Flashcards in Session 5 - Blood Osmolarity Deck (59)
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1
What happens in terms of plasma osmolality when water intake is less than water excretion?
• Plasma osmolarity increases
2
In what situation does plasma osmolarity decrease?
• When water intake is greater than water excretion
3
What is the typical osmolarity of urine?
• 50-1200 Osm/l
4
Do changes in water balance affect volume?
• No, they effect osmolarity
• Problems with Na+ balance affect
5
What is the osmolarity of body fluids?
275 - 295 mOsm/kg
6
What do problems with Na+ balance cause?
• Changes in ECF volume
7
Where is plasma osmolality sensed?
• By hypothalamic osmoreceptors in the organum vasculoum of the laminae terminals (OVLT) of the hypothalamus
8
Where is the OVLT found?
• Anterior and ventral to the third ventricle
9
What is special about the OVLT?
• Fenestrated leaky endothelium which exposes it directly to systemic circulation
10
What are the two efferent pathways which regulate osmolarity?
• Thirst
• ADH
11
Which method of regulation is the first line of defence in modulating osmolarity?
ADH
12
What two changes trigger release of ADH?
• Increase in osmolarity of the blood
• Baroceptors detecting decreased stretch (low blood volume)
13
What are the effectors of thirst, and what is affected?
• Brain - Drinking behaviour
14
What is the effector of ADH, and what is the consequence?
• Kidney
• Renal water excretion
15
When is ADH released?
• When water is lost and osmolarity increases by 1%, osmoreceptors in the hypothalamus initiates release of ADH from posterior pituitary
16
How is ADH secretion inhibited?
Decreased osmolarity
17
What is thirst stimulated by?
• Large increase in fluid osmolarity
• <10% changes in osmolarity or decrease in volume
18
What is drinking induced by?
• Increase in plasma osmolarity
• Decrease in ECF volume
19
When is ADH released stimulated?
• When there is a 1% increase in plasma osmolarity due to loss of water
20
Describe ADH
• Small peptide hormone
• 9 AA long
21
What initiates the release of ADH?
• Osmoreceptors in the OVLT of the hypothalamus initiate the release of ADH from the posterior pituitaruy
22
What does ADH act on to regulate volume and osmolarity of urine?
• The kidney
23
What happens if ADH is low?
• Water diuresis will occur due to decreases reabsorption
24
What happens if ADH is high?
• Small volume of urine excreted
25
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)
26
Where is ADH secreted from?
• The posterior pituitary
27
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
28
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
29
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
30
