All life on Earth is considered carbon-based because the building blocks of all the living organisms, including human beings, are the biomolecules based on carbon: proteins, lipids, and carbohydrates.
Oxygen is an element necessary element for various cell processes, so humans may be referred to as oxygen-dependent life forms.
Humans, alongside all other life on this planet, have carbon as the basic element found in every thing in their body. One may say that carbon is omnipresent within living beings. With us, we breathe oxygen as we need it for basic PROCESSES of life, whereas carbon provides the actual structures of the things which take in that oxygen. Without carbon as the base element of life, Earth's creatures and general environment would be very different. I honestly hope that this answer makes sense to you.
When blood is rich in carbon dioxide it leaves the heart through the pulmonic valve, into the pulmonary artery and to the lungs. (All of which have to do with the circulatory system)
