ENDOCRINE SYSTEM

Lecture Notes:

Along with nervous system, the endocrine system's major function is to regulate body functions via cell to cell communication. Nervous system feedback loops are faster and short-lived, but localized while the endocrine system feedback loops can affect many cells in the body. Both systems are interrelated, especially at the hypothalamus. In this section, we will only cover those hormones that are NOT uniquely part of another system. So.. no reproductive or digestive hormones

click on this image for an audio message

I. Introduction to endocrine system
A. Endocrine organs have cells that produce hormones. The organ may have non-endocrine functions as well.

B. Comparison between gland types.
      Endocrine glands                           (vs Exocrine glands)

1. Secreted and transported in blood vessels                (into ducts outside body)

2. Produce hormones                                 (produce oil, sweat, enzymes, mucus)      

click on this image for an audio message
Describe the difference between endocrine and exocrine glands.

C. Chemistry of hormones

1. Local hormones
produced and used locally in one cell or between nearby cells
aspirin reduces local hormone (prostaglandin) so less-inflammation

2. Circulating hormones
produced in endocrine gland and affects distant target cell

a. Water soluble hormones (Amino acid-based)
Examples: most non-sex hormones and catecholamines (epinephrine and NE)

b Lipid soluble (Cholesterol-based =steroids) hormones Examples: Testosterone, estrogens, progesterone and Thyroid hormone.

1. Membrane-associated receptors (protein-based hormones)a. Circulating water soluble hormone (1st messenger) attaches to receptor activating enzyme adenylate cyclase

b. Adenylate cyclase converts ATP to cAMP

c. cAMP activates cellular enzymes (kinases) which promote production of other enzymes (thereby altering roles of protein production and cellular physiology)

d. Hormone's affect is a cascade affect so only a few molecules of hormones can change number and kinds of many proteins in cell

e. Hormones change cell's function by changing type and number of pre-existing cellular enzymes

2. Intracellular receptors (cholesterol based hormones)a. Lipid-soluble hormones diffuse through lipid membrane

b. React with receptor to form hormone-protein complex

c. Complex binds to chromosome and regulates gene expression, e.g., controls rate of RNA replication and protein synthesis

d. steroid hormones change cell function by altering rates of protein synthesis via direct gene control

click on this image for an audio message
Contrast and compare how the different hormone chemical types acts to signal changes in cell function.

II. Physiology of hormones

A. Hormonal feedback loops
A change in condition (stimulus) is often damaging to homeostasis so feedback loops are used. As such, hormones are circulate through the bloodstream between production cells (endocrine gland cells) and target cells to alter their function.. The overall result of these changed functions is to alter the condition (response).
click on this image for an audio message
Remember:
In negative feedback loops the response always opposes the stimulus.
In positive feedback loops, the response increases the stimulus.

click on this image for an audio message on studying hormones

NOTE: Most hormones have more then one target cell.CLICK TO VIEW AND PRINT THE COMPLETE HORMONE CYCLE TEMPLATE..

B. Hormone production (endocrine cell function) triggered by different "stimuli"

1. Negative feedback (most common)

a. Releasing or regulating hormones (e.g., hypothalmic stimulating or inhibiting hormones) that affect pituitary gland hormone secretions

b. Direct nervous control of secretion (e.g., NE from medulla and adrenal glands) due to increased sympathetic stimulation (stress activated)

c. Blood chemistry control = humoral ( e.g., PTH, calcitonin, Insulin, ADH) endocrine cells, sensitive to changes in blood chemsitry, produce hormones that affect those changes in levels of blood solutes

click on this image for an audio message

2. Positive feedback (rare) stimulil

suckling (oxytocin promotes labor and milk ejection)

Describe each type of mechanism that stimulates hormone production. Give examples of hormones for each stimulation type.

C . Target cell actions, are variable, including below:

1. Membrane potential/permeability

2. Enzyme production/activation

3. Mitotic/meiotic rates

4. Muscle contraction rates

5. Secretion rates of hormones

Most body cells, and particularly skeletal muscle cells, act to: increase glucose intake via facilitated diffusion (change in membrane permeability)

Fat cells act to: increase Lipogenesis, converting glucose to lipids (change in cell enzymes)

Liver cells act to: increase glycogenesis, converting glucose to glygocen (change in cell enzymes)

Overall Response is decreased blood glucose level click on this image for an audio message

1. hGH (human growth hormone) (not tropic) produced by somatotroph cells of anterior pituritary a. Actions-(through Insulin-like growth factors)
Increases mitotic rates in bones and muscle cells
Increases protein synthesis, hence growth, in body cells-especially bones and muscle cells
Increased use of fats (lipolysis) and amino acids to fuel cellular metabolism in body cells allows decreased use of glucose for cell metabolism (and therefore allowing glucose levels to rise)

b. -production stimulated by hypothalmic hormones which, in turn, are stimulated by: low levels of glucose or fatty acids in blood, or high levels of amino acids in blood

Disorders
-as children hypersecretion=pituitary giant, hyposecretion=pituitary dwarf (abnormal body heights)
as adults=acromegaly (abnormal bone growth)

2. TSH (thyroid stimulating hormone) (tropic) - produced by thyrotroph cells of anterior pituritary a. Actions-increases production of thyroid hormones (T3 and T4) by follicular cells of thyroid

b. -production stimulated by hypothalamic hormones which are, in turn, stimulated by: low blood glucose, low metabolic rate
 
  3. ACTH (adrenocorticotropic hormone) (tropic) -produced by corticotroph cell of anterior pituitary a. Actions- increases production of cortisol hormone by adrenal cortex cells

b.-production stimulated by hypothalamic hormones which are, in turn, stimulated by: low blood glucose, stress/injury/infection
 

1. ADH (antidiuretic hormone) -produced by neurosecretory cells in hypothalamus that extend to posterior pituitary a. Actions
Increases retention of water by nephrons (kidney cells) then decreasing volume of urine but increasing water and blood volume
Increases vasoconstriction of smooth muscle cells in arterioles increasing blood pressure under extreme conditions

b. -production stimulated by high osmotic pressure (dehydration, loss of blood, excessive sweating, diarhea=too little water in blood) as measured by hypothalamic osmoreceptors

Disorder - hyposecretion=diabetes insipidus (high volume, dilute urine)

1. Thyroxine (thyroid hormoens, or T3 and T 4)- produced by follicular cells (iodine necessary)

a. Actions
Increase use of oxygen and glucose (Glycolysis) to elevate cell metabolism of all body cells resulting in increased basal metabolism and body temperature.
Increase protein synthesis to promote normal growth of muscle and nervous tissue
Increase use of fats for cellular metabolism
Increase contraction of smooth muscle cells in arterioles (increase blood pressure) and cardiac muscle (increase heart rate)

b. production stimulated by high levels of TSH (low blood glucose)

Disorders

Hyposecretion-adult= Myxedema ( no retardation, slow BMR, edema)
-children=Cretinism (developmental. dwarf and mental retardation)

Hypersecretion-adulat= Exopthalmic goiter (goiter behind eyes), Grave's Disease (high BMR, autoimmune deficiency )

2. CT (Calcitonin)- produced by parafollicular cells

a. Actions-
Inhibits osteoclast cell activity (bone matrix resorption) so lowers blood calcium level

b. production stimulated by high levels of blood calcium

1. Parathyroid hormone (PTH)- produced in parathyroid gland near thyroid

a. Action-
Increases osteoclast cell number and activity increasing bone matrix resorption
increases Ca reabsorption from urine by nephron (kidney cells)
Increases formation of Calcitriol (which increases resorption of Ca from diet into intestine ) by kidney endocrine cells
with overall response being an increase in blood calcium
 
b. production stimulated by low levels of blood calcium

1. Aldosterone - produced by cells in the zona glomerulosa a. Actions
Increases kidney (nephron cells) retention of Na+ (Cl- and water follows osmotic gradient produced by Na +)
Increases secretion of K+ out of body, by nephron cells
Increases secretion of H+ out of the body, by nephron cells
Overall response is increase in Na blood levels, decreased K blood levels and increased blood volume.

b. production stimulated by high levels of blood K
production stimulated by low levels of blood Na , dehydration or blood loss (reduced blood volume) which triggers eventual production of Angiotensin II, a urinary hormone that stimulates Aldosterone production to increase blood volume and constricts arterioles to elevate blood pressure

2. Cortisol -produced by cells in zona fasciculata a.Action-
Increases breakdown of proteins in muscle cells (protein lysis) and breakdown of fats (lipolysis) in adipose cells, resulting in use of these sources for ATP production -providing an alternate fuel source ...so increased blood glucose.
Increases production of new glucose (gluconeogenesis) from amino acids and fatty acids
Promotes breakdown of glycogen (glycogenolysis), by glucagon, at liver cells to increase blood sugar
Reduces histamine and other inflammatory chemicals produced by histiocytes (anti-inflammatory)
Reduces response by lymphocyte cells (immune suppressant)

b. production stimulated by high levels of ACTH (low blood glucose)

Disorders -adult
Hyposecretion=Addisons  (dehydrated, low blood sugar)
Hypersecretion=  Cushings (high blood sugar, edema

F. Adrenal Medulla

1. Epinephrine/Norepinephrine -produced by chromaffin cells in adrenal medula

a. Action-
increase contraction of cardiac muscle cells (increased heart rate)
Alter blood flow to brain, heart, lungs by change in vasoconstrictio/vasodilation of smooth muscle cells in arterioles
Increase glucose release (glycogenolysis) by liver cells
Increased bronchodilation by relaxing smooth muscle cells in bronchioles.

b. production stimulated by high levels of sympathetic nervous stimulation  

Overall response is to produce Flight or Fight response (increased oxygen and glucose in blood, increased blood flow, altered blood flow to organs needed to reduce stress and away from not needed organs)

1. Insulin- produced by beta cells in Isle of Langerhans a. Actions-
Increase glucose uptake (facilitated diffusion) into body cells
Increase Amino acids intake into body cells
Increase glycogenesis (convert glucose to glycogen) at liver cells
I ncrease lipogenesis (produce triglyceride lipids) at adipose cells
Increase Glycolysis (break down glucose to form ATP)

Overall response is decreased blood glucose levels.

b. production stimulated by high levels of blood glucose

Disorder - adult hyposecretion of insulin =diabetes mellitus (hyperglycemia, lots of urine, high triglycerides, small capillary blockage, neuropathy)

2. Glucagon - produced by alpha cells in Isle of Langerhans a. Action-
Increases blood glucose levels increasing glycogenolysis in liver
Increases gluconeogenesis (production of glucose from fatty acids and amino acids) in liver

b. production stimulated by low levels of blood glucose and increased sympathetic activity

H. Kidney

1. Calcitriol or Vitamin D(converted into active form in skin)

a. Action- increase absorption of Ca from food by intestine cells
b. production stimulated by more of precursor of hormone coming from kidney (because of increased production stimulated by PTH).

2 . EPO (Erythropoiten)-produced by kidney cells
a. Action- increase production of red blood cells by red bone marrow cells.
b. production stimulated by low levels of blood oxygen.

3. Renin-Angiotensin-produced by juxtaglomerular appratus cells in kidney
a. Action-increase production of Aldosterone by adrenal cortex cells
Increase vasoconstriction by smooth muscle cells in arteriole
b. production stimulated by low blood volume

 

J. Heart

1 ANP (Atrial Natriuretic Peptide)-produced by atrial cardiac muscle cells
a. Action-
Decreases retention of Na by nephron cells
Decreases production of Aldosterone by zona glomerulosa cells of adrenal cortex
Decreases production of ADH by neurosecretory cells of hypothalamus/posterior pituitary

Overall response is to lower blood volume and pressure

b. production stimulated by high blood volume and pressure in atria of heart.

Construct hormonal negative feedback cycles. CLICK TO VIEW AND PRINT THE COMPLETE HORMONE CYCLE TEMPLATE..

IV. Stress response sequence

Human Anatomy and Physiology Courses 
at St. Petersburg College
St. Petersburg/Gibbs Campus

5/2008