The cardiovascular/circulatory system and respiratory system are two systems in the human body that are extremely vital. They continuously provide the body with a flow of nutrients and oxygen and take care of the removal of waste products. Any breakdown to these systems can result in permanent damage to your body within minutes. In addition, these two body systems are the most influenced by the underwater environment.
Every part of the human body constantly needs oxygen. Oxygen is needed for what is called the oxidative metabolism: the oxidation of nutrients in order to supply energy required for living. All body cells need oxygen. The respiratory system takes care of bringing oxygen into the body and removing carbon dioxide from the body via the lungs. The cardiovascular system takes care of the circulation of blood and transport of oxygen and carbon dioxide. It mainly consists of the heart, arteries, veins, and capillaries.