Endocrine System and Homostasis

Question 1

Learning outcomes

Answer 1

• Define homeostasis.
• Describe the endocrine system and it’s role in the maintenance of homeostasis.
• Describe negative and positive feedback pathways used in homeostasis and give examples of each.

Question 2

Homeostasis - constancy of the internal environment

Answer 2

• Homeostasis is a state of overall internal chemical and physical
  stability that is required for survival of cells and the body.
• Homeostatic mechanisms maintain a constant internal environment despite a variable external environment.
  Short term (minute to minute)
  blood pressure, body temperature
  Medium term (hours to days)
  food intake, sleep
  Long term
  body weight, blood pressure, growth, sexual
  maturation
  Deviation from homeostasis indicates disease.
  homeo = similar, stasis = stable, homeostasis = steady state

Question 3

Set point

Answer 3

• Set point = the physiological value around which the normal range fluctuates.
  > The set point for normal human body temperature is ~37°C
• A normal range is the restricted set of values that is optimal and stable.
  *The normal range for body temperature is 36.5 - 37.5°C
• Negative feedback is the mechanism that reverses a deviation from the set point.

Question 4

Control of homeostasis

Answer 4

• Homeostasis requires the interaction of sensors, integrators & effectors.
• Negative feedback: control mechanism that reverses changes to a body system if
  it exceeds a set point.
• Negative feedback loops are the predominant mechanism used in homeostasis.

Question 4

Set point

Answer 4

Set points can change and feedback loops can maintain new
settings
E.g. blood pressure: over time, the set point for blood pressure can increase as a result of continued increases in blood pressure. The body no longer recognizes the elevation as abnormal and no attempt is made to return to the lower set point. The result is the maintenance of an elevated
blood pressure.

Answer 4

Acclimatization: Changes can occur in organ systems in order to maintain a different set point.
E.g. high altitude athletic training: in order to adjust to the lower oxygen levels at the new altitude, the body increases production of red blood cells to ensure adequate oxygen delivery to the tissues. (*Human Ecology lectures)

Answer 4

Answer 5

Question 5

Short term homeostatic mechanisms

Answer 5

• E.g. blood pressure, blood pH (chemical regulation), body temperature (thermoregulation), osmoregulation.

Answer 6

Question 7

Homeostasis requires communication

Answer 7

Neurotransmitters:
> Released at synapses
between neurons &
target cells.

Hormones:
* Carried by blood to
distant targets

Question 8

Hormones - signalling molecules

Answer 8

What are hormones?
* Secreted by endocrine glands, endocrine cells and some neurons
* Travel through the circulation to target cells
* Target cells have receptors for hormones
* Hormones change the activity of their target cells
* horme = “to excite”
The endocrine system = the glands, organs and cells that produce
hormones.
Generally regulate slower, long term changes in growth or functioning
of body systems.
Regulated by negative feedback.

Question 9

Types of hormones

Answer 9

• 2 types of hormones
• Steroid hormones
• Lipids derived from cholesterol
• Produced in adrenal glands & reproductive organs (ovaries & testes)
• Non-steroid hormones
  : epid ormonesine, a peptide or a protein
• Interact with receptors:
• in the nucleus
• or on a target cell’s membrane
• Change the cell’s activity
  > Increase or decrease target cell’s synthesis of proteins, or
  > Increase or decrease activity of enzymes in the target cell

Question 10

Transport and action of steroid & protein hormones

Answer 10

Receptors for hormones can be on the cell membrane or inside the cell.
* Lipid soluble hormones (steroids)
bind to receptors inside of the cell
* Water soluble hormones (peptide hormones) bind to receptors on
the outside of the cell
* Activate second messenger
systems

Answer 10

Question 11

Steroid hormones

Answer 11

• Lipid soluble
• Diffuse directly across the lipid bi-layer of
  the target cell membrane
  > Enter the cytoplasm
  > Enter the nucleus
• Form a hormone-receptor complex (transcription factor)
• Binds to promotor regions of genes
  *Stimulate or inhibit transcription from
  those genes
• Changes activity of the target cell

Some steroid hormones bind with receptors on target cell membrane to change the membrane properties which
affects the cell’s function

Question 12

Protein hormones

Answer 12

• Water soluble (can’t cross the lipid membrane)
• Don’t enter the target cell.
• Act indirectly by using “second”
  messengers to relay signals

> Hormone binds to receptors in plasma
membrane
activate an enzyme system
changes the activity of the cell
(the hormone acts as the first messenger)

Answer 13

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Question 14

Target Tissues

Answer 14

Question 15

Negative feedback mechanisms & the endocrine system

Answer 15

Question 16

Anterior Pituitary:

Answer 16

Question 16

Hypothalamus &
Pituitary gland

Answer 16

Hypothalamus & pituitary gland interact as a major centre controlling the activity of
other organs.
Hypothalamus:
* Secretory neurons deliver hormones to
the pituitary
Posterior Pituitary:
* Stores & releases hormones made in the
hypothalamus
* ADH & Oxytocin

Answer 17

Question 18

Hypothalamus - Posterior pituitary relationship

Answer 18

Hypothalamus:
* Axons of neurons extend down into the
posterior pituitary.
* Secretory neurons in the hypothalamus synthesize and store, ADH & Oxytocin.
* ADH & Oxytocin move down in the axons and accumulate in the axon endings.
* Action potentials trigger release of these
hormones which enter the blood
capillaries in the posterior lobe.
* The hormones move into the circulation
and travel to target cells.

Answer 19

Question 20

Hypothalamus - Anterior pituitary relationship

Answer 20

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* Cell bodies of secretory neurons in the hypothalamus secrete hormones.
* Hormones are picked up by the capillary bed
at the base of the hypothalamus.
* Hormones travel to capillary bed of the AP.
* Hypothalamic hormones act on the AP
endocrine cells to produce hormones.
* AP hormones enter the circulation.
* Adrenocorticotropic hormone (ACTH)
* Thyroid stimulating hormone (TSH)
* Growth hormone (GH)
* Follicle stimulating hormone (FSH)
* Luteinising hormone (LH)
prolactin PRL

Answer 21

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Answer 23

Question 24

• Thyroid gland:

Answer 24

> Produces Thyroid Hormone (TH: T3 & T4)
TH required for metabolism, growth and development, and
the nervous system
Sets basal metabolic rate, enhances production of GH
Produces calcitonin (lowers the level of calcium in blood)

Question 24

Hypothalamus - pituitary - thyroid axis.

Answer 24

• Hypothalamus > Thyrotropin Releasing Hormone (TRH)
• Anterior pituitary > Thyroid Stimulating Hormone (TSH)
  > Thyroid gland > Thyroid hormones (TH: T3 & T4)
• Thyroid hormones have negative feedback effect on hypothalamus & anterior pituitary

Question 25

Hypothalamus - pituitary - thyroid axis.

Answer 25

Answer 25

Question 25

Thyroid hormone dysfunction

Answer 25

• Hypothyroidism = low blood levels of TH
• 16 million people in the US have hypothyroidism
• Symptoms = fatigue, weight gain, intolerance to cold temperatures, dry skin
• It is most commonly caused by endemic iodine deficiency.
• Autoimmune thyroid disease (Hashimoto’s disease) is the most common cause when
  iodine intake is adequate.
• Hyperthyroidism = excess TH in blood
• Symptoms = high heart rate and high blood pressure, heavy sweating, heat
  intolerance, weight loss
• Causes: autoimmune or excessive exposure to iodine

Question 26

Anterior pituitary hormones - Growth hormone (GH)

Answer 26

• Growth Hormone Releasing Hormone (GHRH) from
  hypothalamus.
• Stimulates Growth Hormone (GH) release from
  anterior pituitary.
• Important for normal body growth.
• Stimulates growth of bone, cartilage & skeletal muscle.
• Increases muscle mass.
• Dysfunction of GH causes major abnormalities.

Answer 27

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Question 28

Adrenal glands - and the stress response

Answer 28

Adrenal cortex (outer part):
* Secretes steroids:
* Glucocorticoids
* Secreted at times of stress
* Cortisol
* Raises blood glucose
* Promotes breakdown of muscle protein
* Stimulates liver to take up amino acids to synthesise glucose
(gluconeogenesis)
* Breakdown fats for energy
* Reduce inflammation (> cortisone treatment for injuries)
* Mineralocorticoids
* Aldosterone regulates blood pressure by adjusting
reabsorption of K*, Nat in the kidneys
* Sex hormones in the fetus & in early puberty
Adrenal medulla (inner part):
* Contains neurons that release adrenaline & noradrenaline
* Sympathetic nervous system

Question 29

Hypothalamus-pituitary-adrenal axis

Answer 29

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Question 31

Pancreas

Answer 31

Exocrine function: secretion of digestive enzymes.
Endocrine function: secretion of glucagon & insulin.
Pancreatic islets:
* Alpha cells
> Secrete glucagon
* Raises blood glucose levels
* Beta cells
> Secrete insulin
> Lowers blood glucose levels
* Glucagon & insulin work antagonistically to
maintain blood glucose levels.

Answer 32

Question 33

The pancreas: Regulating blood sugar

Answer 33

Why is regulation of blood glucose so important?
* Low blood glucose levels compromise
normal brain function.
* High blood glucose levels damage
blood vessels & nerves
* Homeostatic responses ensure
appropriate glucose levels despite
intermittent fuel supply and variable
rates of glucose utilization.

Answer 34

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