Circulatory and Respiratory Anatomy - Online Tutor, Practice Problems & Exam Prep

Gas Exchange and Circulation

Hi. In this video, we're going to talk about gas exchange and circulation, and look at the anatomy of the respiratory system and the cardiovascular system. Now the job of the respiratory system is to bring in gases from the environment, and specifically to take in O₂ to the body. And it's going to output waste, CO₂, from the body. And we'll talk about in a second where these gases are coming from. Now the circulatory system kind of has a hand in everything. It does a lot of stuff. It's involved in a lot of processes. We're just going to focus on its role in terms of gas exchange here. So while the circulatory system transports oxygen and carbon dioxide, it also transports nutrients from digestion, hormones in the endocrine system, and blood cells, including white blood cells for the immune system. But we're not going to focus on any of that, we're just going to focus on the transport of these gases. So the circulatory system is going to be responsible for delivering that oxygen to cells, which they need for cellular respiration. It's also going to pick up and remove waste carbon dioxide, which is a byproduct of cellular respiration. So these gases that you are breathing in and exhaling are needed for cellular respiration and waste from cellular respiration. Pretty incredible to think about what's coming in and out of our lungs as being involved in chemical reactions at the subcellular level. Now, ventilation is going to be sort of the first step of this larger process of gas exchange, gas exchange and circulation rather. So ventilation is when air moves into, you know, the organ of gas exchange, like the lungs, with some organisms, and we're not going to cover this here, we'll cover it in a different lesson, they'll actually be taking in water and passing it through their gills. But for our purposes, we're going to be using the example of taking air into the lungs. So then gas exchange is going to occur, which is when oxygen will diffuse, you know, through the lung tissue, basically, into the bloodstream, and carbon dioxide will diffuse out of the bloodstream and into the lungs. And this is, of course, all going to happen, as it says here, at the respiratory tissue surface. Again, that's going to be our lungs. Right? That's our respiratory system. You know, we don't have gills. Now, circulation is the transport of those diffused gases. Right? So oxygen is going to be transported to the tissues, where it'll be used for cellular respiration. Right? It's the final electron acceptor of the electron transport chain. And CO₂, carbon dioxide, is going to make its way into the circulatory system, and from there to the lungs. And this CO₂, again, is a byproduct of cellular respiration, specifically glycolysis and the citric acid cycle, which are going to be the components that break down glucose. So each of the carbons in glucose is going to be turned into CO₂ and exhaled. So this is a very complicated process, and it involves two organ systems working in conjunction. We have the circulatory system, which sometimes is called the cardiovascular system, and we have the respiratory system, which is sometimes called the respiratory system. Just getting really, you know, it only has the one name. Now, the circulatory system and respiratory system are going to function in conjunction, as you can see right here in this figure. And, basically, there's going to be, two loops of circulation. What we call pulmonary circulation, which is when, blood that needs oxygen, blood that doesn't have oxygen goes from the heart into the lungs. So here's our heart, these are our lungs. So the deoxygenated blood, as it's called, is going to go into the lungs, or it's going to pick up oxygen and make its way back into the heart full of oxygen now. So just, for reference, when you see diagrams, deoxygenated blood is often shown in blue, and the oxygenated blood is shown in red. That's what these colors represent. So once that oxygenated blood comes back into the heart, it's going to be pumped out into the bodies, into the body, only one. And in the body's tissues the, oxygen is going to be picked up and the CO₂ is going to be unloaded into the blood, and then that deoxygenated blood is going to make its way back to the heart. We call this systemic circulation. So pulmonary circulation takes deoxygenated blood to the lungs and brings it back to the heart. Systemic circulation takes oxygenated blood out into the body, gases diffuse there, and then it brings the deoxygenated blood back to the heart. And of course, of course, I keep emphasizing this point, those gases are being used for and byproducts of cellular respiration. With that, let's flip the page.

Vasculature

Vasculature is what carries the blood around the body. It's going to be lined with a special type of epithelial tissue we call endothelium, and that's going to line the interior surface of these blood vessels. It actually also lines the interior surface of lymphatic vessels, but we'll talk about those a little later. Now there are basically three types of vasculature you need to know. There are arteries, which are going to transport blood away from the heart. That is what determines an artery. The direction of blood flow is going away from the heart. Veins transport blood to the heart. Now the reason I emphasize the direction is because it's a common oversimplification that people just say, oh, veins transport deoxygenated blood, and arteries transport oxygenated blood. That's true in the case of systemic circulation, but don't forget there's also pulmonary circulation. So in the systemic loop, arteries are carrying oxygenated blood, but in the pulmonary loop, the arteries carry deoxygenated blood. Remember, that's going to be the part where the heart pumps the deoxygenated blood to the lungs. So an artery is transporting that. Now, arteries have these elastic walls, and you know, a lot of smooth muscle, and this allows them to change their diameter, which is going to be important when we talk about blood pressure. And as arteries start to branch and get smaller as they make their way to the tissues, and ultimately become capillaries, we call these branches arterioles. And they have smooth muscle just like arteries, but they have a smaller diameter. So veins, kind of like the opposite scenario here, they are going to carry deoxygenated blood in the systemic loop, but in the pulmonary loop, they are going to bring oxygenated blood from the lungs to the heart. Right? Veins carry blood to the heart. When the blood's coming from the lungs to the heart, it has oxygen, and because the naming of veins and arteries is about the direction of flow, that's why veins are carrying oxygenated blood there. Now, veins are kind of different from arteries. They don't have as much smooth muscle, they have some. But they can compensate for this because they actually will run through skeletal muscles, and we'll talk about the significance of that when we talk about blood pressure. Now, veins also have valves in them. Basically, there's going to be these flaps of tissue, and they'll allow flow in one direction but will prevent backflow. And the reason this is so important is because the pressure in veins is lower than in arteries. Right? Arteries are going to have a lot of pressure coming from the heart to keep the blood moving in the right direction. Veins, not so much, because they are going to be coming after the capillary beds. So in order to ensure that the blood keeps flowing in the right direction, veins have these valves in them. Now, just like arteries branch into arterioles before they become even smaller and are considered capillaries at that point. As capillaries start to converge together, they form what are called venules. So these are going to be little veins that are going to converge together and form veins, like the main big veins. And, of course, venules come from converging capillaries. So what are capillaries? These are the sites where the magic happens, really. This is where gas exchange is going to occur between blood and tissues. And they are really small, they are tiny. They have walls that are only one cell thick, and their diameter is about the size of a red blood cell. So they can basically, as you see here, let through one red blood cell at a time. They are super thin. I am sorry. They have a super small diameter, and they are super thin to allow for easy exchange. Now they are found in tissues as what are called capillary beds. Basically, it is like a very branched network of capillaries that sort of diffuse through a tissue, and this helps maximize the surface area and exchange. Now, because capillaries lack smooth muscle, they can't control blood flow like veins and arteries can. They can't constrict and dilate. However, there are what are called precapillary sphincters. Basically little sphincter muscles that will control blood flow into the capillary beds. So this is a capillary's way of compensating and being able to have some control over blood flow. And again, it is through these precapillary sphincters that the blood moving into the capillary beds is controlled. So looking at our diagram here, tracing our loops again, we have the heart and this here, remember, oops, is an artery because it is going where the blood is going away from the heart. And this is a vein because it is going to the heart. These are arteries, and they are going to branch, as you can see here, and turn into arterioles, or be considered arterioles. And when they get to the tissue they are going to become super branched and diffuse and form a capillary bed. Sounds comfortable, doesn't it? Capillary bed. I could take a nap there. Now capillary beds, the capillaries in a capillary bed are going to converge and form venules, and those will converge and form veins, and those veins will lead back to the heart. So that is the basic rundown of the vasculature. With that, let's flip the page.

Heart Anatomy

The heart is a muscular organ that contracts to pump blood through the body, and it contracts just like you'd contract a muscle, like the one in your arm. Of course, it's a different type of muscle, and we'll get into the details of that, but it's a similar idea. Now the heart has various chambers in it, and these chambers are called atria and ventricles. The singular of atria is atrium, if you're curious. And ventricles would be ventricle. No special Latin, singular, plural there. So the job of atria is to receive blood from veins, and ventricles receive blood from atria, and also pump that blood into arteries. And separating atria, and ventricles are what are known as atrioventricular valves. These are valves similar to what we saw in veins, and their job is to prevent backflow from the ventricle to the atrium. Blood needs to flow from the atrium into the ventricle, not the other way around. In fact, if blood moves across a valve, this is a bad thing. It's known as a heart murmur, and usually it's due to some type of damage or infection in the valve.

Now between the right atrium and right ventricle is the tricuspid valve. Don't worry about memorizing this name. And on the left, between the left atrium and left ventricle, we have the mitral valve. Again, you don't need to memorize this name. Now there are also valves that prevent backflow from the ventricles to the arteries, and these are called semilunar valves. These, again, are going to be one on the right and one on the left. The valve that separates the right ventricle from the pulmonary artery is known as the pulmonary valve, and the valve on the left that separates the left ventricle from the aorta is the aortic valve. Again, don't worry about memorizing these names. Just know semilunar valves and atrioventricular valves. You just need to understand, sort of, the basic idea behind what their purpose is.

Now, looking at the heart, you can see that in this diagram, right and left are backwards. That's going to be fairly typical because these diagrams are set up as if you are looking at someone's heart. Right? Like you're facing them and looking at their heart. So everything is going to be mirrored. Right? That's why the stuff labeled right is technically on the left side of the page, and the stuff labeled left is on the right side of the page. So the heart has various arteries and veins connected to it that will lead to pulmonary circulation and systemic circulation. The pulmonary artery is going to be part of the pulmonary loop, and it's going to deliver deoxygenated blood from the heart to the capillary beds in the lungs. The pulmonary veins take that oxygenated blood from the capillary beds in the lungs, and bring it back to the heart. So this is all part of our pulmonary loop, or pulmonary circulation, whatever term is easier for you to remember.

Now the aorta is going to deliver that oxygenated blood that's coming from the lungs to the tissues of the body. And the vein that's going to deliver deoxygenated blood from the capillary beds and the body tissues back to the heart, it's actually going to be 2 veins, their plural name for the two of them is vena cava. It's kind of a mouthful. They're basically broken down into what's called the superior vena cava and inferior vena cava. And it's not because one's better than the other; the names superior and inferior come from the fact that one is found above the other. The superior one is located above the inferior one.

So looking at our diagram here, let's just go ahead and trace the path of each loop of circulation. So deoxygenated blood is going to be delivered by the vena cavae. Right here, it's written singular, but, you know, because it it doesn't really matter which one, we're talking about for our purposes. Inferior, superior, both are going to bring that deoxygenated blood into the remember, this is going to be the right side, so it's going to deliver it to the right atrium. The right atrium is going to move that blood into the right ventricle, and the right ventricle is going to send it through the pulmonary artery to the capillary beds in the lungs. From there, the oxygenated blood will be delivered by the pulmonary vein to the left atrium, which is going to move the blood into the left ventricle. And from the left ventricle, it's going to be pumped through the aorta and delivered to the tissues, and that is going to be our pulmonary loop.

And, of course, our systemic loop takes us from the aorta all the way down through these tissues, through all these capillary beds, back up all these veins, and delivers our deoxygenated blood from the vena cava to the right atrium. So these are our two circuits of blood circulation, and they basically each have a purpose to fill. The pulmonary circuit, as we've seen, is there to oxygenate the blood, absorb oxygen from the lungs, and to get rid of the waste CO 2 that gets picked up in the tissues. So the systemic circulation's job is to deliver that oxygen to the tissues, and to pick up that waste CO 2 from cellular respiration, and bring it to the lungs so that the body can get rid of it. So those are our two circulatory loops. Let's flip the page and look at what's going on in the blood.

Blood

Blood is the fluid that moves through the vasculature and performs gas exchange with the tissues. It also plays a role in transporting nutrients, hormones, and wastes. Blood is composed of three main components: plasma, white blood cells or leukocytes, and red blood cells or erythrocytes. Plasma constitutes the majority of blood volume. It is the liquid portion of blood and sorts itself above other components when spun in a test tube due to being less dense. It mainly comprises water, dissolved electrolytes, organic compounds, and gases. The smallest portions of blood consist of white blood cells and platelets. White blood cells aid in fighting and identifying infections in the immune system. Platelets are small cell fragments essential for blood clotting, providing a rapid response to plug any holes and assist other factors in sealing the wound. However, problematic clots, known as thrombi (singular: thrombus), can form and obstruct blood flow, leading to severe consequences.

Red blood cells are notable for their role in oxygen transport, attributed to the presence of the protein hemoglobin. Mature red blood cells lack nuclei and organelles to maximize space for hemoglobin. These cells have a characteristic donut-like disk shape with a central depression observable from both sides, and they are produced in the bone marrow. The production of red blood cells is stimulated by erythropoietin, a hormone secreted by the kidneys. Hemoglobin's quaternary structure consists of four polypeptide subunits, each containing a heme group bound to an iron molecule within a porphyrin ring. The iron alternates between reduced and oxidized states to bind and release oxygen. This porphyrin structure is a notable example of nature's tendency to conserve functional structures across different biological systems, as seen with chlorophyll in plants which utilizes a similar structure for photosynthesis.

Another important respiratory pigment is myoglobin, found primarily in skeletal muscles. It binds oxygen more tightly than hemoglobin and contains only one heme group. Myoglobin's function is crucial in muscles, particularly in oxygen dissociation and binding, which will be explored further in later discussions.

Last, a mention of sickle cell disease, caused by a mutation in the hemoglobin protein which leads to abnormal aggregation in red blood cells, distorting their shape and impairing function. Although sickle cell disease can be life-threatening, it has persisted in certain populations because heterozygotes show increased resistance to malaria, providing a genetic advantage in malaria-endemic areas.

With that, let's flip the page.

Lung Anatomy

The throat at the back of the mouth is known as the pharynx, and it's a shared passageway for food, air, and water, which is why I'm sure we've all had the nasty experience of having some food or maybe some water go down our windpipe. And the windpipe is the trachea. This is what brings air from the pharynx to the lungs, and it's actually supported by these rings of cartilage that are kind of C-shaped. Basically, if you have the tube, the ring of cartilage kinda runs around it like such. Now the beginning of the trachea is called the larynx, and this is sometimes called, like, the voice box or the vocal cords, because it contains what are known as the vocal folds, which is how I'm talking to you right now. Now the trachea is going to branch when it gets to the lungs, and these two branches are known as the primary bronchi. Now bronchi are branches from the primary bronchi, so those primary bronchi are going to branch off into many, smaller bronchi that will diffuse throughout the lungs, and they're going to be supported by cartilage, similarly to the trachea. Now the thing about these bronchi is they're going to be getting smaller and smaller. Think of it like a tree. There's going to be thick branches, and then smaller and smaller branches will come off of that. The smallest branches of these bronchi are called bronchioles, kind of like arteries and arterioles or veins and venules. Got bronchi and bronchioles. So these are the smallest branches, and these guys are not supported by cartilage, they're supported by smooth muscle, and this means they can collapse, which is a bad thing. Now the lungs are the organs of respiration in humans and mammals, So their job is going to be to inhale air, and absorb that oxygen, and exhale the waste carbon dioxide. Now, the ends of these bronchioles are known as alveoli. They kind of look like bunches of grapes, and this is where the gas exchange between air and blood is going to occur. This is the thin layer of respiratory tissue that's going to act as the transfer surface. And not only is this a thin layer, it's also aqueous. And it's going to be that aqueous interface between the air and the tissue that is going to be the surface that the gases pass through, and they're going to make their way to these capillary beds that surround the alveoli. So here, you can see the alveoli, these little pink sacs that kinda look like bunches of grapes, and they're surrounded by capillary beds, as you can see in this particular image right here. And I said that they have an aqueous layer. Now remember that water has surface tension, so in order to avoid these alveoli collapsing—right, remember bronchioles can collapse—these alveoli are even more prone to collapsing, or they would be, except they have this stuff called surfactant, which is a mix of phospholipids and proteins that are produced by some alveoli, and what they do is reduce surface tension, so they're going to help prevent the alveoli from collapsing. And we need the alveoli not to collapse, so that gas exchange can keep occurring, at that surface of the respiratory tissue. So that is the basic anatomy of the respiratory system. However, there's one important piece to the puzzle here, and it's this muscle called the diaphragm. It's kind of like a sheet of muscle that runs through the middle of your chest, and it separates the top half of your torso from the bottom half, and we call that top half the thoracic cavity, like the area in there is the thoracic cavity. You know, that's where your lungs and heart are going to be located, for example, And the bottom half underneath the diaphragm is called the abdominal cavity, and that's, like, where your guts are. Right? Your intestines and that sort of stuff. So the diaphragm runs across, and it's this sheet of muscle, and it's going to be what's responsible for pulling air into the lungs. It's going to contract and pull down, and this is going to create negative pressure that pulls air into the lungs. We'll talk about the physiology of this, later in a different lesson, but those are the main components of the respiratory system. The trachea, the bronchi, the lungs, and the alveoli are that important place where gas exchange is going to happen.

Lymphatic System

The lymphatic system, in a similar manner to the circulatory system, is a network of vessels. However, these are lymphatic vessels instead of blood vessels, and they carry lymph rather than blood. Now the lymphatic system picks up, what's called interstitial fluid. It actually drains plasma from the interstitial fluid, and the interstitial fluid is basically the fluid that surrounds and bathes the cells in our body. So it's going to pick up fluid from outside of cells, and the lymphatic system is going to bring this fluid toward the heart.

Now, this fluid, as we said, is lymph, it's a clear fluid, and it forms again from interstitial fluid entering lymphatic ducts. It's going to be filtered through what's known as lymph nodes, which are organs of the lymphatic system found all over the body and play a supercritical role in the immune system. In addition, the lymphatic system includes the organs, the spleen, and the thymus. Their roles are also going to have to do with the immune system. So in terms of circulation, the lymphatic system's main job is basically to drain any excess plasma from the interstitial fluid and bring it towards the heart. It will get filtered along the way. Make sure there are no pathogens in there that are going to get into your bloodstream, and then it's going to actually add that plasma back into the bloodstream.

And you can see here how the lymphatic vessels and the vasculature are associated very closely and how they can exchange fluids. That's all I have for this video. I'll see you guys next time.