Why do red blood cells collect at the bottom of a tube when centrifuged?

When a tube of blood is centrifuged, the red blood cells collect at the bottom of the tube due to the force of gravity. Red blood cells are heavier than plasma and other components of the blood, so when the tube is spun at a high speed, the red blood cells move to the bottom of the tube. This process is called sedimentation.

The rate of sedimentation depends on several factors, including the size, density, and shape of the red blood cells. Larger, denser red blood cells sediment faster than smaller, less dense red blood cells. This is why red blood cells from people with certain conditions, such as anemia, sediment more slowly than red blood cells from healthy people.

The sedimentation rate is also affected by the temperature and viscosity of the plasma. Higher temperatures and lower viscosities increase the sedimentation rate. This is why red blood cells sediment faster at room temperature than they do in a cold environment.

Centrifugation is a useful technique for separating red blood cells from other components of the blood. It is used in a variety of laboratory tests, including blood typing and hemoglobin measurement.