The Endocrine System
The Endocrine System and Hormone Function
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Along with the nervous system it controls and coordinates the activity of the body (helps to maintain homeostasis).
Instead of nerve impulses, it uses hormones (chemical messengers) to regulate body processes.
Hormones act on target cells or organs in four ways:
1. Change the plasma membrane permeability
2. Synthesize proteins (enzymes) in the cell
3. Activate or inactivate enzymes
4. Stimulate mitosis
Control of Hormone release
 Negative Feedback- hormone secretion is triggered by hormonal stimulus; rising hormones inhibit further
hormone release.
 Three types of stimuli that activate endocrine organs:
1. Hormonal- endocrine system is activated by other hormones
Ex:
2. Humoral- changing levels of blood ions causes release of hormones
Ex:
3. Neural- (rare) nerve fibers cause release of hormones
Ex:
Major Endocrine Organs
Pituitary Gland- size of a grape, inferior side of hypothalamus.
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Surrounded by Turks saddle of sphenoid bone. Anterior pituitary is made
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up of glandular tissue and the posterior pituitary is made up of nervous
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Anterior Pituitary
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1. Growth Hormone (GH) - stimulates growth in childhood and
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maintains a healthy body composition in adults.
-Homeostatic Imbalance- Hyposecretion can lead to pituitary
dwarfism. Hypersecretion can lead to gigantism. If
hypersecretion occurs after bone growth has stopped the result is
acromegaly, or abnormal bone growth (usually in facial bones).
2. Adrenocorticotropic hormone (ACTH)-stimulates the
production of cortisol (stress hormone) by the adrenal glands.
3. Thyroid stimulating hormone (TSH)/Thyrotropic hormone (TH)
- stimulates the thyroid to make thyroid hormones
4. Follicle Stimulating Hormone (FSH)-In men it promotes the
production of sperm. In women, FSH will stimulate development
of a follicle. Works with LH to allow normal function of ovaries
and testes.
5. Luteinizing hormone (LH)- Triggers ovulation of an egg and
then causes ruptured follicle to become a corpus luteum which then produces progesterone and estrogen
(promotes pregnancy)
6. Gonadotropic hormones-regulates hormonal activity in ovaries and testes
Posterior Pituitary
1. Oxytocin-stimulates contractions in women and causes milk let down in nursing mothers
2. Antidiuretic hormone (ADH)-helps maintain water balance by causing kidneys to reabsorb more water
from urine.
- Urine volume will decrease, blood volume will increase. In large amounts it increases blood pressure by
constricting arterioles. Because of this, it is often referred to as a vasopressin.
- Alcohol inhibits ADH.
-Diuretics increase urine production.
- Homeostatic imbalance: Hyposecretion of ADH will lead to diabetes insipidus (excessive urine)
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Thyroid Gland
-Located in front of trachea just below the Adam’s apple.
-T3 and T4 contain iodine
1. Thyroid hormone (T3 and T4) -body’s major metabolic hormone. Controls the rate at which glucose is
oxidized and converted to body heat and chemical energy.
- Without iodine (food we eat contains enough iodine) a goiter can develop. A goiter is an enlarged thyroid.
TSH calls for T3, thyroid expands trying to put T3 out but without iodine the body only makes the nonfunctional part, failing to inhibit (stop) TSH
-Homeostatic imbalance: Hyopthyroidism: Cretinism-deficiency of TH (dwarfism, occurs in infants)
Myxedema- physical and mental sluggishness in adults; results in puffiness of face, fatigue, poor muscle tone,
low body temperature
-Homeostatic Imbalance: Hyperthyroidism: Graves disease-rapid heartbeat, weight loss, nervous and
agitated behavior and inability to relax
2. Calcitonin-decreases blood calcium levels by causing calcium to be deposited into bones
Parathyroid Gland
-Tiny tissues found on posterior surface of thyroid gland. Usually there are two glands on each side of the lobe
(totaling four) but as many as eight have been found.
1. Parathyroid hormone (PTH)-Increases blood levels of calcium by stimulating osteoclasts to break bone
down and release the calcium into blood. It also stimulates kidneys and intestines to absorb more calcium.
-Homeostatic imbalance: If blood calcium levels fall too low, neurons become overactive and cause tatany,
or uncontrolled spasm.
Adrenal Gland
-Two bean shaped glands that curve over the top of the each kidney. The outer portion is the adrenal cortex; the
inner portion is the adrenal medulla.
Adrenal Cortex
1. Aldosterone- helps kidneys conserve sodium and excrete potassium, maintain blood pressure
2. Cortisone and Cortical- Promote normal cell metabolism and helps body resist long term stressors by
increasing blood glucose levels. (Keeps blood glucose levels stable)
3. Sex hormones-produces both sex hormones regardless of sex (in low levels). Mostly produces androgens
(male) but does produce trace amounts of estrogens (female)
-Homeostatic imbalance: Hyposecretion of all adrenal cortex hormones will result in Addison’s disease,
which causes a bronze color of skin, decreased sodium levels (and water retention as a result), low blood
pressure
-Homeostatic imbalance: Hypersecretion: Cushing’s syndrome, blood glucose will remain high, too much
sodium, puffy skin, and masculinizing effects in women
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Adrenal Medulla
1. Epinephrine (adrenaline) and Norepinephrine-sympathetic nervous system, increase blood pressure,
heart rate, blood glucose levels, dilate passageways of lungs
Pancreas
-Located behind the stomach, contains specialized cells called islets, or islets of Langerhans which secrete glucagon
and insulin.
1. Insulin-high glucose levels will cause beta cells of islets to release insulin. Insulin will decrease blood
glucose concentrations by causing cells to uptake glucose
-Homeostatic imbalance: Diabetes mellitus (means sweet) - blood glucose is too high, spilling into urine
because kidneys cannot absorb it fast enough. As glucose leaves, water follows causing dehydration. Type I
juvenile, Type II mature onset. Hypoglycemia-low blood sugar
2. Glucagon-low levels of blood glucose will cause alpha cells of islets to stimulate the liver to break down
glycogen to glucose, raising blood sugar levels
Pineal Gland
-Found in brain
1. Melatonin-maintains Circadian rhythm (sleep cycles). Levels of melatonin are higher at night
Thymus
-Located in upper thorax, posterior to sternum
1. Thymosin-important to immune function. Larger in children, shrinks in adults
Gonads
-Ovaries and Testes
Ovaries
1. Estrogen-stimulates development of secondary sex characteristics, stimulate uterine lining during
pregnancy
2. Progesterone-promote growth of uterine lining
Testes
1. Testosterone-development of adult male sex characteristics, necessary for sperm production
Placenta
-Temporarily formed in uterus, produces hormones to maintain pregnancy.
1. Human chorionic gonadotropin (hCG)-stimulates ovary to continue producing estrogen and progesterone
so that lining of uterus is not sloughed off. Over the counter pregnancy tests test for increased presence of
hCG.