endo 4 jho
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Endocrine SystemAdrenal. Thyroid. Parathyroid.
Adrenal GlandsPaired organs that cap the superior borders of the
kidneysEach consists of:
Adrenal cortex (outer)– derived from neural crest ectoderm– does not receive neural innervation so it must be
stimulated hormonally- Consists of three zones: zona glomerulosa, zona
fasciculata and zona reticularisAdrenal medulla (inner)– derived from mesoderm– innervated by preganglionic sympathetic nerve fibers
Structure of Adrenal Gland
Functions of Adrenal CortexSecretes steroid hormones (corticosteroids
or corticoids)Mineralocorticoids – regulate Na+ and K+
balanceGlucocorticoids – regulate the metabolism of
glucose and other organic moleculesSex steroids – weak androgens
(dehydroepiandrosterone or DHEA)
AldosteroneMost potent mineralocorticoidSecreted by the zona glomerulosaStimulate the kidneys to retain NaCl and
water while excreting K+ in the urineThese actions help to increase the blood
volume and pressure and to regulate blood electrolyte balance
CortisolPredominant glucocorticoidSecreted by zona fasciculata and perhaps
also the zona reticularisSecretion is stimulated by the ACTH from
the anterior pituitary glandhave effects on metabolism:
Stimulate gluconeogenesisInhibit glucose utilizationPromote lipolysis and consequent release of
free fatty acids into the blood
Exogenous GlucocorticoidsTaken as pills, injections, sprays and topical
creamsUsed medically to suppress the immune
responseVery useful in treating inflammatory diseases
(asthma, rheumatoid arthritis)Side effects:
HyperglycemiaDecreased glucose toleranceDecrease synthesis of collagen and other
extracellular matrix proteins and increased bone resorption (osteoporosis)
Cushing’s SyndromeHypersecretion of ACTH (glucocorticoid)Can also result from a tumor of the adrenal
cortexCharacterized by:
Changes in carbohydrate and protein metabolism
HyperglycemiaHypertensionMuscular weakness
Functions of Adrenal MedullaSecrete epinephrine and norepinephrineEffects of catecholamine hormones secreted by the
medulla:Increase cardiac outputDilate coronary blood vesselsIncrease mental alertnessIncrease respiratory rateElevates metabolic rate
Supported by the metabolic actions of E and NE:Rise in blood glucose (due to stimulation of hepatic
glycogenolysis)Rise in blood fatty acids (due to lipolysis)
Stress and the Adrenal Gland1936 – Hans Selye discovered that injections
of cattle ovary extract into rats:1. stimulated growth of the adrenal cortex2. caused the atrophy of the lymphoid tissue of the spleen, lymph nodes and thymus, and3. produced bleeding peptic ulcers
Same things happened when he subjected the rats to cold environment and when he dropped them into water and made them swim until they were exhausted
Stress and the Adrenal GlandStress – reaction of the organism to stimuli
(stressors) which may produce damaging effects
Stressors stimulate the pituitary-adrenal axisUnder stressful conditions, there is a
increased secretion of ACTH therefore there is the increased secretion of glucocorticoids
Stress and the Adrenal GlandThere is a nonspecific response of the body to
readjust itself following any demand made upon itA rise in the plasma glucocorticoid level results
from the demands of the stressors (General Adaptation Syndrome or GAS)
3 stages in response to stress:Alarm reaction (adrenal glands are activated)Stage of resistance (readjustment occurs)Stage of exhaustion (if readjustment is not
complete – this leads to sickness and possibly, death)
Thyroid GlandLocated just below the larynxTwo lobes are positioned on either side of
the trachea and are connected anteriorly by a medial mass of the thyroid tissue (isthmus)
Largest pure endocrine gland (20-25 grams in wt.)
Thyroid Gland
Thyroid GlandConsists of numerous spherical hollow sacs
(thyroid follicles) lined with simple cuboidal epithelium composed of follicular cells (secrete thyroxin)
The interior of follicles contain colloid (protein-rich fluid)
Also contains parafollicular cells (secrete calcitonin or thyrocalcitonin)
Thyroid Gland
Production and Action of Thyroid HormonesThyroid follicles actively accumulate iodide (I-)
from the blood and secrete it into the colloidIodide will be oxidized to form iodine once it
enters the colloid then it will be attached to amino acids (tyrosines) within the polypeptide chain of protein (called thyroglobulin)
The attachment of:One iodine to tyrosine => monoiodotyrosine
(MIT)Two iodines => diiodotyrosine (DIT)
Production and Action of Thyroid HormonesWithin the colloid, enzymes modify the structure
of MIT and DIT and couple them togetherTetraiodothyronine (T4) or thyroxine –
produced when two DIT molecules that are appropriately modified are coupled together
Triiodothyronine (T3) – combination of one MIT with one DIT
Upon stimulation of TSH, the cells of the follicle take up a small volume of colloid by pinocytosis, hydrolyze the T3 and T4 from the thyroglobulin, and secrete free hormones into the blood
Production and Action of Thyroid HormonesThrough the activation of genes, thyroid hormones:
Stimulate protein synthesisPromote maturation of the nervous systemIncrease the rate of cell respiration
Through this, thyroxin elevates the basal metabolic rate (the resting state of calorie expenditure by the body)
Calcitonin– works with parathyroid hormones to regulate
calcium levels of the blood- Inhibits dissolution of the calcium phosphate crystals
of bone and stimulates the excretion of calcium in the urine by the kidneys
HyperthyroidismIodine-deficiency (endemic) Goiter– abnormal growth of the thyroid gland– due to the absence of the dietary iodine, the
thyroid cannot produce adequate amounts of T4 and T3
– the resulting lack of negative feedback inhibition causes hypersecretion of TSH that stimulates abnormality
HypothyroidismCondition of people who have inadequate
secretion of the thyroid hormonePeople who have hypothyroidism have an
abnormally low basal metabolic rate and they experience weight gain and lethargy
In adults, it causes:Myxedema– accumulation of mucoproteins and fluid in
subcutaneous connective tissues– symptoms: swelling of the hands, face, feet and
tissues around the eyes
HypothyroidismCan also be caused by insufficient
thyrotropin-releasing hormone (TRH) secretion from the hypothalamus, or insufficient secretion of TSH from the anterior pituitary or insufficient iodine in the diet
Parathyroid GlandsSmall flattened glands that are embedded in
the posterior surfaces of the lateral lobes of the thyroid gland
Parathyroid Glands
Parathyroid hormoneOnly hormone secreted by the parathyroid
glandsImportant in the control of calcium levels of
the bloodPromotes rise in blood calcium levels by
acting on the bones, kidneys and intestine