Blood

Blood is one of our most important bodily fluids. As blood flows around our bodies, it carries essential substances to our cells, and removes wastes.

Blood is pumped around the body by the heart, which is a specialised muscle. It flows through wide tubes called arteries and veins, and narrower ones called capillaries. There are also specialised blood vessels that take blood in and out of bones. Artificial blood vessels are being developed for medical purposes.

The pumping of the heart means the blood is always under pressure. Having high blood pressure or hypertension increases a person’s risk of heart attack or stroke. Medications are available to reduce blood pressure, and exercise can also help.

All backboned animals have blood, including birds and fish, but other kinds of animals do not. For example, insects have a fluid called haemolymph that does not travel in specialised tubes.

Human blood contains several kinds of cell. The red blood cells give blood its colour and carry oxygen to the body’s cells and organs. There are also white blood cells, which are part of the immune system and help fight off infection. Finally, platelets cause blood to clot and help heal wounds.

Not everyone’s blood is identical. Each of us has a blood type, which is determined by the chemicals on the outer surfaces of the red blood cells. There are dozens of possible combinations. The most well-known are A, B, AB and O. These refer to two sugars called A and B: we each either have one of them (A, B), both (AB), or neither (O).

Blood type is crucial when a person needs a blood donation, for example to undergo major surgery. Type-O blood can be given to anyone, but is in short supply. The other three types can only be given to people with the same type: otherwise, the recipient’s body will reject the blood. One possible solution is to transform these other bloods into type-O blood. It should also be possible to make artificial blood – for example by harvesting stem cells from blood.

There are other potential risks from blood donations, even if the blood type is correct. In the 1980s, many people became infected with HIV – the virus that causes AIDS – after receiving blood donations contaminated with the virus. Donor blood is now treated to prevent this. However, in recent years evidence has emerged that Alzheimer’s disease may be spread through blood transfusions. This idea remains controversial.

Beyond blood type, our blood contains an enormous amount of information about us. This is why doctors often take blood samples, and why blood at crime scenes can be used to identify who was present. By examining the chemical makeup of blood, it is possible to tell if a pregnant woman is likely to suffer pre-eclampsia, whether a person has cancer, and even to diagnose genetic disorders in unborn fetuses. Scientists have also studied the blood of extremely elderly people to track the effects of ageing and identify possible limits to human lifespan.

Studies of ageing blood have revealed that young people’s blood is significantly different to that of older people. This has raised the possibility that a transfusion of young blood could partially rejuvenate an elderly person, for instance reversing the symptoms of diseases of ageing like Parkinson’s. Tests in mice have been promising, and some of the proteins involved have been identified. Rather than relying on donors, it may be possible to make old blood young again.

Does it work? In recent years studies have been performed in which people with Alzheimer’s disease were given blood from young people to see if it reversed the brain degeneration. There have been some seemingly promising results, but some of the studies have been criticised for poor design, and there is concern that private companies are rushing to exploit the idea before the facts are in.