USMLE Step 1: Physiology 6. Infusion of isotonic saline FIgure 1-2-7 The infusion of isotonic saline would increase the volume but not the effective osmolar. ty of the extracellular space. Because there was no change in osmolarity, the intracellular volume is unchanged. An additional point is that most of the saline would enter the inter stitial space. a much smaller volume would remain in the intravascular compartment. If plasma, which does contain protein, was infused, however, almost all of the fluid would remain in the vascular space because the proteins do not easily cross capillary membranes 7, Infusion of hypertonic saline(or hypertonic mannitol; mannitol does not cross cell mem branes easily) The hypertonic saline would increase both the volume and effective osmolarity of the extracellular compartment. The increased osmolarity would cause a fluid shift from the intracellular to the extracellular space, reducing intracellular volume and further increas- Ing extracelular volume dical....- USMLEStep1: Physiology 16 KAPLAN .. . . me leaI 6. Infusion of isotonic saline: Figure 1-2-7 The infusion of isotonic saline would increasethe volume but not the effective osmolari￾ty of the extracellular space.Becausethere was no changein osmolarity, the intracellular volume is unchanged. An additional point is that most of the salinewould enter the inter￾stitial space.A much smaller volume would remain in the intravascular compartment. If plasma, which does contain protein, was infused, however, almost all of the fluid would remain in the vascularspace because the proteins do not easilycrosscapillary membranes. 7. Infusion of hypertonic saline (or hypertonic mannitol; mannitol does not cross cellmem￾branes easily): ~-------------- Figure 1-2-8 The hypertonic saline would increase both the volume and effective osmolarity of the extracellular compartment. The increased osmolarity would cause a fluid shift from the intracellular to the extracellular space, reducing intracellular volume and further increas￾ing extracellular volume