Body Fluids

Question 1

Describe the major fluid compartments and the forces involved in fluid movement between them

Define Osmolarity and Osmolality

Answer 1

Total body water = 60%
ICF = 40% of total body water
ECF = 20% of total body water

ECF is divided into the plasma and interstitial compartments.

Osmosis determines the movement of water between compartments.

Osmolarity - the concentration of solute particles per L solution (Sodium is the main determinant of the ECF osmolarity)
Osmolality - the concentration of solute particles per Kg water

Question 2

How is salt and water balanced?

Answer 2

Osmoregulation and volume regulation. Osmoregulation is the control of the ECF osmolarity, and the body does this by adding/removing water, instead of Sodium. With volume regulation, wherever Sodium goes, water will follow.

The kidneys also work to control salt balance, using the RAAS (Na retention) and Atrial Natiuretic Peptides/ANP (Na excretion).