A red blood cell, or erythrocyte, has a hardworking but finite lifespan of about 120 days. Over this period, it endures constant stress while squeezing through the body's narrowest capillaries. This repeated mechanical strain takes a toll on its cell membrane, gradually reducing its flexibility and deformability.

This process of ageing is called senescence. As the erythrocyte becomes more rigid and its surface proteins change, it's marked for removal. The primary site for this cellular retirement is the spleen, an organ that acts as a sophisticated quality-control filter for the blood.

The spleen's red pulp contains a network of narrow passages called the splenic cords. Healthy, flexible red blood cells can easily deform to pass through these tight spaces and re-enter circulation. However, older, stiffer erythrocytes are unable to make the squeeze. They become physically trapped, a process known as splenic sequestration.

Once trapped in the splenic cords, the senescent red blood cells are cleared from the body by specialized immune cells.

Resident macrophages within the spleen are responsible for identifying and destroying these trapped, ageing erythrocytes. These large scavenger cells recognise specific signals on the surface of senescent red blood cells, triggering their engulfment through a process called phagocytosis.

Inside the macrophage, the old red blood cell is broken down. Its components, most importantly the iron-rich haemoglobin molecule, are released. This marks the end of the erythrocyte's life cycle and the beginning of a new process: the recycling of its valuable parts.

Now let's check your understanding of how red blood cells are retired from service.

The breakdown of haemoglobin is a critical step in recycling iron and other components, which we will explore next.