Endocrine System

Chapter 11

Endocrine System - Overview

• Endocrine System – the body’s second great controlling system which influences metabolic activities of cells by means of hormones

• Endocrine glands – pituitary, thyroid, parathyroid, adrenal, pineal, and thymus

• The pancreas and gonads produce both hormones and exocrine products

Endocrine System - Overview

• The hypothalamus has both neural functions and releases hormones

• Other tissues and organs that produce hormones – adipose cells, pockets of cells in the walls of the small intestine, stomach, kidneys, and heart

Endocrine vs. Exocrine

• Endocrine secretions – secrete hormones directly into body fluids (ductless)

• Exocrine secretions – reach some internal or external body surface through ducts (contain ducts)

Endocrine System• It is a communication system• Relies completely on hormones released into

the blood• Cells have receptors for the hormones to act

on• Controls rates of certain chemical reactions• Aids in transport of substances across

membranes• Helps regulate water & electrolyte balances• Plays vital roles in reproduction, development,

& growth

Hormone Action• Hormone – biochemical that a cell secretes

that affects the functions of another cell• Each particular hormones physiological action

is restricted to the hormone’s target cells -cells with specific receptors for the hormone molecule

• Target cells have receptors that other cells lack• Made up of proteins or glycoproteins w/

binding sites for a specific hormone

Hormone Action

• Even in extremely low concentrations, hormones can stimulate changes in target cells

• Either steroids or steroid-like substances• Synthesized from cholesterol OR…• They are amines, peptides, proteins, or

glycoproteins synthesized from amino acids

Control of Hormonal Secretions

• Negative feedback systems control many hormonal secretions

• The endocrine gland (or system controlling the release of the hormone) is sensitive to the concentration of the substance

• When the concentration reaches a certain level, the gland is inhibited and the secretion decreases

Control of Hormonal Secretions

• As the concentration of the hormone decreases, the inhibition of the gland ceases so the hormone can be secreted again

• Hormone secretion is controlled in 3 ways all of which employ negative feedback

Control Sources• Hypothalamus – Controls the anterior pituitary gland’s release of

hormones that stimulate other glands to release hormones

• Nervous system– Adrenal medulla secretes its hormones in

response to sympathetic nerve impulses• Changes in the internal environment– Ex) when blood glucose levels rise, pancreas

secretes insulin; when they fall, glucagon is secreted

Pituitary Gland

• Also known as the hypophysis, located at the base of the brain

• Consists of an anterior pituitary & a posterior pituitary

Anterior Pituitary Hormones

• Growth Hormone• Prolactin• Thyroid-stimulating Hormone• Adrenocorticotropic Hormone• Gonadotropins

Growth Hormone (GH)

• Stimulates cells to increase in size and more rapidly divide

• Enhances the movement of amino acids across cell membranes– Speeds the rate that cells utilize carbohydrates &

fats

Prolactin (PRL)

• Stimulates and sustains a woman’s milk production following birth

• In males – unknown function

Thyroid-stimulating Hormone (TSH)

• Stimulates the normal development and secretory activity of the thyroid

• Rising blood levels of thyroid hormones act on the pituitary and hypothalamus to block the release of TSH

Adrenocorticotropic Hormone (ACTH)

• Stimulates secretion of glucocorticoids by the adrenal gland

Gonadotropins• Follicle-stimulating hormone (FSH)• Luteinizing hormone (LH)• In males:– LH stimulates interstitial cells of the testes to

produce testosterone– FSH stimulates production of sperm

• In females:– FSH stimulates follicle development in ovaries– LH triggers ovulation– LH promotes synthesis & release of estrogens and

progesterone

Posterior Pituitary Hormones

• Oxytocin (OT)– Strong stimulant of uterine contraction– Triggers milk ejection– Synthetic & natural oxytocic drugs are used to

induce or hasten labor• Antidiuretic Hormone (ADH)– Helps to avoid dehydration or water overload

Thyroid Gland

• Largest endocrine gland, located in the anterior neck (around trachea & below neck)

• Produces and releases:– Thyroid hormone– Calcitonin

Thyroid Hormone (TH)

• Regulates tissue growth• Developing skeletal & nervous systems

Calcitonin

• Lowers blood calcium levels in children• Targets the skeleton where it:– Inhibits osteoclast activity and release of calcium

from the bone matrix– Stimulates calcium uptake and incorporation in

the bone matrix

Parathyroid Glands

• Tiny glands embedded in the posterior side of the thyroid, usually there are four (superior & inferior gland on each of the thyroid’s lobes)

• Secretes parathyroid hormone (PTH)– Increases Ca2+ in the blood– Helps break down bone to add Ca2+ into the blood

Adrenal Glands

• Paired, pyramid-shaped organs above the kidneys

• Structurally & functionally, they are two glands in one– Adrenal medulla– Adrenal cortex

Hormones of the Adrenal Medulla

• Epinephrine (adrenaline)• Norepinephrine (noradrenaline)• Secretions of these hormones causes:– Blood glucose levels to rise– Blood vessels to constrict– The heart to beat faster– Blood to be diverted to the brain, heart, and

skeletal muscle

Hormones of the Adrenal Cortex

• Mineralocorticoids – regulate electrolytes in extracellular fluids (salt concentration)– Aldosterone: maintains Na+ balance

• Glucocorticoids (Cortisol)– Helps the body resist stress by:• Keeping blood sugar levels relatively constant• Maintaining blood volume and preventing water shift

into tissue

Hormones of the Adrenal Cortex

• Gonadocorticoids (Sex Hormones)– Most secreted are androgens (male sex

hormones), and most important one is testosterone

– Androgens contribute to:• Onset of puberty• Sex drive in females• Can be converted into estrogens after menopause

Pancreas

• Has both exocrine and endocrine cells• Located behind the stomach• Produces an enzyme-rich juice used for

digestion (exocrine product)• Pancreatic islets produce hormones

(endocrine products)• Islets contain 2 major cell types:– Alpha (α) cells – produce glucagon– Beta (β) cells – produce insulin

Glucagon

• Major target is the liver where it promotes:– Breakdown of glycogen to glucose– Release of glucose to the blood– Increases blood-glucose levels

Insulin

• Lowers blood-glucose levels• Acts on the liver • Enhances the transport of glucose into body

cells

Pineal Gland

• Small gland hanging from the roof of the third ventricle of the brain

• Secretory product is melatonin• Melatonin is involved with:– Day/night cycles– Physiological processes that show rhythmic

variations (body temperature, sleep, appetite)

Thymus Gland

• Located deep to the sternum, above the heart• Major hormonal products are thymopoietins

& thymosins• These are essential in the development of the

T lymphocytes (T cells) of the immune system

Gonads: Female

• Paired ovaries produce estrogens and progesterone

• Responsible for:– Maturation of the reproductive organs– Appearance of secondary sexual characteristics

(Only 1 ovary produces hormones at a time, so if 1 is damaged, the other will take over)

Gonads: Male

• Testes that produce testosterone• Testosterone:– Initiates maturation of male reproductive organs– Necessary for sperm production– Appearance of secondary sexual characteristics

Stress

• The result of the body’s response to a change in the internal environment that threatens life

• Stressor – a factor that can stimulate such a response

Types of Stress

• Physical factors– Exposure to extreme heat or cold, decreased

oxygen concentration, infections, injuries, and loud sounds

• Psychological factors– Thoughts about real or imagined dangers,

personal losses, & unpleasant social interactions– Feelings of anger, fear, grief, anxiety, depression,

and guilt can also produce psychological stress

Responses to Stress• The hypothalamus activates mechanisms that

prepare the body for “fight or flight”– Raising blood concentrations of glucose, glycerol,

and fatty acids– Increasing heart rate, blood pressure, and

breathing rate– Dilating air passages– Shunting blood from the skin and digestive organs

to the skeletal muscles– Increasing epinephrine secretions from the

adrenal medulla