Cardiovascular system anatomy and physiology
The circulatory system is also called the cardiovascular system, where “cardi” refers to the heart, and “vascular” refers to the blood vessels.
So, these are the two key parts: the heart, which pumps blood, and the blood vessels, which carry blood to the body and return it back to the heart again.
Ultimately, this is how nutrients like O2, or oxygen, get pushed out to the organs and tissues that need it, and how waste like CO2, or carbon dioxide, which is the main byproduct of cellular respiration, gets removed.The heart is about the size of a person’s fist, which makes sense: a bigger person has a bigger fist and, therefore, a bigger heart.And it’s shaped like a cone, and sits slightly shifted over to the left side, in the mediastinum, which is the middle of the chest cavity, or thorax.It sits on top of the diaphragm, which is the main muscle that helps with breathing, behind the sternum, or breastbone, in front of the vertebral column, squished in between the two lungs, and protected by the ribs.
If you look more closely, you can see that the heart sits inside a sac of fluid that has two walls, called the serous pericardium.
The outer layer is called the parietal layer. It gets stuck tightly to another layer called the fibrous pericardium, which is made of tough, dense connective tissue, which holds the heart in place and prevents it from overfilling with blood.
The inner layer is called the visceral layer, and it gets stuck tightly to the heart itself, forming the epicardium, or the outer layer of the heart.
The cells of the serous pericardium, both the parietal and visceral layer -- secrete a protein-rich fluid that fills the space between those layers and serves as a lubricant for the heart, allowing it to move around a bit with each heartbeat without feeling too much friction.
So, moving from the outside to the inside of the heart, after the epicardium, there’s the myocardium, which is the muscular middle layer.
This forms the bulk of the heart tissue because those cardiac muscle cells contract and pump blood. In addition to cardiac muscle cells, there are crisscrossing connective tissue fibers, which are made of collagen, that together form the fibrous cardiac skeleton, which helps supports the muscle tissue.
The myocardium also has dedicated blood vessels - called coronary vessels - which lay on the outside of the heart and then penetrate into the myocardium to bring blood to that layer because it needs a lot of energy to pump blood.
Finally, there’s the innermost layer of the heart, called the endocardium, which is made of a relatively thin layer of endothelium, which is the same layer of cells that line the blood vessels.
This endocardium lines the heart chambers and heart valves. All right, so on the right side of the heart, deoxygenated blood enters either through the top, through a blood vessel called the superior vena cava, or the bottom, through another blood vessel called the inferior vena cava, in the right atrium, where “atrium” means “entryway.”.
Both vena cavas are veins, which bring blood towards the heart. There’s also a tiny third opening into the right atrium called the coronary sinus, which collects blood from coronary vessels returning from the myocardium.
Now, all of that blood then goes through the first of two atrioventricular valves that separate the atria from the ventricles.
This one is called the tricuspid valve, and it allows blood into the right ventricle. The tricuspid valve has three little flaps or ‘cusps’, and each cusp looks kind of like a parachute because it has tiny little strings called chordae tendinae coming off of it that tether the cusp to a small muscle called a papillary muscle.
When the heart contracts, that papillary muscle keeps the chordae tendinae taut, and both of these help to prevent regurgitation of blood back into the atrium, allowing it to only flow out next valve.That being said, that contraction pumps the blood out the pulmonary valve which like the tricuspid valve has three cusps and also prevents blood from going backwards - but unlike the tricuspid valve, the pulmonary valve doesn’t have any of those chordae tendinae.
Once it’s past the pulmonary valve, the blood goes into the pulmonary arteries which carry the blood away from the heart to the left and right lung.
Just remember that arteries start with “a” and carry blood “away” from the heart. The blood goes from the pulmonary artery into a pulmonary arteriole, which is a bit smaller, and finally into a capillary, which is the smallest.
Up until now the blood has been loaded with carbon dioxide, which makes the blood look dark red rather than blue, which is how it’s usually drawn, and how we’ll still draw it to stay consistent.
Now, at this point in the journey, the carbon dioxide moves from the capillary to the alveolus and oxygen moves from the alveolus to the capillary, giving the blood that nice bright red color.
Now, in the blood, each red blood cell has millions of hemoglobin proteins, and each of these hemoglobins can bind to four oxygen molecules, so each red blood cell can carry millions of oxygen molecules when fully loaded!
The oxygen-rich blood moves into a venule and then eventually into a pulmonary vein that dumps the blood into the left atrium.
This trip -- from the right ventricle of the heart through the pulmonary artery to the lungs and back to the left atrium of the heart -- is called the pulmonary circulation.
After entering the left atrium, the blood goes through the second atrioventricular valve, called the mitral valve, into the left ventricle.
The mitral valve has only two cusps or leaflets, one in front called the anterior leaflet that’s a little smaller and one behind it called the posterior leaflet.
Both of these have chordae tendinae coming off of them that tether the valve to papillary muscles in the left ventricle.
Similar to the right side of the heart, when it contracts, this prevents blood from going backwards. Finally, blood in the left ventricle gets pumped out through the aortic valve, which normally has three cusps, out to the aorta, the largest artery in the body.
Just like in the lungs, the aorta branches into arterioles which are smaller arteries and finally into capillaries which are the smallest, and at that point they’re at the organs and tissues.
In the organs, the red blood cells line up alongside tissue cells and drop off oxygen and pick up carbon dioxide, basically the reverse of what happened with the alveolus in the lung.
Loaded up with carbon dioxide, the blood turns that dark red color again, shown as blue, and starts the return journey to the heart by going into small venules and then larger veins.
Now, the lower half of the body drains into the inferior vena cava, and the upper half drains into the superior vena cava, both of which dump blood back into the right atrium.
So this trip -- from the left ventricle of the heart to the body and back to the right atrium of the heart -- is called the systemic circulation.
Now, relative to the pulmonary circulation, the systemic has a lot more blood vessels, which means there’s about a 5 times greater resistance to blood flow, which essentially meaning it’s a lot harder to pump blood through, even though it’s the same amount of blood being pumped as the pulmonary side.
Because of this difference, the left ventricle needs to be stronger, and so the muscular layer of the left ventricle wall - or its myocardium - is three times thicker than the right ventricle’s myocardium.
Okay so let’s talk a little bit about that pumping. Every heartbeat, sounds something like, “lub dub, lub dub, lub dub.” So the first heart sound - “lub”, is called S1, and the noise comes from the tricuspid and mitral valves snapping shut when the left and right ventricles contract which happens at about the same time.
Right after the S1 sound, the aortic valve and pulmonic valve open up, allowing blood to get pushed out to the body, and this period of time is called systole.
The second heart sound - “dub”, is called S2, and the noise comes from the aortic and pulmonic valves snapping shut to prevent blood from flowing backwards after it leaves the ventricles - effectively ending systole.
Right after the S2 sound, the tricuspid and mitral valves open back up, allowing blood to fill up the ventricles again, and this period of time is called diastole.
That’s it, each heartbeat can be broken into systole and diastole. So a systolic blood pressure is the pressure in the arteries when the ventricles are squeezing out blood under high pressure, and diastolic blood pressure is when the ventricles are filling up with more blood, so it’s going to be slightly lower pressure.
Alright so if we simplify this guy a bit, then the amount of blood that is pumped out by either ventricle over a period of time is called the cardiac output.
And the rate at which the veins return blood back to the atria is called venous return -- pretty easy to remember, right?
Since the circulatory system is a closed loop, cardiac output and venous are equal. Okay so let’s use some numbers to make this a little more concrete.
Let’s say that about 70ml is ejected per squeeze, with a heart rate of 70 beats per minute or 70 squeezes per minute. Then 70 x 70 is 4900ml per minute, which in liters is 4.9 L per minute.
Meaning this heart’s pumping abut 4.9 L per minute. Now, in an average adult there’s about 5 L of blood total in the body, which just to be clear is different from the cardiac output we just found, which was the amount pumped per minute.
Now, taking this total volume, 10% of that (or about 0.5 ml) is in the pulmonary arteries, capillaries, and veins which make up the pulmonic circulation, and 5% (or .25L) is in one of the four chambers of the heart itself.
Another 15% (or 750 ml) is in the systemic arteries, travelling away from the heart, 5% (or 250 ml) is in the systemic capillaries, and the remaining 65% (or 3.25 L) is in the systemic veins returning to the heart.
Now, in terms of where all of that systemic arterial blood goes - about 15% goes to the brain, 5% nourishes the heart itself, 25% goes to the kidneys, another 25% goes to the gastrointestinal organs, yet another 25% goes to the skeletal muscles, and the the last 5% goes to the skin.
These numbers can change, for example during exercise, but it gives you a general sense of things. Alright so that was the systemic arterial blood, but you’ll notice that there’s a lot more blood in the systemic veins, with that being said, arteries are generally lower volume while also being under much higher pressure.
In contrast, veins are high volume, low pressure vessels. This explains why arteries and veins have different structures.
For example, veins often have valves to help fight gravity and keep blood flowing in one direction back to the heart, whereas arteries don’t need these valves because they’re under higher pressure.
If we take a closer look at blood vessels, they have three layers, also called “tunics,” or coverings, that surround the vessel lumen, which is the hollow part of the vessel that contains the blood.
The innermost tunic is the tunica intima, which includes the endothelial cells which create a slick surface that minimizes friction for blood moving through.
Next, there’s the tunica media, or middle tunic, which is mostly made of smooth muscle cells and of sheets of elastin protein.
Both the cells of the tunica intima and tunica media generally get the nutrients they need from the blood in the lumen. Finally, there’s the tunica externa, or outside tunic, which is made up of loosely woven fibers of collagen protein that protect and reinforce the blood vessel and anchor it in place.
The tunica externa also has nerve fibers, lymphatic vessels, and, in the biggest vessels, the protein elastin. To make things really wild, some huge vessels have a tunica externa that is so thick that it needs its own blood supply!
So there are tiny blood vessels, called the vasa vasorum, which means “vessels of the vessels,” that creep along the tunica externa to bring nutrients to that layer of the blood vessel wall.
So largest arteries closest to the heart, which are the aorta, its main branches, and the pulmonary arteries -- have so much elastin in their tunica externa and media that they’re called the elastic arteries.
These guys are really stretchy -- kinda like spandex -- and this is what allows these arteries to keep their shape as well as absorb and even out the systolic and diastolic pressures.
These arteries eventually branch into the arterioles, the smallest of the arteries. In the arterioles, the tunica media is bulky, and can contract in response to hormones and the autonomic nervous system, which is called vasoconstriction, where the lumen gets a lot smaller, which decreases the blood to a capillary bed and ultimately organ or tissue.
Also these vessels can relax, or vasodilate and the lumen’s diameter increases, which allows more blood flow. This process can also help with temperature control, or thermoregulation, because blood carries a lot of heat and when it gets near the surface of the skin, more of that heat is lost.
So with vasodilation of arterioles, more heat’s lost, which therefore helps to lower body temperature, likewise if you wanted to raise body temperature, vasoconstriction reduces blood flow, which means less heat gets lost through the surface of the skin.
Finally, the capillary walls are generally only one cell thick - just the tunica intima layer, with some larger vessels that also have a subendothelium basement membrane layer, which is a layer of protein just outside of the endothelial cells that provides extra support.
In addition to allowing oxygen and carbon dioxide to flow back and forth, capillaries are also where nutrients like glucose can get delivered, and where fluid can move out of the blood vessel and into the interstitial space - which is the space between the blood vessels and cells.
Water-soluble substances, like ions, cross the capillary wall either through water-filled spaces, called clefts, between the endothelial cells, or through large pores in the walls of fenestrated capillaries.
Meanwhile, lipid-, or fat-soluble, molecules like oxygen and carbon dioxide can dissolve and then diffuse across the endothelial cell membranes.
On the other end of the capillary bed, there’s a venule, and the arteriole and venule are usually directly connected by a vessel called the metarteriole.
###SUMMARYAll right, as a quick recap, the pulmonary circulation goes from the right ventricle which delivers blood to the lungs, which then sends fresh oxygenated blood to the left atrium, at which point it enters the systemic circulation starting with the left ventricle which pumps blood to the body, and then deoxygenated blood comes back to the right atrium and it starts all over again!
sends fresh oxygenated blood to the left atrium at which point it enters the systemic circulation starting with the left ventricle which pumps blood to the body And then deoxygenated blood comes back to the right
- "Medical Physiology" Elsevier (2016)
- "Physiology" Elsevier (2017)
- "Human Anatomy & Physiology" Pearson (2018)
- "Principles of Anatomy and Physiology" Wiley (2014)
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