New blood entering
the right atrium is pumped by the tricuspid valve, to the right ventricle. Then, the
pulmonary valve opens to the pulmonary artery. This artery has to carry the blood to
the lungs to get oxygen. This is the only artery in the body that carries
deoxygenated blood. Once the blood reaches the lungs, carbon dioxide is diffused
into the lungs. Carbon dioxide is a cell's waste product after using oxygen. Then the
blood must get its oxygen. This is where the circulatory system and the respiratory
system
intertwine.
The oxygen in the lungs is diffused through
the alveoli sacs and then through the wall of the lungs into the bloodstream. The
blood carries the oxygen to the various cells in the body. To get the oxygen to the
actual cells, the arteries branch off into smaller arterioles. These even branch off
to capillaries, the smallest of blood vessels. Their walls are extremely thin and
elastic. In these vessels, the red blood cells must travel single file to pass
through. The oxygen is released from the hemoglobin and diffuses across the
capillary wall. It then travels to a nearby cell and enters through the cell
membrane. Once inside the cell, the oxygen molecule travels into the mitochondrial
matrix where it plays a role in ATP synthesis.
The carbon dioxide that returns to the blood
is taken back to the lungs through the capillaries. These then fork into venules,
which then fork into veins. The veins carry the carbon dioxide along with the blood
back to the heart. This completes the cycle of the circulatory system.
