Chapter 45 Hormones and the Endocrine System

Bio 1270 Molloy University

Define hormone

• A hormone is a secreted chemical signal that travels through the bloodstream and acts on specific target cells to regulate physiology and behavior.

List the five types of intercellular communication by secreted molecules

• Endocrine → long distance (bloodstream)

• Paracrine → nearby cells

• Autocrine → same cell

• Synaptic → neurons → target cell via synapse

• Neuroendocrine → neuron → bloodstream

Define local regulators and list two examples

• are molecules that act over short distances, reaching target cells solely by diffusion

ex:

1. Paracine

2. Autocrine

• Both play a role in process such as blood pressure and regulation, nervous system function, and reproduction

Specify nitric oxide signaling

• Nitric oxide (NO) is a gas molecule that acts as a signaling molecule (a gas hormone).

• When oxygen levels in the blood drop (low O₂), cells in the blood vessel walls (endothelium) release NO.

• NO quickly diffuses into nearby smooth muscle cells in the blood vessel wall.

• Inside these smooth muscle cells, NO activates an enzyme called guanylyl cyclase.

• This increases levels of cGMP (cyclic guanosine monophosphate), an important signaling molecule.

• cGMP causes smooth muscle relaxation, which leads to:
  

  • Vasodilation (widening of blood vessels)

  • Increased blood flow to tissues that need more oxygen

Define synapses, neurotransmitters and neurohormones

• Synapses: Neurons communicate with target cells via specialized junction

• Neurotransmitters: secrete molecule at synapses that diffuse short distances and bind to receptors on target cells

• Neurohormones: secreted by specialized neurosecretory cells; diffuse from nerve endings to the bloodstream.

Specify pheromones

• Members of some animal species use it to communicate

• Chemicals that are released into the environment

List the three major chemical classes of hormones and specify the solubility of each

Class	Solubility	Examples
Peptide/Protein	Water-soluble	Insulin, glucagon
Amine	Usually water-soluble (some lipid-soluble)	Epinephrine, thyroid hormones(lipid soluble)
Steroid	Lipid-soluble	Cortisol, estrogen, testosterone

Specify the cellular hormone response pathways for water-soluble and lipid-soluble hormones

Water-Soluble Hormones

• Travel freely in blood

• Bind to receptors on cell membrane

• Use second messengers (like cAMP)

• Cause quick, short-term responses

Lipid-Soluble Hormones

• Travel bound to proteins

• Enter the cell and bind inside

• Act on DNA to control gene expression

• Cause slow, long-lasting responses

Define signal transduction

• the process by which a signal from a hormone is converted into a cellular response

Specify multiple responses to a single hormone

• one hormone can trigger different effects in different target cells

Ex: epinephrine increases heart rate, dilates airways, and breaks down glycogen in liver cells.

Define endocrine and exocrine glands

Endocrine glands:

-       Endocrine cells are often grouped in ductless organs, such as the thyroid and parathyroid glands and testes or ovaries. secrete hormones directly into blood (no ducts).

Exocrine glands:

• Such as salivary glands, have ducts to carry secreted substances onto body surfaces or into body cavities secrete substances through ducts to the outside or digestive tract.

• Endocrine → no ducts

• Exocrine → ducts (sweat, saliva, digestive enzymes)

List human endocrine glands

• Hypothalamus

• Pituitary gland

• Pineal gland

• Thyroid gland

• Parathyroid glands

• Adrenal glands

• Pancreas

• Ovaries (female)

• Testes (male)

• Thymus

Specify simple emdocrine pathway

Simple Endocrine Pathway

• Stimulus → endocrine cells detect a change

• Endocrine cells secrete a hormone

• Hormone travels through the bloodstream

• Hormone binds to specific receptors on target cells

• Signal transduction occurs inside the target cells

• Leads to a physiological response

Example: Secretin Pathway

• Stimulus: Acidic chyme enters the duodenum (low pH)

• Response: Endocrine cells in the duodenum release secretin

• Target: Pancreas

• Effect: Pancreatic cells secrete bicarbonate (HCO₃⁻) into ducts

• Result: Bicarbonate neutralizes acid → pH in the duodenum increases

Specify simple neuroendocrine pathway

Simple Neuroendocrine Pathway

• Stimulus is detected by a sensory neuron

• Signal is sent to a neurosecretory cell (in the brain)

• Neurosecretory cell releases a neurohormone into the bloodstream

• Hormone travels to target cells

• Leads to a physiological response

Example: Oxytocin & Milk Release

• Stimulus: Infant suckling

• Detection: Sensory neurons send signals to the hypothalamus

• Response:

  • Hypothalamus sends nerve impulses to the posterior pituitary

  • Posterior pituitary releases oxytocin

• Target: Mammary glands

• Effect: Milk is ejected (released)

Define negative and positive feedback

-       In a negative feedback loop, the response reduces the initial stimulus

• Negative feedback = most common in homeostasis

-       For example, the increase in pH in the intestine caused by secretin release shuts off further secretin release

-       Positive feedback reinforces a stimulus to produce an even greater response

-       For example, in mammals oxytocin causes the release of milk, causing greater suckling by offspring, which stimulates the release of more oxytocin

List the functions of hypothalamus, posterior pituitary and anterior pituitary

Hypothalamus

• Coordinates the endocrine system

• Receives input from the nervous system

• Sends signals to the pituitary gland

• Produces releasing and inhibiting hormones (control anterior pituitary)

• Produces hormones stored in the posterior pituitary (ADH & oxytocin)

Posterior Pituitary

• Does NOT make hormones

• Stores and releases hormones made by the hypothalamus

• Releases hormones into the bloodstream when stimulated by nerve signals

• Posterior pituitary = controlled by nerves

Anterior Pituitary

• Makes and secretes its own hormones

• Controlled by hormones from the hypothalamus

• Regulates many body processes:

  • Growth

  • Metabolism

  • Reproduction

  • Osmoregulation

• Secretes tropic hormones (control other endocrine glands)

• Anterior pituitary = controlled by hormones

List the two posterior pituitary hormones

• Antidiuretic Hormone (ADH)

  • Promotes water reabsorption in kidneys

  • Helps control blood pressure and hydration

• Oxytocin

  • Stimulates milk ejection from mammary glands

  • Triggers uterine contractions during childbirth

Extra: Anterior Pituitary Example

• Hypothalamus releases prolactin-releasing hormone (PRH)

• → Stimulates anterior pituitary

• → Releases prolactin (PRL)

• → Promotes milk production

Define tropic hormones

-       Set of hormones from the hypothalamus, anterior pituitary, and a target endocrine gland are often organized into a hormone cascade

-       Hormones that stimulate other endocrine glands to release their hormones.
Example: TSH → thyroid gland

Specify growth hormone

Growth Hormone (GH)

• Secreted by: Anterior pituitary gland

• Functions: Has both tropic and nontropic effects

How GH Works

• GH targets many tissues, especially the liver

• The liver releases insulin-like growth factors (IGFs)

• GH also increases protein synthesis and fat breakdown

• IGFs stimulate:

  • Bone growth

  • Cartilage growth

Effects of GH Imbalance

• Too much GH (childhood): → Gigantism (excessive growth)

• Too little GH: → Dwarfism (stunted growth)

Specify control of blood calcium

Homeostatic Regulation of Blood Calcium (Ca²⁺)

• Maintaining stable blood Ca²⁺ levels is essential for:

  • Muscle contraction

  • Nerve function

  • Bone health

Parathyroid Hormone (PTH)

• Released by: Parathyroid glands

• Trigger: Low blood Ca²⁺ levels

• Overall effect: Raises blood Ca²⁺

How PTH works:

1. Bone: Stimulates release of Ca²⁺ into the blood

2. Kidneys: Increases reabsorption of Ca²⁺ (less lost in urine)

3. Indirect effect: Promotes activation of vitamin D, which increases Ca²⁺ absorption from the intestines

4. PTH is more important than calcitonin in humans (often asked)

Calcitonin

• Released by: Thyroid gland

• Trigger: High blood Ca²⁺ levels

• Overall effect: Lowers blood Ca²⁺

 

How Calcitonin works:

1. Bone: Promotes Ca²⁺ deposition (storage in bone)

2. Kidneys: Increases Ca²⁺ excretion

List the two hormones produced in the adrenal medulla and their major functions

Adrenal Medulla Hormones

• Secreted by: Adrenal medulla

• Hormones:

  • Epinephrine (adrenaline)

  • Norepinephrine (noradrenaline)

• These are catecholamines

• Function: Control the “fight-or-flight” response

Fight-or-Flight Effects

Epinephrine and norepinephrine:

• Epinephrine = more dominant than norepinephrine in humans

• Increase glycogen breakdown in the liver

• Increase release of glucose and fatty acids into the blood

• Increase oxygen delivery to cells

• Redirect blood flow:

  • Toward: heart, brain, skeletal muscles

  • Away from: skin, digestive system, kidneys

Epinephrine’s Multiple Effects

Epinephrine acts on different cells in different ways:

1. Liver Cells

• Binds to a receptor → activates protein kinase A (PKA)

• → Stimulates glycogen breakdown → glucose release

2. Blood Vessels (Skeletal Muscle)

• Causes vasodilation (widening of blood vessels)

• → Increases blood flow to muscles

3. Blood Vessels (Intestines)

• Causes vasoconstriction (narrowing of blood vessels)

• → Reduces blood flow to the digestive system

Specify the role of the adrenal cortex

Adrenal Cortex

• Activated by: Stressful conditions such as:

  • Low blood sugar

  • Decreased blood volume or pressure

  • Shock

• Produces a group of steroid hormones called corticosteroids

• Controlled by ACTH (from anterior pituitary)

Types of Corticosteroids

1. Glucocorticoids (e.g., cortisol)

• Regulate glucose metabolism

• Help the body respond to stress

• Affect the immune system (often suppress inflammation)

2. Mineralocorticoids (e.g., aldosterone)

• Control salt (Na⁺) and water balance

• Increase:

  • Sodium reabsorption in kidneys

  • Water retention

• Help maintain blood volume and blood pressure

Specify Sex Hormones

Sex Hormones (Gonads)

• Produced by:

  • Testes (males)

  • Ovaries (females)

• Main hormones:

  • Androgens (e.g., testosterone)

  • Estrogens (e.g., estradiol)

  • Progesterone

• All three are present in both males and females, but in different amounts

Androgens (Testosterone)

• Produced mainly by the testes

• Functions:

  • Promote development of male reproductive organs

  • Responsible for male secondary sex characteristics

    • Deep voice

    • Facial hair

    • Increased muscle mass

Estrogens (Estradiol)

• Produced mainly by the ovaries

• Functions:

  • Maintain the female reproductive system

  • Promote female secondary sex characteristics

    • Breast development

    • Wider hips

Progesterone

• Primarily involved in:

  • Preparing and maintaining the uterus

  • Supporting pregnancy

Hormonal Control Pathway

Hypothalamus → Pituitary → Gonads

• Hypothalamus releases GnRH (gonadotropin-releasing hormone)

• → Stimulates anterior pituitary

• → Releases gonadotropins:

  • FSH (follicle-stimulating hormone)

  • LH (luteinizing hormone)

• → Act on gonads to produce sex hormones

Define endocrine disruptor

• A chemical that interferes with the normal function of hormones or hormone pathways.

• It can mimic, block, or alter hormone signals.

Example:

• Diethylstilbestrol (DES) is a synthetic estrogen.

• It disrupted estrogen signaling, leading to reproductive abnormalities and increased cancer risk in daughters of women who took it.

Specify pineal gland

A small gland in the brain that secretes melatonin

Function of melatonin:

• Regulates biological rhythms (sleep–wake cycle)

• Influences daily activity patterns and reproduction

Controlled by the Suprachiasmatic Nucleus (SCN) in the hypothalamus

• The SCN acts as the body’s biological clock

• Melatonin is secreted mostly at night

Specify melanocyte-stimulating hormone (MSH)

• Produced by the pituitary gland

• Function varies across species:

In amphibians, fish, and reptiles

• Controls skin color

• Works by regulating pigment in melanocytes

In mammals

• Plays roles in:

  • Appetite (hunger)

  • Energy balance (metabolism)

  • Minor role in pigmentation