Flashcards in Session 5 - Blood Osmolarity Deck (59)
What happens in terms of plasma osmolality when water intake is less than water excretion?
• Plasma osmolarity increases
In what situation does plasma osmolarity decrease?
• When water intake is greater than water excretion
What is the typical osmolarity of urine?
• 50-1200 Osm/l
Do changes in water balance affect volume?
• No, they effect osmolarity
• Problems with Na+ balance affect
What is the osmolarity of body fluids?
275 - 295 mOsm/kg
What do problems with Na+ balance cause?
• Changes in ECF volume
Where is plasma osmolality sensed?
• By hypothalamic osmoreceptors in the organum vasculoum of the laminae terminals (OVLT) of the hypothalamus
Where is the OVLT found?
• Anterior and ventral to the third ventricle
What is special about the OVLT?
• Fenestrated leaky endothelium which exposes it directly to systemic circulation
What are the two efferent pathways which regulate osmolarity?
Which method of regulation is the first line of defence in modulating osmolarity?
What two changes trigger release of ADH?
• Increase in osmolarity of the blood
• Baroceptors detecting decreased stretch (low blood volume)
What are the effectors of thirst, and what is affected?
• Brain - Drinking behaviour
What is the effector of ADH, and what is the consequence?
• Renal water excretion
When is ADH released?
• When water is lost and osmolarity increases by 1%, osmoreceptors in the hypothalamus initiates release of ADH from posterior pituitary
How is ADH secretion inhibited?
What is thirst stimulated by?
• Large increase in fluid osmolarity
• <10% changes in osmolarity or decrease in volume
What is drinking induced by?
• Increase in plasma osmolarity
• Decrease in ECF volume
When is ADH released stimulated?
• When there is a 1% increase in plasma osmolarity due to loss of water
• Small peptide hormone
• 9 AA long
What initiates the release of ADH?
• Osmoreceptors in the OVLT of the hypothalamus initiate the release of ADH from the posterior pituitaruy
What does ADH act on to regulate volume and osmolarity of urine?
• The kidney
What happens if ADH is low?
• Water diuresis will occur due to decreases reabsorption
What happens if ADH is high?
• Small volume of urine excreted
What are the three main effects of ADH?
• Increases the permeability of the collecting duct to water via the addition of aquaporin
• Increases permeability of collecting duct to urea
• Increases activity of Na/K/Cl- co-transporter in the TAL of the loop of henle (Assists in generation of hypertonic medullary intersticium)
Where is ADH secreted from?
• The posterior pituitary
Where is ADH synthesised?
• As a preprohormone in the
• Supraoptic nuclei of hypothalamus
• Paraoptic nuclei of hypothalamus
• Is then sent to posterior pituitary and stored
Outline the effects of ADH on the nephron
• Vasoconstriction at glomerulus
• Increased Na/K+/2Cl- cotransport absorption at ascending limb of loop of henle
• Increased water reabsorption in late DT and Cortical collecting duct via addition of aquaporin 2 to the apical membrane
• Increased K+ secretion of and Urea reabsorption in cortical collecting duct
• Insert aquaporin 2 in the apical membrane of cells in collecting duct
What exactly does ADH in relation to Urea?
• Increases permeability of medullary region of collecting duct, causing its reabsorption
• Rise in urea concentration in the tissues allows it to passively move down its conc gradient into the ascending limb