Cells come in various sizes and shapes, depending on their function. This diversity can be seen across all forms of life. In unicellular organisms, such as bacteria or yeast, a single cell performs all the required functions. Conversely, in multicellular organisms, such as humans, that consist of over a hundred trillion cells, individual cells work together to perform many specialized functions. For example, macrophages engulf and phagocytose invading pathogens, while endocrine cells produce hormones such as adrenaline and nor-adrenaline. Human cells come in many different shapes. The disc shape of red blood cells allows the cells to flow smoothly through the narrow blood vessel, and the spindle shape of smooth muscle cells increases their surface area leading to increased cell-cell contact. Cells also vary greatly in their size. Sperm cells are only about four micrometers, but neuronal cells may have long cytoplasmic extensions of up to several meters.