What is the shape of red blood cells in human blood? What function do red blood cells perform?

Human red blood cells (erythrocytes) have a distinctive biconcave disc shape, meaning they are round and flat with both sides curved inward like a doughnut without a hole. This unique shape, resembling two saucers pressed together rim-to-rim, provides a large surface area-to-volume ratio that optimizes their primary function. Mature red blood cells in mammals lack a nucleus and most organelles, maximizing internal space for their oxygen-carrying machinery.

The primary function of red blood cells is to transport oxygen from the lungs to all body tissues and carry carbon dioxide back to the lungs for exhalation. They accomplish this through hemoglobin, an iron-containing protein that binds oxygen molecules and gives blood its red color. The biconcave shape enhances gas exchange efficiency by increasing the surface area available for diffusion and allows the cells to deform and squeeze through narrow capillaries without rupturing. Additionally, this shape creates a shorter diffusion distance for gases to move between the cell's center and its membrane. A healthy adult has approximately 25 trillion red blood cells circulating at any given time, each one making the complete circuit through the body about once per minute, demonstrating the remarkable efficiency of this oxygen delivery system.