An educational journey...

I had to write how the heart pumps blood through the circulatory system for a discussion post in my Anatomy and Physiology class and this is what I came up with, well roughly anyways... Enjoy!
(sorry that its 90 years long, but I hope you learn something!!)

Throughout time, man has searched the world over for wonderful and exciting places to visit, made lists of those places, and gone to extremes to prove that there is no place that we couldn’t go; when really we haven’t even begun to scratch the surface of “amazing” places. The body is one of the most amazing formations that you can find on the planet; it really is its own planet by its self. Think about it, every system in our body is like a city living on its own, but we need transportation, government, learning centers, grocery stores, factories, places to live, and everything that makes those comes from different places. Let’s have a look at the transportation side of things.
Like in all things there are different forms of transportation. Endocrine and exocrine glands distribute, and nerves carry the 911 calls of the body’s emergencies, but what about the daily “commute” as it were. The heart is the main hub of circulation, essentially it could be considered the central area, like downtown or a densely populated suburb, which is kind of fitting seeing as the heart is nearly center in your chest. The blood cells are like little cars, they begin their journey for the sake of this story in the bone marrow. Like a car factory, the bone marrow produces cells as the body needs them, usually on a regular pace, but with “depression”, rates of production can slow down. Once into the circulatory system the journey starts. Like most cities the heart has four sections, each area with its own label, and some areas never really interact, and when they do, everyone thinks something is wrong (which in the case of the heart would be true). The first place we will go is the right atrium. Directly off the “highways” so to speak, of the superior and inferior vena cavae. Tired, and a little blue from their long journey through the body, the blood cells wouldn’t mind getting changed and showered from their long day.
From the right atrium the blood hits a very quick red light at the tricuspid valve, so that traffic doesn’t get backed up, and into the right ventricle. With the next green light, the cell travels past the pulmonic semilunar valve, and into the main pulmonary artery, a one way street, that splits into two sections, the right and left pulmonary arteries. Although some may have preference, it doesn’t matter which way you go, the scenery doesn’t change too much, and you will find what you’re looking for. The pulmonary arteries lead to the pulmonary capillaries in another city, so to speak. The pulmonary capillaries are in the lungs, and it is here that they offer everything the cells could ever want.

The lungs are like a trading post of sorts, the tired out blood cells want food, and a change of clothes. You could also say that the lungs are like the spa for a blood cell, seeing as they enter tired and worn out, and leave with renewed vigour to take on the world again. They could also be considered a bank, where you exchange your bills (carbon dioxide) for money (oxygen). It is here that the blood trades all of the debris its collected through the day are scrubbed away, and new oxygen is picked up to be distributed to relatives in other cities (systems) along the way. After getting what they need from the lungs the blood cells leave the pulmonary capillaries and head into the four pulmonary veins. Again, the scenery might vary a little bit along the way but all pulmonary veins lead to the left atrium.

The left side of the heart is kind of like the nicer side of town, it only carries red blood cells, not the tired, dirty blue blood cells, while some would call this segregation, there really would be trouble if there was interaction! It can also be mentioned that the left side of the heart has thicker walls. This is because the pressure exerted to move the cells through the entire body, is much greater than that needed to move the blood to the lungs and a thinner muscle wall would wear out. The atrium goes through the green light of the bicuspid (or mitral valve), and into the left ventricle, where there is one last burst of energy as the cells pass the aortic semilunar valve, spin through the aorta, and onto the freeway as it were.

But what causes the green and red lights at the valves? The electrical company of the heart is known as the sinoatrial (SA) node. It has different sub companies known as the atrioventricular node (AV node), and the His-Purkinje system, these are all connected by the atrial conducting fibres, the bundle of his, and the perkinje fibres. The electrical impulse starts in the upper atrium, originating in the SA node, this impulse is called the cardiac impulse, and spreads from here to both atria through the atrial conducting fibres, to its sub company the AV node. The AV node is on the floor of the right atrium near the septum (or dividing wall of the heart). From the AV node, the impulse travels down the bundle of his, a slower conducting fibre, which gives the heart a chance to slow down for just a moment to allow the ventricle time to fill with blood from the atrial contraction. Once the signal has gone from the bundle of His, it separates into the Perkinje fibres where it moves incredibly fast to contract the ventricles.

Now back to our blood cell that’s cruising down the highway of the abdominal aorta. There are many ways the blood can travel, it can go to the brain via the carotid arteries, it can go into the arms through the axillary artery, it can branch off the abdominal aorta and into the renal artery to see the kidneys, the lumbar artery to go the lumbar region of the back, or the gonadal artery to the reproductive organs. These are just a few options but for this, were going to go to the common iliac artery and down into the leg, past the femoral artery, into the posterior tibial artery, and into the dorsalis pedis artery, which would be in the top of the foot. From the main artery would branch arterioles, smaller less elastic arteries, then into even smaller capillaries. Capillaries have many functions, not only are they the connection between arteries and veins, the way to the destination and the way home, they are also where the cells visit distant family and bring them the present of oxygen, and nutrients. Oxygen is kind of like money, you can’t do much without it these days, and if your body’s cells can’t get oxygen then they will die. The tissue our blood cell is visiting in the foot would give the blood carbon dioxide as a gift, since there really isn’t anything the blood can do with it, except give it to the lungs, it could be considered bills. Bills are something no one has a use for, so we carry them around until we get rid of them (or until we have to pay them). With that, the cell says good bye and leaves the foot tissue with its new, load, and just like bills, the cell is looking a little blue; although it’s because of the carbon dioxide and waste product more than anything. Like any good traveler the blood doesn’t take the same rout back to the heart but takes a more scenic route with more stops on the way back. From the capillaries, the blood travels through the venules, and then to the veins, the more main transportation of our return route. The veins are not quite as big as the arteries, and they are a little different, there are more stops in the veins because the blood cell is working its way back. Lining the veins are flaps that when the heart beats the pressure pushes the blood cell through, but when it contracts, the blood stops for just a second to snap a picture as it goes past the sciatic nerve. Without the flaps lining the veins the blood would just kind of slosh around, going forward and then back when we walked around, because gravity would not allow it to move back to the heart, the flaps cause the blood to stop and stay instead. The veins take different paths, but they all lead to the same place, the superior or inferior vena cavae, and into the right atrium. There are other things that travel through the blood, hormones, leukocytes, and platelets do as well, and they all go to different parts of the body, but their stories are for a different day!