Outline:
General Characteristics of the Endocrine System
Steroid Hormones, Nonsteroid Hormones, and Prostaglandins
Control of Hormonal Secretions
Negative Feedback Systems and Control Sources
Anterior Pituitary Hormones and Posterior Pituitary Hormones
Structure of the Gland and Thyroid Hormones
Structure of the Glands and Parathyroid Hormone
Structure of the Glands, Hormones of the Adrenal Medulla, and Hormones of the Adrenal Cortex
Structure of the Gland and Hormones of the Islets of Langerhans
Types of Stress and Responses to Stress
The endocrine system includes the organs of the body that secrete hormones directly into body fluids such as blood. Hormones help to regulate chemical reactions in cells. Therefore they control the functions of cells and the tissues and organs the cells make up. This chapter covers major endocrine organs.
Click here to see the locations of major endocrine glands in the body
Hormones can very simply be defined as chemicals secreted by a cell that affect the functions of other cells. Once secreted, most hormones enter the blood stream where they are carried to their target cells. The target cells of a hormone are the cells that contain the receptors for the hormone. A hormone cannot affect a cell unless the cell has receptors for the hormone.
Many hormones in the body are derived from steroids. Steroids are soluble in lipids and can therefore cross cell membranes very easily. Once a steroid hormone is inside a cell, it binds to its receptor. The hormone receptor complex then enter the nucleus to turn on a gene or to turn off a gene. This is ultimately how steroid hormones affect their target cells.
Nonsteroid hormones are those that are made of amino acids or proteins. Proteins cannot cross the cell membrane easily. Therefore nonsteroid hormones bind to receptors on the surface of the cell. The hormone-receptor complex in the membrane usually activates a protein called a G-protein. The G-protein then causes enzymes inside the cell to be turned on. Now different chemical reactions can take place inside the cell since the enzymes have been turned on. This is how nonsteroid hormones affect their target cells.
Click here to see how steroid hormones and nonsteroid hormones exert their actions on cells
Prostaglandins can be thought of as local hormones. They are derived from lipid molecules and typically do not travel in the blood stream to find their target cells as their target cells are close by. They have the same affects as other hormones and are produced by many body organs including the kidneys, stomach, uterus, heart, brain, etc.
Hormones are very potent in small concentrations so their release must be regulated. The release of most hormones if regulated by negative feedback control systems.
In a negative feedback control system and endocrine gland is turned off by its own secretions or it is turned off by a product of a process it controls.
For example, let's say gland A secretes hormone A. When the levels of hormone A rise to a certain level, it will turn off gland A. This way hormone A concentrations do not get too high. When the levels of hormone A fall, the gland is no longer inhibited and can start secreting hormone A again.
Or let's say that gland A secretes hormone A. Hormone A increases calcium in the blood stream. When calcium levels increase, this turns off gland A. Now hormone A is not secreted and calcium levels fall in the blood stream. When calcium levels fall, gland A is no longer inhibited and can start secreting hormone A again.
Endocrine glands or hormone secretion is controlled in one of three ways, all of which use negative feedback processes:
1. The hypothalamus turns the endocrine gland on or off with its hormones.
2. The nervous system turns the endocrine gland on or off with nerve impulses.
3. Chemicals surrounding an endocrine gland stimulate the gland to be on or off.
The pituitary gland is located at the base of the brain. It is divided into two glands - the anterior pituitary and the posterior pituitary.
The anterior pituitary secretes the following hormones:
1) Growth hormone (GH) - stimulates an increase in the size of the body (very important in childhood for growth); also stimulates repair of tissues; its secretion is controlled by the hypothalamus.
2) Prolactin (PRL) - stimulates milk production by mammary glands; its secretion is controlled by the hypothalamus
3) Thyroid-stimulating hormone (TSH) - stimulates the thyroid gland to release its hormones; its secretion is controlled by the hypothalamus.
4) Follicle-stimulating hormone (FSH) - in females, it stimulates production of estrogen by ovaries; in males, it stimulates sperm production; its secretion is controlled by the hypothalamus
5) Luteinizing hormone (LH) - in females, stimulates ovulation (release of egg from ovary); in females it also stimulates the production of estrogen; in males it stimulates the production of testosterone; its secretion is controlled by the hypothalamus.
The posterior pituitary secretes the following hormones:
1) Antidiuretic hormone (ADH) - stimulates the kidneys to conserve water; therefore it decreases urine output; helps to maintain blood pressure; its secretion is controlled by the hypothalamus.
2) Oxytocin (OT) - in the female, it causes contraction of uterine wall during childbirth; it also causes the ejection of milk from mammary glands during breast feeding; its secretion is controlled by the hypothalamus
Click here to see a table summarizing all pituitary hormones.
The thyroid gland consists of two lobes and sits below the larynx (voice box). The thyroid gland is covered by a capsule that also invades the gland to divide it up into follicles. The follicles store the hormones produced by the thyroid gland.
There are two hormones produced by the thyroid gland:
1) Thyroid hormone - Thyroid hormone is made of thyroxine and triiodothyronine; thyroid hormone increases energy production by cells, increases protein synthesis by cells, accelerates repair of tissues and stimulates the nervous system; its release is controlled by TSH from the anterior pituitary gland
2) Calcitonin - acts to lower blood calcium levels and blood phosphate levels; its secretion is controlled by the amount of calcium in the blood stream.
Most people have four parathyroid glands. They are small glands that are embedded into the posterior surface of the thyroid gland.
The only hormone secreted by the parathyroid glands is called parathyroid hormone (PTH). This hormone acts to raise blood calcium levels. Its secretion is controlled by blood calcium levels.
The adrenal glands sit on top of the kidneys. It is divided into two glands - 1) the adrenal cortex (outmost layer of it) and 2) the adrenal medulla ( the central portion of it).
The adrenal medulla secretes epinephrine and norepinephrine. They produce the same effects that the sympathetic nervous system produced. In other words, they increase heart rate, increase breathing rate, increase blood pressure, etc. The secretion of these hormones is controlled by the nervous system.
The adrenal cortex secretes the following hormones:
1) Aldosterone - stimulates the body to retain sodium which helps it to retain water; important for maintaining blood pressure; its secretion is controlled by concentration of ions (such as sodium ions) in body fluids.
2) Cortisol - decreases protein synthesis so it slows down the repair of tissues; decreases inflammation of tissues so it decreases pain; its secretion is controlled by the hypothalamus.
The pancreas is located behind the stomach. It is an endocrine gland as well as an exocrine gland. It is considered an exocrine gland because it secretes digestive enzymes into a duct that leads to the small intestine. It is considered an endocrine gland because it contains structures known as Islets of Langerhans that secrete hormones into the blood stream.
The islets of Langerhans secrete two hormones:
1) Insulin - promotes the uptake of glucose by cells; therefore it reduces the glucose concentration in the blood stream; promotes the transport of amino acids into cells, increases protein synthesis; its secretion is primarily controlled by glucose concentrations in the blood stream.
2) Glucagon - opposite effect of insulin; increases glucose concentrations in the blood stream; its secretion is controlled by glucose concentrations in the blood stream.
Click here to see Islets of Langerhans
The pineal gland is a small gland located between the cerebral hemispheres. It secretes a hormone called melatonin. Melatonin helps to regulate your circadian rhythms which is your biological clock. Your biological clock helps you decide when you should be awake or asleep. Melatonin is also thought to play a role in the onset of puberty.
The thymus gland lies between the lungs. It secretes a hormone called thymosin. Thymosin promotes the production of certain lymphocytes.
The following reproductive organs secrete hormones:
1) Ovaries - secrete estrogen and progesterone
2) Placenta - secretes estrogen and progesterone during pregnancy
3) Testes - secrete testosterone
The linings of the stomach and small intestines also secrete hormones. These will be covered in the digestive system chapter.
The heart secretes a hormone called atrial natriuretic peptide and kidneys secrete a hormone called erythropoietin. These hormones are covered in later chapters.
Any stimulus that produces stress is termed a stressor. Stressors include physical factors such as being exposed to extreme heat or cold, infections, injuries, heavy exercise, and loud sounds. Stressors can also include psychological factors such as personal loss, grief, anxiety about tests, depression and guilt. Even positive stimuli such as sexual arousal, joy, and happiness can induce stress.
The body's physiological response to stress consists of a group of reactions called the general stress syndrome. Reactions of this syndrome include an increase in heart rate, increase in blood glucose concentrations, increase of fatty acids in blood stream, increase of blood pressure, increase of epinephrine release by adrenal medulla, increase in breathing rate, and an increase of blood supply to skeletal muscles. Prolonged stress causes the release of cortisol. Cortisol slows down body repair because it prevents protein synthesis.
Click here to see a table summarizing the effects of stress