Attention passengers! The Pulmonary Circulation Train is leaving Right Atrium Station. All Aboard!
The trip of the body’s blood from all distant points of the body has now returned. Blood cells carrying their load of CO2 have converged in two main tracks. The vena cavae and now the blood has been pushed into the right atrium of the heart.
That’s right, Sir, just put that CO2 luggage in the overhead. Lub Dub, Lub Dub. Final Call! The whistle blows – Lub Dub, Lub Dub. The right atrium contracts and the blood is pushed through a one-way valve into the right ventricle. Stay awake, Switchman! We’ve got important passengers onboard!
There’s little conversation onboard as oxygen-depleted cells are tired. But like a morning jolt of coffee, their reviving nourishment will soon be here. Lub Dub, Lub Dub. The right ventricle contracts and the blood train is on its way into the pulmonary artery which leads to the lungs. It is in the pulmonary capillaries that the blood train will pick up its new load of oxygen. It’s a lot more complex and miraculous than baggage handlers slinging your suitcases onto carts. The oxygen comes from the outside. It is up to you to deliver it to your lungs through the act of breathing. Inspiration (breathing in) brings oxygen-rich air into your lungs where it enters the alveoli, tiny branchings inside the lungs.
The alveoli lie next to pulmonary capillaries. The wall between them is thinner than in a cheap motel (actually about one micron if you can imagine something that thin). Oxygen diffuses into the capillaries and CO2 comes back through into the alveoli where it is released into the lungs for your next expiration (breath out). This entire operation puts Fed Ex to shame. About 6 to 10 liters of air per minute are brought in and out of the lungs and about three tenths of a liter of oxygen is transferred from the alveoli to the blood each minute even when you are at rest; much faster if you are exercising.
Once the transfer is done, everybody really is onboard and the blood train returns to the left side of the heart through pulmonary veins. The oxygen-rich blood passes through a one-way valve, Lub Dub, into the left ventricle where it exits the heart through the main artery called the aorta. The blood train is then on its way delivering its life-giving payload to every cell in your body.
There are many things that can slow the journey faster than a cow on the railroad tracks. If the valves of the heart don’t open and close efficiently, there may be back flow into the heart chambers. If the heart is weakened, it may lack the muscle strength to push the blood through efficiently. If the alveoli of the lungs are damaged by disease, the oxygen-carbon dioxide exchange will be less than desired.
Many of these roadblocks to good circulation can be prevented through healthy living. Surgery can often replace malfunctioning valves. Understanding the pathway of the blood between the heart and lungs is like witnessing an ongoing miracle. With every breath you take, the train leaves the station with your life onboard. Lub Dub, lub dub. All Aboard!