The circulatory or cardiovascular system's ability to deliver oxygen throughout the body depends on proper functioning of the respiratory system. The interactions between the cardiovascular and respiratory systems are best demonstrated by following the path of a red blood cell starting in the heart and traveling through the lungs.
A red blood cell that has just returned from delivering oxygen and that has brought back carbon dioxide would be in the right upper chamber of the heart or in the right atrium. When the atrium contracts, the cell is pumped into the right lower chamber of the heart, or the right ventricle. When that ventricle contracts, the red blood cell is pumped out of the heart through the pulmonary artery to the lungs.
In the lungs, the red blood cell enters tiny blood vessels that come into close contact with the walls of the alveoli air sacs of the lungs. The carbon dioxide in the red blood cell passes through the walls into the alveoli while the oxygen in the alveoli air passes into the red blood cell. The red blood cell then returns to the heart via the pulmonary vein.
From the pulmonary vein, the red blood cell enters the left atrium of the heart and then the left ventricle. The part of the heart muscle powering the left ventricle is very strong because it has to push the blood out to the whole body. The red blood cell is pumped out of the left ventricle via the aorta artery and eventually reaches the capillaries leading to the individual cells. There the cells absorb the oxygen from the red blood cell and pass on their waste carbon dioxide. The red blood cell returns to the right atrium of the heart via the veins to complete the cycle.
These circulatory and respiratory system interactions are ones that humans and higher animals such as mammals and birds share and that represent one of the basic functions of their bodies. Only when these two systems work and interact properly can the human or animal carry out other functions such as looking for food or reproducing.
