ENODCRINE SYSTEM PART 2 - ENGLISH
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ENDOCRINE SYSTEM PART 2
ADRENAL GLAND-
• The
two adrenal glands are found on the top of each of the two kidneys (one on each
kidney). The right gland is roughly triangular in shape, and the left, which is
commonly the larger of the two, is crescent‐shaped. Both glands are encased in
a connective tissue capsule and embedded in an area of fat.
• The
glands are composed of two parts which have different structures and functions.
The outer part is the cortex and the inner part the medulla.
• The
adrenal cortex produces three group of steroid hormones from cholesterol. They
are collectively called adrenocorticocoids (corticosteroids). They are:
• Glucocorticoids
• Mineralocorticoids
• Sex
hormones (androgens).
• Glucocorticoids-
The glucocorticoid hormones include:
• 1.
Cortisol 2.
Cortisone 3.Corticosterone.
• The
glucocorticoid hormones have several effects:
• They
influence the metabolism of most body cells;
• They
promote glycogen storage in the liver;
• During
fasting they stimulate the generation of glucose;
• They
increase blood glucose levels;
• They
are involved in providing resistance to stressors;
• They
promote the repair of damaged tissues by promoting the breakdown of stored
protein to create amino acids;
• They
suppress the immune system;
• They
suppress inflammatory processes.
• Mineralocorticoids-
Aldosterone is the major mineralocorticoid. Mineralocorticoids regulate
homeostasis of two mineral ions, namely sodium ions (Na) and potassium ions
(K), and helps adjust blood pressure and blood volume. Aldosterone also
promotes excretion of H in the urine; this removal of acids from the body can
help prevent acidosis
• Sex
hormones- Sex hormones secreted by the adrenal cortex are known as androgens (dehydroepiandrosterone(DHEA)
and testosterone.) All have weak effects, but play a role in early
development of the male sex organs in childhood, and female body hair
during puberty.
• The
medulla is completely surrounded by the adrenal cortex. It develops from
nervous tissue in the embryo and is part of the sympathetic division of the
autonomic nervous system. It is stimulated by its extensive sympathetic nerve
supply to produce the hormones adrenaline (epinephrine) and noradrenaline
(norepinephrine).
• Epinephrine
and norepinephrine increase the output of the heart, which increases blood
pressure. They also increase blood flow to the heart, liver, skeletal muscles,
and adipose tissue; dilate airways to the lungs; and increase blood levels of
glucose and fatty acids.
PANCREAS-
• The
pancreas is a pale grey gland weighing about 60 grams. It is about 12 to 15 cm
long and is situated in the epigastric and left hypochondriac regions of the
abdominal cavity. It consists of a head, body and tail. The pancreas is both as
:
• Exocrine
gland and
• Endocrine
gland- There are groups of specialised cells called the pancreatic islets
(islets of Langerhans). These islets are distributed throughout the gland. The
islets have no ducts so the hormones diffuse directly into the blood. Each
islet has three major cell types, each of which produces a different hormone:
• Alpha
cells, which secrete glucagon;
• Beta
cells, the most abundant of the three cell types and which secrete insulin;
• Delta
cells, which secrete somatostatin.
• Glucagon-
Glucagon has an important role in maintaining normal blood glucose levels,
especially as the brain and neurones can only use glucose as a fuel. Glucagon
has the opposite effect on blood glucose levels to insulin.
• It
timulates the breakdown of glycogen stored in the liver;
• It
activates hepatic gluconeogenesis .
• It
has a minor effect enhancing triglyceride breakdown in adipose tissue and
providing fatty acidfuel for most cells, and thus conserving glucose for the
brain and neurones.
• Insulin-
Insulin is well known for its effect in reducing the blood glucose levels. It
does this by:
• Facilitating
the entry of glucose into muscle, adipose tissue and several other
tissues.
• Stimulating
the liver to store glucose in the form of glycogen.
• Insulin
also have an effect on protein, lipid andmineral metabolism
• Somatostatin-
Somatostatin is actually released by a broad range of tissues. Its
physiological effect in the pancreas is to inhibit the release of insulin and
glucagon; it does this in a paracrine fashion; that is, the hormone is released
and has its effect locally
PINEAL GLAND-
• The
pineal gland is a small body attached to the roof of the third ventricle of
brain and is connected to it by a short stalk containing nerves, many of which
terminate in the hypothalamus. The pineal gland is about 10 mm long, is reddish
brown in colour and is surrounded by a capsule. The gland tends to atrophy
after puberty and may become calcified in later life.
• Melatonin-
This is the main hormone secreted by the pineal gland. Secretion is
controlled by daylight and levels fluctuate during each 24-hour period, being
highest at night and lowest around midday. Melatonin is believed to be
associated with:
• coordination
of the circadian and diurnal rhythms of many tissues, possibly by influencing
the hypothalamus and inhibition of growth and development of the sex organs
before puberty, possibly by preventing synthesis or release of gonadotrophins.
THYMUS GLAND-
• The
thymus is located behind the sternum between the lungs. It weighs about
10 to 15 g at birth and grows until puberty, when it begins to atrophy. Its
maximum weight, at puberty, is between 30 and 40 g and by middle age it has
returned to approximately its weight at birth.
• The
hormones produced by the thymus—thymosin, thymic humoral factor (THF), thymic
factor (TF), and thymopoietin. These hormones promote the maturation of T cells
(a type of white blood cell that destroys microbes and foreign substances) and
may retard the aging process.
GONADS-
• Gonads
are the organs that produce gametes—sperm in males and oocytes in females.
In addition to their reproductive function, the gonads secrete hormones.
• These
are – ovaries in females and
• Testes
in males.
OVERIES-
• The
ovaries, paired oval bodies located in the female pelvic cavity, produce
several steroid hormones including two estrogens (estradiol and estrone)
and progesterone.
• These
female sex hormones (Projesterone and estrogens), along with FSH and LH from
the anterior pituitary, regulate the menstrual cycle, maintain pregnancy, and
prepare the mammary glands for lactation. They also promote enlargement of the
breasts and widening of the hips at puberty, and help maintain these female
secondary sex characteristics.
TESTES-
• The
male gonads, the testes, are oval glands that lie in the scrotum. The
main hormone produced and secreted by the testes is testosterone, an
androgen or male sex hormone
• Testosterone
stimulates descent of the testes before birth, regulates production of sperm,
and stimulates the development and maintenance of male secondary sex
characteristics, such as beard growth and deepening of the voice. The testes
also produce inhibin, which inhibits secretion of FSH.
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