Download Feedback Control in Homeostasis of Blood Sugar

Chemical Coordination
Endocrine Glands
An endocrine gland is a gland that has no duct to carry its secretions. Rather, these are
carried through the circulatory system. An endocrine gland produces an internal secretion
that is discharged into the blood or lymph to various parts of the body.
Exocrine Glands
An exocrine gland is a gland which lies near the concerned organ and empties its secretions
directly, or has a duct to carry its secretions to its target organ.
Hormones
Hormones are organic compounds manufactured by the endocrine glands from substances
carried in the blood. They are also known as chemical coordinators or messengers.
Hormones are mainly protein in nature, but some are steroids. They are carried by the
blood to their target organ, stimulating it by functional activity to increase functional activity
or to increase the secretions of another hormone.
The glands secrete their hormones continuously but the quantity of secretion can be
increased or decreased according to body needs. Control of hormone secretion occurs in
several ways :
Nerve cells produce chemicals, which are carried to the gland and cause secretion.
The gland may secrete a hormone as a response to impulses from the autonomic
nervous system. e.g. adrenal glands secreting adrenaline
One gland produces a hormone that affects a second gland. The second gland secretes
its hormone, which influences secretion of the first gland. This is called a feedback
mechanism. e.g. release of glucagon or insulin from the pancreas
Video Clip 1 : How Hormones Work
The Endocrine System
The ductless or endocrine glands of the body form the endocrine system. There are five
main endocrine glands in the body.
1. The pituitary gland or the hypophysis
2. The thyroid gland
3. The adrenal or suprarenal glands
4. The pancreas
5. The testes in males or the ovaries or ovaria in females
Video Clip 2 : The Endocrine System
Figure 1 : The Endocrine System
The Pituitary Gland
The pituitary gland is a small, grey, rounded body and is located in a bony cavity of the
skull, the sella turcica, 13 mm wide, beneath the brain and above the roof of the mouth. It
is attached to the base of the brain by the infundibular stalk. The hypophysis is 1.3 cm by
1.0 cm by 0.5 cm in size and 0.55 g to 0.6 g in weight.
Video Clip 3 : The Pituitary Gland and its Hormones
The pituitary gland releases seven hormones.
1. GONADOTROPIN : Hormone gonadotropin is released from the anterior lobe of the
pituitary gland – the adenohypophysis. It acts on the gonads – the testes in males and
the ovaria in females.
In females, gonadotropin stimulates the growth of Graafian follicles, and in males, it
stimulates the growth of the germinal epithelium in the testes – thus causing the
production and secretion of sex hormones.
2. THYROTROPIN : Hormone thyrotopin is released by the adenohypophysis and is also
known as the thyroid stimulating hormone (TSH). It controls the production and the
amount of hormone secreted by the thyroid gland, by altering the number, size, and
rate of hormone production by the thyroid gland.
3. CORTICOTROPIN : Hormone corticotropin is also released by the adenohypophysis, and
is known as the adrenocorticotrophic hormone (ACTH). It controls the production and
the amount of hormone secreted by the cortices of the adrenal glands, by altering the
number and degree of activity of the cells of the adrenal cortex.
4. SOMATOTROPIN : Hormone somatotropin is released by the adenohypophysis, and is
also known as growth or somatotrophic hormone. It increases the rate of growth, and
then maintains size once maturity has been attained. Hormone somatotropin exerts its
influence mainly on the hard tissue of the body, although there is some effect on the
soft tissue. It is secreted by the adenohypophysis throughout life even though most
growth procedures are completed by adolescence.
Oversecretion of hormone somatotropin causes overgrowth of the long bones, resulting
in gigantism. Undersecretion of hormone somatotropin causes dwarfism.
5. PROLACTIN : Prolactin is a female hormone. It is released by the adenohypophysis
during pregnancy and the whole period of milk production in the postpartum period. It is
also known as the lactogenic hormone. This hormone stimulates the development of the
breasts, ad production of milk in the mammary glands.
6. VASOPRESSIN : Vasopressin is secreted by the posterior lobe of the pituitary gland – the
neurohypophysis, and is also known as the antidiuretic hormone (ADH). It is produced in
the hypothalamus and is then carried to the neurohypophysis by the hypothalamohypophyseal tract for storage. This hormone causes an increase in the reabsorption of
water from the glomerular filtrate by the kidney tubules so that less urine is excreted. It
does this by altering the permeability of the kidney tubule epithelial cells. In this way,
ADH plays a major part in osmoregulation and excretion.
Deficient secretion of ADH causes less water to be reabsorbed from the glomerular
filtrate and the consequent production of large amounts of copious urine – a condition
called diabetes insipidus or water diabetes. Excessive secretion causes large amounts of
water to be reabsorbed and consequently increases the concentration of urine.
7. OXYTOCIN : Hormone oxytocin is produced by the hypothalamus and is then carried to
the neurohypophysis by the hypothalamo-hypophyseal tract for storage and secretion.
This is a female hormone, which is released at the very end of pregnancy. It acts mainly
on the unstriped muscle of the pregnant uterus, stimulating it to contract, thus inducing
parturition, i.e. the birth of the baby.
The Thyroid Gland
The thyroid gland is a butterfly-shaped gland that lies in the anterior part of the neck
immediately below the larynx and overlying the trachea. The thyroid gland secretes
hormone thyroxine.
The function of hormone thyroxine is to regulate metabolism in the tissues. It controls
growth and development of the body by affecting energy release by mitochondria.
Thyroxine steps up the rate of reaction of all cells, by increasing the secretion of enzymes.
Video Clip 4 : The Thyroid Gland and its Hormones
Lack of hormone thyroxine or hypothyroidism in children causes thyroid cretinism, which, if
untreated, results in a mentally retarded dwarf. Hypothyroidism in adults causes
myxoedema. In both cretinism and myxoedema, the skin and hair are coarse and dry; the
metabolic rate is slow and so the patient is abnormally obese and sluggish.
Excessive secretion of hormone thyroxine is called hyperthyroidism. It causes
thyrotoxicosis, which is a condition marked by an increased metabolic rate. The individual is
anxious, nervous, and restless, with a fast pulse rate. The skin is fine and moist. They eyes
may or may not protrude from the orbits – a condition known as exophthalmos.
The secretion of hormone thyroxine is completely regulated by hormone thyrotropin
released by the pituitary gland.
The Adrenal Glands
There are two adrenal or suprarenal glands that lie one above and in front of the upper end
of each kidney. The gland has two distinct regions – the outer part called the cortex and the
inner part called the medulla. The adrenal medulla releases hormone adrenaline. This
hormone is released during stress or emergency. It prepares the body for fight or flight.
Video Clip 5 : The Adrenal Gland and its Hormones
Effects of Adrenaline
It increases the heartbeat and the rate and depth of breathing.
It increases blood supply to the heart and skeletal muscles by vasodilation, and by the
constriction of other arteries like those supplying the skin, etc.
Activity of the gut, the kidneys and other organs not related energy release is lowered.
Blood sugar level is heightened by increased breakdown of glycogen into glucose.
The Pancreas
The pancreas is a long gland that lies immediately beneath the stomach. It is both an
endocrine and an exocrine gland. The endocrine part of the pancreas is known as the Islets
of Langerhans. The islets are composed of -cells and -cells. The -cells release hormone
insulin and the -cells secrete hormone glucagon. Hormone insulin causes the liver to
convert extra glucose into glycogen. Hormone glucagon causes the breakdown of glycogen
into glucose.
Video Clip 6 : The Pancreas and its Hormones
The Ovaries
The ovaries or the ovaria are the female sex glands. There are tow ovaries in the female,
one on each side of the pelvic cavity. The ovary produces two hormones – oestrogen and
progesterone.
Hormone oestrogen has two main functions.
1. It controls the development of secondary sexual characteristics in females. These are :
a) Development of the mammary glands
b) Development of pubic hair
c) Onset of the menstrual cycle
2. It causes the lining of the uterus to thicken just before an ovum is released.
Hormone progesterone promotes further thickening and vascularisation of the uterus after
ovulation. It maintains the placenta during pregnancy and prevents the uterus from
contracting until the baby is due to be born. It also helps in the development of the
mammary glands.
The production and secretion of hormones from the ovaries is controlled by hormone
gonadotropin released by the pituitary gland.
The Testes
The testicles or the testes are the male sex glands. There are two testes in the male and are
situated in the scrotum. The testis produces the male sex hormone testosterone.
Hormone testosterone controls the development of the secondary sexual characteristics in
males.
Deepened voice
Increased musculature
Development of pubic hair
Development of facial hair
Production and secretion of testosterone is controlled by the pituitary hormone
gonadotropin.
Homeostasis & Feedback
The mechanism in which the consistency of the internal environment of the body is
maintained by not allowing the temperature and composition of body fluids to fluctuate
beyond certain limits is known as homeostasis.
The mechanism in which information about bodily functions is sent to the central nervous
system and the endocrine system so that consistency of the internal environment of the
body can be maintained is knows as feedback.
Feedback Control in Homeostasis of Blood Sugar Level
EXCESS
DEFICIENT
NORM
If blood sugar level is above
150 mg/100 cm3 of blood
DETECTORS
Detectors present within the blood vessels detect changes in
blood sugar level.
CORRECTIVE
MECHANISM
The hypothalamus instructs
the pancreas to release
hormone insulin.
The hypothalamus instructs
the pancreas to release
hormone glucagon.
NEGATIVE
FEEDBACK
Insulin causes the liver to
convert surplus glucose to
glycogen. Thus, sugar level is
lowered to normal.
Glucagon causes the liver to
convert stored glycogen into
glucose. Thus, sugar level is
brought back to normal.
Lack of insulin causes sugar
level to continue to rise,
resulting in hyperglycaemia.
Lack of glycogen causes blood
sugar level to continue to fall,
resulting in hypoglycaemia.
The person collapses, loses
consciousness and goes into
coma, then dies.
POSITIVE
FEEDBACK
If blood sugar level is below 90
mg/100 cm3 of blood
Feedback Control in Homeostasis of Blood Thyroxine Level
EXCESS
DEFICIENT
NORM
If thyroxine level in blood is above
6 g/100 cm3 of blood
If thyroxine level in blood is
below 3 g/100 cm3 of blood
DETECTORS
Detectors present within the blood vessels detect changes in the
level of thyroxine in blood.
CORRECTIVE
MECHANISM
The pituitary gland stops the
production of secretion of
hormone thyrotropin.
The pituitary gland increases
the production and secretion
of hormone thyrotropin.
NEGATIVE
FEEDBACK
Lack of thyrotropin causes the
thyroid gland to stop the
production and secretion of
hormone thyroxine. Thus,
thyroxine level in blood is brought
back to normal.
Presence of thyrotropin
accelerates production and
secretion of thyroxine. Thus,
thyroxine level is raised to
normal.
Thyroxine level continues to rise,
resulting in hyperthyroidism. Body
metabolism increases.
Thyroxine level continues to
fall, resulting in
hypothyroidism. Body
metabolism decreases
POSITIVE
FEEDBACK