AnaPhy Lecture 13
Endocrine System
Endocrine Characteristic
➔ The endocrine system is composed of endocrine glands.
➔ Endocrine glands and cells secrete minute amounts of chemical
messengers called hormones into the bloodstream,
rather than into a duct.
Endocrine functions
➔ Regulation of metabolism
➔ Control of food intake and digestion
➔ Modulation of tissue development
➔ Regulation of ion levels
➔ Control of water balance
Classes of chemical messenger
Autocrine Paracrine Neurotransmitter and Hormones and neurohormones
released by cells and a have released by cells that affect neuromodulators secreted into blood and bind to
local effect on same cell other cell types in close secreted by nerve cells receptors on target tissues
type proximity
Example - nervous system Example - epinephrine and insulin
Example - eicosanoids Example - somatostatin function
Types of Hormones
Water-soluble hormones Lipid-soluble hormones
• include proteins, peptides, • include steroids and
amino acids eicosanoids
• most common type of hormone
Examples - growth hormone, Examples - Luteinizing
antidiuretic, prolactin hormone, androgens
AnaPhy Lecture 13
Endocrine System
Control of hormone secretion
Humoral Stimuli:
● Blood-borne chemicals that directly stimulate hormone release.
● "Humoral" refers to body fluids, including blood.
Neural Control:
● Hormone release can be controlled by the nervous system.
● Neurons release neurotransmitters that affect hormone-secreting
cells after an action potential.
Hormonal Control:
● Some hormones regulate the release of other hormones (hormonal control).
● A hormone may trigger the secretion of another hormone.
Stimuli for Hormone Release:
● Humoral stimuli: Blood-borne chemicals.
● Neural stimuli: Action potentials and neurotransmitters.
● Hormonal stimuli: Hormones influencing other hormone secretions.
● These types of stimuli can either stimulate or inhibit hormone release.
Inhibition of Hormone Release
Humoral Inhibition: Neural Inhibition: Hormonal Inhibition:
● Involves companion hormones ● Neurons can inhibit as well as ● Some hormones act as
that usually perform opposite stimulate hormone secretion. inhibitory hormones,
functions. ● If the neurotransmitter is reducing the release of other
● Example: Aldosterone raises blood inhibitory, the target hormones.
pressure, while atrial natriuretic endocrine gland will not secrete ● Example: Thyroid
peptide (ANP) lowers blood its hormone. hormones can inhibit the
pressure. release of their own
● These hormones work together to regulating hormone from
maintain homeostasis of blood the anterior pituitary.
pressure.
AnaPhy Lecture 13
Endocrine System
Regulation of Blood Hormone Levels
Negative feedback:
● The most common mechanism, where a
hormone's secretion is inhibited once blood levels
reach a certain point.
● Hormones inhibit their own secretion once
blood levels are within a homeostatic range.
● This helps maintain stable hormone levels over time.
Positive feedback:
● Less common, where the release of a hormone stimulates further secretion, often amplifying the response.
● Tropic hormones can stimulate further hormone release, amplifying the effect.
● Positive feedback is typically seen in processes that need a rapid and strong response (e.g., childbirth).
Hormones Receptors and mechanism of action
● Hormones only stimulate cells with the appropriate receptors.
● The binding site on a receptor molecule is called the receptor site, which has specificity for a
particular hormone.
● Some hormones (e.g., epinephrine) can bind to a family of structurally similar receptors.
● Specificity is determined by molecular shape and chemical characteristics.
● Lipid-soluble hormones bind to nuclear receptors because they can pass through the cell
membrane.
● Water-soluble hormones bind to membrane-bound receptors since they cannot pass through
the cell membrane due to their polarity.
Pituitary Gland
Small gland in brain
Controlled by hypothalamus
Divided into 2 regions: anterior and posterior
Secretes at least 6 hormones
AnaPhy Lecture 13
Endocrine System
Hypothlamic Control Pituitary
Anterior pituitary Posterior pituitary
● Synthesizes hormones controlled by ● Stores/releases hormones from
hypothalamus. hypothalamus.
● Releasing hormones: Stimulate ● ADH & Oxytocin: Two hormones
hormone secretion. released by posterior pituitary.
● Inhibiting hormones: Decrease
hormone secretion.
Anterior Pituitary Hormones
Growth Hormone (GH) Thyroid-Stimulating Luteinizing Hormone (LH)
Hormone (TSH)
Target: Most tissues Females
Functions: Stimulates growth Target: Thyroid gland Target: Ovaries,
of bones, muscles, organs Functions: Regulates thyroid Function: Ovulation,
Abnormalities: secretion progesterone production
Too much: Giantism Abnormalities: Males
Too little: Pituitary dwarfism Too much: Goiter Target: Testes,
Too little: Thyroid shrinkage Function: Sperm &
testosterone production
Follicle-Stimulating Prolactin: Adrenocorticotropic
Hormone (FSH) Hormone (ACTH)
Target: Mammary glands,
Females ovaries Target: Adrenal cortex
Target: Ovarian follicles, Functions: Milk production Functions: Stimulates
Function: Follicle maturation, corticosteroid production
estrogen secretion Melanocyte-Stimulating
Males Hormone (MSH):
Target: Seminiferous tubules
(testes), Target: Melanocytes (skin)
Function: Sperm production Functions: Stimulates melanin
production
AnaPhy Lecture 13
Endocrine System
Hormones of the Posterior Pituitary
Antidiuretic Hormone (ADH) Oxytocin:
Target: Kidneys Target: Uterus
Functions: Water conservation Functions: Increases uterine contractions during
Abnormalities: labor
Low ADH: Diabetes insipidus, large amounts of
dilute urine, dehydration, thirst
Thyroid Gland
Size: One of the largest glands
Needs: Iodine to function
Secretes: Thyroid hormone, calcitonin
Thyroid Hormones (Follicular Cells)
Target: Most tissues
Functions: Regulates metabolic rate, growth
Calcitonin (Parafollicular Cells)
Target: Bone, other tissues
Functions: Reduces blood calcium levels when high
Thyroid Hormone Disorder
Hypothyroidism Hyperthyroidism
Decreased metabolism Increased metabolism
Symptoms: Weight gain, reduced appetite, Symptoms: Weight loss, increased appetite,
fatigue, low temp., dry skin nervousness, high temp., warm skin
Conditions: Condition
Myxedema (adults), Graves' disease (goiter)
Cretinism (infants)
AnaPhy Lecture 13
Endocrine System
Parathyroid Gland
Parathyroid Hormone (PTH)
Target:Bones, kidneys
Functions:Regulates blood calcium levels
● Low Ca²⁺:Osteoclasts break down bone, less Ca²⁺ in urine
● High Ca²⁺: Osteoclasts don’t break down bone, more Ca²⁺ in urine
Adrenal gland action
two small glands located superior to each kidney inner part, called the adrenal medulla, and an outer
part, called the adrenal cortex.
Adrenal medulla hormones
Epinephrine/ norepinephrine
tissue: heart blood vessels liver fat cells
Function: released as part of fight or flight response
Adrenal cortex Hormones
Aldosterone
Type:Mineralocorticoid
Target: Kidneys
Functions: Regulates ion balance; retains Na+ and H2O, secretes K+; indirectly affects blood pressure
and volume.
Cortisol
Type:Glucocorticoid
Target:Most tissues
Functions: Breaks down fat and protein for energy; reduces inflammation and immune responses.
Androgens
Target: Most tissues
Functions:Males: Secondary sexual characteristics
Females:Sex drive
AnaPhy Lecture 13
Endocrine System
Pancreas Actions:
● Mixed Gland: Contains both exocrine and endocrine functions.
● Exocrine Function: Secretes digestive enzymes.
● Endocrine Function: Pancreatic islets (Islets of Langerhans) release hormones.
Pancreatic Hormones
1.Insulin (from Beta Cells):
● Target: Liver, skeletal muscle, adipose tissue.
● Function: Regulates blood glucose levels, stores excess glucose as glycogen.
● Abnormalities: Diabetes mellitus (too little insulin or faulty receptors.
1. Type I Diabetes: Insulin-dependent, requires daily injections.
2. Type II Diabetes: Insulin-independent, related to obesity, treatable with diet but can
progress to Type I.
2. Glucagon (from Alpha Cells)
● Target: Liver.
● Function: Increases blood glucose levels by breaking down glycogen into glucose between
meals.
3. Somatostatin (from Delta Cells):
● Function: Regulates the release of insulin and glucagon.
Testes Hormone
Testosterone:
● Target: Most tissues.
● Function: Aids sperm and reproductive organ development.
AnaPhy Lecture 13
Endocrine System
Ovarian Hormones:
Estrogen:
● Target: Most tissues.
● Function: Involved in uterine and mammary gland development, menstrual cycle regulation.
Progesterone:
● Target: Most tissues.
● Function: Works alongside estrogen in the menstrual cycle.
Thymus Gland Hormone:
Thymosin:
● Target: Immune system tissues.
● Function: Promotes immune system development and function.
Pineal Gland Hormone:
Melatonin:
● Target: Hypothalamus.
● Function: Controls circadian rhythms and the onset of puberty, influenced by light.
