Physiology of Body Fluids

Question 1

What components make up the extracellular fluid? What separates them?

Answer 1

1. Interstitial fluid - minimal protein - 75%
2. Plasma - 20%
3. Transcellular fluid - 5% - minimal, includes synovial fluid, CSF, intraocular fluid, digestive secretions, etc

Interstitial fluid and plasma separated by capillary wall, relatively permeable to small solutes but not large solutes (proteins)

Question 2

What is blood volume vs hematocrit?

Answer 2

Blood volume - total volume of intravascular compartment -> plasma + blood cells

Hematocrit - fraction of blood volume occupied by cells

(Hematocrit*BV) + Plasma = BV

Question 3

Why do women have less body water than men? How do their relative TBW / TBWt’s compare?

Answer 3

They tend to have more adipose tissue due to breasts and buttocks. As adipose tissue increases, percent water content goes down.

TBW/TBWt:
Male - ~60%, about 42L in a healthy man of 70 kg
Female - ~50%

Question 4

What are the relative volumes of ICF / ECF and why?

Answer 4

ICF = 2x ECF, thus ICF makes up 2/3 of TBW (total body water). This is because ICF has about 2x as much solute, and water moves by osmosis

Question 5

How does water content of kidney, muscle, bone, and adipose compare?

Answer 5

Kidney (84%) > muscle > bone > adipose (10%)

Question 6

How do you calculate plasma volume with hematocrit + blood volume?

Answer 6

PV = BV (1-hct)

Question 7

How does total body water / total body weight (TBW / TBWt) change with age?

Answer 7

Decreases, so old people have a problem with heat and staying hydrated -> less water to lose

Question 8

What are the normal sources of daily fluid loss and where might they be severe in each case?

Answer 8

1. Respiration - 1-2L, exercise + extreme cold
2. Perspiration - 1-2L, exercise / hot weather
3. Urination - 1-2L, varies with hydration
4. Defecation - 0.1 L - 25L losses with cholera

Question 9

What are the major cation differences between ICF and ECF?

Answer 9

ICF: High K+, Low Na+. Gradient maintained by Na+/K+ ATPase

ECF: High Na+, Low K+

ICF also has extra Mg+2

Question 10

What are the major anion differences between ICF and ECF?

Answer 10

ICF: Proteins and inorganic phosphates, make the cell swell and give overall negative membrane potential

ECF: Chloride and bicarbonate

Question 11

How are chloride and bicarbonate kept out of the cell?

Answer 11

Negative charge on inside of membrane keeps anions out. Proteins are very negatively charged inside the cell as it is, so the cell has a net negative potential.

Question 12

How does ionized Ca+2 compare in ICF vs ECF?

Answer 12

ECF is four orders of magnitude higher than ICF so you don’t have muscles contracting

Question 13

How does the osmolality of ICF compare to ECF?

Answer 13

This is the total solute concentration. It is the same in both, because the cell membranes are very permeable to water. ~290 mM / kg H20

Question 14

What is the major difference between plasma and interstitial fluid (ISF)? How does this affect ion concentrations?

Answer 14

Plasma contains soluble proteins, whereas ISF does not

These proteins:

1. Occupy space - make all solutes more concentrated in the remaining water
2. Carry negative charge - attracts cations from ISF, repels anions into ISF

Question 15

What percent of plasma volume do proteins + lipids take in plasma solution and how does the relate to protein free plasma?

Answer 15

They take about 7%. Thus, to calculate protein-free plasma, you take the concentrations of ions per liter solution and divide by (1-.07) = 0.93.

Protein-free plasma = plasma water

Question 16

What is an equivalent? Why is it important?

Answer 16

moles * valence of solute. Valence = charge. I.e. calcium would have a valence of two.

It is important for keeping track of net charge.

Question 17

What are normal lab values for Na+ in protein-free plasma and cell?

Answer 17

Protein-Free Plasma: 153 mM

Cell: 15 mM

Question 18

What are normal lab values for K+ in protein-free plasma and cell?

Answer 18

Protein-Free Plasma: 4.7 mM

Cell: 120 mM

Question 19

What are normal lab values for Ca+2 in protein-free plasma and cell?

Answer 19

Protein-Free Plasma: 1.3 mM

Cell: 1 x 10^-7 M (4 orders of magnitude)

Question 20

What are normal lab values for Mg+2 in protein-free plasma and cell?

Answer 20

Protein-Free Plasma: 0.6 mM

Cell: 18 mM (total), 1 ionized

Question 21

What are normal lab values for Cl- in protein-free plasma and cell?

Answer 21

Protein-Free Plasma: 110 mM

Cell: 20 mM

Question 22

What are normal lab values for HCO3- in protein-free plasma and cell?

Answer 22

Protein-Free Plasma: 24 mM

Cell: 15 mM

Question 23

How tightly regulated are phosphates?

Answer 23

Not regulated, about 0.7 mM all around

Question 24

What are normal lab values for protein in protein-free plasma and cell?

Answer 24

Protein-Free Plasma: (None) - 1 mM in regular

Cell: 4 mM

Question 25

What is normal blood glucose and what is it in cells?

Answer 25

5.9 mM in protein-free plasma, it is very low in cells as it is readily used.

Question 26

How does pH differ from ECF to ICF?

Answer 26

ECF: 7.4
ICF: ~7.2, slightly lower

Question 27

What is meant by electroneutrality?

Answer 27

All body fluids are electrically neutral

Question 28

What is the anion gap?

Answer 28

[Na+] + [K+] > [Cl-] + [HCO3-]. This gap normally amounts to about 9-14 mEq/L, but it works out because the proteins + smaller organic ions have a net negative charge

Question 29

What are some problems with clinical extracellular Na+ disorders?

Answer 29

Causes abnormal shifts of water between body fluid compartments, especially in brain which can cause seizures, coma, and death

Question 30

What are some problems resulting from failed K+ gradient?

Answer 30

Coordination problems, muscle weakness

Question 31

What are four places where solutes and water is transferred between body and outside world?

Answer 31

1. Kidney
2. Lung
3. Integument
4. GI tract