Lecture 5: The Physiology and Terminiology of Body Fluid Spaces

Question 1

What are the two key physiologic parameters body spends to regulate?

Answer 1

1. Effective vascular volume

2. Tonicity

Question 2

What is Effective vascular volume?

Answer 2

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

Question 3

What are the elements that go into maintaining end-organ perfusion? Significance?

Answer 3

1. CO (cardiac output)
2. SVR (systemic vascular resistance)
3. Plasma volume
   Significance: the third parameter for end-organ perfusion, plasma volume, is determined by the kidney

Question 4

What is the Total Body Water?

Answer 4

The total amount of water in body

50% of total body weight in women and 60% in men roughly

Question 5

What is TBW (total body water) comprised of?

Answer 5

1/3 of TBW is EXTRAcellular fluid (ECF)

2/3 of TBW is INTRAcellular fluid (ICF)

Question 6

What is the major cation of the extracellular fluid?

Answer 6

Sodium, Na+

Thus Na+ is the major cation for 1/3 of the TBW

Question 7

What is the major cation of the ICF?

Answer 7

Potassium, K+

Thus K+ is major cation for 2/3 of TBW

Question 8

What is the ECF composed of?

Answer 8

¼ plasma
¾ interstitial fluid
ECF volume is the major determinant of plasma volume

Question 9

Why does body care about ECF volume?

Answer 9

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

Question 10

What happens if you don’t get enough plasma volume?

Answer 10

Inadequate organ perfusion aka inadequate effective vascular volume

Question 11

What are the body fluid spaces?

Answer 11

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

Question 12

For an avg 70 kg man,

what is TBW and its components?

Answer 12

TBW = 42 L (70kg * 0.6)
ICF = 28 L (42 * 2/3)
ECF = 14L
Plasma = 3.5 L (1/4 of 14)

Question 13

What happens if you have more fat?

Answer 13

The more fat, the less total body weight of water

Question 14

Why do you have 5L of blood if you only have 3.5L of plasma?

Answer 14

The rest of the volume comes from INTRAcellular volume

Question 15

What is osmolality?

Answer 15

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)

Question 16

What is the calculated osmolality?

Answer 16

Calculated osmolality = 2*Na + Glu/18 + Urea/2.8 all in mOsm

Usually it is 290 mOsm

Question 17

What contributes to tonicity?

Answer 17

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

Question 18

What are effective osmoles?

Answer 18

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

Question 19

How does glucose contribute to tonicity?

Answer 19

An effective osmole without insulin

But once you have insulin, glucose gets taken up into the cell

Question 20

What are examples of effective osmoles?

Answer 20

Na, K, Cl and mannitol

Question 21

What are examples of ineffective osmoles?

Answer 21

Urea, ethanol

Question 22

What is the one compound that act as both an EFFECTIVE or ineffective osmole?

Answer 22

Glucose which depends on insulin

Question 23

What is calculated tonicity?

Answer 23

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

Question 24

Why do we care about Tonicity?

Answer 24

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

Question 25

What do fluctuations of tonicity most effect?

Answer 25

The brain | Won’t affect RBCs first because brain will get fucked first

Question 26

What is the best way to expand volume?

Answer 26

Through sodium | That’s because sodium is stuck outside in the ECF and thus is an “effective osmole”

Question 27

Why is water a poor way to regulate effective vascular volume?

Answer 27

Because most of the water will end up in the ICF (because it can pass through the plasma membrane)

Question 28

What is the best way to regulate effective vascular volume (ECF)?

Answer 28

Sodium | So if you have low ECF, you upregulate sodium retention through renin-angiotensin-aldosterone

Question 29

What is the best way to regulate tonicity?

Answer 29

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

Question 30

How does body upregulate water content in the body?

Answer 30

Through ADH and thirst

Question 31

If you have greater than 10% of effective volume depletion, what happens?

Answer 31

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

Question 32

What is the primary determinant of ECF volume? Why?

Answer 32

Salt This is because salt is an effective osmole and only stays in the ECF And Na is the major osmole in ECF compartment

Question 33

How is Na balance maintained?

Answer 33

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)

Question 34

How do we evaluate effective vascular volume?

Answer 34

``` 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 ```

Question 35

How do we evaluate tonicity?

Answer 35

Clinical exam is unreliable | Lab testing for serum sodium and serum osmolality

Question 36

What are the NON-SPECIFIC symptoms of tonicity changes?

Answer 36

Nausea Mental status change Due to hypo or hypernatremia

Question 37

What is serum sodium concentration?

Answer 37

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

Question 38

Is volume depletion the same thing as dehydration?

Answer 38

NO Volume depletion is not the same as dehydration Dehydration = too little water Volume depletion = too little salt

Question 39

What is the difference between volume and tonicity terminology?

Answer 39

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

Question 40

What do you say when a patient has too little salt (or too little effective vascular volume)?

Answer 40

You say the patient is volume depleted

Question 41

What do you say when a patient has too much salt (or too much effective vascular volume)?

Answer 41

Volume OVERLOAD

Question 42

What do you say when patient has too little water relative to sodium?

Answer 42

You say the patient is dehydrated or hypernatremic

Question 43

What do you say when patient has too much water relative to sodium?

Answer 43

Patient is HYPOnatremic

Question 44

What are types of IV fluids?

Answer 44

1. IVF used to give NaCL | 2. IVF used to give water

Question 45

What are the characteristics of IVF for NaCl?

Answer 45

Isotonic saline Called “normal saline” Tonicity is comparable to the aqueous portion of the blood

Question 46

What are the characteristics of IVF used to give water?

Answer 46

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

Question 47

What happens if you add 1.5L of D5W (water with 5mg/dl of dextrose) to ECF?

Answer 47

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

Question 48

What is the significance of giving someone too much D5W?

Answer 48

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

Question 49

What is the significance of the following findinga for the change in ECF volume?

Answer 49

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)

Question 50

If you say a patient lost ECF volume (diarrhea, blood loss, etc), does that say anything about the patient’s osmolality?

Answer 50

NO because loss of volume can lead to either an increase, decrease or no change in osmolality

Question 51

What is the kidney’s compensatory response if you are given 1.5L of normal saline?

Answer 51

Euvolemic person will pee the salt and water out so ECF goes back to 17L

Question 52

What is the kidney’s compensatory response if you are given 1.5L of water (D5W)?

Answer 52

Hyponatremic person will pee the extra water out

Question 53

What is the kidney’s compensatory response if you are given 420 mEq NaCl?

Answer 53

Hypernatremic person will drink more water and pee out the extra Na

Question 54

What would happen if you give KCl instead of NaCl in isotonic saline?

Answer 54

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