The Endocrine System and Homeostasis

Worth a “looksee”!

Chemical Control Systems“No cell operates in isolation”

Animals must respond to stimuli from inside and outside the body

The nervous system and endocrine system work together to control the various organs

The nervous system reacts quickly to changes in the environment

The endocrine system reacts slower and maintains control over a longer duration (ex. hours – years)

Locations of the Major Endocrine Glands

* The small intestine and stomach have certain cells that produce hormones that affect digestion

Importance of the Endocrine Importance of the Endocrine SystemSystem

Hormones: chemical controls/regulators speed up or slow down certain bodily speed up or slow down certain bodily

processesprocesses produced by cells in one part of the body affect cells in another part of the body Control: a) growth and development

b) maintenance of homeostasis

Glands are specialized organs for secreting substances needed by the

organism Exocrine Glands: Secrete substances

that are needed quickly

Secreted through small ducts

Examples: salivary, sweat, and milk glands

Endocrine Glands: Secrete hormones

directly into the bloodstream, without the use of ducts

Delivered by the circulatory system

Long-term control Examples:

hypothalamus, pituitary, pancreas

HORMONESClassified according to their activation site

NONTARGET TARGET HORMONES

HORMONES

Affect many cells Affect specificthroughout the body cells

*only a small amount is necessary to alter cell metabolism

Chemical Structure of Hormones

Two major types of hormones:

1. Protein-type hormonesex) insulin, adrenaline

2. Steroid hormonesex) testosterone,

Protein-type Hormones Chains of amino acids Soluble in water Combine with specific

receptor protein on the plasma membranes of target tissue cells = recognizes target tissue easily

Never actually enters the cell

2nd messenger is activated inside the cell to cause a series of reactions to perform the necessary action

Ex) insulin, adrenaline

Steroid Hormones Chemical structure =

complex rings of carbon, hydrogen, and oxygen

Soluble in fat; Not in water Actually pass through the

membranes of target cells Combine with receptors in

the cytoplasm The steroid-receptor

complex then moves into the nucleus where it activates a gene that produces the response

Ex) testosterone, estrogen

The Lock and Key ModelThe Lock and Key Model Hormones affect target cells by binding to specific Hormones affect target cells by binding to specific

receptor molecules receptor molecules in the cell membranein the cell membrane This “unlocks” the cell’s responseThis “unlocks” the cell’s response Many Many drugs drugs work by work by mimicking a naturally occurring mimicking a naturally occurring

hormonehormone. . If the drug causes the receptor to If the drug causes the receptor to respond in the same way respond in the same way

as the naturally occurring substance, the drug is referred to as the naturally occurring substance, the drug is referred to as an as an agonist.agonist.

• Ex. Nicotine and morphineEx. Nicotine and morphine

• These drugs “pick the lock”These drugs “pick the lock” Other drugs work in the opposite way as Other drugs work in the opposite way as antagonistsantagonists. These . These

drugs bind to the receptor, but drugs bind to the receptor, but do not produce a responsedo not produce a response. . Because the drug prevents the receptor from binding to the Because the drug prevents the receptor from binding to the normal hormone or neurotransmitter, it has an inhibitory normal hormone or neurotransmitter, it has an inhibitory effect on the naturally occurring substance. effect on the naturally occurring substance.

• Ex. Caffeine and atropine.Ex. Caffeine and atropine.• These drugs “jam the lock”These drugs “jam the lock”

“Lock and Key”

The Pituitary – Hypothalamus Relationship

Homeostasis is maintained by hormone levels responding to internal and external stimuli

The hypothalamus regulates the pituitary gland by nerve stimulation

The pituitary (aka Master Gland) regulates endocrine glands that secrete chemicals that affect the hypothalamus’ nerve activity by negative feedback.

The pituitary releases hormones which stimulate other glands to release hormones that either increase or decrease cell processes

Pituitary Gland

Other Endocrine

Glands

Hypothalamus

The Pituitary GlandThe Pituitary Gland Anterior lobeAnterior lobe Makes and stores its own Makes and stores its own

hormoneshormones Releases these into the Releases these into the

blood to travel to target blood to travel to target glands/cells where glands/cells where secondary hormones will secondary hormones will be “mobilized” to alter be “mobilized” to alter cell activitiescell activities

Posterior lobePosterior lobe Stores hormones Stores hormones

produced by the produced by the HypothalamusHypothalamus

1.1. Oxytocin Oxytocin –regulates –regulates uterine muscle uterine muscle contraction and milk contraction and milk productionproduction

2.2. ADHADH – regulates water – regulates water concentration in bloodconcentration in blood

*a 3*a 3rdrd lobe exists lobe exists (not discussed here)(not discussed here)

Negative FeedbackNegative FeedbackThe rate of hormone secretion is

not constant determined by the needs of the animal at

the particular time

The nervous system may cause a gland to speed up or slow down due to a stimulus; But it is usually another hormone that is

the messenger for the body activities to increase or decrease

Negative feedback Negative feedback maintains maintains body processes body processes at the levels at the levels required by the bodyrequired by the body

These These requirements may vary requirements may vary according to the conditions of the according to the conditions of the body:body: Negative feedback acts to decrease a Negative feedback acts to decrease a

hormone in the blood if it increases hormone in the blood if it increases beyond the level required beyond the level required

If the concentration is too low, negative If the concentration is too low, negative feedback will act to increase the levelfeedback will act to increase the level

Negative Feedback for Response to Extreme Cold

(stimulus)

Negative feedback in Negative feedback in glucose homeostasisglucose homeostasis

A rise in blood A rise in blood glucoseglucose causes release of causes release of insulininsulin from from betabeta cells the cells the pancreaspancreas, , promoting promoting glucoseglucose uptake in cells and uptake in cells and storage as storage as glycogen in the in the liverliver. .

A fall in blood A fall in blood glucoseglucose stimulates stimulates alphaalpha cells in the cells in the pancreaspancreas to secrete to secrete glucagonglucagon, which causes the , which causes the liverliver to break to break down down glycogenglycogen and release and release glucoseglucose. .

Control of Blood Glucose Level 1. High concentration of glucose in blood after eating

2. Liver and skeletal muscle stores some as glycogen

3. Excess glucose is stored as fat

4. When glucose levels in the bloodstream decrease, glycogen is “mobilized” to increase levels as needed

5. Once glycogen reserves are depleted, fat stores are used to supply needs until the next meal can replenish glucose levels in the bloodstream.

6. If animal always has access to glucose and does not need to use stored energy, animal gains weight (mass)

Insulin and Uptake of Glucose

Glucose enters cells through the action of transport proteins in the cell membrane.

Insulin is believed to help glucose enter cells by bringing more transport proteins to the plasma membrane from inside the cell.

When insulin binds to a receptor, vesicles containing transport proteins fuse with the cells plasma membrane, providing more transporters to bring glucose into the cell

Insulin (produced in pancreas) causes excess glucose to enter liver and muscle cells for storage as glycogen.

When glucose levels fall, less insulin is produced causing glycogen to revert to glucose.

Diabetes MellitusDiabetes Mellitus Too much glucose Too much glucose in the blood due to in the blood due to

regulatory problems (insulin is not getting regulatory problems (insulin is not getting its job done for some reason) causes:its job done for some reason) causes: Excess glucose to enter urine Excess glucose to enter urine Water and salts move from body cells in to Water and salts move from body cells in to

the blood due to osmosis and diffusion the blood due to osmosis and diffusion Kidneys work harder to remove excess water Kidneys work harder to remove excess water

and salts and salts Frequent urination/thirst are common Frequent urination/thirst are common

symptomssymptoms Long-term complications – stroke, blindness, Long-term complications – stroke, blindness,

kidney failure, circulatory problemskidney failure, circulatory problems

Adrenal GlandsAdrenal GlandsLocated above kidneys; made of 2 glands in Located above kidneys; made of 2 glands in

one shellone shellAdrenal CortexAdrenal Cortex Outer casingOuter casing Regulated by Regulated by

hormoneshormones Produces 3 Produces 3

different different typestypes of of hormoneshormones GlucocorticoidsGlucocorticoids MineralocorticoidsMineralocorticoids Sex hormones (few)Sex hormones (few)

Adrenal MedullaAdrenal Medulla Inner glandInner gland Regulated by the Regulated by the

nervous systemnervous system Produces 2 Produces 2

hormones:hormones: Epinephrine Epinephrine

(adrenaline) (adrenaline) Norepinephrine Norepinephrine

(noradrenaline)(noradrenaline)

Aldosterone Cortisol AndrogensAldosterone Cortisol Androgens

Using cholesterol as the starting material, the cells of the adrenal cortex secrete a variety of steroid hormones. These fall into three classes:

glucocorticoids (e.g., cortisol) mineralocorticoids (e.g., aldosterone) androgens (e.g., testosterone)

Production of all three classes is triggered by the secretion of ACTH from the anterior lobe of the pituitary.

Adrenal Cortex

These hormones achieve their effects by:

1. Travelling through the body in the blood. Because they are so hydrophobic, they must be carried bound

to a serum globulin.

2. Entering from the blood into all cells

3. Binding to their receptor a protein present in the cytoplasm and/or nucleus of "target"

cells

4. The hormone-receptor complex binds to a second to form a dimer.

The dimer migrates into the nucleus (if it did not form there).

5. The hormone-receptor dimer binds to specific hormone response elements in DNA.

These are specific DNA sequences in the promoter of genes that will be turned on (sometimes off) by the interaction.

6. Other transcription factors are recruited to the promoter and gene transcription begins.

GlucocorticoidsThe most abundant glucocorticoid is cortisol.

(also called hydrocortisone)

1. Raise the level of blood sugar (glucose). 2. Also have a potent anti-inflammatory effect on

the body (They depress the immune response). For this reason glucocorticoids are widely

used in therapy: to reduce the inflammatory destruction of

rheumatoid arthritis and other autoimmune diseases

to prevent the rejection of transplanted organs to control asthma

Mineralocorticoids Affect mineral metabolism. The most important of them is the

steroid aldosterone. Acts on the kidney promoting the

reabsorption of sodium ions (Na+) into the blood Water follows the salt and this helps maintain

normal blood pressure. Acts on sweat glands to reduce the loss of

sodium in perspiration Acts on taste cells to increase the sensitivity

of the taste buds to sources of sodium

Androgens The adrenal cortex secretes precursors to

androgens such as testosterone.

In sexually-mature males, this source is so much lower than that of the testes that it is probably of little physiological significance. However, excessive production of adrenal androgens can cause premature puberty in young boys.

In females, the adrenal cortex is a major source of androgens. Their hypersecretion may produce a masculine pattern of body hair and cessation of menstruation.

Adrenal Medulla The adrenal medulla consists of masses of

neurons that are part of the sympathetic branch of the autonomic nervous system.

Instead of releasing their neurotransmitters at a synapse, these neurons release them into the blood.

Thus, although part of the nervous system, the adrenal medulla functions as an endocrine gland.

The adrenal medulla releases: adrenaline (also called epinephrine) and noradrenaline (also called norepinephrine)

Release of adrenaline and noradrenaline is triggered by

nervous stimulation in response to physical or mental stress. .

“Fight or Flight Response”

Fight or Flight ResponseFight or Flight Response

When activated, the fight or flight response causes a surge of adrenaline and other stress hormones to pump through our body.

This surge is the force responsible for mothers lifting cars off their trapped children and for firemen heroically running into blazing houses to save endangered victims.

Some of the effects are: increase in the rate and strength of the heartbeat

resulting in increased blood pressure blood shunted from the skin and digestive system to

the skeletal muscles, coronary arteries, liver, and brain

rise in blood sugar (increase energy) increased metabolic rate bronchi dilate (increase O2) pupils dilate (improved vision) hair stands on end ("gooseflesh" in humans) reduced pain sensation clotting time of the blood is reduced increased ACTH secretion from the anterior lobe of

the pituitary immune system mobilizes

All of these effects prepare the body to take immediate and vigorous action.

We become prepared— physically and psychologically —

for fight or flight.

When our fight or flight system is activated, we tend to perceive everything in our environment as a possible

threat to our survival.

By its very nature, the fight or flight system bypasses our rational mind—where our more well thought out beliefs

exist—and moves us into "attack" mode.

Water RegulationWater Regulation Antidiuretic hormone binds to receptors on cells in

the collecting ducts of the kidney and promotes reabsorption of water back into the circulation. In the absence of antidiuretic hormone, the collecting ducts are virtually impermeable to water, and it flows out as urine.

Antidiuretic hormone stimulates water reabsorption by stimulating insertion of "water channels" or aquaporins into the membranes of kidney tubules. These channels transport solute-free water through tubular cells and back into blood, leading to a decrease in plasma osmolarity and an increase osmolarity of urine.