Tissue ventilation is a medical term for the normal situation where arterial blood is more oxygenated and venous blood is less oxygenated. The tissues of the body supply oxygen to the tissues through the arterial blood supply, and after the tissues absorb oxygen, the tissues return through the venous blood with lower oxygen content, which is commonly known as tissue ventilation. The skin, subcutaneous tissues, or internal organs such as the heart, liver, lungs, kidneys, spleen, and even the bones include two blood systems, an arterial system that supplies blood and a venous system that returns blood.