Body Fluid Compartments

Question 1

What are the normal values for total body water and what factors affect it?

Answer 1

60% for males, 50% females

Age: Inversely related

Adipose tissue: Inversely related

Gender - less in females

Question 2

What are the three major fluid compartments in the body?

Answer 2

Blood Plasma

Interstitial fluid (ISF)

Intracellular fluid (ICF) - about 2/3 of TBW and 40% of body weight

Question 3

What comprises the extracellular fluid?

Answer 3

Plasma and ISF

About 20% of body weight and 1/3 of TBW

Question 4

What is the fourth, transcellular fluid compartment?

Answer 4

Consists of fluid in transit in the lumina of epithelial organs (e.g. gall bladder, stomach, intestines, and bladder)

Cerebrospinal fluid

Intraocular fluid

Question 5

What four organs communicate the ECF with the external environment?

Answer 5

Alimentary canal

Lungs

Kidneys

Skin

Question 6

What are the major ions in the ECF and ICF?

Answer 6

Na and Cl - ECF

K - ICF

Question 7

What is the difference in protein concentration between the plasma and ICF?

Answer 7

Capillaries are sparingly permeable to proteins so there is a much higher protein concentration in the plasma

Question 8

What is a good clinical approximation for total osmolality?

Answer 8

Osmolality = 2[Na] + [glucose]/18 + BUN/2.8

About 295mOsm/L normally

Question 9

What is the underlying principle of the dilution method of determining distribution of water in body compartments?

Answer 9

V = Q/(Q/V)

By injecting a known quantity of substance (Q) and then measuring steady-state concentration, we can calculate volume (V)

Question 10

What are three characteristics that a substance must have to be used for the dilution method?

Answer 10

Nontoxic

Neither synthesized nor metabolized

Does not cause shifts in fluid distribution among the compartments

Question 11

How is the volume of the ISF calculated?

Answer 11

Visf = Vecf - PV

Question 12

How is the volume of the ICF calculated?

Answer 12

Vicf = TBW - Vecf

Question 13

What occurs in an isosmotic water shift?

Answer 13

Change in ECF volume only

E.g. saline infusion, hemorrhage, burns, GI loss in cholera

Question 14

What occurs in hyperosmotic overhydration?

Answer 14

Cells shrink due to fluid loss caused by hyperosmolality of the ECF

Salt gain > water gain

Question 15

What occurs in hyperosmotic dehydration?

Answer 15

Salt loss < water loss

Causes decreases in both ICF and ECF volume, with an increase in osmolality

E.g. sweating, low ADH, hypodipsia

Question 16

What are some common causes of decreased ADH secretion?

Answer 16

Head trauma, hyposic or ischemic encephalopathy

Question 17

What are some symptoms of hyperosmolarity?

Answer 17

Primarily neurologic

LEthargy, weakness, irritability

Can progress to twitching, seizures, coma and death

Question 18

What is hyposmotic overhydration?

Answer 18

Salt gain < water gain

Increase in volume of ICF and ECF, decrease in osmolality

Cells swell

E.g. Post-op SIADH, Pure water IV, 5% glucose IV

Question 19

What are some common causes of Syndrome of Inappropriate ADH secretion (SIADH)?

Answer 19

Neuropsychiatric disorders, drugs, pulmonary disease, or Post-op patient

Question 20

What is hyposmotic dehydration?

Answer 20

Salt loss > water loss

Question 21

What are the symptoms of hyposmolarity?

Answer 21

Mild - nausea and malaise

Moderate - headache and lethargy

Severe - seizures, coma, permanent neurologic deficits, or death

Question 22

What are the brain’s adaptations to hyposmolarity?

Answer 22

Cerebral swelling will increase the flow of brain ISF toward the cerebrospinal fluid, decreasing the amount of swelling

Solutes move out of brain cells, decreasing the amount of swelling