Summary Human Endocrine System notes - IEB syllabus

HUMAN ENDOCRINE SYSTEM
Chemical co-ordination – slow process of communicating info throughout the body via
hormones.
Hormone – organic chemical substance (usually a protein, sometimes a steroid), secreted by
an endocrine gland, carried in blood to target organs where it regulates metabolic reactions.
They do not last for long in the body, they are broken down by enzymes.
Target organs – hormones only affect certain cells, these cells have receptor sites for that
hormone. Can be a single gland/organ/scattered throughout body.
Exocrine glands – glands that secrete substances onto an epithelial surface by way of a
duct. Sweat/salivary/mammary.

Endocrine glands – vascular (richly supplied with blood vessels) ductless glands, secrete
hormones into blood supply where they travel to target organs. Hypothalamus, pituitary,
thyroid, Islet of Langerhans in pancreas, adrenal.
 Pancreas secretes digestive juices along a tube (digesting proteins, fatty acids etc.)
Can also secrete insulin to control sugar levels. Only organ that can do both.
PITUITARY GLAND (hypophysis)
master gland, chemical coordinator of other endocrinal glands. Produces many tropic
hormones (a hormone that makes another gland make a hormone). Has a posterior and
anterior lobe.




1. Growth hormone (GH, not a tropic hormone)
 Skeletal and muscle growth by stimulating production of proteins
 Controls and regulates the growth of an individual
 Acts very slowly and controls bone and muscles growth
 In kids: too few GH, leads to dwarfism. Too much, leads to gigantism
 HYPERSECRETION – too much GH
Kids – overdevelopment of skeleton (tall), gigantism, caused by tumour in pituitary
gland
Adults – acromegaly. Bones of face, feet, hands, lips, tongue & eyelids enlarged

,  HYPOSECRETION – too little GH (2 types of dwarfs)
Kids – dwarfism
Adults - dwarfism
2. TSH (thyroid stimulating hormone) – stimulates thyroid gland to secrete its hormone,
thyroxin
HYPOTHALAMUS
 Part of brain, above the pituitary gland, secretes ADH
 ADH - anti-diuretic hormone, osmoreceptors in blood vessels detect low water
levels, ADH makes collecting ducts more permeable to water, more water is actively
absorbed back into blood stream – less water lost in urine
THYROID GLAND
 Butterfly shape, either side of trachea below larynx (voice box)
 Thyroid gland secretes THYROXIN
 Iodine (in small quantities) is essential in the production of thyroxin
 A goitre may develop if iodine levels are low
 Thyroxin:
Promotes muscle function, brain development, maintenance of bones, overall
metabolism
controls rate of cellular respiration and therefore production of body heat
promotes normal functioning of heart
promotes normal functioning of the nervous system by increasing nervous
activity and sharpening reflexes and alertness
 HYPOTHYROIDISM
 Too little thyroxin produced, low metabolic rate
 Adults – myxoedema, mental & physical sluggishness, low BP, slow heart, low
body temp, low respiratory rate
 Kids – cretinism, child does not grow physically, mentally retarded, immature
sexual development
 Treatment – iodine supplements, synthetic iodine. If damage is done in a
child – it cannot be reversed
 HYPERTHYROIDISM
 Too much thyroxin produced, high metabolic rate
 Body temp high, sweating increases, heart & respiratory rate and BP
increases
 Muscular tremors and nervousness
 Swollen thyroid gland – exophthalmic goitre
 Thyroxin levels low  TSH secreted from pituitary gland. High levels of thyroxin
prevent production of further TSH. This negative feedback mechanism insure that
the thyroxin remain stable - controlling mechanism.