blood cell

Vampire spleen

The average red blood cell, known as an erythrocyte, rushes round your circulation for an average of 120 days. Over this time it gradually gets damaged, some of this is due to external shear forces from squeezing through capillaries or being blasted along arteries. Most of it is actually internal; carrying oxygen is a hazardous occupation. Although essential for life, oxygen reacts with proteins and other molecules inside the erythrocyte causing oxidative damage. Antioxidant proteins mop up most of this but their cleaning and recycling action requires energy. Physically maintaining their specialised biconcave shape is also energy intensive and together these processes place large demands on erythrocyte metabolism. Unfortunately, in order to carry as much haemoglobin – and therefore oxygen – as possible, erythrocytes have sacrificed their nucleus and other internal organelles, leaving them with very limited capacity for repair. One of the main things that can happen as they get exhausted is losing their shape and flexibility. This is potentially dangerous as it can lead to them getting trapped in capillaries, blocking the blood supply to tissues.

As the blood travels through the spleen it has to pass through sinusoidal passages criss-crossed with fibres. Lurking among these cords are macrophages on the look out for ageing or misshapen erythrocytes. Some of these will be revealed as their changed shape or reduced flexibility leads to them becoming trapped in the meshwork of cables. Others carry tell-tale signs of damage on their surface which are recognised by the macrophages. The role of these macrophages is to devour the defunct erythrocytes and in doing so, carry out two important functions. One is to reduce the risk of deformed cells becoming trapped elsewhere in the body which could have disastrous consequences. The other is to recycle the haemoglobin, in particular the iron contained at its centre. These vampire macrophages are an important reserve for the body’s overall stores of iron.

You can see examples of the different shapes adopted by red blood cells in Drawing Blood. Sometimes, the red blood cells only have a small amount of damage and manage to escape with just that part nibbled off. These ‘bite cells’, looking like a half-eaten biscuit, are called degmacytes.

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