ENDO - Physiology of The Adrenal Gland Flashcards
Describe the layers of the adrenal glands.
- Cortex consisting of the zona glomerulosa, zona fasciculata and zona reticularis
- Adrenal medulla
Describe the different hormones produced by different parts of the adrenal glands.
- Glomerulosa produces aldosterone (a mineralocorticoid)
- Fasciculata produces cortisol and corticosterone (a glucocorticoid)
- Reticularis produces sex hormones (mainly testosterone and its precursors)
- Medulla produces the fight or flight hormones adrenaline and noradrenaline
Describe the anatomy of the adrenal glands. PART 1
- Only the adrenal medulla receives secretomotor innervation from the sympathetic nervous system
- Sympathetic innervation of the adrenal medulla arises from the splanchnic nerve. The splanchnic nerve originates from between T3-L3, but those fibres innervating the adrenal gland arise from T8-T11
- Some of these preganglionic fibres synapse in the coeliac ganglion, which gives rise to post-ganglionic fibres which innervate blood vessels
Describe the anatomy of the adrenal glands. PART 2
- Other preganglionic fibres directly innervate medullary chromaffin cells via a cholinergic synapse to stimulate adrenaline and noradrenaline secretion.
- Each chromaffin cell is innervated by a preganglionic, cholinergic sympathetic fibre.
- Acetylcholine binds to nicotinic receptors on the chromaffin cell, which depolarises the membrane leading to calcium influx (through opening of a voltage-gated calcium channel), and subsequent exocytosis of noradrenaline and adrenaline filled vescicles
Describe some of the general effects of adrenaline and noradrenaline.
- Tachycardia
- FFA mobilisation
- Pupil and bronchial dilation
- Reduced GI motility
- Glycogenolysis - rise in plasma glucose
- Piloerection and cutaneous vasoconstriction
Describe the responses of the sympathetic nervous system.
- Stress-activated
- Response is fast (milliseconds) and targeted (each organ can be directly stimulated by discrete neuronal fibres).
- Sustaining such neural activation for long is energy demanding due to the amount of ATP required for action potential generation
Describe the responses of the adrenal glands.
- Much slower to respond (minutes)
- Effects are not targeted to specific organs, because adrenaline is released into the blood stream.
- Activation of a single nerve fibre can cause the release of adrenaline - much less energy demanding to produce an effect and because adrenaline can be released continuously if needed for very little energy, effects are sustained over longer periods of time
Pheochromocytomas are chromaffin cell tumours that secrete excess adrenaline and noradrenaline. With this in mind, suggest some presenting complaints of someone with this.
- Heat intolerance
- Sporadic tachycardia
- Hypertension
- Pallor
- Hyperglycaemia
- Pani attacks
Describe what the zona glomerulosa and the JGA produce.
- Zona glomerulosa produces aldosterone exclusively
- JGA - collection of cells surrounding afferent arteriole and DCT - produce renin (in low sodium and blood volume)
- Both contribute to RAAS
Describe the mechanism of action of aldosterone. PART 1
- Works on the collecting ducts and DCT to increase sodium reabsorption
- Binds to the mineralocorticoid receptor in the cell nucleus
- Increased expression of ENaC in the apical membrane.
- Influx of sodium channels down their concentration gradient, out of the filtered fluid and into the cell.
Describe the mechanism of action of aldosterone. PART 2
- Assuming the presence of aquaporins (which are inserted into the membrane by vasopressin in response to low blood volume), the electrochemical gradient produced by the influx of sodium also leads to the influx (and thus reabsorption) of water.
- This is how aldosterone helps to maintain blood volume and pressure
- Mineralocorticoid receptor also increases expression of the Na-K-ATPase, which swaps two potassium ions for three sodium ions, thus reabsorbing sodium back into the blood stream
If not well controlled what can aldosterone receptor antagonists do?
- Opposite effects of hyperaldosteronaemia
- If not well controlled, including hyponatraemia, hyperkalaemia and hypervolaemia
In abnormal aldosterone production cases, what can cause alkalosis?
- In the cells of the collecting duct and convulted tuble, potassium-hydrogen exchange channels prevent the body from losing too much potassium by shuttling some potassium back out of the cell and into the extracellular fluid in exchange for hydrogen ions.
- In cases where aldosterone production is too high, and thus the expression of the Na-K-ATPase is high, the exchange of hydrogen ions will increase, leading to alkalosis
Describe Conn syndrome.
Adrenal aldosteronoma
- High secretion of aldosterone
- Manifests as hypertension and hypokalaemia
How can renovascular hypertension affect aldosterone production?
- Narrowing of arteries supplying kidneys
- Increase in renin production
- Increase in aldosterone production
