Endocrine system overview

A gland is one or more cells that makes and secretes an aqueous fluid. 

They are classified by 

site of product release endocrine or exocrine 

Relative number of cells forming the gland unicellular or multicellular 

Exocrine glands are more numerous than endocrine glands  

Secrete their products onto body surfaces (skin) or into body cavities 

Examples includes mucous, sweat, oil, and salivary glands 

Endocrine glands are ductless glands that produce hormones 

steroidal (lipid) or amino acid like molecule release into blood. 

Secretions include amino acids, proteins, glycoproteins, and steroids 

The diverse cells of your body are constantly communicating with one another, coordinated through chemical and electrical signals, traveling by way of two major organ systems. 

In the nervous system, the signals are primarily electrical and are transmitted via neurons. 

The endocrine system is a group of glands and tissues that produce and secrete chemicals to initiate and maintain body functions. 

Hormones are chemical signals released by endocrine cells and carried to all locations in the body, usually via blood. 

The circulatory system carries hormones throughout the body, but a hormone can only bind to a target cell with receptors for that specific hormone. 

Each target cell receptor has a unique shape that functions to receive a specific signal, an example of the relationship between structure and functions. 

Because hormones reach all parts of the body, the endocrine system is important in controlling whole body activities. 

Hormones trigger changes in target cells differently, depending on whether the hormone is water soluble or lipid soluble. 

Water soluble hormones trigger responses without entering the cell. 

They cannot pass through the phospholipid bilayer of the plasma membrane, but they can still bring about cellular changes. 

In contrast, lipid soluble hormones (steroids) pass through the phospholipid bilayer and trigger responses by binding to receptors inside the target cell. 

Natural steroid hormones (testosterone and estrogen) as well as artificial anabolic steroids work in this manner. 

The human endocrine system consists of about a dozen major glands. Some of these, such as the thyroid and pituitary glands, are endocrine specialists, secreting hormones into the blood. 

Other organs, for example, the stomach and the pancreas of the digestive system, are primarily non endocrine but have some cells that secrete hormones. 

Teh hypothalamus, a gland in the brain, acts as the main control center of the endocrine system. 

It receives information from nerves about the internal condition of the body and about the external environment and then responds by sending out appropriate nervous or endocrine signals. 

These signals directly control the pituitary gland, a pea sized structure that hangs down from the hypothalamus. 

The endocrine system maintains homeostasis. For example, when the hypothalamus receives information that the body tissues are dehydrated, it makes antidiuretic hormone (ADH). 

ADH is stored and released by the pituitary gland and signals kidney cells to reabsorb more water from blood so that less urine is produced, and serious dehydration is prevented. 

As an individual rehydrates, the hypothalamus slows the release of ADH from the pituitary and the kidneys increase the output of urine. 

Ethanol has the opposite effect of ADH and signals kidney cells to not reabsorb more water which creates more urine output. It is a diuretic (lose water). 

The pituitary gland consists of two distinct parts. 

The posterior pituitary is an extension of the hypothalamus, stores hormones, and secretes hormones made in the hypothalamus. 

Two hormones are released by the posterior pituitary. 

ADH, as just noted, participates in osmoregulation. 

Oxytocin stimulates the contraction of the uterus during childbirth and causes the mammary glands to release milk. 

The anterior pituitary synthesizes and secretes its own hormones directly into the blood. 

The hypothalamus exerts control over the anterior pituitary by secreting two kinds of hormones into short blood vessels that connect the two glands. 

Releasing hormones stimulate the anterior pituitary to secrete specific hormones. 

Inhibiting hormones induces the anterior pituitary to stop secreting specific hormones. 

One of the several anterior pituitary secretions is human growth hormone (HGH), which promotes development and enlargement of all parts of the body. 

Too much HGH can produce gigantism. 

Too little HGH can lead to dwarfism. 

The thyroid gland is located in the front of your neck and produces thyroid hormone in response to thyroid stimulating hormone (TSH) released from the anterior pituitary. 

Thyroid hormone affects many functions of the body including metabolic rate (the amount of energy expended over a period of time), heart rate, blood pressure, tolerance to cold, and the development and maturation of bone and nerve cells in children. 

And additive to table salt, iodine(I) is necessary for normal thyroid function. Thyroid hormone cannot be synthesized without iodine. 

Without iodine, and thus a lack of thyroid hormone, goiter eventually develops. 

Goiter still affects many people in less industrialized nations where iodine is not available in the diet. 

Disease or environmental toxins are causes of hypothyroidism. (Underproduction of thyroid hormone) 

Hashimoto's disease is an autoimmune disorder in which the immune system attacks thyroid cells. 

Environmental toxins that adversely affect the endocrine system, termed endocrine disruptors, are commonly found in pesticides and plastic containers. 

For example, BPA (bisphenol A) is an endocrine disruptor found in some plastic water bottles. 

A growing body of evidence links particular endocrine disruptors to thyroid disease in animals. 

Hyperthyroidism (overproduction of thyroid hormone) is characterized by weight loss, high blood pressure, rapid heart rate, and increased sensitivity to heat.  

It is treated by using low doses of radioactivity to kill some thyroid cells. 

The pancreas secretes two hormones, insulin and glucagon, that play an important role in managing the body's energy supplies. 

When blood glucose levels are low, glucagon levels increase. This hormone causes stored glycogen to be catalyzed into glucose which increases blood sugar levels. 

When blood glucose levels are high (after a meal), insulin levels increase. This hormone causes glucose to be anabolized into glycogen within the liver which decreases blood glucose levels. 

Diabetes mellitus is a serious hormonal disease that affect about 1 in every 12 Americans. Millions of them do not even know they are ill. If current trends continue, 1 in every 3 Americans will be affected by 2050. 

In diabetes, body cells are unable to absorb glucose from the blood, either because  

there is not enough insulin in the blood (as in type 1, or insulin dependent, diabetes) or 

In diabetes, body cells are unable to absorb glucose form the blood, either because 

the target cells do not respond normally to the insulin in the blood (as in type 2, or non insulin dependent, diabetes). Of those Americans who have diabetes, more than 90% have type 2. 

A third type of diabetes, called gestational diabetes, can affect any women during pregnancy, even one who has never shown symptoms of diabetes before. 

There are treatments for diabetes but no cure. 

Diabetics measure their blood glucose routinely throughout the day. 

Type 1 patients require injections of insulin. 

Type 2 is almost always associated with being overweight and underactive, although whether obesity causes diabetes remains unknown. 

Low income appears to be a factor in developing type 2 diabetes, and low income diabetics often suffer greater complications from the disease. 

Scientists from the University of California, San Fransisco, hypothesized that diabetics with low income would have more health problems at the end of a calendar month (compared to the beginning) when they have little to no money for food. 

Women and men produce the same sex hormones. 

The gonads, or sex glands (ovaries in the female and testes in the male), produce three categories of sex hormones 

1)estrogen, 2) progestin, and 3) androgens. 

The proportions of these hormones are what differentiate the sexes. 

The sex hormones are steroid hormones (lipid soluble) that are secreted under the direction of the hypothalamus and anterior pituitary gland (LH FSH). 

Growth and development are affected by sex hormones, as are the reproductive cycle and sexual behavior. 

Estrogens and progestins are found in higher concentrations in women than in men. The remnant of what formed the egg is called corpus luteum. 

It produces estrogen and progesterone. 

Estrogens maintain the female reproductive system and promote the development of specific female features, such as breast and wider hips. 

Progestins, such as progesterone, are primarily involved in preparing the uterus to support a developing embryo. 

Both hormones are critical to the menstrual cycle. 

The menstrual cycle (uterine cycle) consists of 3 phases (menstrual, proliferative, and secretory). 

Days 1 to 5 the Menstrual phase, and this is when the uterus sheds all but the deepest part of the endometrium. 

Days 6 to 14 is the proliferative phase, and this is when the endometrium rebuilds itself. 

Days 15 to 28 is the secretory phase, and this is when the endometrium prepares for implantation of the embryo. 

If fertilization does not occur, progesterone levels fall, depriving the endometrium of hormonal support. 

Spiral arteries kink and go into spams and endometrial cells begin to die. 

Men have high levels of androgens (the main one being testosterone), which stimulate the development and maintenance of the male reproductive system. 

Endocrine disruptors affect reproductive function by altering the balance of sex hormones. 

In one study, scientists examined the effects of an endocrine disruptor on frogs. They raised male frogs in the absence or presence of a chemical found in weed killers called atrazine. 

When male frogs reached adulthood, they found that atrazine exposed males were far less successful at mating behaviors and had lower testosterone level. It feminized and chemically castrated some male. 

Furthermore, 10% underwent complete sex reversal. They became females capable of producing eggs! 

Other endocrine disruptors are included below in the image. The most common are the BPAs that are in plastic water bottles. 

Chemicals that affect the balance of sex hormones are not always detrimental.  

A drug used in breast cancer treatment decreases estrogen's effects, and synthetic hormones are useful for boosting function in individuals deficient in particular hormones. 

These chemicals have similarities in structure and function to the natural hormones they mimic. 

Approximately 70% of all breast cancers rely on estrogen to grow and divide. Inhibiting estrogen binding or its effects is a successful way to treat many breast cancers. Tamoxifen is a drug that inhibits estrogen. Once in the body, it breaks down into molecules that bind to estrogen receptors. 

As individuals age, sex hormone levels decline. 

One treatment for the side effects of menopause is synthetic estrogen. 

Anabolic steroids, artificial hormones that are similar in structure and function to testosterone, are sometimes used to treat the gradual decline of testosterone in men. 

Testosterone and other androgens of the human male plays a similar role in all vertebrates. 

Androgens are also widely associated with aggressive behavior. For example, in male elephant seals, androgens promote the development of bodies weighing more than 2 tons and aggressive behavior toward other males. These males fight by slamming their bodies against each other.