Blood - Copyright Cesar Astudillo © César Astudillo

By Royal Society Rosalind Franklin Award Winner Sunetra Gupta

What is blood?

Blood is a vital fluid that carries a fleet of specialized cells through our bodies’ canals. Its numerous roles include transporting food and oxygen to where it’s needed, distributing a variety of essential substances (such as hormones), regulating temperature, removing the waste products of metabolism, and detecting and fighting infections.

Why is it red?

The distribution of oxygen is performed by "red" blood cells; when these cells trap oxygen in molecules of haemoglobin, they turn red. All vertebrates use haemoglobin, but some invertebrates (such as octopuses) use haemocyanin, which makes their blood turn blue when oxygenated. Very small invertebrates (such as insects) don't have to transport oxygen, so their blood has no colour at all.

What problems can we have with our blood?

Blood is a very attractive medium for many dangerous human pathogens like malaria and HIV, as they can hide in its nutrient rich cells and get a free ride through the body to some of their target organs. There are also many different types of harmful blood disorders (like haemophilia). However, some blood disorders can actually defend against infectious diseases, like sickle cell anaemia which protects against death from malaria.

When did our understanding of blood improve?

Until the 19th century it was assumed that the body was composed of four basic substances, or ‘humors’; blood, black and yellow bile, and phlegm. These ‘humors’ were believed to determine our temperament - our physical and mental make-up. In Robert Boyle’s 17th century account of a blood transfusion between dogs, published by the Royal Society, a major concern was that the procedure would change the behavior of the recipient animal!