Fortunately, the circulatory system doesn't have such a wide variety of organs to talk about, so this should be done relatively quickly. (Note the emphasis on the word "relatively.")
Basically the circulatory system is made up of:
- the heart;
- arteries and arterioles (very small arteries);
- veins and venules (very small veins); and
- capillaries.
In a nutshell, the arteries take the blood away from the heart. The arteries then branch out into smaller arteries in arterioles, which then branch out into capillaries, which take blood to each individual cell. Then the capillaries join up to venules, which then join up to larger and larger veins, before arriving back in the other side of the heart to start the process again. (The two sides of the heart send the blood to different parts of the body, as we shall soon see.)
Let's first take a look at the heart, since the heart is after all what drives the blood through all of those arteries and veins and capillaries.
The heart is located in the middle of the chest, between the lungs. Sometimes it feels like your heart is a bit more over to the left, but that's only because the left side beats harder than the right side- more on that later.
The heart is made up of four main chambers, two on each side. The two halves are separated by a wall called the septum, and the entire heart is surrounded by a membrane called the pericardium which holds the heart in place and prevents it from overbeating while still giving it some freedom to move while beating. The wall of the heart is made up of cardiac muscle (protip: words containing cardi-, cardia- or cardio- are all related to the heart).
Each side of the heart has an atrium and a ventricle. The atrium is the first chamber that the blood enters after entering the heart through the veins. It has relatively thin walls as the atrium doesn't need a lot of muscle to hold that low-pressure blood coming in from the veins. Next the blood flows to the ventricle, which has thick muscular walls which pump the blood out of the heart and to other parts of the body. Between the two chambers is a valve called the atrioventricular valve which allows blood to flow from atrium to ventricle, but not the other way. These valves consist of flaps as well as fibrous chords, or chordae tendinae. The flaps swing shut when the ventricle contracts due to blood pressure (I think), and the chordae tendinae prevent the flaps from swinging too far. Once the blood leaves the ventricle, the valve at the bottom of each artery, called the semilunar valve, which consists of three cusps, hold any blood that tries to flow back, preventing backflow.
Now that we've looked at what each side of the heart has in common, let's take a look at what's different about each side of the heart. IMO, the main difference lies in where each side pumps the blood.
The left side of the heart receives blood from the lungs and delivers blood to all over the body (except for the lungs). The circulation of blood through the body is known as systemic circulation.
The right side of the heart, on the other hand, receives blood from all over the body (except for the lungs), and delivers it to the lungs. The flow of blood through the lungs is then known as pulmonary circulation (pulmo = lungs).
Let's see how this fundamental difference relates to other differences between the two sides of the heart:
- Since the left side pumps all over the body, while the right side only pumps to the lungs, the left ventricle's muscular wall is much thicker and stronger than the right ventricle's muscular wall in order to pump blood more forcibly. This increased force required to pump blood on the left side is why, if you put your hand on your chest, your left side feels like it's beating harder than your right side.
- The names of the arteries and veins leading into and out of each side of the heart are different.
On the left side, the vein that flows in from the lungs is called the pulmonary vein. The artery that flows to the body is called the aorta. It's the main artery in the body IIRC, and it splits into many other arteries, some of the main ones being the carotid arteries (to head and neck), the coronary artery (to the heart itself), the subclavian arteries (to the arms), the hepatic artery (to the liver. Also has a branch going to the stomach), the mesenteric artery (to the intestines), the renal arteries (to the kidneys) and the femoral arteries (which take blood to the legs).
On the right side, on the other hand, there are two veins flowing in from the body: the superior vena cava (from the upper body) and the inferior vena cava (from the lower body). The artery flowing out towards the lungs is called the pulmonary artery, which splits into the left and right pulmonary arteries (one for each lung obviously). - The left side's atrioventricular valve has two cusps, and is called the bicuspid valve. The right side's atrioventricular valve has three cusps, and is called the tricuspid valve. I don't know why one side has more cusps than the other. Maybe it's because the left side has more muscle to hold the blood in the ventricle?
Oh and a fun fact for you: since the blood gets pumped by the heart twice in each circuit, this type of circulation is known as double circulation. This keeps the blood moving rapidly and stops the blood from losing too much pressure.
Anyway now that I've done talking about all the bits and pieces of the heart, let's see how the heart does its job!
The heartbeat also goes by a more fancy name (just like everything in human biology)- the cardiac cycle. It consists of a sequence of events:
- For a short time (roughly 0.4 seconds), the atria and ventricles are in diastole- that is to say, the heart muscles are relaxed, allowing the blood to flow into the atria and ventricles (the atrioventricular valves are open at this point).
- Atrial systole occurs, lasting roughly 0.1 seconds. During this phase, the atrium contracts (that's what's meant by "systole," by the way), forcing blood into the ventricles. Meanwhile, the ventricle stays relaxed, ready to receive the blood rushing in.
- Ventricular systole then occurs, lasting roughly 0.3 seconds. This forces blood into the arteries (aorta on left side, pulmonary artery on right side).
Although all this beating is happening in the heart, why can we feel a pulse when we put our fingers on particular points on our bodies (such as our wrists or our neck)? This is a question I'll answer very shortly.
Blood Vessels
The heart is very important and all, but equally important are the tubes that take the blood to where it's needed. I'm referring, of course, to arteries, veins and capillaries. Let's have a closer look at what each kind of blood vessel does, and how it serves its function.
Once the blood leaves the heart, it's pumped into arteries, which are branched into smaller arteries, which are branched into very small arteries called arterioles, which then supply blood to the capillaries. Arteries and arterioles have thick, strong walls complete with smooth muscle to help the arteries stretch and relax in order to maintain blood pressure as the blood is pumped through. (Take note that this stretching and relaxing does NOT force the blood along, but rather just maintains the pressure.) The expansion and contracting of the arteries occurs in time with your heartbeat, giving you the ability to feel your pulse at certain points on your body. Other great aspects of having this kind of muscle is that the stretching gives the arteries some flexibility when being assaulted with relatively high pressure blood pumped out from the heart, and that the contracting and relaxing can manipulate the diameter of arteries, which can serve to either reduce or increase blood flow to particular organs, depending on the needs of each organ. These muscles also help regulate blood flow through the capillaries. Speaking of which...
Capillaries are microscopic blood vessels which form complex networks, allowing them to carry blood to nearly every body cell. Their walls are only one cell thick, just like villi, making it easier for things to pass in and out of them. Once blood has gone through the capillaries, it goes through the...
Venules, which are small veins (like how arterioles are small arteries), which later join up to larger and larger veins. Veins carry blood back to the heart. The walls of veins are much thinner than those of arteries, since the blood flowing through them isn't high pressure any more- blood loses a lot of pressure during its journey through the capillaries. Instead, the low pressure causes backflow to be a possible problem- but fear not! Many veins have several valves that prevent backflow of blood.
But if the blood is such low pressure, when does the pressure ever increase enough for any of the valves to open? you may ask. Well, when you move around, your muscles contract, squeezing your veins and providing the pressure required to push your blood along. Due to the valves, the blood can only go in one direction.
There's also more stuff about heart sounds and how those blood pressure monitors and electrocardiograph things work. I'm not going to go into these today, as I'm already quite tired, but maybe at some other point. Or not. I really don't know.
If you really want me to add this stuff though, please let me know. If you're even reading this, that is.
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