What happens when red blood cells are placed in normal saline?

When red blood cells are exposed to normal saline (also called isotonic saline or 0.9% NaCl solution), depending on the concentration difference of ions on both sides of the cell membrane, water movement may occur due to the process of osmosis. Here's what happens:

1. Hypoosmolality: Normal saline has an osmolality of about 300 milliosmoles per liter (mOsm/L). If red blood cells are placed in normal saline, the concentration of solutes outside the cells is balanced with that inside the cells, creating an isotonic environment. In this case, the net movement of water across the cell membrane is minimal. The cells maintain their normal size and shape, preserving their integrity.

2. Osmolality Differences: If red blood cells are exposed to normal saline with a higher or lower osmolality compared to the cells' internal environment, water movement occurs due to osmosis:

- Hyperosmolality (Outside > Inside): In a hypertonic solution (osmolality outside the cells is higher than inside), the red blood cells will lose water through osmosis. Water moves from the inside of the cells to the surrounding saline to balance the concentrations. This process causes the red blood cells to shrink and become crenated, appearing as small, irregular, spiky cells.

- Hyposmolality (Inside > Outside): Conversely, if red blood cells are placed in a hypotonic solution (osmolality outside the cells is lower than inside), water will enter the cells due to osmosis. The cells swell and expand as they absorb water. As the cells reach their maximal capacity, their membrane may rupture, leading to hemolysis—the bursting and breaking down of red blood cells.

It's important to note that the rate and extent of water movement and the resulting changes in red blood cell shape depend on the specific concentrations and conditions of the saline solution.