Fluid Dynamics in Physiology

Measurement of Interstitial Fluid

• Indicators for measuring interstitial fluid include 3H2O and Na or 125I-albumin.
• 125I-albumin is a specific marker used in assessing interstitial fluid concentrations.

Effects of Isotonic Saline Infusion

• Infusion of isotonic saline alters extracellular fluid volume, having no significant change to intracellular fluid volume.
• Extracellular fluid osmolarity remains stable during isotonic saline infusion.

Decrease in Interstitial Protein Concentration

• Increasing capillary hydrostatic pressure decreases interstitial protein concentration.
• Increasing interstitial hydrostatic pressure or capillary oncotic pressure also affects protein concentration.

Plasma Osmolarity Increases

• The greatest increase in plasma osmolarity is caused by increasing plasma Na+ by 10 mEq/L.
• Other solutes like K+, glucose, and urea produce smaller osmolarity changes.

Fluid Shifts and Infusions

• Hypotonic fluids cause fluid shifts from extracellular to intracellular compartments.
• Isotonic and hypertonic fluids do not lead to significant fluid shifts in this direction.

Outcomes of Isotonic Saline Infusion

• Infusion of 15 L isotonic saline leads to increased plasma Na+ after equilibration.
• It results in fluid shifts from intracellular to extracellular fluid compartments and expands extracellular fluid volume.

Effects of Glucose Addition

• Adding 500 mEq of glucose to plasma increases extracellular fluid volume after equilibrating.
• It also causes a decrease in plasma Na+ and elevates plasma osmolarity.