Fluid Compartments

Question 1

Describe TBW

Answer 1

TBW varies depending upon age, size, gender and fat content.

60% body weight male, 50% female

Divided into extracellular and intracellular.

Intracellular - 2/3 TBW, contained within phospholipid bilayer,

Extracellular - 1/3 TBW. Further divided into:

• interstitial fluid (75%)
• plasma (25%)
• trans cellular fluids (third spaces ie. CSF, sweat, saliva etc)

Total blood volume = 5-6L

Question 2

How is body water compartment volumes estimated?

Answer 2

Dilutional techniques are used to estimate this;

Known mass of indicator dye administered to compartment, it should distribute but remain within it.

Conc dye is measured.

Vd = dose / conc.

Can derive other compartments.

Ie inulin stays in ECF

Radiolabelled albumin stays in plasma

Question 3

What factors regulate TBW?

Answer 3

Intracellular water balance regulated by amount of water present- mainly via ADH. Hyperosmolarity detected by osmoreceptors in hypothalamus, outside BBB stimulates thirst + ADH.

Sodium balance governs ECF