=Blood= Blood is a body fluid that acts as a transport medium, carrying nutrients, gasses and other molecules around the body. '''Blood consists of the following components:''' [image:] • '''Red Blood Cells''' (Erythrocytes) - contain a protein called haemoglobin that binds and transports oxygen. Mature red blood cells lack a nucleus and have a bi-concave shape that increases their surface area. Humans have around 4-6 million red blood cells. Red blood cells make up about 45% of the blood volume. • '''Plasma''' - a "straw yellow" coloured fluid that transports dissolved nutrients and carbon dioxide. Blood plasma is mostly water but also carries important proteins such as albumin which regulates blood volume / pressure and antibodies (immunoglobulins) that help help the immune system detect foreign substances and invaders. Plasma makes up about 55% of the blood volume. [image:] • '''White Blood Cells''' (Leukocytes) - are part of the immune system and play an important role in removing foreign substances and pathogens (disease causing micro-organisms). They also remove old and damaged body cells. White blood cells make up about 0.7% of blood volume. • '''Platelets''' (Thrombocytes) - Play an essential role in blood clotting, the the conversion of blood from a liquid to a semi-solid gel to stop bleeding from a damaged blood vessel. They make up less than 0.1% of blood volume. [image:] An animation of a typical human red blood cell cycle in the circulatory system. This animation occurs at real time (20 seconds of cycle) and shows the red blood cell deform as it enters capillaries, as well as changing colour as it alternates in states of oxygenation along the circulatory system.