What is a red blood cell?

A red blood cell is described as a biconcave disc - a disc that has been pushed in on both sides. They are around 7 to 8 micrometers wide. In 1 millilitre of blood there are typically around 5 billion red blood cells reference.

Each red blood cell contains several hundred million molecules of haemoglobin see note.

Image of red blood cells from Wikimedia commons link