian rhythms in humans depend?
two groups of cells in the hypothalamus called the suprachiasmatic nuclei (SCN)
What do the cells from the suprachiasmatic nuclei (SCN) do?
Set a daily rhythm even if grown in culture with no external cues about the time of day
In the brain:
* control the secretion of the hormone melatonin by the pineal gland
What is melatonin secreted by?
Pineal gland
When does melatonin secretion increase and decrease?
Increase in the evening and drops to a low level at down
Blood concentrations rise and fall rapidly in response to these changes in secretion
What feelings are caused with high and low melatonin levels?
High: feelings of drowsiness and promote sleep through the night
Low: encourage waking at the end of the night
What does melatonin contribute in terms of temperature?
Blocking the rise in melatonin levels reduces the drop in core body temperature
Giving melatonin artificially during the day causes a drop in core temperature
What does the melatonin receptors found in the kidney suggest?
Decreased urine production at night may be another effect of this hormone
Without light cues to indicate the time of day, what maintains the rhythm and how long?
SCN and pineal gland maintain a rhythm of slightly longer than 24 hours
How is melatonin secretion responding to light?
The retina of the eye detects the light and passes impulses to cells in the SCN and it adjusts the melatonin secretion so that it corresponds to the day-night cycle
What aer the symptoms of jetlag?
- difficulty sleeping through the night
- fatigue
- irritability
- headaches
- indigestion
What causes jet lag?
the SCN and pineal gland are continuing to set a circadian rhythm to suit the timing of the day and night at the point of departure rather than the destination
How to combat jet lag?
It should only last for a few days
or melatonin can be taken orally at the time when sleep should ideally be commencing. It has shown to be effective
What is human reproduction a fusion of?
Involves the fusion of a sperm from a male with an egg from a female
What is the intial development of the embryo like?
It is the same in all embryos and the embryonic gonads that develop can either become ovaries or testes
What does the developmental pathway of the embryonic gonads and thereby the whole baby depend on?
the presence or absence of one gene - gene SRY
What do the embryonic gonads develop into in the presence of gene SRY?
Testes
Where is the gene SRY located?
On the Y chromosome, so only present in 50% of embryos
What does gene SRY code for?
A DNA-binding protein called TDF (testis determining factor)
What does TDF stimulate?
the expression of other genes that cause testis development
Why do embryos with 2 X chromosomes develop as ovaries?
Because there is no Y and so no SRY gene. TDF is therefore not produced and the embryonic gonads develop as ovaries
In the 8th week, what happens to the testes?
They develop from the embryonic gonads
At an early sage, what does the testes develop?
The testes develop testosterone-secreting cells
Until when do the testosterone-secrceting cells produce testosterone?
until about the 15th week
During the weeks of secretion of testosterone, what does testosterone cause?
During the weeks of secretion, testosterone causes male genitalia to develop
When does the secretion of testosterone increase?
During purberty
What does the increase in tetosterone stimulate at puberty?
- sperm production in the testes
What do testosterone also cause the development of besides sperm?
Also causes the development of secondary sexual characteristics during puberty such as:
* enlargment of the penis
* growth of pubic hair
* deepening of the voice due to growth of the larynx
Why do female reproductive organs develop?
Due to the absence of fetal testosterone, the presence of maternal estrogen and progesterone that were always present in pregnancy and secreted by the mother’s ovaries and later by the placenta, develop female reproductive organs
What occurs during puberty for females?
- increased secretion of estrogen and progesterone causing the development of female secondary sexual characteristics
- those include enlargement of the breasts and growth of pubic and underarm hair
What are the labels
What are the labels
What is the function of testis?
produce sperm and testerone
What is the function of scrotum?
Hold testes at lower than core body temperature
What is the function of scrotum?
Hold testes at lower than core body temperature
What is the function of epididymis?
Store sperm until ejaculation
What is the function of epididymis?
Store sperm until ejaculation
What is the function of sperm duct?
transfer sperm during ejaculation
What is the function of seminal vesicle and prostate gland?
secrete fluid containing alkali, proteins and fructose that is added to sperm to make semen
What is the function of urethra?
transfer semen during ejaculation and urine during urination
What is the function of penis?
penetrate the vagina fpr ejaculation of semen near the cervix
What is the function of ovary?
produce eggs, estrogen and progesterone
What is the function of oviduct?
collect eggs at ovulation, provide a site for fertilisation then move the embryo to the uterus
What is the function of oviduct?
collect eggs at ovulation, provide a site for fertilisation then move the embryo to the uterus
What is the function of uterus?
provide for the needs of the embryo and then fetus during pregnancy
What is the function of cervix?
provide the fetus during pregnancy and then diliate to provide a birth canal
What is the function of vagina?
stimulate penis to cause ejaculation and provide a birth canal
What is the function of vulva?
protect internal parts of the female reproductive system
When does the mentrual cycle occur in most women?
from puberty until the menopause, apart from during pregnancies
What are the two purpose of menstrual cycle hormones?
- Mature an oocyte
- Build the endometrium for implantation
What is given a chance each time the mentrual cycle occurs?
pregnancy
What is the first half of the menstrual cycle called?
follicular phase because a group of follicles is developing in the ovary
What is stimulated to grow in each follicle?
an egg
What happens at the same time during the follicular phase?
the lining of the uterus (endometrium) is repaired and starts to thicken
What deterines which follicle releases it egg?
The most developed follicle breaks open, releasing its egg into the oviduct. The other folicles degenerate
What is the second half of the mentrual cycle?
Luteal phase because the wall of the follicle that released an egg becomes a body called the corpus luteum
What happens during luteal phase?
The wall of the follicle that released an egg becomes a body called the corpus luteum
Continued development of the endometrium prepares it for the implantation of an embryo
Why does continued development of the endometrium do?
Prepares for the implantation of an embryo
What happens if fertilization does not occur in the luteal phase?
- the corpus luteum in the ovary breaks down
- the thickening of the endometrium in the uterus also breaks down and is shed during menstration
What happens if fertilization does not occur in the luteal phase?
- the corpus luteum in the ovary breaks down
- the thickening of the endometrium in the uterus also breaks down and is shed during menstration
Which 4 hormones all help to control the menstrual cycle and how?
- LH, FSH, progesterone, estrogen
- all help to control the mentrual cycle by both negative and positive feedback
What are FSH and LH and what are they produced by?
Both are protein hormones produced by the pituitary gland that bind to FSH and LH receptors in the membranes of follicle cells
What are estrogen and progesterone and produced by what?
They are ovarian hormones released from the ovaries, produced by the wall of the follicle and corpus luteum
What do FSH and LH do?
FH: stimulate oocyte development
LH: Matures oocyte and causes releases (ovulation)
What does estrogen and progesterone do?
estrogen: develops endometrium, early: positive feedback on FSH, late: negative feedback on FSH and LH
Progesterone: maintains endometrum, negative feedback on FSH and LH
Where is HCG released from and what does it do?
Released from placenta
* pregnancy hormone: keeps estrogen and progesterone high
What is oxytocin and what does it do?
Birth hormone: contractions of uterus and dilates the cervix
What are estrogen and progesterone absorbed by and what do they influence?
by many cells in the female body
they influence gene expression and therefore development
When does FSH rise to a peak?
Towards the end of the mentrual cycle
What does FSH stimulate?
- the development of follicles, each containing an oocyte and follicular fluid
- secretion of estrogen by the follicle wall
When does estrogen peak?
towards the end of the follicular phase
What does estrogen stimulate?
- repair and thickening of the endometrium after menstruation
- stimulate an increase in FSH receptors that make the follicles mroe receptive to FSH, boosting estrogen production (positive feedback)
- When it reaches high levels, estrogen inhibits the secretion of FSH (negative feedback) and stimulates LH secretion
When does LH peak?
LH rises to a sudden and sharp peak towards the end of the follicular phase
What does LH stimulate?
- the completeion of meiosis in the oocyte and partial digestion of the follicle wall allowing it to burst open at ovulation
- promotes the development of the wall of the follicle after ovulation into the corpus luteum which secretes estrogen (positive feedback) and progesterone
WHen do progesterone levels rise?
At the start of the luteal phase, reach a peak and then drop back to a low level by the end of this phase
What does progesterone promote?
- the thickening and maintenance of the endometrium
- Inhibit FSH and LH secretion by the pituitary gland (negative feedback)
What does in vivo fertilization mean?
The natural method of fertilization, meaning it occurs inside the living tissues of the body
What does in vitro fertilization mean?
Abbreviated to IVF, is fertilization that happens outside the body in carefully controlled laboratory conditions
What is the first stage of IVF?
Down-regulation
* The woman takes a drug each day, usually a nasal spray to stop her pituitary gand secreting FSH or LH.
* Secretion of estrogen and progesterone therefore also stops
* This suspends the normal mentrual cycle
* This allows the doctors to control the timing and amount of egg production in the woman’s ovaries
What occurs in the superovulation stage of IVF?
- intramuscular injections of FSH and LH are then given daily to stimulate follicles to deveop
- the FSH injections give a much higher concentration than during a normal mentrual cycle -> far more follicles develop
What is done to the follicles when they are 18mm in diameter in the IVF?
- stimulated to mature by an injection of HCG
- A micropipette mounted on an ultrasound scanner is passed through the uterus wall to wash eggs out of the follicles
- Each egg is mixed with sperm cells in sterile conditions in a shallow dish
- incubated at 37C until the next day
What is the otder of phases of the mentrual cycle?
Day 1-4 Follicular phase
Day 5-14 Ovulatory phase
Day 14-28 Luteal phase
What occurs during the follicular phase?
- menstruation. Endometirum shed
- FSH increases, stimulating folicle development
What occurs during the ovulatory phase oft he menstrual cycle?
- FSH and follicle stimulate oestrogen release
- Oestrogen stimulate endometrium development
- oestrogen stimulates LH
- Peak in LH causes ovulation
What occurs in the luteal phase of the menstrual cycle?
- Fall in LH. Corpus luteum forms from now-empty follicle
- Corpus luteum releases progesterone
- progesterone maintains the endometrium and inhibits FSH and LH
What happens in the mentrual cycle when n o fertilisation and implantation occurs?
Progesterone and oestrogen drop, triggering menstruation. andFSH release`
Negative and positive feedback mechanisms involving ovarian and pituitary hormones
