The Endocrine System. Hormones Recall that neurons have the ability to communicate However, this is merely cell-to-cell What if a signal needed to be.

The Endocrine SystemThe Endocrine System

HormonesHormonesRecall that neurons have the

ability to communicateHowever, this is merely cell-to-cellWhat if a signal needed to be sent

to more than one cell?This is the role of the endocrine

system◦Releases chemicals into bloodstream

to rest of body

These chemicals are hormonesTwo major classes based on

chemical structure◦Can be water soluble or lipid soluble◦And can be exist as a polypeptide,

steroid, or amineReleased by glands, they have

one or two possible destinations: 1.Onto body surface or into body

cavities (exocrine)2.Into the bloodstream (endocrine)

Although the effects of hormones can be long lasting, they can also take a period of time to reach target cells

Local RegulatorsLocal RegulatorsSimilar to hormones, but have no

need to enter bloodstreamTarget cells are nearbyTwo types:

◦Paracrine – secreting cell is next to target

◦Autocrine – molecules act on secreting cell itself

Neurotransmitters and Neurotransmitters and NeurohormonesNeurohormonesLastly, we have already seen

secreting action through neurotransmitters

Neurohormones are a special class where the hormone is released from a neuron instead of a gland

Major GlandsMajor Glands

10 in total

HypothalamusHormones both used by, and

stored in, pituitary gland

Pituitary GlandOften called master glandControls other glands

Pineal GlandInvolved in circadian rhythm

ThyroidRegulates metabolism with

thyroxine

Parathyroid glands4 in totalParathyroid hormone controls

calcium levels

ThymusReleases hormones to stimulate

immune system

PancreasProduces insulin and glucagonHave opposite effects on blood

sugar levels

Adrenal glandsReleases epinephrineAlso site of cortisol release

Ovaries and TestesProduce sex-specific hormonesResponsible for secondary sex

characteristics.

Maintaining HomeostasisMaintaining HomeostasisThe body can respond to internal

conditions◦Insulin released when blood glucose

levels are highHowever, what is released can

have a effect on the pathway that created it

These are feedback mechanismsHave both positive and negative

Negative (more common) feedbacks involve inhibition of pathways by end products

Positive (less common) feedbacks stimulate pathways by end products

Metabolism ControlMetabolism ControlIncreased thyroxine levels spike

metabolismBut, how is it released?Lowered thyroxine stimulates

hypothalamus to release thyrotropin-releasing hormone (TRH)

Travels to pituitary, releases thyroid-stimulating hormone (TSH)

Now thyroid releases thyroxine

The negative feedback comes in play when high levels of thyroxine inhibits release of both TRH and TSH

Insulin and GlucagonInsulin and GlucagonSometimes hormones act in

opposing mechanismsBlood glucose level needs to be

kept at 90 mg/100mLWhen it drops, alpha cells of

pancreas release glucagonAbsorbed by liver, glycogen

broken down into glucoseLevels go up, alpha cells shut off

When blood glucose levels are high, beta cells release insulin

Body cells absorb glucose, liver synthesizes glycogen

Levels drop, beta cells shut offProblems in these systems lead

to diabetes and hypoglycemia

Homework!Homework!Determine the difference

between the pathways for water-soluble and lipid-soluble hormones

Find and describe an example of a positive feedback mechanism