The cardiovascular/circulatory system and respiratory system are two systems in the human body that are vital. They continuously provide the body with a flow of nutrients and oxygen and remove waste products. Any breakdown of or interruption to these systems can result in permanent damage to your body, sometimes within minutes. In addition, these two systems are the ones 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 to live. Every cell in your body needs oxygen. The respiratory system brings oxygen into the body and removes carbon dioxide from the body via the lungs. The cardiovascular system governs the circulation of blood and transport of oxygen and carbon dioxide. It mainly consists of the heart, arteries, veins, and capillaries.