Cardio- The Pump that Never Stops

Cardio- The Pump that Never Stops

So far, we talked a lot about how muscles work, what fuel they use, and how they are organized. But like every engine, they need something to be put into them (O2, nutrients…) and they need their garbage taken away (acid, broken proteins…). This is the job of blood. Depending on how fast and how well that blood is delivered, the better your muscles will work. And that goes for all types of muscle, fast and slow twitch. For that, blood needs a whole system of pipes and pumps to get from one place to another and that is called the cardiovascular system, or cardio for short.


Sounds simple, but this is one of the most important factors that make up a swimmer’s ability to perform. And just like muscles, it has the ability to be trained… a lot.


The circuit: Heart to lungs to heart to body to heart again

Let’s start with some anatomy and pretend we are a little red blood cell starting out in the left side of the heart and take a ride where ever the blood flows.


Starting from the left heart the sequence goes:

Left heart - body arteries - body capillaries - body veins - right heart - lung arteries – lung capillaries – lung veins – left heart.


This whole cycle repeats endlessly (unless something very bad has happened) and takes about 1 minute to pump all of your blood around the circuit one time. Just to be clear, the heart is the pump, the arteries take blood away from the heart either to the body or to the lungs (it does not matter whether the blood is oxygenated or not, arteries by definition take blood away from the heart), capillaries are where the magic happens, and veins by definition bring blood back to the heart either from the lungs or the body.  

Now that we have a road-map, let’s start from the beginning and see why it matters to swimming.


The heart beats at least once per second and it never stops… for a century or more! That’s a lot of beats. It is also made up of muscle, specifically “cardiac” muscle which has a few differences than “skeletal” muscle. Cardiac muscle is like the ultimate slow twitch fiber and up to 40% of its volume is mitochondria (vs. 6% of volume in the slowest of slow twitch fibers and 1-2% in fast twitch) and is fed by a much bigger capillary network than usual. Cardiac muscle fibers are also more connected to each other than typical motor units and have an auto-rhythmicity to them. That way if you detached all the nerves and took the heart out and set it on the counter, it would beat at about 100 beats a minute. This auto-rhythmicity is slowed down depending on how in shape your heart is, which is why athletes have much lower resting and sub-maximal effort heart rates than fatties… I mean normal people.

Tangent: The biggest problem with creating an artificial heart is that we don’t make mechanics that last long enough. Think about a car engine that is turned on all the time and has been running for the last 100 years… that’s what you’re trying to make.

While it would be fun to dive into the four chambers of the heart, the valves, the atria, the ventricles and the internal electrical system, really all you need to know is that the heart looks and works like a water balloon. Water flows into one side of the balloon, and after it fills for a while the balloon squeezes… pumping the water out the other end of the balloon into the arteries. There are two balloons, one for the body (on the left), and one for the lungs (one the right).


Tangent: atria are “extra reservoir” balloons that improve the efficiency of the heart, but are not required for life. Many older people’s atria actually stop working in a disease called atrial fibrillation.

Next up, blood vessels. As arteries get further away from the heart, they get smaller and smaller as arteries branch off each other and the blood starts to spread out all over the body. The same happens in reverse for veins, which get bigger and bigger as they come back to the heart.

Tangent: Early anatomists dissected dead people and didn’t find anything in arteries so they thought these vessels held air…hence the word artery, which means “windpipe” in the ancient Greek…we have come a long way swimmers.


The special things to know about arteries and veins is that arteries deal with high blood pressure and they can control both where blood travels to throughout the body and the pressure at which it travels. Blood pressure is fairly important in that if it drops too much you’ll die. Even though veins are low pressure systems, they contain most of the body’s blood supply and require muscles to pump the blood back to the heart. The amount of blood returning to the heart is HUGE and we will talk a lot about it soon.

The really big thing to know is that both arteries and veins are surrounded by “smooth” muscle which is a third type of muscle found in the body. This smooth muscle allows these “pipes” to changes size in reaction to different conditions, like workout out! More details coming… let’s talk capillaries now.

Tangent: Most high blood pressure in adults is caused by a loss of the elasticity of these blood vessels, turning them into “lead pipes.”


Capillaries are where arteries and veins hook up. They are the smallest blood vessels in the body, their walls are one cell thick and they allow only one red blood cell in at a time. This brings blood cells right next to the tissues the capillary is next to (muscle, liver, brain…all organs). This “closeness” of the red blood cells to the tissues allows important stuff like oxygen to be transferred from the blood to the tissues and allows garbage like CO2 to be picked up by the blood.


Although capillaries have no smooth muscle around them, they do have “sphincters” at their start points to control which pipes are open at any one time. These sphincters open up if the right signal is sent to them… like acid and nitric oxide from working muscle. That’s one of the big reasons for warmup! These capillaries are usually closed, but warmup opens them up and allows blood to flow to the muscle. Why are capillaries usually closed? If they all opened at once, there wouldn’t be enough blood to fill all the pipes, so your blood pressure would drop and you would die. This means your body has to pick and choose where to send blood throughout the body, and this affects workout a lot, more to come.


Capillaries are also where all of change happens with training. Capillaries can grow and extend in and around tissues that need blood and O2 the most. For example, muscles that work a lot run low on O2 and send out signals that they need more blood flow. Over weeks and months of constant training/signaling, capillaries will grow towards the muscles cells and feed them. So the next time you feel that burn during a 20x100 on 1:15 best average set think about this, that is the feeling of growing capillaries! The feeling of getting better!


VO2: How big is your fire?

Remember that all fires rely on oxygen to burn their fuels. Car engines need O2 to burn gasoline, grills need O2 to burn coal, and mitochondria need O2 to burn sugar and fat. All of these examples of “fires” are limited by how much O2 is getting to them. The more O2 you feed, the bigger and hotter the fire. That means we just have to measure how much and how fast O2 is being used up to know how big the fire is. And we can do that in swimming… kind of.


This measurement is called VO2, or “volume of oxygen” and tells us how much O2 the body is using per minute. For an endurance athlete like swimmers (gonna ignore 50ers for now), the ability to maximize VO2 is one of the most important factors that affects our swimming speed. Let me repeat this again for good measure: VO2 is HUGE in swimming! Your ability to eat up O2 and deliver it to the muscles is a big part in how well your mitochondria and heart are working, and we want them to work real well.

Tangent: For a long time VO2 was thought to be the end all be all of endurance, but now we think it is more of a pre-requisite, and lactate threshold (another show) is probably more important, but there is debate.

A swimmer’s ability to increase their VO2 is based on three things: how much O2 your blood can hold, how well your heart pumps and how well your mitochondria extract O2 from the blood. Let’s break these down one by one and take a closer look.


Blood first. Red blood cells are filled with a protein called hemoglobin that is responsible for picking up O2 and holding it while it is transported in the blood vessels (kind of like rust on metal). So really, the amount of O2 in your blood is only as good as the amount of hemoglobin in it, and that can be improved. Your body will adapt and produce more red blood cells that are bigger and hold more hemoglobin than usual if you are well trained (or if training at altitude or doping, both in other shows). This will increase the blood’s ability to hold O2 and increase VO2.


The heart second. The ability of the heart to pump is called “cardiac output” and this is more complicated than you would imagine, which is why the next section is all about this. For now, understand that the higher the cardiac output, the more blood and O2 that can be delivered to the capillary beds around the body. This is based on heart size, heart rate and squeezing power.


Mitochondria are last but not least. We have spoken about their job before so we won’t kill it again. But the important thing to know is that the blood passing by in the capillaries will unload only as much O2 as needed. That means in muscle that is resting or doesn’t have a lot of mitochondria (fast twitch fibers), a lot of O2 will stay in the blood as it continues to the veins. If the muscles are working overtime or contain lots of mitochondria (like heart muscle), almost all the O2 will be sucked out of the red blood cells before it makes it to the veins.


Tangent: Because you use different motor units when doing different activities (running, swimming, fly, free…), different strokes will have a different ability to use O2, and different VO2s.

VO2 max is the most O2 your body can use doing a certain activity. This is not the limit of that activity. Remember you can always tap into the glycolysis system if you need to go over the VO2 max “limit.” But now you are in the anaerobic realm, and the more you need to be here, the less “good” your endurance ability is. Here are some guidelines that relate “effort” with VO2 max, heart rate, and perceived exertion (scale of 1-10). This is a rough estimate only:


Out of these three components, cardiac output is the most important. Blood hemoglobin will change a little, but without altitude training or doping it won’t make or break you as a swimmer. Mitochondria’s ability to use O2 is mostly based on size and number of mitochondria, which is mostly based on muscle types. We can change this a little, but it is mostly a genetic thing. Cardiac output however can change a lot and can be trained to improve. So let’s take a closer look at the heart, the world’s greatest pump.


Cardiac output: the basis of endurance training

We said before the heart is basically two water balloons, a left and right, which each pump blood to the body and lungs. Since lungs don’t cause motion and have little effect on cardiac output, we can ignore the right side of the heart for now and pretend like our water balloon heart is just one simple pump that pushes blood to the body.

The amount of blood that the heart can pump in one minute is called “cardiac output” and this is affected by four things: 

  1. Pre-load- how much blood is coming into the water balloon
  2. After-load- how hard it is to pump blood out of the balloon
  3. Contractility- squeezing power
  4. Heart rate- how many times this is happening per minute.


We can simplify this further for now, but we will have to complicate it again in a bit if you want to understand even simple ideas like: why does coach keep bugging me about staying hydrated. Pre-load, after-load and contractility can be combined into one thing called “stroke volume.” This is simply the volume of blood being pumped by the heart in every beat. The more it can pump out per/beat, the better your heart is functioning.


Cardiac output (the total amount of blood being pumped per minute) is simply the stroke volume x heart rate. This means that your heart function is based on how many times it is beating a minute, and how much blood it pumps with each beat. Cardiac output really only has one job: keep you alive. You see, you may think the heart beats to keep your muscles working hard, but really it only beats to feed your brain. Your brain needs a certain amount of blood pressure to get its supply of blood, otherwise you would die in like four minutes. If the blood pressure drops because you start working out and your muscles are “stealing” blood by opening capillaries to themselves, then your heart has to work overtime and increase its output to keep the pressure up and your brain happy.


Poor heart…stuck in between a muscle and a brain and a slave to both. But the heart has a couple tricks, that’s why we have to go back to the complicated picture of pre-load, after-load, contractility and heart rate. Heart rate is simple enough, just by taking more beats, the heart can pump more blood and supply more blood to all parts of the body. Heart rate has by far the biggest effect on cardiac output, but it is limited in how fast it can pump (max heart rate), and this CANNOT be improved or changed. In fact, that “can’t catch my breath” feeling you have at the end of a race is because your heart can’t pump any faster or harder, not because your lungs are not getting enough O2.

Tangent: as we age our max heart rate gradually goes down (rough estimate is 220 minus your age +/- 12).

Pre-load is the amount of blood filling the balloon. Because the heart works like any other muscle, it has an “optimum” range of stretch where the myosin and actin are most overlapped and will produce the most power (refer to Muscles talk). For a healthy athlete, the more you fill the heart, the better it will pump. Simply by increasing pre-load, you will be pumping more blood. And the same happens in reverse. The less blood that returns to the heart (low pre-load), the faster your heart needs to pump too keep the same cardiac output going.


That may sound un-exciting, but think of this: if you are dehydrated, if you are working out up-right (running), if you are hot, if you are using your whole body vs. just your legs in workout, if you are having an allergic reaction… all of these things decrease pre-load. So what does your heart do to keep your blood pressure up? It beats more times a minute and with more effort which makes you feel like you are working harder and harder.


Contractility next. This is simple too, your heart can change how hard it squeezes with every pump, but there are a few things to know, especially for swimmers. When swimming faster and faster, contractility will change first and max out around 50% of VO2 max (or about 140 beats a minute), but lots of things will decrease contractility: hypoxia (low O2 in the blood), hypercapnia (high CO2 in the blood), and acidosis (acidic blood). All of these are a result of holding your breath! Why does this happen? If your body is running low on O2, do you really want your heart to keep going a million miles an hour and use up all its oxygen supply? Nope, that would be bad…that’s what we call a heart attack. So your body defends itself by slowing your heart rate and contractility to save itself.

Tangent: A heart attack is where the blood vessels supplying the heart are blocked, and the muscle cells down the line actually die. They are never replaced, and the heart contractility is permanently decreased.

After-load is bad for heart’s pumping ability. This is the force pushing from the arteries back against the heart when it is trying to pump blood forward. High after-load means the heart has to pump extra hard (higher contractility) to push the same amount of blood out into the arteries, or it will push less blood out, decreasing cardiac output. High after-load is a big problem in medicine, but in athletics high after-load is caused by one thing: muscle contraction. See, when muscles contract they squeeze the blood vessels inside them shut. This causes a back pressure to build all the way back to the heart, which now has to squeeze against closed blood vessels. This only happens if the muscle is contracting constantly (unlike swimming, which is rhythmic), and it is worse depending on how many muscles are contracting (full body contraction vs. just the calves). This is why coach harps on you to stay relaxed during your races.


If swimmers don’t contract their muscles constantly like weight lifters and don’t build big after-loads, then why are we talking about it? Because a lot of dryland involves those kinds of activities. I’m not saying that’s bad, and there is definitely a place and reason for those activities, but remember this: Your heart will change shape and size depending on after-load, and not all changes are swimmer friendly.

Tangent: Elite Olympic weight-lifters have had blood pressures up to 400 (normal is 120) during a max lift! Imagine your heart pumping against that kind of after-load.

With endurance exercise, the balloon will grow bigger to hold more volume and pump more blood per beat (also why heart rate goes down when you’re in shape). With after-load exercises like weight-lifting, the balloon thickens so that it can increase contractility. You might think this is a good thing, but it isn’t. A thick heart with high contractility will eat up a lot more O2, which will give you that achy feeling when you are working very hard (a sign your heart is low on O2, but not dangerous in a healthy person). Also, thick hearts are not as “pliable” and have difficulty filling, which decreases pre-load… bad bad bad.

Tangent: Elite endurance athlete’s hearts are so large that at maximum effort, their stroke volume actually decreases because there isn’t enough time between beats to fill the heart up fully. It seems to have no effect on performance.

The best way to understand how this all works is to put it in action, so get up on the blocks, you have a mile for time coming up.


Cardio in the real world: explaining all your worst feelings

I say “mile for time” and you heart rate jumps right away…it’s almost as if your heart knows what’s coming. Not only does your body prepare for the coming race by upping the cardiac output, it even opens up capillaries to muscles fibers it knows it will need, like slow twitch for distance races, and fast twitch for sprint races (that’s a little freaky to me…how does it know)? What your body is trying to do is up your O2 delivery to the muscles by maximizing cardiac output (and all its components we spoke of earlier). Let's see how this works during a race…


As soon as the muscles start working, they make all kinds of nasty stuff like acid and urea (yeah…pee). These directly signal the capillary sphincters to open up and increase blood flow to the muscle fibers that are working. That’s good for the muscle, but the blood pressure drops because now there are more pipes open with the same amount of blood in the system, otherwise known as a drop in after-load. Your heart instantly reacts by increasing heart rate and contractility to maintain blood pressure.

Tangent: same thing happens when you stand up quickly and your heart beats real fast for a few seconds.

After a few minutes of working hard some more things start to change in order to improve cardiac output and O2 delivery to the muscles. Remember how we said most of the body’s blood is sitting in the veins? This blood starts to get pumped forward by the contraction of muscles. This “adds” to the blood returning to the heart, increasing pre-load and increasing the stroke volume (kind of like being super hydrated). This is also why your veins start popping like it’s hot when you’re working out. Yet another reason why you need to warm up, because blood that is pooling in your veins gets put to work only when the muscles are working.


As workout starts getting harder, the body runs into a problem. There is only so much blood to go around, and the body needs to start choosing where to send it. The first thing that happens is that organs that don’t involve swimming start getting less blood. Mainly these are the stomach, intestines, and kidneys. That’s not a problem if your bowels don’t have much to do, but if you ate just before working out it causes a big problem. Now you just sent a whole lot of blood flow to your stomach to help digest food, and there isn’t enough left over for the muscles to use. That is why you feel so crappy and weak if you try to swim hard right after a meal, there isn’t enough blood to go to both your stomach and your muscles, at least…not if you want to kill it at practice.


Skin is another organ that can be responsible for “stealing” blood, especially in a hot pool. Your body doesn’t like to send blood to the skin during workout. Adrenaline, and hormones like it, do a good job of causing the sphincters around capillaries to squeeze shut and 8  cut off blood supply to the skin during workout in order to force more blood into circulation, increasing pre-load. But, if your body temperature gets too high (which will melt your brain btw… literally), the capillaries will open back up and the body will choose to supply the skin more than then muscles, because to the body: a slow swimmer is better than a dead one.


Muscles that are not involved in the swimming set also get cut off, that would arms during kick sets, legs while pulling, and calves pretty much all the time. This is why shifting from pulling sets to kicking sets is so hard. Those muscles were not getting any blood, and now you are asking them to work overtime without “warming up”. Without blood and O2, the muscles now have to rely on glycolysis for energy, producing a lot of acid which can’t even be shipped out because there is no blood flow (that’s a lot of pain for little gain). That is one of the reasons for doing a short “re-warmup” of specific muscles before changing sets.


So how much can blood flow change based on what muscles you are using? An experiment showed that even working out one leg vs. both legs at one time will increase the flow to the one leg almost twice as much as when you are using two legs. In theory, that means the more focused your training, the better you are able to use that muscle and the more it will adapt since you can train it at a higher level. At the same time, you are not really challenging your heart because now it only has to send blood one place instead of everywhere (which is what is going to happen in a swimming race). The more places your heart is forced to send blood (arms and legs and abs… during IM sets for instance), the better your cardio will be trained and the better your VO2 max will be. This is the trade-off: Train muscles one at a time or train the heart. Understanding this concept can have a big impact on how your train.


Tangent: While this hasn’t been tested, if I had full control of my team, I would train certain sets in a hot pool (endurance IM or combos of kicking, free and IM) to focus on developing the heart (since I’m forcing the heart to pump to all muscles and skin), and other sets in a cold pool (sprint, pure kick or pull) in order to focus on those muscle groups. In theory, that would maximize the training of both muscle and heart.

The hotter the pool and the more muscles you use at the same time will increase this effect, and your body adapts to that too. After starting a new season and training again, your body will “hang on” to more water and your blood volume will jump up to 10% in one week! This is actually the fastest and a very effective way to improve endurance ability. By having more blood in the pipes, it is easier for the heart to send it to multiple locations at the same time. This is why you feel so much better even after just a week of training (even though little has changed in your muscles or heart). This is also why you feel so bad after taking a few days off. As quick as your blood volume goes up, it goes down.

Tangent: next time you start swimming after being off for a few weeks, take note that the first few days you will be drinking a lot more, and peeing a lot less. You might even see a jump in your body weight (a good increase).


Being dehydrated will have the opposite effect by decreasing the amount of blood in the pipes and forcing the heart to pump faster and harder to maintain the same cardiac output. This happens to you every single day. As workout goes on and on, and you don’t drink enough water to replace it, your blood volume slowly goes down, and by the end of workout your heart rate is way higher even if you are doing the same intensity workout. Want to measure how well you do at staying hydrated during practice? Measure your body weight right before and after a workout. The weight you lose is sweat.


The extra blood volume at the beginning of training is all water which dilutes out the red blood cells (the main cause of “athlete’s anemia”). But over a few weeks, the extra water gets filled in with new blood cells that are bigger and hold more hemoglobin. This helps to improve VO2 even more! Increases in blood volume are related to how much endurance training you do, and how many much you challenge your cardio system. So next time you’re working out in a hot pool and you feel like dying, instead of complaining, try to understand you are getting your heart in better shape than someone who is living the life of luxury in a cold pool.

Tangent: another idea I would like tested is whether taper would be better in a hot pool vs. a cold one, or at least slow stuff in the hot pool and high intensity in the cold one (a diving well near a competition pool would be ideal). This way you can maintain blood volume even though your work load has gone down, improving pre-load during tapered races.

Ok…that was a lot, but it is the foundation of understanding how all of this improves with training, which we will talk about later. For now I leave you with one more Tangent:

Tangent: About half of the entire world’s population undergoes these changes at least once in their life without training at all. The increased blood volume, the larger heart, the growing of red blood cells… can you guess who? Pregnant woman! Women undergo the same adaptations during pregnancy that a training athlete undergoes. This makes sense since being pregnant and giving birth is kind of like its own competition.


Karl Hamouche- Swim Smart founder
© 2017 Swim Smart, ALL RIGHTS RESERVED

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