6.6 Hormones, homeostasis and reproduction Flashcards
The Endocrine System
A stimulus is received and processed.
Hormones are secreted directly into the blood.
They are carried to the target tissues.
The action of the hormone changes the condition of the tissue.
This change in monitored through feedback.
Most hormonal change results in negative feedback (corrects the imbalance)
Key endocrine glands:
Pineal gland
Pituitary gland
Thyroid gland
Thymus
Adrenal gland
Pancreas
Ovary (female)
Testes (male)
Blood glucose is maintained through the actions of the…
pancreas and the liver
Pancreatic hormones insulin and glucagon regulate …
blood glucose levels
islets of Langerhans
small regions of the pancreas that secrete hormones directly into the blood
If blood glucose levels DROP:
pancreas detects drop
𝛂 cells create/secrete glucagon hormone
glucagon stimulates the liver to convert stored glycogen into glucose and its release into the blood
blood glucose increases (to normal)
If blood glucose levels RISE:
pancreas detects rise
β cells create/secrete insulin
Insulin stimulates uptake of glucose by various tissues (skeletal/muscle)
Insulin stimulates liver to convert blood glucose into stored glycogen
blood glucose drops (to normal)
Diabetes
consistently elevated blood glucose levels
Damages tissues and their proteins
Prevents water reabsorption in kidneys 🡪 increase in volume of urine & body dehydration
Symptoms: constant urination, constant thirst, tired, craves sugar, glucose in urine
Type I diabetes (early onset)
Cannot produce enough insulin
Autoimmune disease
Destruction of β cells in islets of Langerhans by immune system
Treatment
Testing blood glucose levels regularly and injecting insulin when too high
Implant devices to release insulin
Stem cell treatment to create new β cells
Type II diabetes (late onset)
Cannot respond to insulin
Lack of insulin receptors or glucose transporters on target cells
Risk factors: sugary/fatty diets, prolonged obesity, genetic factors that affect metabolism
Treatment
Adjusting diet
Frequent, smaller meals
Avoid sugary foods
Eat foods with low glycemic index (digested slowly
High fiber foods
Exercise
Type II diabetes (late onset)
Cannot respond to insulin
Lack of insulin receptors or glucose transporters on target cells
Risk factors: sugary/fatty diets, prolonged obesity, genetic factors that affect metabolism
Treatment
Adjusting diet
Frequent, smaller meals
Avoid sugary foods
Eat foods with low glycemic index (digested slowly
High fiber foods
Exercise
Thyroxin
Produced by: thyroid gland
Structure: four iodine atoms
Targets: most body cells (liver, muscle, brain)
Effects:
increases metabolic rate/rate of protein synthesis
increases heat production when body temp. is low (e.g. increased respiration)
Thyroxin deficiency (hypothyroidism)
Tired/lack of energy
Forgetfulness
Depression
Decrease appetite & weight gain (less glucose/fat broken down by cell respiration)
Feel cold
Constipation (muscle contractions in gut slow down)
Impaired brain development in children
Leptin
Produced by: adipose cells (fat storage cells)
Targets: appetite control centre of the hypothalamus (in brain). Leptin binds to receptors in cell membranes
Effects:
Increase in adipose tissue increases leptin secretions into the blood
Appetite inhibition → reduced food intake
Mice and leptin
Mice with two recessive alleles ob/ob were found to be obese
Wild-type allele supports synthesis of leptin
ob/ob mice injected with leptin showed decline in appetite
Melatonin
Produced by: pineal gland in darkness
Controlled by: suprachiasmatic nuclei (SCN) in the hypothalamus
Targets: pituitary and other glands
Effects:
synchronization of circadian rhythms (24-hr cycle)
Circadian rhythms
Controlled by two groups of cells in the hypothalamus (brain)
Control secretion of melatonin by pineal gland to regulate sleep cycle → increases at night, decreases at dawn
increase in melatonin at night drops body temp. and possibly reduces urine production
Light detected by retina helps to set circadian rhythm
Even if kept in environment without light cues, body’s circadian rhythm will be mostly maintained
Sex determination
in embryos the first appearance of the gonads is essentially the same in the two sexes. Gonads could become either ovaries or testes.
SRY gene
encodes for a protein known as testis determining factor (TDF).
TDF
is a DNA binding protein which acts as a transcription factor promoting the expression of other genes that cause testis development..
In the absence of TDF
the gonads become ovaries and the developing fetus becomes female.
Testosterone
The testes develop from the embryonic gonads when the the embryo is becoming a fetus. They develop testosterone-secreting cells.
The testes secrete testosterone which causes the male genitalia to develop.
At puberty the secretion of testosterone increases causing:
The primary sexual characteristic of sperm production in the testes
Development of secondary sexual characteristics such as enlargement of the penis, growth of pubic hair and deepening of the voice
Estrogen and progesterone
n the absence of fetal testosterone and the presence of maternal estrogen and progesterone, female reproductive organs develop (ovaries develop from the embryonic gonads) due to:
estrogen and progesterone
No testosterone
Estrogen and progesterone are present. At first they are secreted by the first by the mother’s ovaries and later by her placenta.
At puberty the secretion of estrogen and progesterone increases causing:
Primary sexual characteristic of egg release
Development of female secondary sexual characteristics such as enlargement of the breasts and growth of pubic hair
uterus
provides protection, nutrients and waste removal for the developing fetus
Muscular walls contract to aid birthing process
oviduct (fallopian tube)
Connects the ovary to the uterus
Fertilisation of the egg occurs here
ovary
Produce eggs, estrogen and progesterone
endometrium (lining of the uterus)
develops each month in readiness for the implantation of a fertilised egg
cervix
Protect the fetus during pregnancy
Opens to form the birth canal
vagina
Accepts the penis/sperm during sexual intercourse
With the cervix forms the birth canal
sperm duct (vas deferens)
transfer sperm during ejaculation
prostate gland
secrete fluid containing base, proteins, and fructose into semen
urethra
transfer semen during ejaculation and urine during urination
penis
penetrate vagina
delivers sperm to top of the vagina
seminal vesicle
secrete fluid containing base, proteins, and fructose into semen
epididymis
store sperm until ejaculation
testis
produce sperm and testosterone
scrotum
holds testes outside the body to keep low temp.
pituatary hormones in menstrual cycle
FSH, LH
ovaria hormones in menstrual cycle
estrogen and progesterone
Other hormones
HCG, oxytocin
Follicular phase
follicles developing in ovary
growing egg in each follicle
lining of the uterus (endometrium) thickens
most developed follicle breaks open → egg released into oviduct
other follicles degenerate
Luteal phase
follicle wall becomes corpus luteum (in ovary)
endometrium develops for implantation of embryo
if no fertilization
corpus luteum breaks down
endometrium shed during menstruation
FSH
rises at end of cycle → stimulates development of follicles
stimulates secretion of estrogen by follicle wall
Estrogen
peaks near end of follicular phase
stimulates thickening of endometrium after menstruation
stimulates increase in FSH receptors in follicles (produce more estrogen → positive feedback)
high levels of estrogen inhibit secretion of FSH (negative feedback) and stimulates LH secretion
LH
sudden, sharp peak near end of follicular phase
stimulates completion of meiosis in oocyte
stimulates partial digestion of follicle wall to allow bursting during ovulation
promotes development of follicle wall into corpus luteum after ovulation → corpus luteum secretes estrogen (positive feedback) and progesterone
Progesterone
rises at start of luteal phase, then peaks and drops off
promotes thickening of endometrium
inhibits FSH and LH secretion by the pituitary gland (negative feedback)
Follicular phase
Day 1-4
menstruation: endometrium shed
FSH increases, stimulating follicle development
Follicular phase
Day 5-14
FSH and follicle stimulates estrogen release
estrogen stimulates endometrial thickening
estrogen stimulates LH
peak in LH causes ovulation (day 14)
Luteal phase
Day 14-28
fall in LH → corpus luteum forms from now-empty follicle
corpus luteum releases progesterone
progesterone thickens endometrium and inhibits FSH and LH
If no fertilisation/implantation occurs,…
progesterone and estrogen drop, triggering menstruation and FSH release.
Some causes of infertility:
Female:
Ova not maturing or being released
Abnormality in uterus prevents implantation
Antibodies in cervical mucus impair sperm
Blocked fallopian tubes
Male
Unable to achieve an erection or normal ejaculation
Low sperm count or sperm are abnormal with low motility
Blocked vas deferens
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
Mnemonic: SHE’S FIT
Hormonal treatments involved in IVF
- Down-regulation = shutting down of the menstrual cycle by stopping secretion of the pituitary and ovarian hormones; two weeks and allows better control of egg production; done with a drug, commonly in the form of a nasal spray.
- Superovulation collects multiple eggs from the woman. High doses of FSH are injected over approximately a ten day period to stimulate the development of multiple follicles.
- When follicles reach 15-20mm in diameter an injection of HCG is given to start maturation process. Approximately 36 hours later, under a general anesthetic, follicles (typically 8-12) are collected from the ovaries.
- Prepared eggs (removed from the follicles) are combined with sperm in sterile conditions. Successfully fertilised eggs are then incubated before implantation.
- If fertilized, multiple embryos are placed into the uterus after 48 hours.
- For approximately two weeks before implantation the woman takes progesterone (which maintains the endometrium), usually in the form of a suppository, to aid implantation. This treatment is continued until pregnancy test, and if positive, until 12 weeks of gestation.
Testosterone
Pre-natal development of male genitalia, sperm production, development of male secondary sexual characteristics during puberty.
FSH
Stimulates the growth and development of ovarian follicles (bodies containing eggs).
LH
Triggers ovulation, the release of the oocyte (egg) from the ovary
Estrogen
Pre-natal development of female reproductive organs and female secondary sexual characteristics during puberty. Causes the uterine lining to thicken.
