Ashlee BlackKelsey HunterMelanie O’Bar

Composed of cells, tissues, and organs (collectively called endocrine glands) that secrete hormones. As the hormones diffuse into the bloodstream they act on target cells.

The glands of the endocrine system should not be confused with paracrine secretions, which affect neighboring cells, autocrine secretions, which affect only the secreting cell itself, or exocrine glands, which secrete outside the body through ducts or tubes that lead to the skin surface.

Only target cells will respond to a hormone. Endocrine glands help regulate metabolic

processes, control rates of certain chemical reactions, help regulate water and electrolyte balances, and aid in diffusion of substances across membranes.

Reproduction, development, and growth.

The major endocrine glands are the pituitary glands, thyroid gland, parathyroid glands, adrenal glands, pancreas, pineal gland, thymus gland, and reproductive glands (ovaries and testes).

Pituitary glands: located at the base of the brain and attached to the hypothalamus. It has an anterior and posterior lobe.

The posterior pituitary gland releases hormones when nerve impulses from the hypothalamus signal the axon ends of the neurosecretory cells of the posterior gland. “Releasing hormones” from the hypothalamus control the secretion from the anterior gland. The hormones travel through a capillary network associated with the hypothalamus. These capillaries merge to form the hypophyseal portal veins, which pass down the pituitary stalk and arise into a capillary network in the anterior gland. Via this pathway the hypothalamus releases substances that the blood carries to the anterior pituitary.

Hormones of the anterior pituitary: growth hormone (GH) stimulates cells to increase in size and divide more frequently, prolactin (PRL) stimulates and sustains the production of milk after childbirth, thyroid-stimulating hormone (TSH) controls thyroid gland secretions, adenocorticotropic hormone (ACTH) controls the manufacture and secretion of certain hormones from the cortex of the adrenal gland, follicle-stimulating hormone (FSH), luteinizing hormone (LH).

Hormones of the posterior pituitary (composed mostly of nerve fibers and neuroglial cells): antidiuretic hormone (ADH) decreases urine formation by reducing the volume of water the kidneys excrete, thus it regulates the water concentration of body fluids. Oxytocin (OT) contracts smooth muscles in the uterine wall and stimulates contractions in later stages of childbirth. It also controls the lactation process.

Thyroid gland: located just below the larynx on either side and in front of the trachea.

Hormones: thyroxine and triiodothyronine. Have similar actions but T3 is five times more potent. They help regulate the metabolism of carbs, lipids, and proteins and increase the rate at which cells release energy from carbs, the rate of protein synthesis, and stimulate breakdown and mobilization of lipids. Calcitonin regulates the concentrations of blood calcium and phosphate ions. Blood concentration of calcium ions regulates its release and as the concentration increases so does the secretion of calcitonin.

Parathyroid: located on the posterior surface of the thyroid gland, there are 4 glands- a superior and inferior gland for each of the thyroid’s lateral lobes.

Hormones: parathyroid hormone (PTH) increases blood calcium concentration and decreases blood phosphate ion concentration. It affects the bones, kidneys, and intestine.Calcitonin and PTH maintain stable blood

calcium concentration.

Adrenal glands: closely associated with the kidneys, a gland is atop each kidney and is embedded in the adipose tissue that encloses the kidney.

Hormones: epinephrine (adrenaline) and norepinephrine (noradrenaline) increase heart rate, force of cardiac muscle contraction, breathing rate, and blood glucose level, elevate blood pressure, and decrease digestive activity. Aldosterone helps regulate the concentration of

mineral electrolytes, causes the kidney to conserve sodium ions and excrete potassium ions. By conserving the sodium, the hormone stimulates water retention helping to maintain blood volume and blood pressure.

Cortisol (hydrocortisone) affects glucose metabolism, it also influences protein and lipid metabolism.

Pancreas: located near the small intestine and gallbladder.

Hormones: Glucagon stimulates the liver to break down glycogen and certain noncarbs such as amino acids, into glucose, raising blood sugar concentration. Insulin stimulates the liver to form glycogen from glucose and inhibits the conversion of noncarbs into glucose.

Pineal Gland: located between the brain’s hemispheres; secretes the hormone melatonin in response to light conditions outside the body. In the dark, nerve impulses from the eyes decrease and melatonin increases. (Circadian rhythms)

Thymus Gland: located posterior to the sternum and between the lungs; shrinks with age. It secretes hormones called thymosins which affect the production and differentiation of leukocytes (WBCs!).

Two main types: steroid and nonsteroid. Steroid hormones are from cholesterol

and made of rings of carbon and hydrogen with some oxygen atoms.They are insoluble in water but soluble in

lipids.B/c lipids are the bulk of cell membranes,

steroidal hormones diffuse into cells easily and may enter any cell in the body.

What happens when a steroid enters a target cell? The lipid-soluble hormone diffuses through the

membrane. It binds to a specific protein molecule - the

receptor for the hormone. The resulting unit binds within the nucleus to

certain areas of the cell’s DNA and activates transcription of specific genes into mRNA molecules.

The mRNAs leave the nucleus and enter the cytoplasm.

The mRNA molecules then associate with ribosomes to direct the synthesis of specific proteins.

The new proteins then carry out the effects associated with the specific hormone.

Examples of steroid hormones: GnRH- produced by the hypothalamus, “tells” the

pituitary to make FSH & LH FSH- follicle stimulation hormone; aids in egg and

sperm development. Made by the pituitary gland and is under the control of GnRH levels.

LH- luteinizing hormone, tells the gonads to either release estrogen or testosterone.

Progesterone- it is secreted by the corpus luteum and by the placenta and is responsible for preparing the body for pregnancy and, if pregnancy occurs, maintaining it until birth (thickens the uterine lining).

Estrogen- secreted by ovaries, develops eggs and is responsible for the secondary sexual characteristics in females. Targets female reproductive organs.

Testosterone- helps sperm develop; responsible for the secondary sexual characteristics in males. Targets male reproductive organs.

Thyroxine- thyroid gland, metabolic rate increased.

Nonsteroid Hormones: these are amines, peptides or proteins synthesized from amino acids. Glycoproteins are also included though they are formed from protein and carbs. They are insoluble in lipids.

How do they work? Usually bind receptors in target cell membranes. Each of the receptors is a protein with a binding and

activity site. A hormone molecule delivers its message to a target cell

by joining with the binding site of its receptor. This junction stimulates the activity site to interact with

other membrane proteins. This action may alter the function of transport

membranes or enzymes thereby changing concentrations of other cellular components.

The hormone that induces this entire process is called a “first messenger” while the biochemicals in the cell that start the changes in response to the hormone’s binding are called “second messengers.”

Examples of Nonsteroid Hormones: ADH- “tells” the kidney to reabsorb more water. Made by

hypothalamus Angiotensin- makes the kidney reabsorb more NaCl and water.

Lowers blood pressure Aldosterone- absorbs Na+ and water. Produced by adrenal

gland, it increases blood pressure (adrenaline) Glucagon- increases glucose levels in the blood, by hydrolyzing

glycogen in the liver. It is produced by the pancreas. Targets liver

Insulin- decreases glucose levels in the blood, by storing glucose as glycogen in the liver. Produced by the pancreas. Targets liver

Calcitonin- produced by the thyroid, it decreases Ca+ levels in the blood by storing Ca+ in bone and allowing the kidney to urinate more Ca+ out.

PTH- parathyroid hormone, produced by the parathyroid. This increases blood Ca+ levels by accessing stored Ca+ in bone and telling the kidney to reabsorb (keep) more Ca+ ions.

Adrenaline- produced by the adrenal glands. Acts as the “fight” or “flight” hormone. Targets liver, heart, etc.

Oxytocin- produced by the hypothalamus, released by the posterior pituitary gland. It causes uterine contractions during labor.