Blood is the body’s internal transport system. It flows continuously through a network of arteries, veins and tiny capillaries, delivering essential substances to every cell and carrying away waste products. Without blood, organs like the brain, heart and kidneys would stop working within minutes because they would no longer receive oxygen and nutrients.

The main jobs of blood

Blood has four key functions:

• Transport
  Red blood cells carry oxygen from the lungs to all parts of the body and bring carbon dioxide back to the lungs to be exhaled. Blood also transports nutrients from the digestive system, hormones from glands, and waste products to organs that remove them (kidneys, liver, lungs).
  
  
• Protection
  White blood cells and antibodies travel in blood and are a major part of the immune system. They help detect and fight infections caused by bacteria, viruses, fungi and parasites. When a germ enters the body, specific white cells and antibodies act to destroy or control it.
  
  
• Clotting and repair
  When a blood vessel is damaged (for example from a cut or injury), platelets and clotting proteins in the plasma come together to form a clot. This clot helps stop bleeding and creates a temporary “plug” so that healing can begin.
  
  
• Regulation
  Blood helps regulate body temperature by distributing heat, and it helps maintain the balance of fluids, salts (electrolytes) and pH (acidity) across the body. This stable internal environment is called homeostasis.