Endocrine System

Chapter 30AP Biology 2012

Endocrine System

• Consists of cells that produce and release chemical signals called hormones.

Endocrine vs. Exocrine

Endocrine Signals• Secrete substances into the

extracellular fluid• Done by endocrine cells or

secretory organs called endocrine glands (testes, thyroid)

• Example; Signaling molecules

Exocrine Signals• Secrete substances into a

duct or internal body cavity that communicates with the external world

• Example: saliva, sweat

Hormones

• “long distance” endocrine signals that are released into the bloodstream and circulate through the entire body

• 3 chemical groups– Peptide and protein hormones– Steroid hormones– Amine hormones

Peptide and protein hormones

• Large• Water soluble – therefore easily transported

in the blood• Packaged in vesicles and released by

exocytosis• Most hormones are this type• Examples: insulin and growth hormone

Steroid Hormones

• Made from cholesterol (look similar too!)• Lipid soluble• Do not dissolve well in blood plasma so they

are bound to carrier proteins for transport in the blood

• Found inside target cells• Example: aldosteroids, cortisol, testosterone

Amine hormones

• Small molecules• Made from single amino acids• Could be polar or nonpolar depending on the

R group of the amino acid it was synthesized from

• Example: Tyrosine epinephrine and thyroxine

30.2 Review

That same hormone could cause very different responses in different types of cells

Fight or Flight Response

• Rapid physiological response to sudden danger or highly stressful situations

• Involves epinephrine (adrenaline) and norepinephrine (noradrenaline)

What’s really happening

• Epinephrine and norepinephrine bind to a set of adrenergic receptors (G proteins!)

• 2 categories–Alpha–Beta

Have different affinities for ephinephrine and norepinephrine!

Adrenergic Receptors

Alpha• It causes blood vessels in

the skin to constrict, causing cold hands and feet

• Shut off the secretion of digestive enzymes and decrease blood flow through the gut (digesting lunch waits)

Beta• In heart cells, it causes a faster

and stronger heartbeat (thus pumping more blood)

• Cause arterioles in skeletal muscles to enlarge so increased blood flow goes there enabling you to run

• Breakdown of glycogen in liver for quick energy!

• Breakdown of fats to yield fatty acids – another source of energy!!

Hormone receptors are regulated

• Downregulation – makes the cells less sensitive to the hormone, causing cells to ignore the excess hormone– Example in diabetes

• High levels of insulin downregulates the production of insulin receptors in body

Hormone receptors are regulated

• upregulation – occurs if hormone secretion is chronically low; cell makes more receptor for that hormone– Example in high blood pressure

• People who are regular medication (beta blockers)• Let’s say they go off the medication suddenly, the

effects of the receptors are amplified, resulting in heightened anxiety and blood pressure

• Explains why dosages for medications are carefully supervised.

30.3

• Nervous system and Endocrine System are linked.

• This is done by the Pituitary Gland!

Pituitary Gland

• Endocrine gland• Attached to a region of the brain called the

hypothalamus• Has 2 parts

– Anterior Pituitary– Posterior Pituitary

Anterior Pituitary• Releases 4 tropic hormones

» Hormones that direct and control the activities of other endocrine glands1. TSH – thyroid stimulating hormone2. LH – luteinizing hormone 3. FSH – follicle stimulating hormone4. ACTH – adrenocorticotropic hormone

• Produces peptide hormones– Prolactin (stimulates milk production)– Growth hormone (promotes growth)

• Overproduction causes GIGANTISM• Underproduction causes pituitary dwarfish

Posterior Pituitary

• Releases 2 neurohormones– ADH

• Increases amount of water conserved by the kidneys• When high, kidneys produce little volume of highly

concentrated urine• When low, kidneys produce large volume of dilute urine

– Oxytocin• Stimulates uterine contractions during birth• Stimulates flow of milk from mother’s breasts• Promotes pair bonding and trust; “cuddle hormone”

Negative Feedback Loop

• Corticotropin-releasing hormone (CRH) from hypothalamus stimulates release of adrenocorticotropic hormone (ACTH) from anterior pituitary, which in turn stimulates release of hormone cortisol from adrenal glands.

• Once cortisol is released, it circulates thru body, where it directly inhibits further release of both CRH and ACTH

• This maintains a constant level of cortisol

MAMMAL HORMONES30.4

Thyroid Glands

• Wrap around trachea (windpipe)

• Produce two hormones– Thyroxine– Calcitonin

Thyroxine

• Amine hormone made from amino acid tyrosine• Known as T4 – for 4 iodine atoms (T3 is a nearly

identical hormone)• Raises metabolic rate• Lipid soluble• Critical during growth and development• Insufficient amount during growth results in

physical and mental retardation - cretinism

Thyroxine continued…

• Levels are controlled by hypothalamus and anterior pituitary

• Hypothalamus releases thyrotropin-releasing hormone (TRH) which stimulates the anterior pituitary to release thyroid-stimulating hormone (TSH) which stimulates thyroid gland to produce and release thyroxine

• Regulated by a negative feedback loop (like cortisol)

Goiters

• Is an enlarged thyroid gland associated with– Hyperthyroidism

(too much thyroxine)

– Hypothyroidism (too little thyroxine)

Hyperthyroidism

• Caused by an antibody binding to and activating TSH receptors, causing uncontrolled production and release of thyroxine

• Symptoms:– High metabolic rate– Nervous and jumpy– Hot– Eyes bulge due to buildup of fat behind eye

Hypothyroidism

• Caused by a lack of circulating thyroxine to turn off TSH production

• Caused by lack of iodine in diet• Symptoms:

– Low metabolism– Intolerance of cold– Sluggish

• Greatly reduced by widespread use of iodized table salt

Homeostasis of Calcium is VITAL!!

• Why?– trigger

neurotransmitter release

– Muscle contraction

Calcium levels

Too little Ca• Muscle spasms• Seizures

Too much Ca• Nervous system to be

depressed• Muscles and heart to

weaken or stop

99% of Calcium is in bone1% in cells – so this tiny pool of Ca must be maintained!!

3 Mechanisms the body does to maintain Ca levels

1. Deposition or resorption of bone2. Excretion or retention of calcium by kidneys3. Absorption of Calcium from digestive tract

Calcitonin• Released by thyroid gland• Lowers the concentration of Ca in the blood by

regulating bone turnover– Bone is continuously being made (resorption of old bone

and synthesizing new bone)– Osteoclasts break down bone and release Ca into blood– Osteoblasts take up Ca and put it in new bone

• Decreases activity of osteoclasts• Does not play major role in adults, but does in

younger actively growing individuals

Calcitriol• Made from Vitamin D in the liver and kidneys

– Vitamin D is obtained from the sun (UV waves in sunlight convert cholesterol into vitamin D)

– Otherwise Vitamin D must be obtained from diet

• Hormone that stimulates the cells of the digestive tract to absorb calcium from ingested food

Parathyroid Hormone• Most important for homeostasis• Glands located to the rear of the thyroid• Synthesis is triggered when blood calcium fall below

set point• Increases concentration of calcium 3 ways

1. Stimulates bone turnover activities of osteoclasts and osteoblasts

2. Stimulates kidney to reabsorb Calcium3. Increases body absorption of calcium from food by

activitating the synthesis of calcitriol from vitamin D

Adrenal Glands

• Sit above the kidneys • 2 of them• Has a core, called the adrenal medulla, which

produces epinephrine and norepinephrine– This release is under control of the nervous

system– Both hormones increase heart rate and bp– Cause fat and liver cells to release metabolic fuel

Arenal Cortex•Surrounds the adrenal medulla•Produces 2 corticosteroid hormones

1. Mineralocorticoids2. Glucocorticoids

Mineralocorticoids•Increase salt and water balance of the extracellular fluid•Example: aldosterone – stimulates the kidneys to conserve sodium and excrete potassium

Glucocorticoids•Increase blood glucose concentrations and affect macromolecule metabolism•Example: cortisol – critical for mediating long term metabolic responses to stress

Gonads

• Male – testes; Female – Ovaries• Both produce hormones as well as the gametes

(sperm and egg)• Male hormones produced are called androgens,

Example: testosterone• Female hormones produced are estrogen and

progesterone• ** Both sexes produce both testosterone and

estrogen

Sexual Differentiation

• Y chromosome – embryonic glands begin producing testosterone in 7th week of development and a peptide hormone called MIS (Mullerian-inhibiting substance)

• Testosterone causes testes and ducts develop and MIS induces apoptosis of cells of female reproductive ducts, so they disappear

• So androgens are required for male development in humans

Puberty

• Time of sexual maturation and dramatic physical transformation around age 12-13

• Controlled by gonadotropins– Contain two hormones, LH (luteinizing hormone)

and FSH (follicle-stimulating hormone)– Under control of a hypothalamic neurohormone

called gonadotropin-releasing hormone GnRH

• Before puberty, the hypothalamus is producing very low levels of GnRH

Puberty Continued…

• Initiated by a reduction in the sensitivity of GnRH producing cells by negative feedback

• The reduced sensitivity overcomes the negative feedback and so GnRH release increases, stimulating production of gonadotropins and hence production of sex steroid hormones

Puberty Continued…

• In females, increased levels of LH stimulates the ovaries to increase estrogen production– This initiates development of enlarged breasts,

vagina and uterus, broadened hips, increased fat, pubic hair, and menstrual cycle

• In males, increased levels of LH stimulates the testes to produce testosterone– This initiates the voice to deepen, facial and body

hair, muscle mass increase, testes and penis grow larger