Lecture 5: The Physiology and Terminiology of Body Fluid Spaces Flashcards
What are the two key physiologic parameters body spends to regulate?
- Effective vascular volume
2. Tonicity
What is Effective vascular volume?
Refers to how well the arterial space is being loaded with blood in such a way that there is adequate end-organ perfusion
Determined by
i. CO
ii. SVR
iii. plasma volume
-ECF volume which is regulated by the kidney
What are the elements that go into maintaining end-organ perfusion? Significance?
- CO (cardiac output)
- SVR (systemic vascular resistance)
- Plasma volume
Significance: the third parameter for end-organ perfusion, plasma volume, is determined by the kidney
What is the Total Body Water?
The total amount of water in body
50% of total body weight in women and 60% in men roughly
What is TBW (total body water) comprised of?
1/3 of TBW is EXTRAcellular fluid (ECF)
2/3 of TBW is INTRAcellular fluid (ICF)
What is the major cation of the extracellular fluid?
Sodium, Na+
Thus Na+ is the major cation for 1/3 of the TBW
What is the major cation of the ICF?
Potassium, K+
Thus K+ is major cation for 2/3 of TBW
What is the ECF composed of?
¼ plasma
¾ interstitial fluid
ECF volume is the major determinant of plasma volume
Why does body care about ECF volume?
Because plasma volume makes up about ¼ of the ECF or 1/12th of TBW
Changes in ECF generally leads to proportional changes in plasma volume
What happens if you don’t get enough plasma volume?
Inadequate organ perfusion aka inadequate effective vascular volume
What are the body fluid spaces?
2/3 ICF 1/3 ECF -1/4 Plasma and ¾ interstitial fluid So composed of i. ICF ii. Plasma volume (smallest layer) iii. Interstitial fluid
For an avg 70 kg man,
what is TBW and its components?
TBW = 42 L (70kg * 0.6) ICF = 28 L (42 * 2/3) ECF = 14L Plasma = 3.5 L (1/4 of 14)
What happens if you have more fat?
The more fat, the less total body weight of water
Why do you have 5L of blood if you only have 3.5L of plasma?
The rest of the volume comes from INTRAcellular volume
What is osmolality?
Number of particles/ kg of water
Measures body’s electrolyte-water balance
Particles in a solution (chair in swimming pool = 1 chair / kg of water)
What is the calculated osmolality?
Calculated osmolality = 2*Na + Glu/18 + Urea/2.8 all in mOsm
Usually it is 290 mOsm
What contributes to tonicity?
Must fulfill two criteria:
1. Must be osmotically active (have particles in a solution)
2. Must be confined to one side of the cell membrane or the other (doesn’t distribute through cell) such as Na is extracellular while K is intracellular
This means solution is tonically active
What are effective osmoles?
Particles that stay one side of the plasma membrane vs the other
Are considered effective osmoles because they contribute to the particle count over kg of water
How does glucose contribute to tonicity?
An effective osmole without insulin
But once you have insulin, glucose gets taken up into the cell
What are examples of effective osmoles?
Na, K, Cl and mannitol
What are examples of ineffective osmoles?
Urea, ethanol
What is the one compound that act as both an EFFECTIVE or ineffective osmole?
Glucose which depends on insulin
What is calculated tonicity?
Calculated tonicity 2*Na + Glu/18
Same as osmolality except for urea removed since urea is not tonically active
This is our “best guess” since it cannot be calculated precisely
Why do we care about Tonicity?
Because tonicity dictates water distribution
Water follows these effective osmoles (such as water moving with Na)
Concept of hypertonicity, hypotonic and isotonic
Determines which direction water is going to move
What do fluctuations of tonicity most effect?
The brain
Won’t affect RBCs first because brain will get fucked first
What is the best way to expand volume?
Through sodium
That’s because sodium is stuck outside in the ECF and thus is an “effective osmole”
Why is water a poor way to regulate effective vascular volume?
Because most of the water will end up in the ICF (because it can pass through the plasma membrane)
What is the best way to regulate effective vascular volume (ECF)?
Sodium
So if you have low ECF, you upregulate sodium retention through renin-angiotensin-aldosterone
What is the best way to regulate tonicity?
With water
If you have too high of an osmolality of the blood plasma, then your body will upregulate water retention and intake through
i. ADH
ii. Thirst
How does body upregulate water content in the body?
Through ADH and thirst
If you have greater than 10% of effective volume depletion, what happens?
Body will BOTH upregulate salt and water retention
Usually effective vascular volume and tonicity are SEPARATELY regulated, unless you have a greater than 10% effective volume depletion
What is the primary determinant of ECF volume? Why?
Salt
This is because salt is an effective osmole and only stays in the ECF
And Na is the major osmole in ECF compartment
How is Na balance maintained?
Intake (dietary) = output (renal and extrarenal)
If extrarenal loss of Na outpaces intake (through diarrhea, blood loss and burns), then you get hypovolemia (abnormally low ECF volume)
How do we evaluate effective vascular volume?
No reliable lab tests Use of clinical evaluation i. Jugular venous pressure ii. lung exam for crackles iii. peripheral edema iv. acute change in weight v. axillary sweat vi. patient’s HISTORY
How do we evaluate tonicity?
Clinical exam is unreliable
Lab testing for serum sodium and serum osmolality
What are the NON-SPECIFIC symptoms of tonicity changes?
Nausea
Mental status change
Due to hypo or hypernatremia
What is serum sodium concentration?
It is NOT total body sodium
Serum sodium concentration reflects the RATIO between salt to water
So serum sodium is an osmolality measurement
Serum sodium concentration = Salt/Water = OSMOLALITY….does not say shit about total body Na which is a measurement of effective vascular volume and NOT osmolality
Is volume depletion the same thing as dehydration?
NO
Volume depletion is not the same as dehydration
Dehydration = too little water
Volume depletion = too little salt
What is the difference between volume and tonicity terminology?
Normal total body Na = Euvolemia
Too much Na = Volume overload
Too little Na = Volume depletion
Vs.
Normal tonicity = Eunatremia
Low tonicity = hyponatremia (too much water relative to solute)
High tonicity = hypernatremia (too little water relative to solute)
Hypo and hyper natremia are measures of WATER relative to salt even though the name itself unfortunately puts the emphasis on salt
What do you say when a patient has too little salt (or too little effective vascular volume)?
You say the patient is volume depleted
What do you say when a patient has too much salt (or too much effective vascular volume)?
Volume OVERLOAD
What do you say when patient has too little water relative to sodium?
You say the patient is dehydrated or hypernatremic
What do you say when patient has too much water relative to sodium?
Patient is HYPOnatremic
What are types of IV fluids?
- IVF used to give NaCL
2. IVF used to give water
What are the characteristics of IVF for NaCl?
Isotonic saline
Called “normal saline”
Tonicity is comparable to the aqueous portion of the blood
What are the characteristics of IVF used to give water?
Water is given with 5 mg/dl dextrose (so the RBCs don’t lyse)
Often called “D5W”
Giving pure water IV would lyse red cells
5mg/dl dextrose is close to iso-osmolar initially but dextrose gets metabolized
This leaves behind water
What happens if you add 1.5L of D5W (water with 5mg/dl of dextrose) to ECF?
Tonicity will DROP because glucose eventually metabolized
ECF volume will increase (but only by a little bit because most of that water is leaking into ICF but there is still enough remaining to increase total ECF volume)
ICF volume will increase
What is the significance of giving someone too much D5W?
Water will expand BOTH ICF and ECF
This has negative consequences on the brain, which will expand and occlude vessels, etc.
Will expand ICF much more since 2/3 of water will go into cell
What is the significance of the following findinga for the change in ECF volume?
1.5L of normal saline = 17L 18.5L
1.5L D5W = 17L 17.6L
-but Na decreased from 140 mEq/L 135 mEq/L
420mEq NaCl = 17L 17.9L
-but Na increased from 140mEq/L 145 mEq/L
The ECF volume in all 3 cases has been increased
But
This increase occurs independently of changes in osmolality!
Takehome point: change in ECF volume is INDEPENDENT of change in osmolality
Example: If you give hypertonic solution to ECF vs. Hypotonic solution to ECF, you would change the ECF volume by different amounts AND change the osmolality by different amounts and the change in both would be unrelated (as seen in the above example)
If you say a patient lost ECF volume (diarrhea, blood loss, etc), does that say anything about the patient’s osmolality?
NO because loss of volume can lead to either an increase, decrease or no change in osmolality
What is the kidney’s compensatory response if you are given 1.5L of normal saline?
Euvolemic person will pee the salt and water out so ECF goes back to 17L
What is the kidney’s compensatory response if you are given 1.5L of water (D5W)?
Hyponatremic person will pee the extra water out
What is the kidney’s compensatory response if you are given 420 mEq NaCl?
Hypernatremic person will drink more water and pee out the extra Na
What would happen if you give KCl instead of NaCl in isotonic saline?
K+ does the EXACT same thing as adding Na+
K does the same thing to serum sodium concentration
This is because the Na/K ATPase will take up the K into the cell but will push more Na out into the cell
Thus, the ECF gets more Na into the ECF as a SECONDARY effect through increased Na/K ATPase
