D.

5 Hormones
Option D Human Physiology
 Hormones are not secreted at a
Essential Idea uniform rate and exert their effect
at low concentrations
Understandings
■ Endocrine glands secrete hormones directly into the bloodstream
■ Steroid hormones bind to receptor proteins in the cytoplasm of the target cell to
form a receptor-hormone complex
■ The receptor-hormone complex promotes the transcription of specific genes
■ Peptide hormones bind to receptors in the plasma membrane of the target cell
■ Binding of hormones to membrane receptors activates a cascade mediated by a
second messenger inside the cell
■ The hypothalamus controls hormone secretion by the anterior and posterior lobes
of the pituitary gland
■ Hormones secreted by the pituitary control growth, developmental changes,
reproduction and homeostasis
Applications
■ Some athletes take growth hormones to build muscles
■ Control of milk secretion by oxytocin and prolactin
The Endocrine System
The endocrine system is comprised of ductless glands that release
chemicals into the blood to regulate body functions
■ A hormone is a chemical messenger that is transported indiscriminately
via the bloodstream to act on distant target cells
■ Hormones are specific and will only activate cells or tissues that possess
the appropriate target receptor
■ The endocrine system is slower to initiate, but has a more prolonged
response when compared to the nervous system
Endocrine Signalling
Endocrine Glands
Endocrine glands secrete their product (hormones) directly into the
bloodstream, rather than through a duct (e.g. exocrine gland)
■ Major endocrine glands include the pancreas, adrenal gland, thyroid
gland, pineal gland and the gonads (ovaries and testes)
■ The hypothalamus and pituitary gland are neuroendocrine glands and
function to link the nervous and endocrine systems
■ Some organs may also secrete hormones despite not being endocrine
glands (e.g. adipose tissue secretes leptin)
Endocrine Glands
Types of Hormones: Steroid
■ Steroid hormones are lipophilic (fat-loving) – meaning they can freely
diffuse across the plasma membrane of a cell
■ They bind to receptors in either the cytoplasm or nucleus of the target
cell, to form an active receptor-hormone complex
■ This activated complex will move into the nucleus and bind directly to
DNA, acting as a transcription factor for gene expression
■ Examples of steroid hormones include those produced by the gonads (i.e.
estrogen, progesterone and testosterone)
Steroid Hormones
Types of Hormones: Peptide
■ Peptide hormones are hydrophylic and lipophobic (fat-hating) – meaning they cannot freely
cross the plasma membrane
■ They bind to receptors on the surface of the cell, which are typically coupled to internally
anchored proteins (e.g. G proteins)
■ The receptor complex activates a series of intracellular molecules called second messengers,
which initiate cell activity
■ This process is called signal transduction, because the external signal (hormone) is transduced
via internal intermediaries
■ Examples of second messengers include cyclic AMP (cAMP), calcium ions (Ca2+), nitric oxide
(NO) and protein kinases
■ The use of second messengers enables the amplification of the initial signal (as more molecules
are activated)
■ Peptide hormones include insulin, glucagon, leptin, ADH and oxytocin
Peptide Hormones
Hormone Comparison
Hypothalamus
The hypothalamus is the section of the brain that links the nervous and endocrine systems in
order to maintain homeostasis
■ It receives information from nerves throughout the body and other parts of the brain and
initiates endocrine responses
■ It secretes hormones (called releasing factors) into a capillary blood system which target the
anterior lobe of the pituitary gland
■ It also secretes neurochemicals directly via neurosecretory cells that extend into the posterior
pituitary lobe
Pituitary Gland
The pituitary gland lies adjacent to the hypothalamus and is in direct contact due to a portal blood
system
■ The pituitary gland receives instructions from the hypothalamus and consists of two lobes
(anterior and posterior lobe)

Anterior Lobe
■ The anterior lobe is also called the adenohypophysis (‘adeno’ = relating to glands)
■ The hypothalamus produces releasing factors, which are released into portal vessels by
neurosecretory cells
■ The releasing factors cause endocrine cells in the anterior pituitary to release specific
hormones into the bloodstream
■ An example of a releasing factor is GnRH, which triggers the release of LH and FSH from
the anterior pituitary
Pituitary Gland
Posterior Lobe
■ The posterior lobe is also called the neurohypophysis (‘neuro’ = relating to nerves)
■ The posterior lobe releases hormones produced by the hypothalamus itself (via neurosecretory
cells)
■ These neurosecretory cells extend into the posterior lobe from the hypothalamus and release
hormones into the blood
Pituitary Gland
The pituitary gland is often referred to as the ‘master gland’, as it controls the secretion of a
number of other endocrine glands
■ Pituitary hormones will often target endocrine glands in other organs (e.g. gonads, pancreas,
thyroid, mammary gland)

Pituitary hormones hence control many vital body processes, including:

■ Metabolism (e.g. TSH activates thyroxin)
■ Adult Development (e.g. LH / FSH trigger puberty)
■ Reproduction (e.g. LH / FSH control menstruation)
■ Growth (e.g. growth hormone promotes growth)
■ Equilibrium / Homeostasis (e.g. ADH and water balance)

Mnemonic: MARGE
Hypothalamus & Pituitary Roles
Feedback Loops
Physiological processes are commonly moderated via two distinct feedback mechanisms –positive
and negative feedback
■ Homeostatic processes are controlled by negative feedback and hence these systems occur
more commonly within the body
Negative Feedback
Negative feedback involves a response that is the reverse of the change detected (it functions to
reduce the change)
■ A change is detected by a receptor and an effector is activated to induce an opposite effect –
this promotes equilibrium

Examples of processes that utilize negative feedback loops include homeostatic systems, such as:
■ Thermoregulation (if body temperature changes, mechanisms are induced to restore normal
levels)
■ Blood sugar regulation (insulin lowers blood glucose when levels are high ; glucagon raises
blood glucose when levels are low)
■ Osmoregulation (ADH is secreted to retain water when dehydrated and its release is inhibited
when the body is hydrated)
Negative Feedback
Positive Feedback
Positive feedback involves a response that reinforces the change detected (it functions to amplify
the change)
■ A change is detected by a receptor and an effector is activated to induce the same effect – this
promotes further change
■ Positive feedback loops will continue to amplify the initial change until the stimulus is removed

Examples of processes that utilise positive feedback loops include:

■ Childbirth – stretching of uterine walls cause contractions that further stretch the walls (this
continues until birthing occurs)
■ Lactation – the child feeding stimulates milk production which causes further feeding
(continues until baby stops feeding)
■ Ovulation – the dominant follicle releases oestrogen which stimulates LH and FSH release to
promote further follicular growth
■ Blood clotting – platelets release clotting factors which cause more platelets to aggregate at the
site of injury
Positive Feedback
Growth Hormone
Growth hormone (also known as somatotropin) is an anabolic peptide hormone that stimulates
growth
■ It acts directly to reduce the formation of adipose cells (ex. less nutrients stored as fat)
■ It acts indirectly via insulin growth factor (IGF) – produced by the liver – to increase muscle
mass and bone size

Due to its role in promoting growth and regeneration, it is used by some athletes as a
performance enhancer
■ The use of human growth hormone is banned in sports, with proven cases of doping strictly
punished
■ Traditional urine testing could not detect doping, which historically made bans difficult to
enforce
■ Recent blood tests can now identify between natural and artificial variants of growth hormone
Lactation
The production and secretion of milk by maternal mammary glands following birth is called
lactation
■ It is predominantly controlled and regulated by two key hormones – oxytocin and prolactin

Prolactin is responsible for the development of the mammary glands and the production of milk
■ It is secreted by the anterior pituitary in response to the release of PRH (prolactin releasing
hormone) from the hypothalamus
■ The effects of prolactin are inhibited by progesterone, which prevents milk production from
occurring prior to birth
Lactation
Oxytocin is responsible for the release of
milk from the mammary glands (milk
ejection reflex)
■ It is produced in the hypothalamus and
secreted by neurosecretory cells that
extend into the posterior pituitary
■ Oxytocin release is triggered by
stimulation of sensory receptors in the
breast tissue by the suckling infant
■ This creates a positive feedback loop
that will result in continuous oxytocin
secretion until the infant stops feeding
Iodine Defficiency
Certain hormones may require specifc Iodine deficiency is common in many
precursor molecules in order to be countries, as iodine is not a common
synthesized by the body component of most diets (sea food
excepted)
■ Thyroxin contains iodine within its
chemical structure and cannot be ■ The International Council for the Control
produced if iodine is deficient in the diet of Iodine Deficiency Disorders works to
■ Iodine deficiency will therefore effect eliminate the harm of iodine deficiency
the thyroid gland – where thyroxin is ■ One strategy employed is to add iodine
produced to common dietary products (ex. iodized
■ Individuals with an iodine deficiency will table salt)
develop an enlarged thyroid gland – a
condition known as goitre