Endocrine System Flashcards
Define “gland”
The term ‘gland’ refers to any organ that produces a secretion. These secretions are produced by specialised cells in glands, from various components in the blood.
Distinguish between endocrine glands and exocrine glands.
The glands in the body fall into two general categories: the exocrine glands and the endocrine glands. Exocrine glands are very diverse and include the salivary glands, mammary glands, sweat glands, pancreas, stomach, accessory glands, and several others. Their secretions are also varied - saliva, milk, sweat, digestive enzymes and fluids to accompany gametes - just from the glands mentioned above. All of these glands are called exocrine glands because they have tubes or ducts to carry their secretions from the gland to another part of the body. These ducts may be simple tubes or complex, tree-like groups of ducts. Because of these tubes, the exocrine glands are also known as the ducted glands. On the other hand, endocrine glands have no ducts. Their secretions are carried to various body tissues by the blood and lymph. Endocrine glands are sometimes called the ductless glands, and they all produce substances similar in nature, in that they are all hormones. Some organs in the body have both endocrine tissue and exocrine tissue. These include the pancreas, stomach and small intestine, all of which produce both hormones and enzymes.
Describe the purpose and regulation of hormone secretion.
Hormones are defined as chemical substances manufactured by endocrine glands. Their overall function is to regulate the activities of various body organs. A hormone, then, is a chemical messenger.
Describe stimuli for hormone secretion.
There are three main categories of stimulus for the secretion of hormones.
One is humoral, meaning the blood level of various non-hormone chemicals in the blood (e.g. the level of glucose in the blood can stimulate the release of the hormone insulin).
Other hormones can also stimulate the release of particular hormones. For example a hormone called thyroid stimulating hormone from the pituitary gland can promote the release of thyroxin from the thyroids gland.
Stimulus from the nervous system can also stimulate the release of hormones from endocrine glands. For example, as a response to stress, stimuli from the sympathetic nervous system can promote the release of the hormones epinephrine and norepinephrine from the adrenal cortex.
Give 6 functions of hormones
Although each hormone has its own specific effects, hormones generally have the following functions:
a) Some hormones stimulate exocrine glands to produce their secretions
b) Some stimulate other endocrine glands to action
c) Some affect the growth, development and personality of an individual
d) Some regulate body chemistry such as the metabolism of cells
e) Some regulate the contraction of muscle tissues and nervous stimulation
f) Some control reproductive processes
Compare and contrast the nervous and endocrine systems.
As messengers, hormones work to allow communication between various parts of the body. In this way, the endocrine is similar to the nervous system. However, compared to nervous actions, hormones are relatively slow to act because they travel through the bloodstream, and not along structures specialised for rapid transmission, as nervous signals do. A second difference between the endocrine system and the nervous system in regard to their roles as communicators is that the effects of hormones last longer than a nervous stimulation.
Sometimes the endocrine and nervous systems work together in carrying out a particular activity. For example, the hormone epinephrine works together with the autonomic nervous system when the body experiences an emergency situation.
The activities of the endocrine and nervous systems are directly linked through the hypothalamus in the brain. The hypothalamus is part of both systems and it functions as an endocrine gland which controls a large part of the endocrine system.
Be able to identify on a diagram of the human body the locations of each of the following glands (or parts of glands):
- Pineal gland
- Hypothalamus
- Pituitary gland (anterior & posterior)
- Pancreas (Islets of Langerhans)
- Adrenal glands (cortex & medulla)
- Thyroid gland
- Parathyroid glands
- Thymus
- Ovaries
- Testes
Describe the hypothalamus and pituitary glands and their interrelationship.
The hypothalamus is located near the base of the brain, extending below the thalamus. It is part of the nervous system, but, as mentioned before, it is also part of the endocrine system. Four of the hormones it produces act as releasing factors which stimulate the secretion of five separate hormones from the anterior pituitary gland. These four releasing hormones are named after the pituitary hormones whose secretions they stimulate.
Releasing hormones:
a) Adrenocorticotrophic hormone releasing hormone (ACTHRH (also called CRH))
b) Thyroid stimulating hormone releasing hormone (TRH)
c) Gonadotropin releasing hormone (GnRH)
Note: GNRH stimulates release of the gonadotropins (FSH and LH)
d) Growth hormone releasing hormone (GHRH)
The hypothalamus also produces an inhibiting factor, Prolactin Inhibiting Hormone (PIH),
In addition to the releasing and inhibiting hormones, the hypothalamus produces the hormones oxytocin and antidiuretic hormone, both of which are transported to the posterior pituitary gland. These two hormones are stored in and released from the posterior pituitary gland into the blood.
Describe the function and secretion of hormones released by the pituitary gland.
The anterior pituitary gland is attached to the posterior pituitary gland. It produces six hormones whose secretion are controlled by releasing or inhibiting hormones from the hypothalamus. The gland itself consists of several different cell types which vary in size, shape and function.
Adrenocorticotrophic hormone (ACTH)/Corticotropin releasing Hormone (CRH)
stimulates the adrenal cortex to release its hormones (the glucocorticoids in particular), and thus controls body metabolism.
Thyroid stimulating hormone (TSH) also effects body metabolism by stimulating the thyroid gland to release thyroxine.
Two hormones of the anterior pituitary gland regulate reproduction. One is follicle stimulating hormone (FSH), which stimulates the growth and activity of the ovarian follicles in females. In males, FSH promotes the development of sperm and stimulates the growth of the testes.
Luteinising hormone (LH) causes the growth of an ovarian structure called the corpus luteum, from an ovarian follicle which has ruptured and released an egg. In males, LH is called interstitial cell stimulating hormone, and it stimulates the secretion of testosterone from interstitial cells of the testes.
A fifth hormone produced by the anterior pituitary gland is growth hormone. This has the effect of promoting growth of all tissues in the body, by promoting an increase in cell size, the synthesis of proteins and an increase in body metabolism.
Finally, prolactin is a hormone produced by the anterior pituitary gland, which stimulates the breasts of a woman who has given birth to produce milk.
Describe the function and secretion of hormones released by the thyroid gland.
The thyroid gland consists of two lobes which are joined together by an isthmus across the bottom of the larynx. It produces the hormones thyroxine and calcitonin.
Thyroxine has the effect of raising body metabolism. It causes a breakdown of fats and carbohydrates, as well as a buildup of some proteins and a breakdown of others.
Calcitonin plays an important role in regulating blood calcium levels. It causes a reduction in blood calcium and phosphate levels, and inhibits the release of calcium and phosphate from bone tissue.
Describe the function and secretion of hormones released by the parathyroid glands.
The parathyroid glands have an opposite effect to calcitonin from the thyroid gland in maintaining blood calcium levels. That is, their hormone, called parathormone (or parathyroid hormone, PTH), raises blood calcium levels. Parathormone stimulates the release of calcium and phosphate from bone tissue. It also increases the absorption of calcium and secretion of phosphate in the kidneys.
Describe the function and secretion of hormones released by the adrenal gland
The adrenal glands are each divided into two regions: an outer cortex and an inner medulla, each with a separate endocrine function
The adrenal cortex itself is divided into three zones, each producing its own groups of hormones categorized as the mineralocorticoids, glucocorticoids and cortical sex hormones
Aldosterone accounts for 95 % of the mineralocorticoid production of the adrenal cortex. This hormone results in an increase in water reabsorption in the kidneys by promoting an increased sodium reabsorption in the tubules of the nephron.
Cortisol is a hormone which accounts for 95 % of the glucocorticoid production of the adrenal cortex. This hormone increases the body’s resistance to stress. It increases metabolism within muscles and maintains the excitability of nerves. It increases the amount of sugars in the body by promoting the conversion of fats and proteins into sugars.
The sex hormones produced by the adrenal cortex are called androgens and oestrogens - testosterone being the androgen or male sex hormone, and oestrogen the female sex hormone. The adrenal cortex serves as the source of sex hormones in an individual until the gonads mature at puberty. The sex hormones of the adrenal cortex play a role in the development of secondary sex characteristics in both males and females. In males, they further increase muscle development. It is interesting to note that the production of the cortical sex hormones is under the influence of adrenocorticotrophic hormone from the anterior pituitary gland, and not follicle stimulating hormone or luteinizing hormone.
The adrenal medulla produces two hormones: epinephrine and norepinephrine, which are also sometimes called adrenalin and noradrenalin. These two hormones are both involved in the response to fear, excitement and danger. To a certain extent, they have the same action by increasing blood pressure, and the rate and depth of breathing. Epinephrine, however, acts somewhat differently by increasing heart rate and blood sugar levels. On the other hand, norepinephrine reduces the blood flow to the gut and skin.
Describe the function and secretion of hormones released by the pancreas.
The pancreas is both an endocrine and an exocrine gland. The endocrine tissue within the pancreas is located in areas called the islets of
Langerhans. The islets of Langerhans produce insulin from beta cells, and glucagon from alpha cells. The exocrine functions of the pancreas occur in a different tissue of the organ composed of acinar cells.
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nsulin has the effect of lowering blood sugar levels. It causes glucose in the blood to be converted into the storage form of sugar in the body, glycogen, which occurs mainly in the liver but also in muscle tissue. Insulin also increases the transport of glucose from the blood into cells.
Glucagon has a reverse effect. It raises blood sugar levels by promoting the breakdown of glycogen into glucose.
Name the hormones produced by the following glands and describe their actions: ovaries, testes, stomach, duodenum, thymus and pineal gland.
The ovaries produce two hormones: oestrogen and progesterone.
Oestrogen is produced by an ovarian follicle. It stimulates the growth of both primary and secondary sex characteristics. Primary sex characteristics in the female include the growth of the uterus and vagina, and the secondary characteristics include the development of body hair, enlarged breasts and a wider pelvis.
Progesterone is produced by the corpus luteum. It stimulates the development of milk- secretory tissue in the breasts, prepares the uterine lining for the implantation of a fertilised oocyte, and helps to maintain pregnancy.
The testes in the male produce the hormone testosterone. This hormone stimulates the development of primary sex characteristics, such as the accessory glands and the penis, and secondary sex characteristics such as body hair and a deepening of the voice.
The principal function of the stomach is, of course, digestion. We are most familiar with the capacity of the stomach to store food, and its role in the mechanical and chemical digestion of food. However, there is also some endocrine tissue in the stomach. The secretion of hydrochloric acid in the stomach, and some enzymes, is under the control of a hormone called gastrin, which is produced by glandular tissue in the wall of the stomach. This hormone is produced by the stomach, travels through the bloodstream, and stimulates the exocrine tissue of the stomach.
The duodenum also has glandular tissue in its walls. One of the hormones it produces is called secretin. Secretin travels through the blood and stimulates the pancreas to produce pancreatic juice, which then enters the duodenum and aids in digestion.
The thymus gland was once thought to be a vestigial organ. However, it has been determined that it is a central gland of the lymphatic system, which is involved in the body’s immune system. As part of this activity, the gland produces a hormone called thymosin, which is involved in the maturation and development of the immune system. This gland is larger in infants and decreases in size through adulthood. As a result of the change in size of the gland, the amounts of thymosin produced similarly decrease throughout adulthood.
The pineal gland, situated in the brain, produces the hormone melatonin. This hormone acts on the hypothalamus inhibiting the release of luteinizing hormone, thus affecting the activity of the gonads. The pineal gland is an interesting structure, because it is responsive to light and may be involved in the seasonal behaviour changes by some animals in response to changes in day length.
