Regulation of Osmolarity Flashcards
What type of hormone is ADH?
Peptide hormone - neurohormone
Where is ADH synthesised?
Supraoptic and Paraventricular nuclei of the hypothalamus
Where is ADH secreted?
Posterior pituitary
What is the half life of ADH?
10 minutes
Why is the half life of ADH so short?
so can rapidly be adjusted depending on the body’s needs for H2O conservation
What is the primary control of ADH secretion?
Plasma osmolarity
How does increased osmotic pressure effect ADH secretion?
Increased supraoptic/paraventricular nuclei discharge causing increased ADH release
Where are neuronal discharge changes mediated?
Osmoreceptors in the anterior hypothalamus
Where is thirst mediated?
Lateral hypothalamus
How do osmoreceptors detect increased osmolarity?
↑ H2O outflow from cell
Cell shrinks → stretch sensitive ion channel activated
↑ neural discharge
↑ ADH secretion
How do osmoreceptors detect decreased osmolarity?
H2O enters cells
Cells swell
↓ Neural discharge
↓ ADH secretion
What is the effect on ADH when osmoreceptors detect a stretch?
Increased H2O levels, decreased ADH secretion
What is the normal plasma osmolarity?
280-290mOsm/kg
2.5% increase in osmolarity causes what increase in ADH?
10x increase
Why does urea concent not effect osmolarity?
Urea passes freely with water through membranes
What is the maximum concentration of urine?
1200-1400mOsm
Why does ingestion of hypertonic solutions cause dehydration?
The increased solute load increases urine flow as more H2O is needed to excrete than was ingested with it
Where is the main site of water regulation in the kidney?
Collecting duct
Via what mechanism does ADH cause increased water absorption?
ADH binds with basolateral membrane
Vesicles containing aquaporins exocytose onto brush border membranes membranes
Increased permeability to water
ADH levels affect aquaporins where?
On the brush border membrane of the collecting duct
What is the effect of ADH on aquaporin levels on the basolateral membrane of the collecting duct?
None - aquaporins here are constant
What is the collecting duct concentration if ADH is present?
Proportional to the cortical interstitium (300mOsm), then with the hypertonic medullary interstitial gradient
What is the collecting duct concentration if ADH levels are at maximum?
Contents equilibrates with medullary interstitium via osmotic efflux and become highly concentrated at the tip
How would urine present with maximal [ADH]?
Small volume, highly concentrated
How is H2O reabsorbed in the presense of Maximal [ADH]?
Via oncotic pressure of the vasa recta which will be high in the presence of a H2O deficit
How would urine present with absent ADH? What osmolarity?
Large volume
Very dilute
30-50mOsm
Why are urea concentrations high in the presence of ADH?
Water leaves the collecting duct and urea remains
How does urea permeability vary in the collecting ducts?
More permeable to urea towards the medullary tips
What is the role of urea reabsorbed into the interstitium from the collecting ducts?
Acts to reinforce the interstitial gdt in the region of the thin ascending loops of Henle
What happens to urea during maximum anti-diuresis?
Urea is retained to save water and reinforce the medullary gradient in the thin ascending limb
Why is the reabsorption of urea important in maximal ADH?
Too much urea in the collecting duct would have an osmotic effect and impede potential rehydration
Conservation of H2O more important than Urea excretion
What is the effect of increased extracellular fluid volume on ADH?
Increased ECF volume = decreased ADH
What is the effect of decreased extracellular fluid volume on ADH?
Decreased ECF volume = increased ADH
Why does low blood volume cause increased ADH secretion?
Low blood volume = low stretch receptor afferent discharge = increased ADH secretion
Where are the low pressure receptors located?
L and R atria
Great veins
Where are the high pressure receptors located?
Carotid
Aortic arch baroreceptors
Moderate decreases in ECF volume primary affect what?
Atrial - low pressure receptors
How do atrial receptors effect ADH release?
Presence of reduced ECF volume
Decreased discharge (via vagus nerve)
Increased ADH release
When do carotid/aortic baroreceptors engage to contribute to ADH secretion?
When volume changes are significant enough to affect MBP
What type of cells secrete ADH?
Neurones
What stimuli can increase ADH?
Pain, emotion, stress
Exercise
Nicotine, morphine
Surgery
What stimuli can decrease ADH?
Alcohol
What is the effect of Alcohol on ADH?
Suppresses ADH release
Why is ADH an important factor post-surgery?
Surgery causes inappropriate ADH secretion - monitor H2O intake
An osmolarity above what will trigger vasopressin secretion?
280mOsM
Changes in blood pressure are detected where?
Carotid/aortic baroreceptors
Changes in blood volume are detected where?
Atrial stretch receptors
How do messages from the hypothalamic osmoreceptors reach the hypothalamus?
Interneurons to hypothalamus
Changes in osmolarity are detected where?
Hypothalamic osmoreceptors
ADH acts where?
Collecting duct epithelium
What is central Diabetes insipidus?
ADH deficiency
What factors can cause diabetes insipidus?
Meningitis
Tumours
Surgery
What is peripheral diabetes insipidus?
Collecting ducts insensitive to ADH
How are diabetes insipidus characterised?
Polyuria (>10L/day)
Polydipsia
How is central diabetes insipidus treated?
Give patient ADH
What is the typical cause of peripheral diabetes insipidus?
Hypercalcaemia
Hypokalemia
What is the genetic cause of diabetes insipidus?
Gene for aquaporins
Gene for ADH receptors
What is the normal fluid osmolarity in the bowmens capsule?
300mOsM
What is the normal fluid osmolarity at the end of the proximal tubule?
300mOsM
What is the normal fluid osmolarity at the end of the loop of henle?
100mOsM
What is the normal fluid osmolarity at the end of the collecting duct?
50-1200mOsM
How much fluid passes the bowmans capsule a day?
180L
How much fluid passes the ends of the proximal tubules a day?
54L
How much fluid passes the ends of the loop of henle a day?
18L
How much fluid passes the ends of the collecting ducts a day?
1.5L (average)
