Blood is the fluid in our bodies that carries nutrients and oxygen to cells and takes away waste products such as carbon dioxide. It makes up about 7% of the weight of the human body! Blood contains three different types of cell:
Red and white blood cells look pretty different don’t they? Image by WhatamIdoing for Wikimedia
Red blood cells carry oxygen around the body
White blood cells are part of the immune system and help to fight infection
Platelets help blood to clot to stop bleeding when your skin is cut
All of these cell types float in a liquid called blood plasma, which is mostly water but also contains nutrients, gases, proteins, glucose and hormones. Blood is 60% plasma and 40% blood cells.
Red blood cells contain a protein called haemoglobin, which contains iron. This iron combines with oxygen to make our blood red, and without it our red blood cells wouldn’t be able to carry oxygen. So why do our veins look blue? Well apparently this is all to do with the way light is reflected back to the eye.
There are around 30 recognised blood groups, though we usually use the simplified ABO system with 4 different types, O, A, B and AB. In the UK, O is the most common (apparently Elvis Presley was in this group) and AB is the rarest (Barack Obama is this type).