HPA Axis: Hormones and assessment of function Flashcards
What are the core components of the HPA axis?
Hypothalamus
Pituitary gland
Adrenal glands
Which hormone does the hypothalamus secrete that stimulates the pituitary gland?
Corticotrophin Releasing Hormone
Which hormones does the pituitary gland secrete that stimulates the adrenal glands?
Adrenocorticotrophic Hormone (ATCH)
Which hormone does the adrenal glands release in response to stimulation of ATCH by the pituitary gland?
Cortisol
The hypothalamus forms the lateral walls of which ventricle?
Third ventricle
What is the inferior aspect of the hypothalamus called?
Median eminence
What gives rise to the pituitary stalk (Infundibulum)?
Median eminence
What gives rise to the posterior pituitary gland?
Pituitary stalk
Which nuclei is contained in the base of the hypothalamus?
Tuberal nuclei
What is the role of the tuberal nuclei?
To regulate the anterior pituitary gland function
What are the main components of the hypothalamus?
Mamillary bodies
Autonomic centres
Supraoptic nucleus
Tuberal nuclei
Preoptic areas
Paraventricular nucleus
Suprachiasmatic nucleus
What are mamillary bodies involved in?
Episodic memory and regulating feeding reflexes
What do the autonomic centres in the hypothalamus control?
Nuclei which alter HR + BP
What does the supraoptic nucleus secrete?
ADH
What does the preoptic areas control?
Thermoregulation
What does the paraventricular nucleus synthesise?
Oxytocin
What does the suprachiasmatic nucleus control?
Circadian rhythm
Which gland is the ‘master endocrine gland’?
Pituitary gland
What are the two lobes of the pituitary gland?
Anterior
Posterior
What are the anterior and posterior pituitary glands also called?
Anterior - adenohypophysis
Posterior - neurohypophysis
What is the posterior pituitary gland connected to the hypothalamus (median eminence area) by?
Pituitary stalk
What is the anterior pituitary derived from?
The embryonic pharynx
What are the six anterior lobe hormones?
Adrenocorticotropic hormones (ACTH)
Follicle-stimulating hormone (FSH)
Luteinizing hormone (LH)
Growth hormone (GH)
Prolactin (PRL)
Thyroid-stimulating hormone (TSH)
What is the role of the follicle stimulating hormone and luteinizing hormone?
They work in partnership to ensure functionality of the ovaries and testes.
What is growth hormone important for?
Growth. Affects fat distribution/muscle mass in adults.
What does prolactin stimulate?
Breast milk production
What does the thyroid stimulating hormone stimulate?
The thyroid gland to produce hormones.
What are the two posterior pituitary hormones?
ADH
Oxytocin
What is the role of ADH?
Increases the permeability of collecting ducts to absorb water
What is the role is oxytocin?
Smooth muscle contraction facilitating birth and/or milk ejection.
ADH and oxytocin can both be synthesised by which nuclei in the hypothalamus?
Supraoptic nucleus and paraventricular nucleus
Does the posterior pituitary gland synthesis any hormones?
No, only secretes them. ADH and Oxytocin are synthesised in the hypothalamus and stored in the median eminence before being released into posterior pituitary gland.
Capillary beds and veins of the pituitary gland form what type of system for hormones?
A ductal system.
Where does the arterial supply of the pituitary gland originate from?
Internal carotid arteries via the hypophyseal branches.
What does the superior hypophyseal artery supply and where does it drain into?
A primary capillary bed in the wall of the pituitary stalk, which drains into portal vessels.
What does the inferior hypophyseal artery supply and where does it drain into?
Posterior pituitary lobe, which drains into the cavernous sinus.
What percentage of the adrenal glands is the inner adrenal medulla and the outer adrenal cortex?
90% - outer adrenal cortex
10% - inner adrenal medulla
What response is the medulla concerned with?
Acute stress response
What is the adrenal cortex concerned with?
Chronic stress
Sodium/glucose homeostasis.
The hormones synthesised in the adrenal cortex are derived from what?
Cholesterol in our diet or from within the gland itself.
What type of hormones does the adrenal cortex synthesise?
Steroid hormones
What are the three layers of the adrenal cortex?
Outer zona glomerulosa
Middle zone fasciculata
Inner zona reticularis
What hormones are synthesised in the outer zona glomerulosa?
Mineralocorticoids (aldosterone)
What hormones are synthesised in the middle zona fasciculata?
Glucocorticoids (cortisol)
What hormones are synthesised in the inner zona reticularis?
Adrenal androgens (DHEA - Dehydroepiandrosterone - converts to sex hormones)
What chemicals does the adrenal medulla synthesise?
Catecholamines (Adrenaline and Noradrenaline)
What type of feedback loop is involved in the HPA axis?
Negative.
Hypothalamus - CRH
Anterior pituitary gland - ACTH
Adrenal cortex - Cortisol.
Cortisol feeds back to hypothalamus and anterior pituitary to stop secreting hormones.
When does ACTH secretion peak and when is it at its lowest?
Peaks early morning at time of waking
Lowest in the middle of the night
When would there be increased secretion of ACTH?
At times of prolonged stress.
What does cortisol secretion peak and when is it at its lowest?
Same as ACTH - peaks early morning and lowest in the middle of the night BUT peak and lowest happens 2 hours after those of ACTH.
What is the cortisol awakening response?
The increase of cortisol levels peaking around 35-40minutes after waking. Also get an increase in cortisol before waking.
Thought to be a response to anticipated stress.
Can be impaired in those who have chronic stress.
How is cortisol transported in the blood?
10% is in free active form.
90% bound to plasma proteins
What plasma proteins is cortisol bound to?
75% - Corticosteroid blinding globulin (CBG or transportin)
15% - Albumin
Pregnancy increases which plasma protein and what does this result in?
CBG - Corticosteroid binding globulin.
Results in a compensatory increase in circulating plasma cortisol concentrations.
Which system controls the secretion of aldosterone?
RAAS
What can aldosterone secretion be directly stimulated by?
Trauma, anxiety, hyperkalaemia, hyponatraemia
What can aldosterone secretion be directly inhibited by?
Atrial natriuretic peptide (ANP)
Is aldosterone protein bound?
50% is protein bound.
Aldosterone has specific intracellular receptors which cause expression of what?
Ion channels that transport Na+ and K+ ions across cell membrane.
Aldosterone stimulates the reabsorption of what in the distal tubule?
Na+.
Also has some lesser effects in the collecting duct, proximal tubule, and ascending loop of henle.
Na+ reabsorption in the kidneys stimulated by aldosterone, is exchanged for what ions?
K+ or H+
How does aldosterone affect blood pressure?
Aldosterone stimulates reabsorption of K+, thereby influencing plasma Na+ concentration. This in turn affects water reabsorption in the collecting duct via an effect on ADH secretion.
The interaction of the RAAS, aldosterone and ADH therefore controls blood volume and thus blood pressure.
What is acromegaly?
Too much growth hormone in adults.
Results in enlargement of hands, feet, forehead, jaw and nose as the common early creatures.
What is the commonest cause of acromegaly and how is it treated?
Pituitary adenoma
Remove adenoma/radiotherapy
What it pituitary cushings?
Excessive amounts of cortisol.
Endogenous usually due to excessive secretion of cortisol from elevated levels of ACTH. High ACTH usually due to pituitary tumour.
What are the common features of pituitary cushings and how is it treated?
Weight gain
Fatty tissue deposits (midsection, upper back and face)
Fragile skin - easily bruised, slow healing, acne.
Tx - Removal of tumour via surgery and/or radiotherapy.
What is prolactinoma?
Excessive secretion of prolactin due to pituitary tumour.
Can change menstrual cycle, mood disturbances and have galactorrhoea (breast milk production with no pregnancy/baby).
What is hypopituitarism?
Reduced secretion of one or more of the pituitary hormones.
Can happen due to tumours, infections, vascular failure etc.
Treatment involves reversing what’s caused it.
