endocrine system

what is the endocrine system?

the second major communication system of the body

what is the general function of the endocrine system?

allows the body to adapt to both internal and external change

what are hormones of the endocrine system?

chemical messengers that are released by endocrine tissue, transported through the bloodstream and alter activity of target cells in other tissues

what are the general functions of hormones?

1. help regulate: chemical composition and volume of internal fluids, metabolism and energy balance, contraction of smooth and cardiac muscle fibers, glandular secretions, some immune system activities
2. control growth and development
3. regulate operation of reproductive systems
4. help establish circadian rhythms
5. mobilise body's defense against stressors

what are the cellular functions of hormones?

1. altering membrane permeability
2. activating or inactivating key enzymes
3. changing genetic activity

compare the nervous system to endocrine system

nervous:
direct communication
quick response
responds to physical and chemical stimuli
capable of secreting

endocrine:
indirect communication
slow acting, slow response
responds to chemical stimuli
capable of secreting

describe endocrine glands

ductless
well vascularised
secrete hormones into extracellular fluid

what is hormone action?

hormones move through blood stream into the ECF of tissues

target tissue has specific receptors

how is biological activity of target tissue determined?

1. concentration of the hormone
2. receptor
3. receptor affinity

how is effective plasma concentration of a hormone regulated?

rate of secretion
transport
metabolism
excretion

what causes endocrine disorders?

shifts in hormonal excess and hormonal deficiency

decreased responsiveness of the target tissue

what is acromegaly and its symptoms?

increased production of GH
pronounced brow protrusion
enlargement of mandible, tongue, teeth gapping
growth of hands and feet
soft tissue swelling

what are the classes of hormones?

amino acid derivatives
peptide hormones
lipid derivatives

what are amino acid derivatives?

small, water soluble molecules structurally related to amino acids

synthesized from the amino acids: tyrosine and tryptophan

what are peptide hormones?

water soluble chains of amino acids

synthesized as prohormones: inactive molecules converted to active hormones before or after secretion

what are the 2 groups of peptide hormones?

1. glycoproteins
2. short polypeptide chains and small proteins

what are the 2 types of lipid derivatives?

1. eicosanoids (derived from arachidonic acid)
2. steroid hormones (derived from cholesterol)

how are hormones transported?

circulate freely or bound to transport proteins in the blood

when do free hormones become inactive?

remain functional for less than 1 hour, become inactive when:
1. diffuse out of bloodstream: bind to receptors on target cells
2. are absorbed: broken down by cells of liver or kidney
3. are broken down by enzymes: in plasma or interstitial fluids

describe the transport of thyroid and steroid hormones

remain in circulation much longer

enter bloodstream: more than 99% of them bind to special transport proteins

how do water soluble hormones cross the plasma membrane?

cannot cross plasma membrane so act via second messengers

what do lipid soluble hormones do?

can diffuse across plasma membrane

bind to and activate receptors in cytoplasm or nucleus
- activated receptor-hormone complex alters gene expression and directs synthesis of new proteins

alters cellular metabolism

what is hormone secretion regulated by?

1. central nervous system
2. chemical changes in blood
3. other hormones

how does hormone secretion occur and how is it regulated?

secretion of some hormones occurs in short pulsatile bursts (some hypothalamic and pituitary hormones)

secretion regulated by negative feedback for most (some operate via positive feedback)

what are the functions of the pineal gland?

secretes melatonin
sets diurnal rhythm
may contribute to seasonal affective disorder
inhibits reproductive functions
effective antioxidant

what are the organs with endocrine functions?

heart
thymus
adipose tissue
digestive system
kidneys
gonads

what does the hypothalamus master?

master endocrine gland and neuroendocrine control centre of the body

what are the functions of the hypothalamus?

controls release of hormones by pituitary
regulates temperature
controls food and water intake
circadian rhythms
emotional response

what is the role of hypothalamic nuclei?

each nuclei controls specific functions (some through the production of regulatory hormones)

how does hypothalamus integrate activities of nervous and endocrine systems?

1. secretes regulatory hormones
2. acts as an endocrine organ
3. contains autonomic centers

2 types of hypothalamic hormones

releasing hormones
inhibiting hormones

describe the pituitary gland

also called hypophysis
lies within sella turcica
hangs inferior to hypothalamus
releases 9 peptide hormones
divided into anterior and posterior lobes

in the anterior pituitary gland, what is the secretion of hormones controlled by?

negative feedback
releasing and inhibiting factors from hypothalamus which are released into the hypophyseal portal system

what hormones do anterior pituitary cells secrete?

growth hormone
thyroid stimulating hormone
follicle stimulating hormone
luteinizing hormone
prolactin
adrenocorticotropic hormone
melanocyte stimulating hormone

when are growth hormones secreted?

secreted in bursts every few hours
(especially during sleep)

what are the functions of growth hormones?

stimulates general cellular growth and regulates metabolism
increase protein synthesis
decrease protein catabolism
increase uptake of amino acids

how do growth hormones stimulate cellular growth and regulate metabolism?

by stimulating production of somatomedins/IGFs (Insulin-like growth factors)

what is the function of thyroid stimulating hormone (TSH)?

controls secretions and activities of thyroid gland

what is the function of follicle stimulating hormone (FSH)?

in males stimulates sperm maturation
in females stimulates follicle development and estrogen secretion

what are the functions of luteinizing hormone (LH)?

in males stimulates leydig cells to synthesise androgens
in females stimulates ovulation and luteinisation and estrogen and progesterone production

what is the function of prolactin (mammotropin)?

stimulates mammary gland development
in pregnancy: initiates and maintains milk production by mammary glands

what is the function of adrenocorticotropic hormone (ACTH)?

stimulates adrenal cortex to release steroid hormones: glucocorticoids

what is the function of melanocyte stimulating hormone (MSH)?

stimulates melanocytes of the skin, increasing melanin, which causes darkening of skin
produced locally within sun-exposed skin
normally in circulation during fetal development, childhood and pregnancy

describe the posterior pituitary

also called neurohypophysis
contains unmyelinated axons of hypothalamic neurons
does not synthesise hormones
manufacture ADH and oxytocin

antidiuretic hormone functions

released in response to increase solutes in blood or decrease blood pressure or volume
decrease amount of water lost at kidneys
increase blood pressure

what is the antidiuretic hormone release inhibited by?

alcohol

what is the function of oxytocin?

stimulates smooth muscle in uterus (sperm duct and prostate in male)
stimulates contractile cells in mammary glands
secretion and milk ejection are part of neuroendocrine reflex

gender differences in stress response

fight or flight stress response for males
tend and befriend response for females