Body Fluid Compartments, Osmotic & Oncotic Pressure

Question 1

What is Osmosis

Answer 1

is the movement of fluid through a semipermeable membrane towards higher osmolar concentration of solutes. Water goes where the solute is more concentrated.

Question 2

What factors drive osmosis

Answer 2

Osmosis is driven by differences in concentration of dissolved particles between two solutions

Question 3

Explain Osmolarity of a solution

Answer 3

Osmolarity is the concentration of osmotically active particles. It is a measure of the concentration of particles in a solution, calculated as mM/L x g, where g is the number of particles associated with a molecule of solute in solution

Question 4

Explain the importance of osmolarity in body fluids

Answer 4

1. Intracellular osmolarity must be carefully maintained, thus fluid fluxes are kept at a minimum.
2. Extracellular osmolarity (including plasma) must be carefully maintained, or cellular osmolarity will be disrupted.

Question 5

What ions make up the intracellular and extracellular fluid, and how are concentrations controlled?

Answer 5

Ions are Na+ and K+
Ions are controlled by the Na+-K+ pump.
Intracellular Na+ is about 10 mEq/L, K+ is 150 mEq/L
Extracellular Na+ is 140 mEq/L, K+ is about 4 mEq/L

Question 6

Define Osmotic pressure

Answer 6

Definition: The external pressure that would be required to prevent solvent flow across a membrane.
Higher solute concentrations with higher osmotic pressures “draw” water

Question 7

what is Colloid osmotic pressure

Answer 7

Less permeable solutes exert more effective osmotic pressure. Albumin is impermeable to cell membranes and contributes to the osmotic pressure of capillary fluid as “colloid osmotic pressure”, or “oncotic pressure”, favoring fluid movement into the circulation at capillaries as opposed to out.

Question 8

Explain osmotic pressure of sodium vs urea

Answer 8

Sodium exerts greater osmotic pressure than urea, which exerts no osmotic pressure, Urea is a rapidly permeating solute across the cell membrane; glycerol is a slowly permeating solute.

Question 9

What is “Tonicity” of Solutes

Answer 9

Tonicity is the effective osmolarity of a solution, a measure of a solution’s ability to create an osmotic pressure gradient relative to another solution

Question 10

What effect does impermeant solutes have on a cell

Answer 10

Impermeant solutes such as sucrose: will stay in ECF. So a 300 mosm/L solution of sucrose is isotonic (cell volume won’t change).

Question 11

What effect does permeant solutes have on a cell

Answer 11

Permeant solutes such as urea will move through the membrane, increasing the ICF osmolality. This causes water to enter the cell, making the cell swell. So a 300 mosm/L solution of urea is hypotonic, even though the osmolality is the same as the sucrose.

Question 12

Relative osmolarity

Answer 12

is isosmotic, hyperosmotic, hypoosmotic – refers to the concentration of osmotically active particles in a solution relative to a reference solution

Question 13

Relative tonicity

Answer 13

is isotonic, hypertonic, hypotonic – considers only the particles that are impermeant to the biological membrane separating two solutions.

Question 14

what are the Body Fluid Compartments

Answer 14

The major body fluid compartments are intracellular fluid and extracellular fluid, with extracellular fluid being further subdivided to interstitial fluid and plasma

Question 15

In general, total body water (TBW) is what percent of body weight, in 70 kg person.

Answer 15

~60% or 42 L
Intracellular fluid (ICF) = 2/3 TBW (28L)
Extracellular fluid (ECF) = 1/3 TBW (14 L)
Interstitial fluid (ISF) = ~3/4 ECF (11 L)
Plasma vol (PV) = ~1/4 ECF (3L)

Question 16

Cations and anions of ICF

Answer 16

Cations in quantity from most to least are: K+ Mg++, Na+, Ca++
Anions in quantity from most to least are:
Hydrogen Phosphate HPO4–, Proteins, Bicarbonate HCO3-, Sulfate SO4–, Cl-

Question 17

Cations and anions of ECF

Answer 17

Cations in quantity from most to least are: Na+, K+, Mg++, Ca++
Plasma has > Na+, Ca++, and Mg++ than Interstitial fluid.
Anions in quantity from most to least are: Cl-,proteins, Bicarbonate HCO3-, Hydrogen Phosphate HPO4–, SO4-

Question 18

Importance of ECF Osmolarityto Homeostasis

Answer 18

1. The extracellular fluid (ECF) is the bathing medium for cells and therefore needs to be maintained.
2. Proper fluid and electrolyte levels are needed for normal cellular functions (pH balance, membrane potentials, second messenger systems, nutrient absorption, etc).
3. ECF osmolarity is under tight control, which allows the cellular environment to remain constant.
4. Small increases in plasma osmolarity (>2%) will elicit a thirst response, as well as a systemic response to retain fluid (i.e., decrease urine, sweat and saliva production), to keep cells from “shrinking” and the intracellular electrolyte concentrations from rising.

Question 19

To measure the fluid compartments, you can use different substances as indicators….

Answer 19

Evans Blue for plasma
Inulin for ECF
Tritiated Water for TBW
By extrapolation: Intracellular Fluid = TBW-ECF
Interstitial Fluid = ECF- Plasma

Volume (L) = Amount of indicator injected (mg)/ Final concentration of indicator (mg/L)

Question 20

Primary solute constituents of body fluid compartments: ICF

Answer 20

ICF
K+ 140
Misc. Phosphates 85
Albumin (A-) 54
Na+ 12
HCO3- 10
Cl- 3

Question 21

Primary solute constituents of body fluid compartments: ECF- interstitial fluid

Answer 21

ECF
Na+ 140
Cl- 117
HCO3- 27
K+ 5

Question 22

Primary solute constituents of body fluid compartments: Plasma

Answer 22

Na+ 140
Cl- 104
HCO3- 24
Albumin(A-) 14
K+ 5