The mammalian circulatory system is a closed double circulation system where blood is pumped from the heart to lungs and throughout the body in separate loops. Tissue fluid leaks from capillaries into tissues and is almost identical to blood plasma but contains fewer proteins and no red blood cells. About 90% of tissue fluid returns to capillaries while the remaining 10% is collected by lymphatic vessels containing one-way valves and transported via lymph nodes, which help protect the body from disease. Blood itself is composed of plasma and three main cell types.