Lecture Endocrine System Part II
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What is the thyroid gland?
- the largest endocrine gland, ocated in the anterior neck
- consists of two lobes attached by the ithsmus
composed of folicles that produce the glycoprotein thryoglobulin
Colloid (thyroidglobulin + iodine) fills the lumen of the follicles and is the precursir if the thyroid hormone
other endocrine cells, the parafollicular cells, produce the hormone calcitonin
What is the Thyroid hormone?
The major metabolic hormone!
- Consists of 2 related iodine containing compounds
- -T4 (thyroxine)- it has 2 tyrosine molecules plus four bound iodine atoms (major hormone secreted by the thryoid follicles)
- -T3 (triiodothryonine)- has two trosines with three bound iodine atoms (formed at target cells from T4)
What are the effects of the thyroid hormone?
- glucose oxidation which leads to
- *increasing metabolic rate
- *heat production
- (these are called the calorigenic effect)
- TH plays a role in:
- -maintaining blood pressure
- -rgulating tissue growth
- -developing skeletal and nervous systems
- -maturation and reproductive capabilities
How is Thyroid Hormone Synthesized?
1. thyroglobin is synthesized and discharged into the lumen
) are actively taken into the cell
3.Iodide is oxidized to iodine (I2
) and released into the lumen
4. Iodine attached to tyrosine, mediated by peroxidase enzymes, forming T1
5. Iodinated tyrosines link together to form T3
6. colloid is then endocytosed and combine with a lysosome
- 7. T3 and T4 are cleaved and diffused into ethe blood stream
How is TH regulated and Transported?
T4 and T3 bind to thyroxine-binding globulins (TBGs) produced by the liver
both bind to rager receptors. but T3 is ten times more active in binding than T4
peripheal tissues convert T4 to T3
mechanisms of activity are similar to steroids
regulation is by a negative feedback system
hypothalamic thryotropin-releasing hormone (TRH) can overcome the negative feedback, such as when more energy is needed for pregnancy
What is calcitonin?
A peptide hormone produced by the parafolliclar, or C cell of the thyroid gland
lowers the blood calcium levels in children
antagonist to the parathyroid hormone (PTH)
- calcitonin targets the skeleton where it:
- -inhibits osteoclast activity (stops bone absorption) and release of calium from the bone matrix
- -stimulates calcium uptake and incorporation into the bone matrix
-regulated by a humoral (calcium ion concentration in the blood) negative feed back system)
What are the parathyroid glands?
Tiny glands embedded in the posterior aspect of the thryoid
cells are arrabged in cords containing oxyphil and chief cells
chief cells secrete parathyroid hormone (PTH)
- PTH regulates calcium balance in the blood and is the most important calium regulating hormone
What are the effects of PTH?
- PTH release increases Ca2+ in the blood as it:
- -stimulates osteoclasts to digest bone matrix
- -enhances the reabsorption of Ca2+ and the secretion of phosphate by the kidneys
- -increases the absorption of Ca2+ by intestinal mucosa by activatin Vitamin D
- rising Ca2+ in the blood inhibits PTH release
What are the adrenal glands?
pairs, pyramin shaped organs atop the kidneys
structurall and functionally they are two glands in one
1. Adrenal medulla (inner)- neural tissue that acts as part of the sympathetic nervous system
2. Adrenal cortex (outter)- glandular tissue dericed from embryonic mesoderm
What does the adrenal cortex of the adrenal glands do?
synthesizes and releases steroid hormones called corticosteroids
- different corticosteroids are produced in each of the three layers
- 1. Zona glomerulosa-mineralcorticoids (chiefly aldesterone) (control the balance of minerals and water)
- 2.Zona fasciculata-glucocorticoids (metabolic hormones)
- 3. Zona reticularis -gonadocorticoids (produces small amounts of sex hormones)
What are mineralcorticoids?
regulate electrolutes in extracellular fluids
- aldosterone- most important mineralcorticoid
- -maintains sodium balance by reducing excretion of sodium from the body
- -stimulates reabsorption of sodium by the kidneys (can effect blood volume and bp)
- Aldosterone secretion is regulated by:
- rising blood levels of potassium
- low blood sodium
- decreasing blood volume or bp
What are the four mechanisms of aldosterone secretion in the adrenal cortex?
1. Renin-angiostensin mechanism-
kidneys release renin which is converted into angiotensin II that in turn stimulates aldosterone release
2. plasma concentration of sodium and potassium
- directly influences the zona glomerosa cells (more K+
meas more aldosterone release and vice versa)
causes small increases of aldosterone during stress
- 4. Atrial natriuretic peptide (ANP) - inhibits activity of the zona glomerosa and allows Na+ to leave the urine (this is secreted by the heart when bp rises)
What are glucocorticoids (cortisol)?
- help the body resist stress by:
- -keeping blood sugar levels relatively constant
- -maintaining blood volume and preventing water shift into tissue
- cortisol provokes:
- -gluconeogensis (formation of glucose from noncarbs) (esp to save sugar for thr brain)
- -rises in blood glucose, fatty acids,a nd amino acids
What happens where there are excessive levels of cortisol?
can end in cushing disease
- depress cartilage and bone formation
- inhibits inflammation
- depresses the immune system
- promote changes in cardiovacular, neural and gastrointestinal function
What are gonadocarticoids (Sex hormones?)
most gonadocarticoids secreteed are androgens (male sex hormones) and the most important one is testoserone
- androgens contribute to:
- -the onset of puberty
- -the appearance of secondary sex characteristics
- -sex drive in femals
androgens can be converted into estrogens after menopause
What happens int eh adrenal medulla?
made up of chromaffin cells that secrete epinephrine and norepinephrine
- Secretion of these hormones causes (fight or flight response)
- -blood glucose levels to rise
- -blood vessels to constrict
- -the heart to beat faster
- -blood diverted to the brain, heart, and skeletal muscle
epinephrine is the more potent stimulator of the heart and metabolic activities
- norepinephrine is more influential on peripheal vasoconstriction and blood pressure
Describe the pancreas
- Pancreatic islets produce hormones
- -Alpha cells that produce glucagon
- -Beta cells that produce insulin (more nuermous)
*glucagon and insulin work against each other
What is glucagon?
a 29 amino acid polypeptide hormone that is a potent hyerglycemic agent
- its a major target of the liver, where it promotes
- 1. glycogenolysis- the breakdown of glucogen into glucose
- 2. gluconeogensis- synthesis of glucose from lactic acid and non carbs
- 3. release of glucose to the blood from liver cells
- *hyperglycemic effects
what is insulin?
A 51 amino acid protein consisting of two amino acid chains linked by disulfide bonds
sytheiszed as part of proinsulin and then excised by enzymes, releasing functional insulin
- -lowers glucose levels
- -enhances transport of glucose into body cells
- -counters metabolic activity that would enhance blood glucose levels
- *hypoglycemic effects
What are the effects of insulin binding?
the insulin receptor is a tyrosine kinase enzme
- after glucose enters a cell, insulin binding triggers enzymatic activity that:
- -catalyzes the oxidation of glucose for ATP production
- -polymerizes glucose from glucagon
- -converts glucose to fat (esp in adipose tissue)
What is diabetes mellitus?
results from the hyposecretion or hypoactivity of insulin
- three cardinal signs are:
- polyuria-huge urin output
- poludipsia-excessive thirst
- polyphagia-excessive insulin secretion resulting in hypoglycemia
What is the pineal gland?
small gland hanging fromt he roof of the third ventricle of the brain
- secretion is primarily melatonin
- -day/night cycles
what is the Thymus?
lobulated gland located deep to the sternum
major hormonal products are paracrines (thymopoietins ad thymosins)
these hormones are essential for development of the T lymphocytes of the immune system
Name the other hormone producing structures and their functions
Heart- produces ANP which reduces blood pressure, blood volume, and blood sodium concentration
gastrointestinal tract: enteroendocrine cells release local acting digestive hormones
placenta- releases hormones that influence the course of pregnancy
kidneys- secretre erythropoietin, which singnals the production of red blood cells in bone marros, also secretes renin
skin- produces the precursor for Vit D
Adipose tissue: releases leptin, which is involved in tthe sensation of satiety and stimulates increases energy expenditure
What are the developmental aspects of the endocrine system?
HORMONE PRODUCING GLANDS ARISE FROM ALL THREE GERM LAYERS!
Endocrine glands derived from the mesoderm produce steroid hormones
endocrine organs operate smoothly throughout life and while they may show structural change the hormone production may or may not be effected
How do enviormental aspects effect the endocrine system?
exposure to pesticides, industrial chemical, arsenic, dioxin, and soila nd water pollutants disrupts hormone function
sex hormones, thyroid hormones, and glucocorticoids are volnerable to the effects of pollutants
intereference with glucocorticoirds may help explain high cancer rates in certain areas
How do the sex hormones change during lifetime?
ovaries undergo significant changes with age and become unresponsive to gonadotropins
female hormone production declines, the ability to bear children ends, and problems associated with estrogen deficiency begin to occur
testosteribne also dimishes with age but effect is usually not seen until very very old age
What happens to GH as we get old? What about TH and PTH?
Gh levels decline with age and this accounts for muscle atrophy with age
supplmental Gh may spur muscle growth, reduce body fat, and help physique
Th declines with age, causing lower basal metabolic rates
PTH levels remain fairly constant with age, the lack of estrogen in women makes them more vulnerable to the effects of PTH
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