The
Endocrine System

INTRODUCTION TYPES OF GLANDS HORMONES AND TYPES ENDOCRINE GLANDS OTHERS


HORMONES AND TYPES



A hormone is a type of chemical signal. They are a means of communication between cells.
The endocrine system produces hormones that are instrumental in maintaining homeostasis and regulating
reproduction and development. A hormone is a chemical messenger produced by a cell that effects specific change in
the cellular activity of other cells (target cells). Unlike exocrine glands (which produce substances such as saliva,
milk, stomach acid and digestive enzymes), endocrine glands do not secrete substances into ducts (tubes). Instead,
endocrine glands secrete their hormones directly into the surrounding extra cellular space. The hormones then
diffuse into nearby capillaries and are transported throughout the body in the blood.

The endocrine and nervous systems often work toward the same goal. Both influence other cells with chemicals
(hormones and neurotransmitters). However, they attain their goals differently. Neurotransmitters act immediately
(within milliseconds) on adjacent muscle, gland, or other nervous cells, and their effect is short-lived. In contrast,
hormones take longer to produce their intended effect (seconds to days), may affect any cell, nearby or distant, and
produce effects that last as long as they remain in the blood, which could be up to several hours.

In the following table there are the major hormones, their target and their function once in the target cell.

Endocrine Gland Hormone Released Chemical Class Target Tissue/Organ Major Function of Hormone
Pineal Gland Melatonin Modified amino
acid		 Brain Controls circadian and circannual rhythms,
possibly involved in maturation of sexual organs
Hypothalamus Hypothalamic releasing and
inhibiting hormones		 Peptide Anterior pituitary Regulate anterior pituitary hormone
Posterior
Pituitary		 Antidiuretic (ADH) Peptide Kidneys Stimulates water reabsorption by kidneys
Oxytocin Peptide Uterus, mammary
glands		 Stimulates uterine muscle contractions and
release of milk by mammary glands
Anterior Pituitary Thyroid stimulating (TSH) Glycoprotein Thyroid Stimulates thyroid
Adrenocorticotropic (ACTH) Peptide Adrenal cortex Stimulates adrenal cortex
Gonadotropic (FSH, LH) Glycoprotein Gonads Egg and sperm production, sex hormone
production
Prolactin (PRL) Protein Mammary glands Milk Production
Growth (GH) Protein Soft tissue, bones Cell division, protein synthesis and bone growth
Thyroid Thyroxine (T4) and
Triiodothyronie (T3)		 Iodinated amino
acid/td> 		All tissue Increase metabolic rate, regulates growth and
development
Calcitonin Peptide Bones, kidneys and
intestine		 Lowers blood calcium level
Parathyroids Parathyroid (PTH) Peptide Bones, kidneys and
intestine		 Raises blood calcium level
Adrenal Cortex Glucocorticoids (cortisol) Steroid All tissue Raise blood glucose level, stimulates breakdown
of protein
Mineralocorticoids (aldosterone) Steroid Kidneys Reabsorb sodium and excrete potassium
Sex Hormones Steroid Gonads, skin, muscles
and bones		 Stimulates reproductive organs and brings on sex
characteristics
Adrenal
Medulla		 Epinephrine and norepinephrine Monified amino
acid		 Cardiac and other
muscles		 Released in emergency situations, raises blood
glucose level, "fight or flight" response
Pancreas Insulin Protein Livers, muscles
adipose tissue		 Lowers blood glucose levels, promotes formation
of glycogen
Glucagon Protein Liver, muscles,
adipose tissue		 Raises blood glucose levels
Testes Androgens (testosterone) Steroid Gonads, skin, muscles
and bones		 Stimulates male sex characteristics
Ovaries Estrogen and Progesterone Steroid Gonads, skin, muscles
and bones		 Stimulates female sex characteristics
Thymus Thymosins Peptide T lymphocytes Stimulates production and maturation of T
lymphocytes
Hormones can be chemically classified into four groups:
  1. Amino acid-derived: Hormones that are modified amino acids.
  2. Polypeptide and proteins: Hormones that are chains of amino acids of less than or more than about 100 amino
    acids, respectively. Some protein hormones are actually glycoproteins, containing glucose or other carbohydrate groups.
  3. Steroids: Hormones that are lipids synthesized from cholesterol. Steroids are characterized
    by four interlocking carbohydrate rings.
  4. Eicosanoids: Are lipids synthesized from the fatty acid chains of phospholipids
    found in plasma membrane.
Hormones circulating in the blood diffuse into the interstitial fluids surrounding the cell. Cells with specific receptors
for a hormone respond with an action that is appropriate for the cell. Because of the specificity of hormone and target
cell, the effects produced by a single hormone may vary among different kinds of target cells.

Hormones activate target cells by one of two methods, depending upon the chemical nature of the hormone.
  • Lipid-soluble hormones (steroid hormones and hormones of the thyroid gland) diffuse through the cell
    membranes of target cells. The lipid-soluble hormone then binds to a receptor protein that, in turn, activates a
    DNA segment that turns on specific genes. The proteins produced as result of the transcription of the genes and
    subsequent translation of mRNA act as enzymes that regulate specific physiological cell activity.
  • Water-soluble hormones (polypeptide, protein, and most amino acid hormones) bind to a receptor protein on the
    plasma membrane of the cell. The receptor protein, in turn, stimulates the production of one of the following
    second messengers:
Cyclic AMP (cAMP) is produced when the receptor protein activates another membrane-bound protein
called a G protein. The G protein activates adenylate cyclase, the enzyme that catalyzes the production of cAMP from ATP.
Cyclic AMP then triggers an enzyme that generates specific cellular changes.
Inositol triphosphate (IP3) is produced from membrane phospholipids. IP3, in turn, triggers the release of CA2+
from the endoplasmic reticulum, which then activates enzymes that generate cellular changes.

Endocrine glands release hormones in response to one or more of the following stimuli:
  1. Hormones from other endocrine glands.
  2. Chemical characteristics of the blood (other than hormones).
  3. Neural stimulation.


Most hormone production is managed by a negative feedback system. The nervous system and certain endocrine
tissues monitor various internal conditions of the body. If action is required to maintain homeostasis, hormones are
released, either directly by an endocrine gland or indirectly through the action of the hypothalamus of the brain,
which stimulates other endocrine glands to release hormones. The hormones activate target cells, which initiate
physiological changes that adjust the body conditions. When normal conditions have been recovered, the corrective
action - the production of hormones - is discontinued. Thus, in negative feedback, when the original (abnormal)
condition has been repaired, or negated, corrective actions decrease or discontinue. For example, the amount of
glucose in the blood controls the secretion of insulin and glucagons via negative feedback.

The production of some hormones is controlled by positive feedback. In such a system, hormones cause a condition
to intensify, rather than decrease. As the condition intensifies, hormone production increases. Such positive feedback
is uncommon, but does occur during childbirth, where hormone levels build with increasingly intense labor
contractions. Also in lactation, hormone levels increase in response to nursing, which causes an increase in milk
production. The hormone produced by the hypothalamus causing the milk let down and uterine
contraction is oxytocin.