What's Actually in Your Blood

Blood may look like one red liquid, but it contains several components with different roles. Understanding plasma, red cells, white cells and platelets makes common blood test language easier to follow without turning one result into a diagnosis.

Blood contains plasma and formed elements Blood is made from a liquid called plasma and suspended formed elements. The formed elements include red blood cells, white blood cells and platelets, which are small cell fragments. In a typical blood sample, plasma makes up a little over half of the volume. Red cells make up most of the remainder. White cells and platelets together usually form a very thin layer. Plasma carries dissolved substances Plasma is mostly water. It carries proteins, salts, nutrients, hormones, waste products and clotting factors around the body. Plasma is therefore more than a background liquid. Its composition helps maintain blood volume, transport substances and support clotting and other physiological processes. Red blood cells carry oxygen Red blood cells contain haemoglobin , an iron containing protein that binds oxygen in the lungs and releases it in body tissues. Red cells also contribute to carbon dioxide transport. Some carbon dioxide binds to haemoglobin, while much of it is carried in the blood as bicarbonate after chemical conversion. Red blood cells are produced in bone marrow and normally circulate for about 120 days before being removed and replaced. Anaemia means the haemoglobin concentration is below the expected range. It has many possible causes. Tiredness, breathlessness and paler skin can occur, but those symptoms are not specific to anaemia. White blood cells support immune defence White blood cells are a group of immune cells with different roles in defence and inflammation. Some respond rapidly to infection, while others coordinate or specialise in different parts of the immune response. A raised white blood cell count can occur with infection, but it is not an infection test by itself. Inflammation, medicines, physiological stress and blood disorders can also change the count. A low white blood cell count can increase infection risk, depending on which cells are reduced and how low the count is. Platelets help form a haemostatic plug Platelets are small fragments of larger bone marrow cells. When a blood vessel is damaged, platelets adhere to the injured area, become activated and join together. They work with the vessel wall and clotting proteins to limit blood loss. The next lesson follows this process in more detail. What a full blood count measures A full blood count , or FBC, measures several features at the same time. These include haemoglobin, red cell measurements, white cell counts and platelet count. The pattern matters more than one number in isolation. A low haemoglobin establishes anaemia, for example, but it does not reveal why the anaemia developed. Blood results are therefore interpreted alongside symptoms, medical history, medicines, examination findings and other tests when needed.

Blood is a mixture of plasma, red blood cells, white blood cells and platelets. Each contributes to transport, defence or haemostasis, and blood test results need to be interpreted as a pattern rather than as isolated diagnoses.