Endocrine System

Question 1

What is the endocrine system?

Answer 1

The endocrine system is a series of glands that produce and secrete hormones that the body uses for a wide range of functions.

These glands are often widely separated throughout the body with no physical connections, and yet they work together or independently to perform a great variety of functions in order to sustain homeostasis within the body.

Question 2

What is a hormone?

Answer 2

A hormone is a molecule that is released in one part of the body, but regulates the activity of cells in another part of the body.

Most hormones enter interstitial fluid and then the bloodstream. The circulating blood then delivers the hormones to cells throughout the body where they are needed.

Question 3

What is the difference between exocrine and endocrine glands?

Answer 3

Exocrine glands secrete their hormones into ducts that carry the secretions into body cavities and onto the surface of an organ.

Endocrine glands secrete their hormones into the interstitial fluid surrounding the secretory cells rather than into ducts. From the interstitial fluid, hormones diffuse into blood capillaries and blood carries them to target cells throughout the body

Question 4

How do hormones work?

Answer 4

Hormones, like neurotransmitters, influence their target cells by chemically binding to specific protein receptors. Only the target cells for a given hormone have receptors that bind and recognize that hormone.

Question 5

down regulation

Answer 5

If a hormone is present in excess, the number of target cell receptors may decrease in a process called down regulation.

Question 6

up-regulation

Answer 6

In contrast, when a hormone is deficient, the number of receptors may increase in a process called up-regulation which makes target cells more sensitive to a particular hormone.

Question 7

circulating hormones

Answer 7

Most endocrine hormones are circulating hormones, which means they pass from the secretory cells that make them and into interstitial fluid and then into the blood.

circulating hormones may linger in the blood and exert their effects for a few minutes or sometimes for a few hours.

In time, circulating hormones are inactivated by the liver and then excreted by the kidneys, or other metabolic waste pathways.

Question 8

local hormones

Answer 8

Other hormones known as local hormones, act locally on neighbouring cells or on the same cells that secrete them without entering the blood stream.

Local hormones are usually inactivated quickly when the need arrives.

Question 9

what are the two broad classes of hormones?

Answer 9

• soluble in lipids
• soluble in water.

Question 10

lipid-soluble hormones

Answer 10

The lipid-soluble hormones include steroid hormones, thyroid hormones, nitric oxide, and eicosanoid hormones.

most lipid-soluble hormone molecules are bound to transport proteins.

Question 11

water-soluble hormones

Answer 11

The water-soluble hormones include amine hormones and protein and peptide hormones.

Most water-soluble hormone molecules circulate in the water blood plasma and are termed “free form”

Question 12

what are the 3 general functions of transport proteins ?

Answer 12

• They make lipid-soluble hormones temporarily water-soluble, increasing their solubility in the blood.
• They decrease or slow the passage of small hormone molecules through the filtering mechanism in the kidneys, thus slowing the rate of hormone loss in urine.
• They provide a ready reserve of hormone, already present in the blood stream, solubility in the blood stream.

Question 13

response to a hormone can be

Answer 13

• the synthesis of new molecules (insulin)
• changing the permeability of the plasma membrane
• stimulating transport of a substance into or out of the target cells
• altering the rate of specific metabolic reactions
• causing contraction of smooth muscle or cardiac muscle.

Question 14

where are receptors for most lipid-soluble hormones?

Answer 14

located inside target cells.

Question 15

where are receptors for most water-soluble hormones?

Answer 15

usually are part of the plasma membrane of target cells.

Question 16

how are most hormones released?

Answer 16

in short bursts, with little or no secretion between bursts.

Question 17

Hormone secretion is regulated by:

Answer 17

• Signals from the nervous system
• Chemical changes in the blood
• Other hormones

Question 18

how do hormonal regulatory systems work ?

Answer 18

Most hormonal regulatory systems work via negative feedback, but a few operate via positive feedback.

Question 19

List the glands of the endocrine

Answer 19

• pituitary
• thyroid
• parathyroid
• pineal glands

and several organs
- hypothalamus
- thymus
- pancreas
- adrenal glands
- ovaries (in females)
- testes (in males)
- the placenta in pregnant females

Question 20

what do the Hypothalamus and Pituitary Gland do?

Answer 20

The pituitary gland and the hypothalamus act as a coupled unit, regulating the activity of most of the other endocrine glands.

Question 21

where is the pituitary gland?

Answer 21

The pituitary gland lies within the brain in the hypophyseal fossa or the sphenoid bone below the hypothalamus and is attached by a stalk.

roughly the size of a pea, weighs 500mg

Question 22

what does the pituitary gland consist of?

Answer 22

the pituitary gland consists of three distinct parts that come from different cell types

• anterior pituitary, which is a grouping of glandular epithelium cells from the pharynx
• posterior pituitary, which is a grouping of nervous tissue from the brain.
• There is a network of nerve fibers between the hypothalamus and the posterior pituitary. Between these two lobes is a thin strip of tissue called the intermediate lobe and its function is not currently understood by science.

Question 23

how does the hypothalamus influence the release of hormones in the anterior pituitary?

Answer 23

In the anterior pituitary gland, blood supply is indirectly supplied with arterial blood that has already passed through a capillary bed in the hypothalamus.

This network of blood vessels forms part of the pituitary portal system which transports blood from the hypothalamus to the anterior pituitary where it enters thin-walled sinusoids that are in close contact with the secretory cells.

As well as providing oxygen and nutrients, this blood transports releasing and inhibiting hormones secreted by the hypothalamus.

These hormones influence secretion and release of other hormones formed in the anterior pituitary.

Question 24

how does the hypothalamus influence the release of hormones in the posterior pituitary?

Answer 24

The posterior pituitary is formed from nervous tissue and consists of nerve cells surrounded by supporting cells called pituicytes.

These neurons have their cell bodies in the supraoptic and paraventricular nuclei of the hypothalamus and their axons form a bundle known as the hypothalamohypophyseal tract.

Posterior pituitary hormones are synthesized in the nerve cell bodies, transported along the axons and stored in vesicles within the axon terminals of the posterior pituitary.

Question 25

what do the hormones that are secreted by the anterior lobe do?

Answer 25

Some of the hormones secreted by the anterior lobe stimulate or inhibit secretion
of other endocrine glands while others have a direct effect on specific tissues

The release of an anterior pituitary hormone follows stimulation of the gland by a specific releasing hormone produced by the hypothalamus and carried to the gland through the pituitary portal system of blood vessels.

The whole system is controlled by a negative feedback mechanism. That is, when there is a low level of hormone in the blood supplying the hypothalamus it produces the appropriate releasing hormone that stimulates release of a trophic hormone by the anterior pituitary.

This in turn stimulates the target gland to produce and release its hormone. As a
result, the blood level of that hormone rises and inhibits the secretion of releasing
factor by the hypothalamus.

Question 26

what hormones are released by the anterior pituitary gland?

Answer 26

• growth hormone (GH)
• thyroid stimulating hormone (TSH)
• adrenocorticotrophic hormone (ACTH, corticotrophin)
• prolactin
• gonadotrophins

Question 27

Growth Hormone (GH)

Answer 27

• the most abundant hormone synthesized by the anterior pituitary
• stimulates growth and division of most body cells, but especially those
  in the bones and skeletal muscles.
• Body growth in response to the secretion of GH is most evident during childhood and adolescence.
• regulates aspects of metabolism in many organs such as the liver, intestines, and pancreas, and also stimulates protein synthesis, especially tissue growth and repair, and lastly, promotes the breakdown of fats and increases blood glucose levels.
• Its release is stimulated by growth hormones releasing hormone (GHRH) and suppressed by growth hormone release inhibiting hormone (GHRIH), also known as somatostatin, both of which are secreted and controlled by the hypothalamus.

Question 28

• Thyroid Stimulating Hormone (TSH)

Answer 28

• synthesized by the anterior pituitary and its release is stimulated by thyrotrophin releasing hormone (TRH) from the hypothalamus. It stimulates growth and activity from the thyroid gland, which secretes the hormones thyroxine (T4), and tri-iodothyronine (T3).
• Secretion is regulated by a negative feedback mechanism. When the blood level of thyroid hormones is high, secretion of TSH is reduced, and vice versa.

Question 29

Adrenocorticotrophic Hormone (ACTH)

Answer 29

Corticotrophin releasing hormone (CRH) from the hypothalamus promotes the synthesis and release of ACTH by the anterior pituitary.

This increases the concentration of cholesterol and steroids within the adrenal cortex and the output of steroid hormones, especially cortisol.

Secretion is regulated by a negative feedback mechanism, being suppressed when the blood level of ACTH rises. Other factors that stimulate secretion include
hypoglycemia, exercise and other stressors such as emotional states and fever

Question 30

• Prolactin

Answer 30

• secreted during pregnancy to prepare the breasts for
  lactation after childbirth.
• The blood level of prolactin is stimulated by prolactin releasing hormone (PRH) from the hypothalamus, and it is lowered by prolactin
  inhibiting hormone (PIH, Dopamine) and by an increased blood level of prolactin

Question 31

Gonadotrophins

Answer 31

• generally produced in larger amounts just before puberty.
• During this time two gonadotrophins (sex hormones) are secreted in gradually increasing amounts by the anterior pituitary in response to luteinizing hormone
  releasing hormone (LHRH), also known as ganodotrophin releasing hormone (GnRH).
• At puberty secretion increases, further enabling normal adult functioning of the reproductive organs; in both males and females the hormones responsible are:
• Follicle Stimulating Hormone (FSH)
• Luteinizing Hormone (LH)