Imagine being smaller than the period at the end of this sentenced. Imagine being microscopic! Suppose you are almost as tiny as a red blood cell. Picture yourself inside a human heart. You are about to take a guided tour of a human circulatory system.
Your tour begins in the right atrium of the heart. It is a dark chamber with thin walls. Wow! Is it crowded! Millions of blood cells are in the right atrium with you. Most of the cells are about your size. The guide says that they are red blood cells. “Each one is shaped like a doughnut with no hole. Most have no nucleus. Every red blood cell carries a load of carbon dioxide.”
“The large cells with nuclei are white blood cells,” the guide continues. “What are the disk shapes?” you ask. “They are platelets,” the guide answers, “and the yellowish, watery substance around you is plasma.”
Uh-oh! What is happening? The walls of the right atrium seem to be closing in! You bob up and down in the plasma as you flow through an opening that is like a door. The guide says, “Welcome to the right ventricle.” It is a larger chamber with thick walls. It is not as crowded.
Thud! The door you came through closes. “Each door in the circulatory system will close after you go through it,” the guide explains. “You can never turn back…” You begin to wonder if this tour was a good idea after all. The guide senses your concern and tells you not to worry. “Each time one door closes, another door will open.” As the walls of the right ventricle close in, another door opens just like the guide side. Swoosh! You are forced through the door in to an artery. You feel dizzy.
The farther you go in this artery, the narrower it becomes. “We are in the lungs capillaries now,” the guide says. “I am not sure I can make it through this narrow passage,” You remark “It is a tight squeeze, but you will make it,” the guide reassures you. The red blood cells ahead of you squeeze through the capillaries one at a time. The guide explains, “The red blood cells are unloading the carbon dioxide they have been carrying. They will pick up a load of oxygen from the lungs.”
Finally, it is your turn to go through the capillaries. It was a tight squeeze, but you made it. Whew! What a relief! The blood vessel is getting wider now. “What are in a veins that leads back to the heart,” the guide says.
Passing through another door, you find yourself crowded into a small dark chamber again. The guide says it is the left atrium. The walls begin closing in, but you are not worried not just in time, another door opens and you are forced into a larger chamber with very thick walls. “Where are we?’’ you wonder The guide says. We are in the left ventricle. We enter the aorta from here.
The walls of the left ventricle close in with great force. Spurt! You are forced through a door into the aorta. You are moving with great speed. Smaller arteries branch off from the aorta. Plasma, platelets, and blood cells flow into each of the arteries. Looking ahead, you see that the blood vessel is getting narrower. “Here we go again!” the guide says, another tight squeeze through the capillaries. This is where the red blood cells unload the oxygen they are carrying. They pick up a load of carbon dioxide from the cells.
You flow out of the capillaries into a stream of more plasma, platelet and blood cells. “We are in a vein on our way back to the heart,’’ the guide says before you have a chance to ask. Splash! Going through another door, you land in the right atrium. “I hope you have enjoyed your tour of a human circulatory system,” the guide concludes before leaving you. As your tour ends, another tour is already beginning.
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