There are two main types of alternative routes for blood supply. These are arterial and venous. The arterial blood carries oxygen-rich blood. Venous blood carries waste products, such as urea. Both types of blood supply the lung. Although these routes carry a similar amount of blood, arterial blood carries more toxins. Therefore, the arterial blood must be filtered before being used for other purposes.
There are other, smaller, and non-oxygen-rich blood circulation systems. The first type is the venular system. Venous blood flows from the heart to the various organs. Oxygen-rich blood goes from the lungs to the heart, while low-oxygen blood flows back to the heart. The second type of blood supply is via veins. These veins are thin, small vessels that exchange nutrients with tissues.