The heart
Blood vessels are great, and the fact that they manage to get within range of every cell of the body is pretty impressive. But, while they are really, really cool tubes that are specially designed to hold onto blood at the right times and allow for diffusion of nutrients at the right times, they are still, in essence, tubes. The real star of the circulatory system is the heart. I guess you could call it the… center?... crux?... main act?... oh, I know, heart of this lesson! We heart you!
the structure of the heart
Located in the middle and ever-so-slightly to the left of your chest, your heart is about the size of your fist. Every day, it pushes around 10 pints of blood through 60,000 miles of blood vessels. It beats, on average, 60 to 100 times per minute, speeding up when your body needs oxygen and nutrients faster (like when you’re exercising) and slowing down when you’re at rest. It just keeps on beating every second of your life (give or take) until you die. In fact, the very definition of death is when your heart stops beating. The heart is super important and super impressive.
For this special function, the heart has a special structure. Because:
For this special function, the heart has a special structure. Because:
If we were to cut it in half top-to-bottom, so that we could see the inside, it would look like this. The heart has four chambers: the top two are called the atria and the bottom two are the ventricles (you can remember the bottom are the ventricles because the bottom of the heart—the apex—points in a “V” shape). The atria just receive blood: they don’t actually pump it (in a fancy house, the atrium is the entryway). The ventricles are the part that do the pumping, which is why they have thicker muscles.
|
The heart is made up of cardiac muscle, which is somewhat similar to skeletal muscle, but it’s more specialized in structure so that the heart can send a synchronized electrical signal through the entire heart at once (allowing it to beat in unison). This electrical signal comes from a special collection of nerve bundles in the heart called the sinoatrial node or pacemaker.
the pacemaker
The pacemaker sends out an electrical impulse every time your heart beats, keeping the rhythm regular. The pacemaker is completely independent of the brain. This signal gets sent all the way through heart muscle, causing it to contract.
Sometimes, people’s natural pacemakers stop working properly, causing irregular heartbeats. If this happens, they can get an artificial pacemaker, a medical device that keeps the heart beating regularly, surgically placed.
This video gives a good overview of the natural heart rhythm and how an artificial pacemaker can help when the heart isn’t doing its job right:
Sometimes, people’s natural pacemakers stop working properly, causing irregular heartbeats. If this happens, they can get an artificial pacemaker, a medical device that keeps the heart beating regularly, surgically placed.
This video gives a good overview of the natural heart rhythm and how an artificial pacemaker can help when the heart isn’t doing its job right:
the flow of blood
That video just gave you a sneak peak of the controlled flow of blood through the heart. It is very important that oxygenated blood that’s ready to go to the body is separate from deoxygenated, carbon dioxide–rich blood that has just come back from the body and needs to exchange gases with the lungs. Otherwise, the “bad” blood would get all mixed up with our “good” blood and not enough oxygen would get delivered to tissues.
The heart controls this blood flow by separating blood into the left and right halves. The right half (which is actually the left half when we’re looking at a diagram, because it’s the patient’s left as we’re looking at them) receives deoxygenated blood and pumps it to the lungs, where it exchanges bad carbon dioxide for good oxygen. The left half receives freshly oxygenated blood from the lungs and pumps it to the body.
The heart controls this blood flow by separating blood into the left and right halves. The right half (which is actually the left half when we’re looking at a diagram, because it’s the patient’s left as we’re looking at them) receives deoxygenated blood and pumps it to the lungs, where it exchanges bad carbon dioxide for good oxygen. The left half receives freshly oxygenated blood from the lungs and pumps it to the body.
Let’s track this flow of blood, quadrant by quadrant:
The different parts of the heart are separated by valves (those clicky things in the gif above) to keep blood headed in the right direction at the right time.
This video gives a good recap of the flow of blood:
- Deoxygenated blood from the body flows into the right atrium, then
- Deoxygenated blood flows down into the right ventricle, which pumps it to the
- Lungs to get oxygen and drop off carbon dioxide.
- Oxygenated blood then flows back to the heart and into the left atrium, then
- Oxygenated blood flows down into the left ventricle, which pumps it to the
- Body to drop off oxygen at tissues and pick up carbon dioxide and other wastes. Deoxygenated blood then returns to the
- Right atrium, and the cycle repeats.
The different parts of the heart are separated by valves (those clicky things in the gif above) to keep blood headed in the right direction at the right time.
This video gives a good recap of the flow of blood:
It’s more important for you to understand the big picture here than to memorize a pathway: The blood takes separate trips to the lungs to get oxygen and then to the body to drop off that oxygen. This is managed by separate halves of the heart to prevent mixing of “bad” blood and “good” blood.
what about nutrients and waste?
We’ve seen how the blood picks up oxygen and drops off carbon dioxide by going to the lungs. We’ll talk in more detail about what happens at the lungs in the next lesson. But, we also know that the blood transports a lot more than just gases. So, what about nutrients and wastes?
The blood picks up nutrients as it passes through the digestive system, which breaks down food, absorbs it, and passes it to your bloodstream. As for wastes, a lot of them get detoxified as blood passes through the liver, which is basically one big anti-poison machine (among a ton of other functions in the body). Wastes, including some stuff that was just detoxified by the liver, are then mostly excreted through your urine (pee) by the kidneys, which is part of the excretory system. And, blood picks up shiny new blood cells and gets rid of old ones at the spleen. Everything in your body is working towards the same goal, and blood flow through each of these important organs changes the blood somehow.
The spleen is very much a side-note to this lesson, but if you would like a funny video on it, as well as get a recap of some of the other functions of our major organ systems, check out this video:
The blood picks up nutrients as it passes through the digestive system, which breaks down food, absorbs it, and passes it to your bloodstream. As for wastes, a lot of them get detoxified as blood passes through the liver, which is basically one big anti-poison machine (among a ton of other functions in the body). Wastes, including some stuff that was just detoxified by the liver, are then mostly excreted through your urine (pee) by the kidneys, which is part of the excretory system. And, blood picks up shiny new blood cells and gets rid of old ones at the spleen. Everything in your body is working towards the same goal, and blood flow through each of these important organs changes the blood somehow.
The spleen is very much a side-note to this lesson, but if you would like a funny video on it, as well as get a recap of some of the other functions of our major organ systems, check out this video:
Summary
With every heartbeat, with every drop of blood flowing through your blood vessels, you are keeping your cells alive and allowing them to all work together to make you. With every heartbeat, you deliver oxygen and nutrients to every single one of tens of trillions of cells in your body. And your heart will just keep on beating, every second of every minute of every hour of every day, for years and years and years, as long as you live. Take a moment to sit still. Put your hand on the center of your chest, over your heart. Take some deep breaths. Close your eyes. Can you feel it? Can you feel your heart beating?
This video gives a great overview of some of the most important ideas we discussed:
This video gives a great overview of some of the most important ideas we discussed:
If you’d like a deeper dive, this video is great:
You should understand:
- That blood flows through dual circulation, which separates deoxygenated blood from oxygenated blood and pumps deoxygenated blood to the lungs for gas exchange before returning to the heart and sending out the freshly oxygenated blood.
- That the ventricles pump blood and the atria receive it.
- That the sinoatrial node or pacemaker is responsible for maintaining the heartbeat. It is separate from the brain.
- That the liver detoxifies blood, the kidneys excrete wastes, and the spleen recycles blood cells.
Learning Activity
Contributors: Megha Kori, Emma Moulton
Some images made using biorender.com
Some images made using biorender.com