Birgit Woelker, PhD

Chapter 20

Endocrine System

Endocrine Glands and Hormones The endocrine system consists of glands and tissues that secrete hormones

Hormones are chemicals that affect other glands or tissues, many times far away from the site of hormone production

Endocrine glands

•  Secrete hormones into the bloodstream

•  Hormones cause specific changes in target cells

The Endocrine System

hypothalamus

pituitary gland (hypophysis)

thymus gland

thyroid gland

parathyroid glands (posterior surface o f thyroid)

adrenal glands

pancreas

ovary (female)

testis (male)

pineal gland

•  Hormones are secreted by endocrine glands and neurosecretory cells

Secretory vesicles

Endocrine cell

Hormone molecules

Blood vessel

Target cell

Neurosecretory cell

Hormone molecules

Blood vessel

Target cell

•  All hormone-secreting cells make up the endocrine system

–  Which works with the nervous system in regulating body activities

•  A few chemicals ( e.g. adrenaline /epinephrine) serve both as hormones in the endocrine system and as chemical signals in the nervous system

Nerve cell

Nerve signals

Neurotransmitter molecules

Nerve cell

Hormones affect target cells by two main signaling mechanisms

•  Water-soluble hormones such as proteins and amines

–  Bind to plasma-membrane receptors on target cells

Water-soluble hormone (epinephrine)

Receptor protein

Target cell

Relay molecules

Plasma membrane

Signal transduction pathway

Glycogen Glucose

Cellular response (in this example, glycogen breakdown)

1

2

3

Lipid-soluble hormone (testosterone)

Target cell Receptor

protein

Hormone- receptor complex

Nucleus

DNA

mRNA Transcription

New protein

Cellular response: activation of a gene and synthesis of new protein

•  Steroid hormones, such as the sex hormones estrogen and testosterone

–  Diffuse through the plasma membrane of target cells and bind to intracellular receptors

1

2

3

4

Some endocrine glands and their functions Hypothalamus: Hormones released by the posterior pituitary gland and those that regulate the anterior pituitary gland

Pituitary gland posterior lobe : Oxytocin, Antidiuretic Hormone (ADH),

anterior lobe: Growth Hormone (GH), Thyroid stimulating Hormone (TSH), ACTH (Adrenocorticotropic Hormone)

Pineal gland: Melatonin

Thyroid gland: Thyroxine (T4) and Triiodothyronine (T3) Calcitonin

Adrenal gland: Epinephrine (Adrenaline), Norepinephrine

Pancreas: Insulin and Glucagon

Testes: Androgens (Testosterone)

Ovaries: Estrogens

Brain

Posterior pituitary

Anterior pituitary

Bone

Hypothalamus

The hypothalamus, closely tied to the pituitary, connects the nervous and endocrine systems

•  The hypothalamus exerts master control over the endocrine system

–  By using the pituitary gland to relay directives to other glands

•  The posterior pituitary

–  Secretes oxytocin and antidiuretic hormone (ADH)

Hypothalamus

Neurosecretory cell

Hormone

Posterior pituitary

Blood vessel

Oxytocin ADH

Uterine muscles Mammary glands

Kidney tubules

Anterior pituitary

•  The anterior pituitary

–  Secretes TSH, ACTH, FSH and LH, growth hormone, prolactin, and endorphins

Neurosecretory cell

Blood vessel

Releasing hormones from hypothalamus

Pituitary hormones

TSH ACTH FSH and LH

Growth hormone

(GH)

Prolactin (PRL)

Endorphins

Thyroid Adrenal cortex

Testes or ovaries

Entire body

Mammary glands

(in mammals)

Pain receptors

in the brain

Endocrine cells of the anterior pituitary

•  Releasing and inhibiting hormones from the hypothalamus

–  Control the secretion of several other hormones

–  TSH for instance controls the secretion of T4 and T3 from the Thyroid gland

•  Secretion of thyroxine by the thyroid gland

–  Is controlled by a negative-feedback mechanism

Hypothalamus Inhibition

Inhibition

TRH

Anterior pituitary

Thyroid

TSH

Thyroxine

HORMONES AND HOMEOSTASIS

The thyroid regulates development and metabolism •  Two hormones from the thyroid gland, T4 and T3

–  Regulate an animal’s development and metabolism

•  Thyroid imbalance can cause disease like Grave’s disease

Marty Feldman

•  Negative feedback

–  Maintains homeostatic levels of T4 and T3 in the blood

Hypothalamus

Anterior pituitary

Thyroid

No inhibition

No inhibition

No iodine Insufficient T4 and T3 produced

Thyroid grows to form goiter

TRH

TSH

Hormones from the thyroid and parathyroids maintain calcium homeostasis •  Blood calcium level is regulated by a tightly

balanced antagonism

–  Between calcitonin from the thyroid and parathyroid hormone from the parathyroid glands

Calcitonin

Stimulates Ca2+ deposition in bones

Reduces Ca2+ uptake in kidneys

Blood Ca2+ falls

Stimulus: Falling blood Ca2+ level (imbalance)

Parathyroid glands release parathyroid hormone (PTH)

Parathyroid gland

PTH

Increases Ca2+ uptake in intestines

Increases Ca2+ uptake in kidneys

Stimulates Ca2+ release from bones

Active vitamin D

Blood Ca2+ rises

Homeostasis: Normal blood calcium level (about 10 mg/100mL)

Stimulus: Rising blood Ca2+ level (imbalance)

Thyroid gland releases calcitonin

•  Calcium homeostasis

Pancreatic hormones regulate blood glucose levels

•  The pancreas secretes two hormones, insulin and glucagon that control blood glucose

•  Insulin

–  Signals cells to use and store glucose

•  Glucagon

–  Causes cells to release stored glucose into the blood

Body cells take up more glucose

Insulin

Blood glucose level declines to a set point; stimulus for insulin release diminishes

Stimulus: Declining blood glucose level (e.g., after skipping a meal)

Alpha cells of pancreas stimulated to release glucagon into the blood

Glucagon

Liver breaks down glycogen and releases glucose to the blood

Blood glucose level rises to set point; stimulus for glucagon release diminishes

Stimulus: Rising blood glucose level (e.g., after eating a carbohydrate-rich meal) Homeostasis: Normal blood glucose level

(about 90 mg/100mL)

Beta cells of pancreas stimulated to release insulin into the blood Liver takes

up glucose and stores it as glycogen

High blood glucose level

•  Glucose homeostasis

Diabetes is a common endocrine disorder

•  Diabetes mellitus

–  Results from a lack of insulin or a failure of cells to respond to it

•  Diabetes can be detected

–  By a test called a glucose tolerance test

Blo

od g

luco

se (m

g/10

0mL)

0

50

100

150

200

250

300

350

400

1 2

1 2 3 4 5

Diabetic

Normal

0

Hours after glucose ingestion Figure 26.8

The adrenal glands mobilize responses to stress

•  Hormones from the adrenal glands

–  Help maintain homeostasis when the body is stressed

•  Nerve signals from the hypothalamus

–  Stimulate the adrenal medulla to secrete epinephrine and norepinephrine, which quickly trigger the fight-or-flight responses

•  ACTH from the pituitary causes the adrenal cortex to secrete glucocorticoids and mineralocorticoids

–  Which boost blood pressure and blood glucose in response to long-term stress

Adrenal gland

Adrenal medulla

Adrenal cortex

Kidney Spinal cord (cross section)

Nerve signals

Nerve cell

Nerve cell

Adrenal medulla

Epinephrine and norepinephrine

Short-term stress response

1. Glycogen broken down to glucose; increased blood glucose 2. Increased blood pressure 3. Increased breathing rate 4. Increased metabolic rate 5. Change in blood-flow patterns, leading to increased alertness and decreased digestive and kidney activity

Stress

Hypothalamus

Releasing hormone Anterior pituitary

Blood vessel

ACTH

ACTH

Adrenal cortex

Mineralocorticoids Glucocorticoids

Long-term stress response

Mineralocorticoids

1. Retention of sodium ions and water by kidneys 2. Increased blood volume and blood pressure

Glucocorticoids

1. Proteins and fats broken down and converted to glucose, leading to increased blood glucose 2. Immune system may be suppressed

•  How the adrenal glands control our responses to stress

Glucocorticoids offer relief from pain, but not without serious risks

•  Glucocorticoids relieve inflammation and pain

–  But they can mask injury and suppress immunity

Bill Walton

The gonads secrete sex hormones

•  Estrogens, progestins, and androgens are steroid sex hormones

–  Produced by the gonads in response to signals from the hypothalamus and pituitary

•  Estrogens and progestins stimulate the development of female characteristics and maintain the female reproductive system

•  Androgens, such as testosterone

–  Trigger the development of male characteristics

•  Establishing links between androgens and human male aggression

–  Has been found to be problematic

Does testosterone play a role in aggressive behavior?