Bioscience Week 2 - Endocrine System

Learning Objective 1

Describe how the endocrine system maintains homeostasis

homestasis maintained by two communication systems

Nervous system and endocrine system

- Nervous system

• speed

• conducting

• targets

• Quickly within (milliseconds) to a stimulus by generating and conducting electrical signals along highly specialised cells called neurons to alter the activities of an effector - specific muscles (skeletal, cardiac or smooth) or gland

- Endocrine System

• speed

• conducting

• targets

• Responds more slowly (minutes) to a stimulus by producing and secreting chemical messengers called hormones into the bloodstream (blood plasma) to alter the activities of an effector (any cell that has the appropriate hormone receptor

Learning Objective 2

Briefly describe how endocrine glands are stimulated to secrete hormones, the two chemical classifications of hormones and how hormones alter target cell activity.

Hormone are produced and secreted by the various

endocrine glands

endocrine glands are located

• throughout the body and organs

• (organs that contain small populations of endocrine cells)

different locations of glands

hormones are secreted in response to a

stimulus

stimuli that trigger endocrine glands to secrete their hormones can be grouped into 3 main categories

1. Hormonal stimulus

2. Humoral stimulus

3. Neural stimulus

for hormonal stimulus

• one hormone stimulates the secretion of another

Hormonal Stimulus pathway

• one hormone stimulates the release of another

  1. Hypothalamus secretes a hormone that

2. stimulates anterior pituitary to secrete a hormone that

Hormonal Stimulus pathway

2. stimulates anterior pituitary to secrete a hormone that

3. stimulate other endocrine glands to secrete hormones

   eg. Thyroid gland, Adrenal gland, or Gonad (testis)

for humoral stimulus changes in

• ion or nutrient blood levels eg. glucose, Ca2+, Na+, K+

Humoral Stimulus pathway

• changes in ion or nutrient blood levels

• eg.

  1. low blood levels Ca2+ levels…

     • travels through

     • releases

2. stimulates parathyroid gland

   • to secrete parathyroid hormone

For neural stimulus the signals are from

• the nervous system

Neural stimulus pathway

• gets signals from nervous system

  1. preganglionic sympathetic fibres…

     • travel through

2. Stimulates adrenal medulla to secrete adrenalin and noraadrenalin

   • then to capillary

most cases when hormone levels rise to restore homeostasis

• negative feedback returns hormone secretion and blood levels to normal

2 Chemical classes of hormones

• steroid hormones

• amino acid-based hormones

Steroid hormones

• made from

• are lipid ____

• bind to

• made from cholesterol

• are lipid-soluble and can easily diffuse across the plasma membrane

• bind to receptors inside a cell - i.e intracellular receptors

Amino acid-based hormones

• vary in ___

• are lipid ___

• bind to

• vary in size - can be signal amino acids, peptides, or proteins

• are lipid insoluble and cannot easily diffuse across the plasma membrane

• bind to receptors embedded in the plasma membrane - i.e. plasma membrane receptors

Hormone actions

binding of a hormone to its intercellular or plasma membrane receptor activates

the target cell response

the target cell response depends on

both hormone and the target cell

eg. insulin has different effects on liver cells and adipose cells

effects of hormone action

• activating or deactivating enzymes →

eg. enzymes required for metabolic reactions

effects of hormone action

• changing the permeability of the plasma membrane by

• opening or closing ion channels

effects of hormone action

• stimulating protein synthesis →

eg. structural proteins for growth and cell division, enzymes for metabolic reactions, protein channels (aquaporins) for transportation

effects of hormone action

inducing cell secretory activity

→ stimulates hormone secretion

effects of hormone action

stimulating cell division →

eg. for body growth and development

Hormones alter target cell activity to :

• control the rate of chemical (metabolic) reactions

• facilitate the transportation of substances into and out of cells

• regulate fluid and electrolyte balance, blood pressure, reproductive functions, development and growth

• maintain homeostasis

Endocrine glands and their hormones

Learning objective 3 :

Learning objective 4 :

3. Describe how the hormones produced by the hypothalamus reach the anterior and posterior lobes of the pituitary gland.

4. State the site of hormone production and secretion, the stimulus for secretion and the main action(s) of all pituitary gland hormones, thyroid hormone, parathyroid hormone, calcitonin, erythropoietin, cortisol, aldosterone, adrenaline/noradrenaline, insulin, glucagon, oestrogen, progesterone and testosterone.

Hypothalamus is classified as a

Neuroendocrine organ

(neural and endocrine function)

Hypothalamus is connected to the pituitary gland via a stalk called

infundibulum

Pituitary gland is divided into two lobes called

anterior pituitary gland and posterior pituitary

Hypothalamus - Posterior Pituitary

• composed of

• does not produce

• secretes two hormones ____ & ____ produced by the hypothalamus

• composed of neural tissue

• does not produce hormones

• secretes two hormones Oxytocin and antidiuretic hormone (ADH)

Hypothalamus-Posterior Pituitary flow

1. cell bodies of neurons within hypothalamus produce the hormones

oxytocin and antidiuretic hormone (ADH)

2. the axons of these neurons form the :

• which transports these hormones through the _____

• where they are stored in the ____

hypothalamic - hypophyseal tract

• infundibulum to the posterior pituitary

• axon terminals

3. when these hypothalamic neurons are stimulated the stored hormones are

secreted from the posterior pituitary into the bloodstream

Antidiuretic hormone (ADH)

Stimulus for secretion : ADH is secreted by the _____ when blood Na+ levels ______ above normal range and or blood volume and blood pressure ______ below the normal range

• secreted by the posterior pituitary

• when blood Na+ levels increase above normal range

• and or when blood volume and blood pressure decreases below the normal range

Main action :

ADH primarily targets the _____

ADH _____ by stimulating the kidneys to _____

• primarily targets they kidneys

• ADH decreases urine output by stimulating the kidneys to return more water to the blood

ADH _____ the reabsorption of water from the urine being produced and _____ it to the _______

ADH increases the reabsorption of water from the urine being produced and returns it to the bloodstream

ADH increases the reabsorption of water from the urine being produced and returns it to the bloodstream

This effectively

• dilutes the blood plasma, restoring normal blood Na+ levels

• increases blood volume and pressure to normal levels

• maintains normal blood volume and pressure

ADH can be commonly known as _____

Vasopressin as it can also cause vasoconstriction of the arterioles

• helps increase blood pressure

Clinical connection

Conditions such as dehydration or haemorrhage will

decrease blood volume and therefore increase secretion of ADH

Clinical connection

Hypo-secretion of ADH results in

diabetes insipidus - caused by blow to the head that damages the hypothalamus or posterior pituitary

• symptoms include excessive urination (polyuria), leading to dehydration and extreme thirst

• if patient is unconscious following a head injury, their urine output must be monitored

Oxytocin

Stimulus for secretion:

the secretion of oxytocin is stimulated by

stretching the uterus during labour and the mammary glands to stimulate the ejection of milk during breastfeeding

Oxytocin secretion is regulated via a

positive feedback mechanism

increased stretching of the cervix during childbirth, or the suckling action of the infant after birth stimulates the

posterior pituitary to secrete more oxytocin

• this enhances the uterine contractions which move the baby through the birth canal or the ejection of milk from the mammary glands

As suckling promotes the secretion of oxytocin, placing the infant at the mother’s breast after childbirth helps expel

the placenta and return the uterus to its smaller size

Hypothalamus - anterior pituitary

hypothalamic neurons also produce and secrete

releasing and inhibiting hormones

Names of hypothalamic releasing/inhibiting hormones (anterior pituitary hormones)

Thyrotropin-releasing hormone (TRH)

Corticotropin-releasing hormone (CRH)

Growth hormone-releasing hormone (GHRH)

Growth hormone-inhibiting hormone (GHIH or somatostatin)

Gonadotropin-releasing hormone (GnRH)

Prolactin-inhibiting hormone (PIH)

Each releasing or inhibiting hormones reaches the anterior pituitary via the

hypophyseal portal system

the hypophyseal portal system is

a network of blood vessels that connect the hypothalamus to the anterior pituitary as follows

a network of blood vessels that connect the hypothalamus to the anterior pituitary as follows

1. when stimulated hypothalamic neurons secrete ______ into the ______

2. Hormones travel through the ____ via the ____ to the anterior pituitary pituitary

3. releasing/inhibiting hormones stimulate/inhibit the _____ of anterior pituitary hormones into the _____

1. when stimulated hypothalamic neurons secrete releasing/inhibiting hormones into the hypophyseal portal system

2. hormones travel through the infundibulum via the hypophyseal portal system to the anterior pituitary

3. releasing/inhibiting hormones stimulate/inhibit the secretion of anterior pituitary hormones into the bloodstream

The anterior pituitary secretes ___ different hormones, most of which stimulate other endocrine glands to secrete their hormones

6

Gonadotropins - Follicle-stimulating hormone (FSH) and Luteinising hormone (LH)

Stimulus for secretion :

Gonadotropin-releasing hormone (GnRH) stimulates the secretion of FSH and LH from the anterior pituitary

Main actions :

LH and FSH target

the male and female gonads, ovaries in the females and the testes in males

FSH in ovaries

Stimulates:

• the development of oocyte (egg) containing follicles

• production and secretion of oestrogen

FSH in the testes

stimulates

• regulates sperms production

LH in ovaries

stimulate

• Ovulation

• production and secretion of progesterone (mainly) & oestrogen

LH in testes

• stimulates the production and secretion of testosterone

Prolactin (PRL)

Stimulus for secretion :

Prolactin-inhibiting hormone (PIH) primarily controls ____

____ PIH secretion leads to to an ____ in PRL secretion

prolactin-inhibiting hormone (PIH) primarily controls the secretion of PRL from the anterior pituitary

Decreased PIH secretion leads to an increase in PRL secretion

Prolactin (PRL)

Main action:

PRL targets the

the role of PRL in males is

• mammary glands and stimulates mill production

• unknown

Adrenocorticotropic Hormone (ACTH)

Stimulus for secretion :

secretion of ACTH from the anterior pituitary is

Corticotropin-releasing hormone (CRH)

Adrenocorticotropic Hormone (ACTH)

Main action:

ACTH targets the ____ and stimulates

ACTH targets the adrenal cortex (outer part of the adrenal glands) and stimulates the adrenal cortex to secrete the hormone cortisol

Thyroid-stimulating hormone (TSH)

Stimulus for secretion :

the secretion of TSH from the anterior pituitary is stimulated by

• thyrotropin - releasing hormone (TRH)

Thyroid-stimulating hormone (TSH)

Main action:

TSH targets the ____ and _____ the thyroid gland to secrete the thyroid hormones T3 (triiodothyronine) and T4 (thyroxine).

• thyroid gland

• and stimulates the thyroid gland to secrete the thyroid hormones

Growth Hormone (GH)

Stimulus for secretion: the secretion of GH is controlled by

two hypothalamic hormones

Stimulus for secretion: the secretion of GH is controlled by two hypothalamic hormones

• Growth hormone-releasing hormone (GHRH) - stimulates secretion of GH

• Growth hormone-inhibiting hormone (GHIH or somatostatin) - inhibits the secretion of GH

Growth Hormone (GH)

Main action:

GH promotes the growth of ____

GH stimulates cells in the _____

• which stimulate the actions required

• body tissues, especially skeletal muscle and bone.

• liver, skeletal muscles and bones to secrete insulin-like growth factors (IGFs)

• which stimulate the actions required for growth, cell division and protein synthesis

Clinical Connection

the actions of GH are evident in individuals who are either

deficient in GH or produce too much GH

Hypo secretion of GH leads to

dwarfism

Hyper secretion of GH in children leads to

gigantism in children

Hyper secretion of GH in adults leads to

acromegaly (in adults)

Thyroid and Parathyroid glands

The butterfly-shaped thyroid gland lies at the

base of the throat

Thyroid and Parathyroid glands

• It is composed of

• which produce and secrete

• also composed of

• which secrete and produce

1. composed of thyroid follicles

   • which secrete and produce T3 and T4 (aka thyroid hormones)

2. composed of parafollicular cells

   • which secrete and produce the hormone calcitonin

Parathyroid glands are tiny masses of glandular that tissue that

produce and secrete parathyroid hormone (PTH).

Parathyroid glands are located on the

posterior surface of the thyroid gland

Thyroid hormones (TH)

Stimulus for secretion:

Thyroid - stimulating hormone stimulates thyroid hormone

Thyroid hormones (TH)

Main actions:

TH affect virtually every cell in the body

TH affect virtually every cell in the body and primarily

• increase basal metabolic rate (BMR) =

• increases body heat production =

• increases HR and force of contraction by increasing the number of B-adrenergic receptors on cardiac muscle cells -

• Promotes the growth of -

• promotes -

• the amount of energy (ATP) required by body cells to carry out all metabolic reactions at rest

• an increase in metabolism = an increase in heat production) to maintain a normal body temperature

• this helps regulate normal heart function

• promotes growth of muscles and bones (in conjunction with growth hormone)

• promotes nervous system development

the secretion of TH involves a series of interactions between three endocrine glands and this series of interactions is known as the

hypothalamic-pituitary-thyroid axis (HPT)

Clinical connection

deficiencies in TH during fetal development, infancy or childhood

lead to mental retardation and stunted bone growth

Clinical Connection

The hypo-secretion of TH during the adult years can result in

Low BMR, heart rate and blood pressure, cold intolerance, muscular and weight gain

the hypersecretion of TH can result in an

above normal BMR increased body temp (heat tolerance) rapid heart rate, high blood pressure and weight loss

Calcitonin

Stimulus for secretion :

• by the parafollicular cells of the thyroid gland when blood Ca2+ levels increase above the normal range

Calcitonin

Main action:

• targets bones, decreases blood Ca+ to normal levels by

• inhibiting the activity of osteoclasts → specialised bone cells that resorb (digest) the extracellular matrix component of bone release stored calcium into the blood

• stimulating calcium uptake from the blood into bone

Parathyroid hormone (PTH)

stimulus for secretion:

• secreted by parathyroid glands when blood Ca2+ levels decrease below the normal range

Parathyroid hormone (PTH)

Main actions:

PTH targets bone, kidneys, small intestines

Parathyroid hormone (PTH)

Main actions:

PTH targets bone, kidneys, small intestines and increases blood Ca2+ to normal levels by stimulating

• bone-resorbing osteoclasts and the release of stored Ca2+ into the blood

• the kidneys to return more Ca2+ to the blood

• the kidneys to secrete calcitriol (the active form of vitamin D) which increases the absorption of Ca2+ form digested foods in the small intestines

Clinical connection

Hypoparathyroidism can be caused

by parathyroid gland trauma or their removal during thyroid surgery.

The resulting hypocalcaemia (low blood Ca2+ levels) can affect body functions such as blood clotting, neural communication and muscle (skeletal, cardiac and smooth) contraction.

Hyperparathyroidism is the third most common endocrine disorder, and it is often caused by a

parathyroid gland tumour

hypersecretion of PTH leads to the excessive resorption of the bone matrix (via osteoclasts) and the release of stored calcium into the blood.

The loss of calcium from the bone matrix causes bones to become soft, and If left untreated, the continual resorption of the bone matrix can result in a loss of bone mass (thin bones) and osteoporosis.

The Kidneys

Although the kidneys are not classified as an endocrine gland, they do contain cells that produce and secrete the hormone Erythropoietin (EPO).

Erythropoietin (EPO)

Stimulus for secretion:

EPO is secreted by the kidneys when blood oxygen levels drop below their normal range (hypoxemia)

Erythropoietin (EPO)

Main Action:

EPO targets the

bone marrow and stimulates the production of the red blood cells

The adrenal glands

The adrenal glands are paired, pyramid-shaped organs that sit

on top of the kidneys

Structurally and functionally, the adrenal gland has two endocrine glands in one

Consisting of an outer adrenal cortex and an inner adrenal medulla.

the outer adrenal cortex is responsible for the

production and secretion of glucocorticoids (primary cortisol) and mineralocorticoids (primarily aldosterone).

the inner adrenal medulla produces and secretes the hormones

adrenaline and noradrenaline.

Cortisol

Stimulus for secretion:

the secretion of cortisol from the adrenal cortex is stimulated by

the anterior pituitary hormone, ACTH Adrenocorticotropic Hormone

The secretion of cortisol involves a series of interactions between the three endocrine glands

known as the hypothalamic-pituitary-adrenal axis (HPA axis)