Like the respiratory system, the circulatory system serves to transport oxygen throughout the body while removing carbon dioxide. In addition, the circulatory system transports nutrients from the digestive system.

Force is applied to change an object's speed or direction of motion.

Chemical waste like excess water, minerals, and salt are filtered from the blood by the kidneys and secreted into the urine. Urine is transported from the kidneys to the bladder through ureters.

Cells are classified into one of two groups based on whether or not they have a nucleus. Eukaryotic cells have a nucleus, prokaryotic cells do not have a nucleus and therefore have a less complex structure than eukaryotic cells.

A population is a group of organisms of the same species who live in the same area at the same time. A community is a group of populations living and interacting with each other in an area.