To eliminate carbon dioxide (CO2) from the body, the excretory system collaborates with the respiratory and circulatory systems. These three processes work together to maintain oxygen homeostasis by removing CO2. The excretory system is also responsible for fluid equilibrium. It washes waste products including urea from the body through urine and sweat; additionally, it removes toxic substances through feces. The respiratory system performs two important functions in relation to the excretory system. First, it provides oxygen for cells of the body that use it up as they break down chemicals or remove excess hydrogen ions from the blood. Second, it removes carbon dioxide from the body for disposal or conversion into other compounds. Because excessive amounts of CO2 can be harmful, the respiratory system keeps total body concentrations low by absorbing more from the atmosphere than it releases via breathing out. The difference between how much air enters versus leaves the lungs is called the alveolar-arterial gradient. This gradient is used by physicians to determine if a patient is experiencing cardiac failure or not. If the gradient is high, this means there's a lot of CO2 in the blood and therefore heart failure must be present. Finally, the circulatory system carries oxygen-rich blood and CO2-rich blood to different parts of the body to supply them with necessary nutrients and remove waste products.