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The Endocrine System Produces Hormones
Endocrine system: collection of specialized cells, and tissues that secrete hormones
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Endocrine glands: ductless organs that secrete hormones into blood, interstitial fluid,
lymph
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Hormones
– Chemical messengers secreted by endocrine glands
– Circulate in the bloodstream
– Act on specific cells in the body (target cells) that have the appropriate hormone
receptor
Characteristics of the endocrine system:
– Hormones have access to every cell
– Each hormone acts only on specific cells (target cells)
– Only specific cells have receptors for specific hormones
– Endocrine control is slower than the nervous system
– Endocrine and nervous systems interact
Hormones Are Classified as Steroid and Non-steroid
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Steroid hormones
– Structurally related to cholesterol
– Lipid soluble
– Hydrophobic
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Non-steroid hormones
– Structurally related to proteins
– Lipid insoluble
– Hydrophilic
Steroid Hormones Enter Target Cells
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Steroid hormones
– Lipid soluble, chemically derived from cholesterol
– Enter target cells by diffusion across cell membrane
– Bind to intracellular receptor, forming a complex
– Once in nucleus, hormone–receptor complex attaches to DNA, activating specific
genes
– mRNA is produced and then translated
– Protein product carries out cellular response to hormone
– Slower acting than non-steroid hormones; take minutes to hours
Non-steroid Hormones Bind to Receptors on Target Cell Membranes
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Nonsteroid hormones
– Water soluble
– Nonsteroid hormones cannot penetrate the lipid bilayer of cell membranes
– Bind to receptors on target cell membranes
– Work through intermediate mechanisms to activate existing enzymes
– Hormone is considered a first messenger
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– May involve a “second messenger” within the cell, such as cyclic AMP (cAMP)
– Faster acting than steroid hormones; take seconds to minutes
Some Hormones Participate in Negative Feedback Loops
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Many hormones participate in internal homeostatic control mechanisms
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Negative feedback loops involving hormones include the following:
– Endocrine gland serves as the control center
– Hormone is the pathway between the control center and the effectors
– Target tissues or organs are the effectors
The Hypothalamus and the Pituitary Gland
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Hypothalamus
– Homeostatic control center of the brain
– Links nervous system and endocrine system
– Produces two hormones of its own
– Monitors and controls hormone secretions of the pituitary gland
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Pituitary gland
– “Master” gland
– Secretes eight different hormones that regulate other endocrine organs
– Two lobes: posterior and anterior
Posterior Pituitary Stores ADH and Oxytocin
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Posterior pituitary is connected to hypothalamus by neuroendocrine cells
– Hormones (ADH and oxytocin) made in cell bodies in hypothalamus are
transported down axons to axon endings in posterior pituitary for storage and
release
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Posterior pituitary hormones: nonsteroidal
– Antidiuretic hormone (ADH)
– Conserve water in kidneys
– Regulate water balance in body
– Oxytocin
– Causes uterine contractions during labor, and milk ejection through
neuroendocrine reflex
The Anterior Pituitary Produces Six Key Hormones
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Anterior pituitary
– Controlled by hypothalamus
– Releasing and inhibiting hormones from hypothalamus travel to pituitary
through pituitary portal system
– The release of each anterior pituitary hormone is controlled, at least
partially, by the hypothalamus
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ACTH (adrenocorticotropic hormone)
– Stimulates adrenal cortex to release glucocorticoids (cortisol)
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TSH (thyroid-stimulating hormone)
– Acts on thyroid gland, promoting release of thyroid hormones (T3 and T4)
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FSH and LH (gonadotropins)
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– Stimulate growth, development, and function of ovaries and testes
– Not produced until about age 10–13 (puberty)
– Increase in production initiates sexual maturation and development at puberty
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Prolactin
– Stimulates development of mammary glands and milk production
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Growth hormone
– Has widespread effects on body
– Major effects on bone, muscle
– Stimulates cells to enlarge and divide rapidly
– Most of its growth-promoting effects occur during childhood and adolescence
Pituitary Disorders: Hypersecretion and Hyposecretion
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Diabetes insipidus
– Hyposecretion of ADH results in inability to conserve water appropriately
– Causes excessive urination, dehydration, thirst
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Gigantism
– Hypersecretion of growth hormone in childhood
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Acromegaly
– Excessive growth hormone over a long period in adults
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Pituitary dwarfism
– Hyposecretion of growth hormone
– Treated by administration of GH throughout childhood
The Pancreas Secretes Glucagon, Insulin, and Somatostatin
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The pancreas has both exocrine and endocrine functions
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Pancreatic hormones are involved in regulating blood glucose levels
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Endocrine cells in islets of Langerhans within the pancreas secrete the following three
hormones:
– Alpha cells: secrete glucagon
– Beta cells: secrete insulin
– Delta cells: secrete somatostatin
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Glucagon
– Raises blood sugar
– Causes breakdown of glycogen to glucose in liver
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Insulin
– Lowers blood sugar
– Promotes uptake of sugar by cells in liver, muscle, and adipose tissue
– Promotes conversion of glucose into glycogen, proteins, fat
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Somatostatin
– Inhibits secretion of glucagon and insulin, regulates other hormones
The Adrenal Glands Comprise the Cortex and Medulla
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Adrenal cortex: outer layer of adrenal gland
– Glucocorticoids, such as cortisol
– Mineralocorticoids, such as aldosterone
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Adrenal medulla: inner layer of adrenal gland
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– Epinephrine (adrenaline)
– Norepinephrine (noradrenaline)
The Adrenal Cortex: Glucocorticoids and Mineralocorticoids
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Glucocorticoids (Cortisol is an example)
– Secretion mediated through hypothalamus-pituitary secretions
– Maintain blood glucose levels during prolonged fasting
– Suppress inflammatory responses
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Mineralocorticoids (Aldosterone is an example)
– Regulate sodium, potassium, water balance
– Act on kidneys, promoting sodium reabsorption and potassium excretion
The Adrenal Medulla:
Epinephrine and Norepinephrine
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Adrenal medulla
– Neuroendocrine organ
– Secretion stimulated by
– Sympathetic nervous system
– Hormones: nonsteroidal
– Epinephrine and norepinephrine
– Enhance function of sympathetic nervous system (fight-or-flight
response)
Thyroid and Parathyroid Glands:
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Thyroid gland is located just below larynx in neck
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Parathyroid glands embedded in back of thyroid
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Both thyroid and parathyroid are involved in calcium balance
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Thyroid is also involved in regulating metabolism
The Thyroid Gland: Thyroxine Speeds Cellular Metabolism
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Secretion: mediated through hypothalamus-pituitary secretions
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Steroidal hormones
– Thyroxine (T4) and Triiodothyronine (T3)
– Both regulate production of ATP from glucose and modify the metabolic
rate
– Calcitonin
– Lowers blood calcium levels
– Decreases rate of bone resorption by inhibiting osteoclasts
– Stimulates uptake of calcium by bones
Iodine Deficiency Can Cause Goiter
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Iodine is required for the production of active thyroid hormones
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Inadequate dietary iodine leads to underproduction of thyroid hormones
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The feedback response to inadequate thyroid hormone is for the hypothalamus and
pituitary to further stimulate the thyroid gland in a vicious cycle that causes hypertrophy
of the thyroid (goiter)
Calcitonin Promotes Bone Growth
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Calcitonin inhibits osteoclasts
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This decreases bone resorption
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Calcitonin also stimulates the uptake of calcium by bones
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Particularly important for bone growth and development in children
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Combined effects: lower blood calcium levels
Parathyroid Hormone (PTH) Controls Blood Calcium Levels
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Secretion
– Response to lowered blood calcium levels
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Action
– Removes calcium and phosphate from bone
– Increases absorption of calcium by the digestive tract
– Causes kidneys to retain calcium and excrete phosphate
– Combined effects increase blood calcium levels
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Major regulator of blood calcium concentration in adults
Testes Produce Testosterone
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Functions of testosterone and other androgens (other testosterone-like steroid
hormones):
– Before birth, responsible for development of external male genitalia
– At onset of puberty
– Regulates development and normal functioning of sperm, male
reproductive organs, and male sex drive
– Regulates development of male secondary sex characteristics
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Small amounts of androgens produced by adrenal glands in both sexes
Ovaries Produce Estrogen and Progesterone
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Hormones: steroidal
– Estrogen
– Initiates development of secondary sex characteristics
– Regulates menstrual cycle
– Progesterone
– Regulates menstrual cycle
Other Glands and Organs Also Secrete Hormones
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Thymus
– Located in the upper chest, over the heart
– Secretes thymosin and thymopoietin
– Assist maturation of T lymphocytes
– Most active during early development and childhood
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Pineal gland
– Located deep within the brain but receives input from optic nerve
– Secretes melatonin
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– May be important in synchronizing the body’s circadian cycle
Endocrine Functions of the Heart, the Digestive System, and the
Kidneys
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Heart
– Atrial natriuretic hormone (ANH)
– Helps regulate blood pressure by increasing rate of sodium and water
excretion in the urine
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Digestive system
– Gastrin, secretin, cholecystokinin
– Have effects on stomach, pancreas, gall bladder
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Kidney
– Erythropoietin
– Stimulates production of erythrocytes
– Renin
– Stimulates aldosterone secretion and constricts blood vessels
Other Chemical Messengers
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Histamine
– Secreted by mast cells in response to injury or allergy
– Initiates and enhances inflammation
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Prostaglandins
– Local control of blood flow
– Contribute to inflammatory response
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Nitric oxide
– Regulates local blood flow in many tissues
– Controls penile erection
– Regulates smooth muscle contraction in digestive tract
– Antibacterial chemical made by macrophages
– Neurotransmitter in the brain
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Growth factors
– Local acting, modulate development of specific tissues
Disorders of the Endocrine System
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Diabetes mellitus
– Disorder of blood sugar regulation
– Inability to get glucose into cells where it can be used, results in high
blood sugar levels
– Glucose and excess water appear in the urine
– Abnormal metabolism of carbohydrates, proteins, and lipids causes most of the
complications
– Symptoms: dehydration, thirst, fatigue, frequent infections, blurred vision,
tingling of hands/feet
– Two types: type 1 and type 2
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Type 1 diabetes
– Pancreas does not produce enough insulin
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– Also known as insulin-dependent diabetes
– Treated with insulin injections
– Results from autoimmune destruction of beta cells of pancreatic islets
– There may be a genetic component and/or environmental trigger
– Typical onset is during childhood or adolescence
– 5–10% of all cases of diabetes
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Type 2 diabetes
– Non-insulin-dependent
– Characterized by insulin resistance: cells fail to respond appropriately to insulin
– Most often seen in adults over 40
– Lifestyle factors (diet, exercise) play a role in onset
– Treatment: lifestyle changes, variety of medications
– 90–95% of all cases of diabetes
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Hypothyroidism: underactive thyroid gland
– Children: cretinism
– slowed body growth, altered brain development, delayed puberty
– Adults: myxedema
– edema, lethargy, weight gain, low BMR
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Hyperthyroidism: overactive thyroid gland
– Increased BMR, hyperactivity, nervousness, agitation, weight loss
– Graves’ disease: autoimmune form of hyperthyroidism
Adrenal gland disorders
– Addison’s disease
– Failure of adrenal cortex
– Hyposecretion of cortisol and aldosterone
– Lowers blood glucose and sodium
– Slow to develop, chronic fatigue, weakness, abdominal pain, weight loss,
bronzing of the skin
Adrenal gland disorders (continued)
– Cushing’s syndrome
– Excessive cortisol secretion
– Excessive production of glucose from glycogen
– Retention of too much salt and water
– Loss of muscle mass, change in fat distribution
– Similar symptoms seen with use of cortisol and cortisol-like drugs
Hypogonadism
: abnormally low testosterone levels
– Prior to birth: results in ambiguous, undeveloped, or female genitalia
– Prior to adolescence: results in delayed or incomplete male sexual maturation
– Adult males: may result in
– Low sperm count
– Erectile dysfunction
– Unusual fatigue
– Decreased sex drive
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– Depression
– Treatment: testosterone replacement therapy
