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 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 hormones Two major classes based on chemical structure ◦ Can be water soluble or lipid soluble ◦ And can be exist as a polypeptide, steroid, or amine Released 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 Regulators Similar to hormones, but have no need to enter bloodstream Target cells are nearby Two types: ◦ Paracrine – secreting cell is next to target ◦ Autocrine – molecules act on secreting cell itself

Neurotransmitters and Neurohormones Lastly, 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 Glands 10 in total

Hypothalamus Hormones both used by, and stored in, pituitary gland

Pituitary Gland Often called master gland Controls other glands Pineal Gland Involved in circadian rhythm Thyroid Regulates metabolism with thyroxine

Parathyroid glands 4 in total Parathyroid hormone controls calcium levels

Thymus Releases hormones to stimulate immune system Pancreas Produces insulin and glucagon Have opposite effects on blood sugar levels

Adrenal glands Releases epinephrine Also site of cortisol release

Ovaries and Testes Produce sex-specific hormones Responsible for secondary sex characteristics.

Maintaining Homeostasis The body can respond to internal conditions ◦ Insulin released when blood glucose levels are high However, what is released can have a effect on the pathway that created it These are feedback mechanisms Have 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 Control Increased thyroxine levels spike metabolism But, 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 Glucagon Sometimes hormones act in opposing mechanisms Blood glucose level needs to be kept at 90 mg/100mL When it drops, alpha cells of pancreas release glucagon Absorbed by liver, glycogen broken down into glucose Levels 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 off Problems in these systems lead to diabetes and hypoglycemia

Homework! Determine the difference between the pathways for water-soluble and lipid- soluble hormones Find and describe an example of a positive feedback mechanism