Understanding Red Blood Cells in Isotonic Solutions

This question asks about what happens to human red blood cells (RBCs) when they are placed in normal saline. To answer this, we need to understand the concepts of osmosis and tonicity, especially how they relate to cell membranes.

What is Osmosis?

Osmosis is the movement of water molecules across a selectively permeable membrane from an area of higher water concentration (lower solute concentration) to an area of lower water concentration (higher solute concentration). This movement happens until equilibrium is reached.

Tonicity and Solutions

Tonicity describes the effective osmotic pressure gradient between two solutions, separated by a partially permeable membrane. It's basically about how a solution affects cell volume. There are three types of solutions:

• Isotonic Solution: A solution that has the same solute concentration (and thus the same osmotic pressure) as another solution, typically the cell's cytoplasm. When a cell is placed in an isotonic solution, there is no net movement of water into or out of the cell, so the cell size remains unchanged.
• Hypotonic Solution: A solution that has a lower solute concentration (and thus lower osmotic pressure) than another solution. When a cell is placed in a hypotonic solution, water moves into the cell by osmosis, causing it to swell. In extreme cases, animal cells like RBCs can burst (lyse).
• Hypertonic Solution: A solution that has a higher solute concentration (and thus higher osmotic pressure) than another solution. When a cell is placed in a hypertonic solution, water moves out of the cell by osmosis, causing it to shrink (crenate).

Normal Saline and Red Blood Cells

Normal saline is a solution of sodium chloride (NaCl) in water. It is approximately 0.9% NaCl by weight. This specific concentration is used because it has the same osmotic pressure as blood plasma. In other words, normal saline is an isotonic solution to human blood cells.

When red blood cells are placed in normal saline:

• The concentration of solutes inside the RBCs is similar to the concentration of solutes in the normal saline solution.
• Therefore, the water potential inside the RBCs is similar to the water potential outside in the normal saline.
• There is equal movement of water molecules into and out of the RBCs, resulting in no net movement of water.

Since there is no net movement of water, the volume and shape of the red blood cells do not change significantly. Their diameters remain unaffected.

Analyzing the Options

• The RBCs will swell: This would happen in a hypotonic solution, where water moves into the cells. Normal saline is not hypotonic to blood.
• The RBCs will swell and burst: This is an extreme result of being in a hypotonic solution (lysis). Normal saline is not hypotonic.
• The RBCs will shrink: This would happen in a hypertonic solution, where water moves out of the cells. Normal saline is not hypertonic to blood.
• No change in the diameters of the RBCs: This is what happens in an isotonic solution, where there is no net water movement. Normal saline is isotonic to blood.

Therefore, when red blood cells are isolated and diluted in normal saline, there will be no change in their diameters because normal saline is an isotonic solution.

Revision Table: Tonicity Effects on Red Blood Cells

Additional Information: Clinical Significance of Isotonic Solutions

The fact that normal saline is isotonic to blood is clinically very important. It's widely used for intravenous (IV) fluid administration to rehydrate patients because it doesn't cause significant swelling or shrinking of blood cells. Using hypotonic or hypertonic solutions for large volume IV hydration can damage red blood cells and disrupt the body's fluid balance.