The cell is the primary structural unit of life. In humans, each cell is bounded by a membrane which houses the genome and other specialised components that allow the cell to perform its vital functions for the body. In the human body, specialised cells are grouped together to form connective tissues, muscles, organs, nerve systems and the brain. They receive their nutrition via the circulation of the blood which provides nutrients and removes waste products. Certain cells exist in the blood as individual cells, such as the red blood cells which deliver oxygen and remove carbon dioxide. Another example of individual cells are the white blood cells, which provide immune protection against infection.