Oxygen enters through the mouth and flows to the lungs. After it passes through the bronchial tubes, it moves to the alveoli. Then the air passes through the walls and goes into the capillaries.
Blood enters the heart through two large veins, (Inferior and superior vena cava) emptiying the oxygen-poor blood into the right atrium of the heart.
As the atrium contracts, blood flows from your right atrium into your right ventricle then through to the pulmonic valve and back to the lungs.
They flow through the pulmonary artery to go to the lungs to become oxygenated.
The pulmonary veins empty the now oxygen-rich blood from the lungs into the left atrium, the blood then flows into the left ventricle.
When the ventricle is full, the mitral valve shuts, preventing blood flow backwards into the atrium. The ventricle contracts, blood then leaves the heart through the aortic valve, and into the aorta and to the body.