Blood

The main function of red blood cells is to transport oxygen from the lungs to all respiring cells in the body. They contain haemoglobin, which binds to oxygen.

White blood cells defend the body against pathogens. Some, like phagocytes, engulf pathogens by phagocytosis, while others, like lymphocytes, produce antibodies that target specific pathogens.

Platelets are responsible for blood clotting, which prevents blood loss and the entry of pathogens into the body. They initiate the cascade that forms a blood clot when a blood vessel is damaged.

Plasma is the liquid component of blood and transports various substances, including blood cells, ions, nutrients (e.g., glucose, amino acids), urea (waste product), hormones (e.g., insulin), and carbon dioxide.

Blood clotting prevents excessive blood loss from damaged blood vessels and prevents the entry of pathogens into the body through open wounds.

Lymphocytes are a type of white blood cell that produces antibodies. These antibodies are specific to particular antigens found on pathogens, marking them for destruction or neutralizing them.

Phagocytosis is the process by which phagocytes engulf and digest pathogens, cellular debris, and foreign particles. The phagocyte extends its cytoplasm to surround and internalize the particle into a vacuole, where it is then broken down by enzymes.

Blood clotting begins with platelet activation at the site of injury. This triggers a cascade where soluble fibrinogen is converted into insoluble fibrin. Fibrin forms a mesh that traps blood cells and platelets, forming a clot.

Haemoglobin, found in red blood cells, contains iron and binds reversibly to oxygen. It loads oxygen in the lungs, forming oxyhaemoglobin, and then releases oxygen to respiring tissues throughout the body, where oxygen concentration is lower.