Patrick Roisen

M.Ed., Stanford University
Winner of multiple teaching awards

Patrick has been teaching AP Biology for 14 years and is the winner of multiple teaching awards.

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Circulatory System

Patrick Roisen
Patrick Roisen

M.Ed., Stanford University
Winner of multiple teaching awards

Patrick has been teaching AP Biology for 14 years and is the winner of multiple teaching awards.


Whenever I teach about the circulatory system, I think of a girl I knew in junior high named Iris. Not because I had a crush on her and every time I saw her my heart beat. She and I despised each other. Instead, I discovered that she was easily freaked out by the sound of the heart beat. With all the practice I got trying to gross her out, I’ve gotten pretty good at that heartbeat sound. This makes some really entertaining tricks I can play on kids when they are doing their heart dissections.

Besides learning about my knowing habits, studying the circulatory system is pretty important for the AP biology test. One of the 12 official labs is all about how do you assess somebody’s cardiovascular health? And in the multiple choice section, you will often see questions like label the parts of a heart, or how does blood transport oxygen?

Luckily, you probably already know a certain amount about the circulatory system even before you open up your textbook. To make sure we are all starting off with an equal 40 however, I’ll begin by going through what are the parts of the circulatory system? And some of the terms that are used in describing its function. Then, I’ll go into how does blood transport oxygen carbon dioxide, and finish off with some of the factors that affect how your circulatory system functions.

While a lot of people know the parts of the circulatory system; the blood, the blood vessels and the heart, a lot of times it's kind of fuzzy on some of the details. So let’s begin with the blood. Blood is actually considered what’s called connective tissue. Where the liquid that the blood cells are floating in, consider the extra cellular matrix, that is the identifying characteristic of connective tissues.

That liquid is called plasma. And it’s mostly water. It's roughly half of the blood, a little bit more. And what’s in that liquid plasma, are the things like clotting proteins, antibodies, glucose, sodium, hormones, those sorts of things.

The rest of the blood the other roughly 45% are the formed elements or just simply the cells. Most of that are the red blood cells or erythrocytes that we see here, that everybody knows about. As well as the white blood cells, like these guys here, and the thrombocytes or platelets. The proper term for white blood cells is leukocytes where leuk means white.

What blood is basically doing is its transporting the blood gases carbon dioxide and oxygen. That your body needs as producing the nutrients your body needs. As well as giving a transport route for the components of your immune system as well as hormones.

Something that a lot of people don’t think about, but it’s a very important function of the blood, is that its transporting heat from places like your in muscles where its being generated, to places where its needed like your brain. Or out to the surface of your skin so it can dump it out to the environment.

The blood is transporting through blood vessels. While most people have heard of arteries and veins, for some reason a lot of people haven’t heard about the capillaries. The ones that connect arteries and veins together. Let’s take a look at that.

So arteries are these large blood vessels here. And what happens is that, arteries start branching into smaller and smaller ones until you get into what are called arterioles. Finally, you have the single celled wall capillaries. And it's here that you can actually have the exchange of gases; Oxygen and carbon dioxide, as well as nutrients with the tissues of your body.

Ultimately those capillaries start to merge together to form into venules and ultimately then the veins. The arteries have the thickest walls of all the blood vessels. And that’s because they are the ones under the highest pressure.

The capillaries, like I said, their walls are just one cell thick. The veins sort of have thicker walls again, but not quite as thick as the others. Something else that’s different about the veins, is that they have valves. Now why is it that arteries don’t and veins do?

The thing is about arteries is, they are the ones that are picking blood away from the heart. And that’s one of the tricks that I teach my kids. A lot of people think that arteries are the ones that have oxygen and veins are the blood vessels that don’t. And most of the time that’s true, but not always. So what I tell my kids is that, arteries with an 'A' move 'Away' from the heart. And if you think about it, they would be the ones that when the heart beats, they would have to be under the highest pressure.

Once the blood vessels have branched out into the capillaries, the blood pressure is dropped a lot. You can come back in as we start to recollect back into the veins, but the veins will have much little pressure. And a lot of times that means that the blood doesn’t have enough oomph to make its way back to the heart.

So why do you have valves? Well those valves sit there and they only allow the blood to flow in only one direction. So blood can flow here, but if the blood starts to go backwards which is just called reflex, those valves slum shut.

So how do you get the blood back to your heart? Luckily, where are most of these veins? They are going through blood skeleton muscles like in your thigh muscles your biceps. And as you are walking and moving, your muscles are contracting. And that’s squeezes on the veins. And just like if you stomp on a ketchup packet, it squeezes the blood in the veins and then increases the pressure. That boost compression is enough to push it towards the heart, especially since the valves again prevent that back flow.

If for some unknown reason, your grandma was walking around in a bikini, you may notice those spider veins and back of their legs, the varicose veins. That’s often caused by what’s called prolapse when some of those valves flop backwards and that allows the blood to pour and just.

Moving on from the blood vessels, now we are going to take a look at the heart. And again this is one of those types of things that you are often going to see on, whether it’s the AP biology test, or any biology test, people love to ask questions about the parts of the heart.

The trick to remember when you are studying the heart is that, the diagram is the patient’s heart. Kids will look at this and say this is the left side and that’s the right side.

Yes for you, but the patients is the one lying on the table. This is my right this is my left. So here is the right side of the heart, here is the left side of the heart. So I’m going to go through the parts of the heart. And as I do this I’m also going to describe the flow of blood through the heart, because that’s another kind of question that’s often asked about on tests.

So the blood comes to the heart after going through the body and the body's been harvesting the oxygen. So this is low oxygen blood, otherwise known as deoxygenated blood. It’s rich in carbon dioxid,e and very little oxygen. And it comes from the upper body through something called the superior Vena Cava. From the lower body, we use the word that means below inferior. The inferior Vena Cava, these are veins.

How do you remember veins? I told you arteries mean away. Veins, those of you who have taken French or Spanish you’ve often heard the word come in French or Spanish, in French jouvien that means I come. Veins come to the heart. So arteries away, veins come to the heart. So here the blood comes to the heart and enters the right atrium.

The right atrium is a chamber of the heart. It’s the first chamber of the heart, that collects the deoxygenated blood. And then it squeezes it here into the right ventricle. The right ventricle is this large chamber down here. And its job is to then pump the blood through the pulmonary artery here, out to the lungs.

When you listen to somebody’s heart beating, what’s that sound? That sound is not the sound of the heart beating. Because if you move your bicep, that’s contracting. You don’t hear your bicep 'pump' go unless you are like Arnold back when he was really Arnold not Arnold that he is now.

What you are hearing is the slumming of this door right here. These are again, valves, because you don’t want your blood to flow backwards out of the ventricle. And that atrium is a much weaker chamber than the ventricle. Because the atrium really has the pump from here to there. So it’s kind of like here is a kindergartener, here is a sumo wrestler.

So what happens is that, when these guy squeezes, the blood goes this way. When this guy squeezes that make those doors, those valves slum shut. On the right side of the heart, you actually have three flaps to this valve. 1, 2, 3 what’s the root word that means 3? Tri. And these flaps are called cusps, so this is called the tricuspid valve.

Remember, on the right side it’s the tricuspid valve. So when that first contraction by the ventricle starts, you’ll hear that first of the heart beat sound as these doors slum shut. As this squeezes, that blows open these doors here, these valves which are called the semilunar valves. Or in this case, because it's going off to the lungs, it’s called the pulmonary valves.

They open and allow blood to go out the pulmonary arteries away from the heart to the lungs. Notice, it’s an artery moving away from the heart, but it's low oxygen. This is the exception to that rule that most people follow that arteries have oxygen, veins don’t.

Out of the lungs, it drops off the carbon dioxide, picks up oxygen, and returns by way of this pulmonary veins. For those of you who don’t know, pulmon means lung. Like in CPR ,cardiopulmonary resuscitation cardio means you are pushing on the heart to get it going. Pulmonary, you are blowing the person’s mouth to get their lungs to work.

So from the lungs are now oxygenated blood, our rich in oxygen blood comes in to the left atrium. These two helps get this ventricle here ready. So the left atrium squeezes this oxygenated blood, and it goes into the ventricle.

Now the left ventricle squeezes and pushes the blood out the aorta. That aorta is the largest blood vessel in the body. It’s got really thick muscular walls. And the reason for that is because it’s the blood vessel that’s under the highest pressure. Which you are thinking, well the pulmonary arteries also hooked up to a ventricle? Yeah. And both of these pump the same amount of blood, but this guy just has to pump it further. The right ventricle he is pumping to the lungs and back. That’s going from here to there.

The left ventricle on the other hand, when he pumps, he has to push the blood against gravity up to the top of my head, and that’s a lot further than going to my lungs. So the left ventricle has much stronger walls and thus it's putting much higher pressure on the aorta.

The aorta has branches going off to feed my brain and upper body. And then it curves down. This is called the aortic urge, to go feed my lower body.

I’ve told you that the first heartbeat sound is caused by the slumming of these doors. What causes the second sound? That’s when these doors shut because the ventricle start to relax. So the (sound) is slum slum. Now if Iris is watching this for some reason, I really hope you're getting freaked out now. For the rest of you, you now know the basic components of the circulatory system.

So what happened to that blood in the lungs? How did it manage to pick up all that oxygen and carbon dioxide? Let’s take a look. When the blood goes off to the lungs, in the lungs there is this thin walled sacks called alveoli. The singular form for that is alveolus.

And much like the capillaries, they have very thin cells. They are only one cell thick. And what happens is, as the pulmonary artery here comes to the lungs, it starts to form a bed of capillaries that wrap around each one of these alveolus.

And what’s going on is simple diffusion. The air that came into your lungs, assuming everything is okay, has lots of oxygen. But the blood, that’s coming through your pulmonary arteries, that’s lower oxygen since it's been harvested by your body. So what happens is simple diffusion. We have a high concentration in the lungs, low concentration in these capillaries. So the oxygen diffuses into the blood. Then the carbon dioxide, that’s in really high concentration in these arteries, because your body is being generating it, it diffuses out into the air on the lungs. Again form its concentration gradient. Then those capillaries go back and go into the pulmonary veins, goes off to your heart. The left ventricle pumps that out to the aorta, and as very similar process happens with the capillaries, as it passes through your body’s tissue.

Just a little side note, the blood vessels and capillaries that are going off to your lungs, that’s called the pulmonary circuit. The ones going off to the rest of your body, or your body system, is often called the systemic circuit.

But how can the oxygen and carbon dioxide be carried so well by the blood, when simple water can’t dissolve that much carbon dioxide and oxygen? It does this to a number of special tricks and a couple of proteins.

The first of these proteins is a protein called hemoglobin. So let’s take a look at this YouTube video that does a really good job at going through how hemoglobin looks and functions.

So here we see some red blood cells going through the arteries or the capillaries here, picking up the oxygen. You’ll notice they're changing color. That’s because of a special protein called hemoglobin. That’s inside of these erythrocytes red blood cells.

And so the oxygen is going in and what it does is, it sticks to this hemoglobin protein. Let’s take a look at hemoglobin. Hemoglobin is a special protein made out of four chains. Each of these chains holds in it a special group called a haemo-group that has in it an iron atom.

That ion atom as you know, here comes oxygen and oxygen combined to iron, which you know if you’ve ever seen iron rust. The cool thing about hemoglobin is that ,it holds that iron just the a way that it can easily let go off that oxygen if it needs to. Notice how it turns red, and then it turns back to this purply color when it releases the oxygen. That’s the role of hemoglobin. Here we see again their iron inside of that haemo group.

So let’s pause there and now we'll talk about some of the other cool things about hemoglobin. That hemoglobin is all, like I said, made of four chains. And what’s kind of nifty about it, you guys should hopefully know that proteins, their shape and structure can be affected by PH. What happens if the PH starts to drop? If things get a little bit acidic, then the protein changes just a little bit, and the iron falls off.

Why would that happen? What else is going on in the blood? It's that you got carbon dioxide coming in. Now, if you’ve ever drunk soda and your mom is nuggie about what it’s doing to your teeth, you hopefully know that carbon dioxide plus water forms something called carbonic acid.

What does carbonic acid do? It’s an acid, so that hydrogen falls off and becomes H+ ions and bicarbonate ion. That bicarbonate ion is not carbon dioxide. That’s kind of nifty because what happens, carbon dioxide comes in going from an area of high concentration to low concentration in your blood, and then it converts itself into bicarbonate.

So it’s no longer CO2. And so this constantly keeps pulling in additional C02. You can’t do this too far with just plain water. And that’s where your blood plays its second chemical trait with the next protein. Let’s take a look at this YouTube video. We'll go ahead and make it larger.

Here we see red blood cells and here comes carbon dioxide coming in from your body cells, as they are doing their exercise.


And inside of the red blood cells, here we see carbon dioxide combining with water. And it forms the carbonic acid. Then one of the hydrogens falls off as a hydrogen ion. This little gumbie met here, is actually an enzyme called carbonic anhydrase. And what it does, is it can turn carbon dioxide plus water into carbonic acid. And it does this very high rates in your red blood cells and then allows the bicarbonate ions. Then go out into your bloodstream into your plasma.

This is a reversible reaction. So it can do the opposite and that’s where the anhydrase thing is all about. So here we see the carbon dioxide can then go out if it needs to, at the lungs. And this reversible reaction is answering why your blood is really good at buffering pH. But again if there is lots of carbon dioxide, that makes the pH start to drop, so things become acidic.

As we can see as carbon dioxide comes in, it makes things a little bit more acidic. Stop and think that’s pretty cool, why? What will be making lots of carbon dioxide? Your muscles. When? When they are doing a lot of exercise and doing aerobic respiration which breaks glucose into CO2. So right in your muscles, right in that area, as your blood goes through the pH drops. Which means they start letting go of the oxygen because the hemoglobin has been changed. And the CO2 is then diffusing into the bloodstream. The delivery of the oxygen, is going to the muscles that need the most oxygen. Because they are using the oxygen to power aerobic respiration.

This effect of CO2 or pH on the red blood cells' ability to hold on to oxygen, is called the Bohr effect. Let’s take a quick look at that.

Here we see a curve across the bottom is partial pressure of oxygen. It's basically the amount of oxygen and this is the amount of oxy-hemoglobin. The ability of hemoglobin to hold on to oxygen.

When you add on the oxygen is called oxy-hemoglobin. If you add the carbon dioxide something else that hemoglobin can do, is called the carboxymino hemoglobin.

And this blue line here represents the normal saturation curve. Notice what the pH does, pH drops it. Which means as the pH starts to go down, you start having less oxygen carried by the hemoglobin. This is also affected by temperature. So this Bohr effect on hemoglobin is one of the bigger things that can affect how hemoglobin is able to transport oxygen, and carbon dioxide in your bloodstream.

One last thing, there are a few other proteins that are floating in the blood that can help combine to carbon dioxide to give even more of a boost at carrying carbon dioxide away from your body cells, and dropping it off in your lungs.

Besides the Bohr effect, what are some of the other factors that influence what your circulatory system is doing? Obviously if your circulatory system is all about delivering the oxygen to your body’s tissues that they need, it needs to be able to just moment by moment second to second, how much blood is being gotten through your system? And it does that in a couple of ways.

One way it can do this is, by increasing or decreasing a heart rate. The number of beats per minute that your heart is doing, is basically how fast it’s pumping. A good analogy for your heart is a pump, like this one. And you know that if I need to pump up my bike tire quickly, one way I can do it is by going like a little squirrel on caffeine. Or if I need to slow down I can go slower and reduce my heart rate. What’s another way? Instead of going, instead I can do much larger pump per beat. That’s a factor called stroke volume.

And so your heart can adjust its rate, or its stroke volume. Small versus large. Another thing that’s kind of related to this, obviously with the pump, is what’s called blood pressure.

What is blood pressure? That’s the pressure being exerted by your blood against the walls of your arteries and veins. When you go to the doctor and you get your blood pressure taken, you’ll here a couple of numbers being thrown out. One’s bigger one smaller. If you watch any medical drama of course they are tossing one of these numbers all the time. You are likely to have heard things like 120 over 70.

That first bigger number, is what’s called systolic pressure. And here we see the systolic pressure is when the heart is squeezed. When those ventricles are pumping the blood out. That’s like the pressure when I was shoving down on the bike pump. And the pressure will rise in the ventricles until it blows open the aortic valve. And when the aortic valve opens then the pressure in the aorta, which is directly connected to the ventricle increases. Then the heart starts to relax.

That’s called diastole. So systole is when its contracting, diastole is when it’s relaxing. As It starts to relax, the pressure in the ventricle starts to drop. And the pressure in the aorta, as the doors close, as the aortic semi lunar valves close, that makes that second heart beat sound. And then the pressure on the aorta starts to go down. That’s the diastolic pressure of the lower number, the 70 that you'll often hear.

And that pressure drops down until the next sequence of ventricular systole and it starts again. Some people wind up with medical problems if that second number if that diastolic pressure is too high.

This can be caused by clogged arteries, or too much sodium in your diet. Why would too much sodium in your diet cause problems with the diastolic pressure? More sodium in your blood, tends to pull water out of your cells into your blood raising the pressure.

Why is high diastolic pressure a problem? The heart is working, it's pumping and it's pumping and it doesn’t get the blood out of the heart until it rises above the diastolic pressure. And that’s why if you have a long term problem with high blood pressure, then your heart is having to work harder than it should in order to get the blood out.

That’s kind of like if I was trying to use this bicycle pump to pump up the tire not of a bicycle, but I was trying to pump up they tire of a 747 jet. I would have to work really hard to overcome the pressure that’s already in that tire to get the doors open.

So now you understand how your heart can adjust moment by moment to increase or decrease the amount of blood. What are some of the factors to increase or decrease this rate of work? One is exercising.

So if you start to exercise, your muscles need more oxygen and they are making more carbon dioxide. So your heart will start to increase. In the official lab this is one of the ways that they assess somebody’s cardiovascular health. Who’s stronger? Athletes or non athletes? Hopefully the athletes. And because their muscles are stronger, their heart which is made out of muscle, is also stronger. And so a athlete’s heart can often with one beat, pump out more blood than a non athlete.

Many years ago when I had my students going through ovarian on the official lab. I had this one student who while everybody else resting heart rates was around 70 or 80, his was in the 40s. And when they did their maximum exercise, the other kids, their heart rates never got as high as 130 or 140. His, I could hardly get him to go past 60.

But most people, you wont to see that kind of change. But in general, what happens when you exercise, your heart rates increases. When you stop, it slowly returns to normal. There is another thing that they’ll be asking about questions on the AP is, how fast does it recover? That rate of recovery. Again better cardiovascular health, you'll return to normal faster. Poor cardiovascular health, it may take a while. That's all about the fitness. So again, the more fit you are, the better the health, the less fit you are problems. And your heart has to work harder.

The next one that is pretty obvious is your position, your body position. If you are lying down, your heart doesn’t have to work very hard. Because instead of having to fight gravity to get blood to your head, it will just go and it dribbles down to your head and returns. But if you suddenly stand, then your heart has to speed up pretty quickly otherwise you may go offline if you can’t give enough oxygen up to your head. Plus, the gravity is now yanking a bunch of blood down to your legs. You felt this effect if you ever been pretty sick and you are lying down and you suddenly start stand up, what do you feel? You feel kind of light-headed like the room is spinning. Why? Because your heart was having a hard time, because you are sick, of responding to the sudden need. And so it's working much harder than it normally have to get blood to your brain. You felt light headed because your blood wasn’t delivering enough oxygen to your brain to keep your brain properly functioning.

And this is also why there is a lot of people who suffer heart attacks when they first wake up in the morning. If you are older and in poor cardiovascular health, you’ve spent the last 8 hours horizontal. You’ve also not being eating. So the amount of sugar that is in your blood is low. You suddenly rise and all of a sudden that stresses your heart, stresses your brain and problems happen.

The next one is temperature.

You and I are what are called homeotherms, we're warm blooded animals. So temperature doesn’t influence us as much as ectotherms, what most people call cold blooded animals. But even as hometherms, if you start to do a lot of exercise when it’s really a hot day. Remember, one of the things that the blood carries is heat from where it’s being produced to your outer surfaces. So on a hot day, if you are doing a lot of exercise, if your temperature starts to rise, your heart may increase heart beat to try to get the blood to the surfaces so it can dump that excess heat out. Now with ectotherms, things like arthropods, the bugs and in one common creature that they’ll use is something called daphnia or water fleas.

If you increase their temperature, that increases their rate of metabolism. More metabolism, demands more oxygen, and starts producing more carbon dioxide. So as you increase the temperature on an ectotherm, their heart rate will speed up. As you decrease the temperature, they start to slow down. And that’s the factor.

One of the factors that I neglected to mention earlier was things like your emotions. For example, if Iris saw me now, probably we would see her heart rate spike as she plotted how to murder me. Other things could also include your nutrition and such. But in general, you should be really well prepared for the AP test.

If you’ve got a crazy teacher like me, who spent several weeks on the circulatory system, I do recommend that you learn things like how blood clots or, how the electrical signals that cause your heart to contract are conducted. I’ve also included materials in your bonus materials folder to help you go over these sorts of concepts. As well as some other things that I think would really be of interest to you.

Again, for the AP test, you know now that the three parts of the circulatory system are the blood, blood vessel and heart. You understand now how blood can absorb oxygen carbon dioxide and deliver it to where it’s needed. And you also know the factors for the official AP lab in case there is an essay question about it.

You can also go online and do the virtual version of the lab. But be aware that the virtual version kind of glosses over the first part. Because it’s kind of hard to virtually stand up and step off of a stool. But I think you know how that works.

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