6.6 Hormones and Homeostasis Flashcards
Homeostasis
process of maintaining a constant internal environment despite large variations in the external environment
Body homeostatic systems
- body temp
- blood pH
- CO2 conc
- blood glucose conc.
- water balance
- sleep wake cycles
- hunger
- metabolism
endocrine system
consists of glands, which release hormones that are transported in the blood
Feedback system
involves a stimulus from one part of the body invoking a response from another part which is sensed and acts to alter the original stimulus
Negative feedback system
returns back to optimum
Positive feedback system
pushes you further from optimum
e.g. childbirth, allergic reactions and defacation)
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
The primary role of thyroxin is to increase the basal metabolic rate (amount of energy the body uses at rest)
This can be achieved by stimulating carbohydrate and lipid metabolism via the oxidation of glucose and fatty acids
A consequence of increasing metabolic activity is the production of heat – hence thyroxin helps to control body temperature
Thyroxin is released in response to a decrease in body temperature in order to stimulate heat production
Thyroxin is partially composed of iodine and hence a deficiency of iodine in the diet will lead to decreased production of thyroxin
Iodine deficiency will cause the thyroid gland to become enlarged, resulting in a disease known as goitre
Body response to overheating
skin arterioles dilate
skeletal muscles remain relaxed
sweat glands secrete sweat
muscles of hair follicles relax
Body response to chilling
skin arterioles constrict
skeletal muscles do many rapid contractions to generate heat (shivering)
sweat glands do not secrete sweat
muscles of hair follicles contract causing hair to stand up (trapping air as insulation)
Control of blood glucose
High levels of glucose in the blood can damage cells (creates hypertonicity) and hence glucose levels must be regulated
Two antagonistic hormones are responsible for regulating blood glucose concentrations – insulin and glucagon
These hormones are released from pancreatic pits (called the islets of Langerhans) and act principally on the liver
Body response to high glucose levels
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)
Body response to low glucose levels
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)
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 (mainly Type I) or by carefully monitoring and controlling dietary intake (Type II)
Type I diabetes
usually occurs during childhood
body does not produce sufficient energy
caused by destruction of beta cells
required insulin injections
Type II diabetes
during adulthood
not respond to insulin production
caused by the down regulation of insulin receptors
controlled by managing diet and lifestyle
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
Overeating causes more adipose cells to formed and hence more leptin is produced, suppressing further appetite
Conversely, periods of starvation lead to a reduction in adipose tissue and hence less leptin is released, triggering hunger
As obese people are constantly producing higher levels of leptin, their body becomes progressively desensitised to the hormone
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’)
Thryoxin
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
The primary role of thyroxin is to increase the basal metabolic rate (amount of energy the body uses at rest)
This can be achieved by stimulating carbohydrate and lipid metabolism via the oxidation of glucose and fatty acids
A consequence of increasing metabolic activity is the production of heat – hence thyroxin helps to control body temperature
Thyroxin is released in response to a decrease in body temperature in order to stimulate heat production
Thyroxin is partially composed of iodine and hence a deficiency of iodine in the diet will lead to decreased production of thyroxin
Iodine deficiency will cause the thyroid gland to become enlarged, resulting in a disease known as goitre
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
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 secretion is suppressed by bright light (principally blue wavelengths) and hence levels increase during the night
Over a prolonged period, melatonin secretion becomes entrained to anticipate the onset of darkness and the approach of day
Melatonin functions to promote activity in nocturnal animals and conversely promotes sleep in diurnal animals (like humans)
During sleep, necessary physiological changes occur in body temperature, brain wave activity and hormonal production
Melatonin levels naturally decrease with age, leading to changes in sleeping patterns in the elderly
Jet lag and metatonin
Jet lag is a physiological condition resulting from a change to the body’s normal circadian rhythm
This alteration is caused by the body’s inability to rapidly adjust to a new time zone following extended air travel (‘jet’ lag)
The pineal gland continues to secrete melatonin according to the old time zone so that the sleep schedule is not synchronised to the new timezone
As a result of these sleep disturbances, individuals suffering from jet lag will often experience symptoms associated with fatigue
Symptoms of jet lag include headaches, lethargy, increased irritability and reduced cognitive function
Jet lag should only last a few days and symptoms should resolve as the body resynchronises its circadian rhythm
Some health professionals recommend taking melatonin near the sleep time of the new time zone to help recalibrate the body
By artificially increasing melatonin levels at the new night time, the body can respond quicker to the new day-night schedule
