Final answer:
Red blood cells are hypertonic to pure water, which is hypotonic. IV solutions are designed to be isotonic with blood serum to prevent RBCs from swelling or shrinking due to osmotic pressure.
Step-by-step explanation:
Red blood cells are hypertonic compared to an IV bag of pure water. When red blood cells (RBCs) are placed in a hypotonic solution, such as pure water, water will rush into the cells due to osmosis. This can cause the red blood cells to swell and potentially lyse (burst). To avoid this, IV fluids are made to be isotonic to blood serum so that the concentration of solutes inside and outside of the RBCs is the same, thus preventing osmotic pressure from changing the shape of the cells.
Specifically, a hypertonic solution would cause water to leave the RBC, making it shrivel up. An isotonic solution means the external environment has the same concentration of solutes as the RBC's internal environment, which is the desired state for maintaining RBC shape and functionality. In contrast, a hypotonic solution has a lower concentration of solutes than the interior of the RBC, leading to swelling and possible rupture.
Note that the use of isotonic solutions is critical for medical treatments involving IV fluids. A standard isotonic IV solution is 0.9% saline, which approximates the osmolarity of blood serum.