Volume Flashcards
“Volume”
Short hand for extracellular Volume
Note that total water space is 70% of body weight; 2/3 is intracellular and 1/3 is extracellular
Of extracellular, 1/3 is Vascular blood/fluid and 2/3 is interstitial fluid
For extraceullar fluid, how much water: Na?
1L Water + 135-145 mEq Na
“Normal Saline”
Why does Sodium remain extracellularly? Why is it important that sodium remains extracellularly?
If you add Sodium/water solution (normal saline), you will expand the extracellular fluid compartment because the Sodium channels are hard to open; they remain closed & the fluid stays extracellularly
This is important because when we give normal saline, the fluid stays in the extracellular compartment! This would not be true if we gave K+/water – theoretically the solution would go to intracellular compartment
What happens if you add salt alone (either via diet or via injection)?
Water flows from intracellular compartment (which shrinks) to extracellular compartment (which expands)
But you get thirsty and your hormones change = shows you how important salt is in maintaining everything in your body
What is the most specific controller of extracellular volume and why?
Sodium!
Water is not
What happens to intracellular/extracellular compartments if you drink a lot of water?
Both expand in proportion to their starting volumes
Water distributes itself among all spaces
Water is not specific for any compartment
It also decreases ADH so you pee out more water
What is the maximum water you can drink in a day?
20L
Does diet control volume expansion/contraction?
Yes
Intake = Output
But there might be an intrinsic set point controlling Na/water content
Volume Depletion: what is it
A condition where you have reduced extracellular fluid that can have many causes, but the primary event is a loss of sodium: vomiting, diarrhea, fever>sweat, exercise>sweat, not eating, hemorrhage, adrenal sufficiency, diuretics, high urine flow
Volume Depletion: Symptoms and Signs
Symptoms: light headed, weak, tired, cold extremities, thirsty
Signs: low blood pressure, orthostasis, weight loss, dry skin, no sweating, no saliva, loss of skin turgor
Kidney signs: elevated BUN/creatinine rario, low urine Na, concentrated urine, low urine volume
How does your body sense that you have a low volume?
1) Intrathoracic
- Both atria: stretch receptor –> ANP release
- Right ventricle
- Pulmonary capillaries
2) Arterial volume/baroreceptors: carotid & aortic arch
3) Renal artery baroreceptor: juxtaglomerular apparatus
4) CNS volume receptors
5) Hepatic volume receptors: process through GI system that senses sodium load & response that is greater than if it was IV
What is the body’s response to volume depletion?
Body senses volume depletion –> Signalling to CNS –>
(1) Sympathetic activation –> Renin release –> Increases Angiotensin II & Aldosterone –> recovery of urinary Na; maximizes in 2-3 days
(2) Thirst
(3) Salt appetite
• What is the role of angiotensin II on the proximal tubule?
Angiotensin II binds 2 receptors:
• (1) receptor I which signals through Gq, activates IP3 production, which increases diacyl glycerol production & Ca is released from ER/SR
• (2) voltage dependent Ca channel
• The effect of both is to cause vasoconstriction of the efferent arteriole and to regulate gene expression in the proximal tubule cell
Which target genes does AII regulate?
Na/K ATPase, Na solute cotransporter, 2Na/H exchanger
• Both increases the number of these transporters and they are placed at the correct membrane
• Encourages sodium flux into cells & is a positive regulator of sodium recovery
• What is the importance of the Na/H exchanger, besides just Na recovery?
o It has a pivotal role in turning HCO3 into CO2 using carbonic anhydrase
o This is how bicarb can cross the membrane: as CO2
