2.15 - Volume Regulation Flashcards
How is extracellular fluid osmolality controlled?
Via H20 resorption through the action of Anti-Diuretic Hormone (ADH)
Describe the synthesis and release of ADH
Input from osmoreceptors in the brain (most sensitive)
Synthesised as a prohormone in the hypothalamus and later processed by peptidases.
Packaged in to granules and axonally transported to then be released from the posterior pituitary directly in to capillaries
Describe the stimuli for ADH secretion
A small increase (1%) in osmolarity causes ADH secretion above basal.
A decrease in BP also leads to secretion
How does ADH work?
It works to insert aquaporins, stored in vesicles in the tubular cells, in to the apical membrane, allowing for the reabsorption of water
Describe the action of thirst on volume control
An increase in osmolarity of 2-3% induces a strong desire to drink. Blood loss and a decrease in BP ar less sensitive (change of 10-15% needed)
What is the Effective Circulating Volume?
It is the portion of the extracellular fluid contained within the vascular system and is therefore “effectively” perfusing tissues
Describe how Na+ is key to balancing ECF
A decrease in effective perfusion of the kidney –> Na+ retention –> increased ECF
What are the two types of vascular volume sensors?
Low pressure. volume sensed via compliance. In the cardiac atria. Neural and hormonal (ANP) mechanisms
High pressure. Respond to BP. Carotid sinus and aortic arch. Also in JGA (afferent arterioles), hormonal as they release renin.
Describe the Juxtaglomerular Apparatus
composed of macula densa and juxtaglomerular cells as well as mesangial cells
Describe the volume regulating action of Macula Densa cells
Chemoreceptors in distal tubule
Respond to changes in [NaCl].
Fast response (10s).
When ↑ [NaCl] –> ↓ GFR via afferent arteriole constriction (TGF) & ↓ RBF –> ↓ Na+ and water excretion
Describe the volume regulating action of the Juxtaglomerular cells
Specialised endothelial cells in afferent arteriole, where these meet glomerulus (RAA system)
Contain secretory granules (renin)
Mechanoreceptors, sensing BP
A decrease in BP leads to release of renin
How is release of renin regulated?
Beta1 stimulation increases renin secretion.
AngII and aldosterone have negative feedback mechanism on renin release
What effects does AngII have?
Increased craving for salt and increased thirst sensation –> increased fluid and salt intake
Afferent arteriole constriction –> ↓ GFR and ↓ RBF –> reduced salt and water excretion
Systemic vasoconstriction
Secretion of aldosterone –> increased salt and water reabsorption
Increased secretion of ADH from the posterior pituitary
All these increased plasma volume and therefore blood pressure
Describe the response when plasma volume is increased
Volume expansion –> ↑ NaCl excretion & ↓ Sympathetic –> ↓ RAAS and ↑ ANP –> ↑GFR with ↓ NaCl resorption in PT and CD
Which has more control, osmocontrol (ADH), or volume control (RAAS)
Osmocontrol. Has a much stronger and faster response.
Example: after heavy sweating with electrolyte loss, thirst is satiated via drinking H2O and not via NaCl craving. For CVS, ↓ Na+ seems to be smaller problem to deal with than Hypovolaemia
John Segor, 40 year old, had been lost in the snowy mountains during a hiking trip for 3 days until he was found by a rescue team and flown to TCH by helicopter. On arrival, he was severely dehydrated. Compared to normal, which of the following combinations best captures the state that this patient is in?
C
