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The Endocrine System

The Endocrine System
Second controlling system of the body
Nervous system is the fast-control system
Uses chemical messengers (hormones) that are released into the blood

Hormones control several major processes

Reproduction
Growth and development
Mobilization of body defenses
Maintenance of much of homeostasis
Regulation of metabolism

Hormone Overview
Hormones are produced by specialized cells
Cells secrete hormones into extracellular fluids
Blood transfers hormones to target sites
These hormones regulate the activity of other cells

The Chemistry of Hormones
Amino acid–based, which includes
Proteins
Peptides
Amines
Steroids—made from cholesterol
Prostaglandins—made from highly active lipids

Mechanisms of Hormone Action
Hormones affect only certain tissues or organs (target cells or target organs)
Target cells must have specific protein receptors
Hormone-binding alters cellular activity

Effects Caused by Hormones
Changes in plasma membrane permeability or electrical state
Synthesis of proteins, such as enzymes
Activation or inactivation of enzymes
Stimulation of mitosis
Promotion of secretory activity

The Chemistry of Hormones
Two mechanisms in which hormones act
Direct gene activation
Second-messenger system

Direct Gene Activation (Steroid Hormone Action)
Diffuse through the plasma membrane of target cells
Enter the nucleus
Bind to a specific protein within the nucleus
Bind to specific sites on the cell’s DNA
Activate genes that result in synthesis of new proteins

Second-Messenger System (Nonsteroid Hormone Action)
Hormone binds to a membrane receptor
Hormone does not enter the cell
Sets off a series of reactions that activates an enzyme
Catalyzes a reaction that produces a second-messenger molecule (such as cAMP)
Oversees additional intracellular changes to promote a specific response

Control of Hormone Release
Hormone levels in the blood are mostly maintained by negative feedback
A stimulus or low hormone levels in the blood triggers the release of more hormone
Hormone release stops once an appropriate level in the blood is reached

Hormonal Stimuli of Endocrine Glands
Most common stimuli
Endocrine glands are activated by other hormones

Examples:
Anterior pituitary hormones travel to target glands, such as the thyroid gland, to prompt the release of a particular hormone, such as thyroid hormone

Changing blood levels of certain ions stimulate hormone release
Humoral indicates various body fluids such as blood and bile

Examples:

Parathyroid hormone and calcitonin are produced in response to changing levels of blood calcium levels
Insulin is produced in response to changing levels of blood glucose levels

Neural Stimuli of Endocrine Glands
Nerve impulses stimulate hormone release
Most are under the control of the sympathetic nervous system

Examples: 

The release of norepinephrine and epinephrine by the adrenal medulla

Major Endocrine Organs
Pituitary gland
Thyroid gland
Parathyroid glands
Adrenal glands
Pineal gland
Thymus gland
Pancreas
Gonads (Ovaries and Testes)
Hypothalamus

Pituitary Gland
Size of a pea
Hangs by a stalk from the hypothalamus in the brain
Protected by the sphenoid bone
Has two functional lobes
Anterior pituitary—glandular tissue
Posterior pituitary—nervous tissue
Often called the “master endocrine gland”

Hormones of the Anterior Pituitary
Six anterior pituitary hormones
Two affect non-endocrine targets
Growth hormone
Prolactin
Four stimulate other endocrine glands (tropic hormones)
Thyroid-stimulating hormone (thyrotropic hormone)
Adrenocorticotropic hormone
Two gonadotropic hormones

Characteristics of all anterior pituitary hormones
Proteins (or peptides)
Act through second-messenger systems
Regulated by hormonal stimuli, mostly negative feedback

Growth hormone
General metabolic hormone
Major effects are directed to growth of skeletal muscles and long bones
Plays a role in determining final body size
Causes amino acids to be built into proteins
Causes fats to be broken down for a source of energy

Growth hormone (GH) disorders
Pituitary dwarfism results from hyposecretion of GH during childhood
Gigantism results from hypersecretion of GH during childhood
Acromegaly results from hypersecretion of GH during adulthood

Prolactin (PRL)
Stimulates and maintains milk production following childbirth
Function in males is unknown

Adrenocorticotropic hormone (ACTH)

Regulates endocrine activity of the adrenal cortex

Thyroid-stimulating hormone (TSH)

Influences growth and activity of the thyroid gland

Gonadotropic hormones
Regulate hormonal activity of the gonads

Follicle-stimulating hormone (FSH)
Stimulates follicle development in ovaries
Stimulates sperm development in testes

Luteinizing hormone (LH)
Triggers ovulation of an egg in females
Stimulates testosterone production in males

Pituitary–Hypothalamus Relationship
Hormonal release is regulated by releasing and inhibiting hormones produced by the hypothalamus

Hypothalamus produces two hormones

These hormones are transported to neurosecretory cells of the posterior pituitary
Oxytocin
Antidiuretic hormone
The posterior pituitary is not strictly an endocrine gland, but does release hormones

Hormones of the Posterior Pituitary
Oxytocin
Stimulates contractions of the uterus during labor, sexual relations, and breastfeeding
Causes milk ejection in a nursing woman

Antidiuretic hormone (ADH) 
Inhibits urine production by promoting water reabsorption by the kidneys
In large amounts, causes vasoconstriction leading to increased blood pressure
Also known as vasopressin

Thyroid Gland
Found at the base of the throat
Consists of two lobes and a connecting isthmus
Produces two hormones
Thyroid hormone
Calcitonin

Thyroid hormone

Major metabolic hormone
Composed of two active iodine-containing hormones
Thyroxine (T4)—secreted by thyroid follicles
Triiodothyronine (T3)—conversion of T4 at target tissues

Thyroid hormone disorders

Goiters 
Thyroid gland enlarges due to lack of iodine
Salt is iodized to prevent goiters
Cretinism
Caused by hyposecretion of thyroxine
Results in dwarfism during childhood
Myxedema
Caused by hypothyroidism in adults
Results in physical and mental slugishness
Graves’ disease
Caused by hyperthyroidism
Results in increased metabolism, heat intolerance, rapid heartbeat, weight loss, and exophthalmos

Calcitonin
Decreases blood calcium levels by causing its deposition on bone
Antagonistic to parathyroid hormone
Produced by parafollicular cells
Parafollicular cells are found between the follicles

Parathyroid Glands
Tiny masses on the posterior of the thyroid
Secrete parathyroid hormone (PTH)
Stimulate osteoclasts to remove calcium from bone
Stimulate the kidneys and intestine to absorb more calcium
Raise calcium levels in the blood

Adrenal Glands

Sit on top of the kidneys
Two regions
Adrenal cortex—outer glandular region has three layers
Mineralocorticoids secreted by outermost layer
Glucocorticoids secreted by middle layer
Sex hormones secreted by innermost layer
Adrenal medulla—inner neural tissue region

Hormones of the Adrenal Cortex
Mineralocorticoids (mainly aldosterone)
Produced in outer adrenal cortex
Regulate mineral content in blood
Regulate water and electrolyte balance
Target organ is the kidney
Production stimulated by renin and aldosterone
Production inhibited by atrial natriuretic peptide (ANP)
Glucocorticoids (including cortisone and cortisol)
Produced in the middle layer of the adrenal cortex
Promote normal cell metabolism
Help resist long-term stressors
Released in response to increased blood levels of ACTH
Sex hormones
Produced in the inner layer of the adrenal cortex
Small amounts are made throughout life
Mostly androgens (male sex hormones) are made but some estrogens (female sex hormones) are also formed

Adrenal cortex disorders
Addison’s disease
Results from hyposecretion of all adrenal cortex hormones
Bronze skin tone, muscles are weak, burnout, susceptibility to infection
Hyperaldosteronism
May result from an ACTH-releasing tumor
Excess water and sodium are retained leading to high blood pressure and edema

Cushing’s syndrome

Results from a tumor in the middle cortical area of the adrenal cortex
“Moon face,” “buffalo hump” on the upper back, high blood pressure, hyperglycemia, weakening of bones, depression
Masculinization
Results from hypersecretion of sex hormones
Beard and male distribution of hair growth

Hormones of the Adrenal Medulla
Produces two similar hormones (catecholamines)
Epinephrine (adrenaline)
Norepinephrine (noradrenaline)
These hormones prepare the body to deal with short-term stress (“fight or flight”) by
Increasing heart rate, blood pressure, blood glucose levels
Dilating small passageways of lungs

Pancreatic Islets
The pancreas is a mixed gland and has both endocrine and exocrine functions
The pancreatic islets produce hormones
Insulin—allows glucose to cross plasma membranes into cells from beta cells
Glucagon—allows glucose to enter the blood from alpha cells
These hormones are antagonists that maintain blood sugar homeostasis

Pineal Gland
Found on the third ventricle of the brain
Secretes melatonin
Helps establish the body’s wake and sleep cycles
Believed to coordinate the hormones of fertility in humans

Thymus Gland
Located posterior to the sternum
Largest in infants and children
Produces thymosin
Matures some types of white blood cells
Important in developing the immune system

Gonads
Ovaries
Produce eggs
Produce two groups of steroid hormone
Estrogens
Progesterone
Testes
Produce sperm
Produce androgens, such as testosterone

Hormones of the Ovaries
Estrogens
Stimulate the development of secondary female characteristics 
Mature female reproductive organs
With progesterone, estrogens also
Promote breast development
Regulate menstrual cycle
Progesterone
Acts with estrogen to bring about the menstrual cycle
Helps in the implantation of an embryo in the uterus
Helps prepare breasts for lactation

Hormones of the Testes
Produce several androgens
Testosterone is the most important androgen
Responsible for adult male secondary sex characteristics
Promotes growth and maturation of male reproductive system
Required for sperm cell production

Other Hormone-Producing Tissues and Organs

Parts of the small intestine
Parts of the stomach
Kidneys
Heart
Many other areas have scattered endocrine cells

Endocrine Function of the Placenta
Produces hormones that maintain the pregnancy
Some hormones play a part in the delivery of the baby
Produces human chorionic gonadotropin (hCG) in addition to estrogen, progesterone, and other hormones

Developmental Aspects of the Endocrine System
Most endocrine organs operate smoothly until old age
Menopause is brought about by lack of efficiency of the ovaries
Problems associated with reduced estrogen are common
Growth hormone production declines with age
Many endocrine glands decrease output with age