Our chapter today is going to cover the cardiovascular system, so cardio for heart, vascular for transportation inside of vessels. And there are three types of vessels. We have arteries, capillaries, and then veins. Arteries always carry blood away from the heart. Capillaries, these are in between the arteries and the veins, and it's in the capillaries where the diffusion in and out of the tissues is is going to take place.
And so only in the capillaries normally do we have exchange of materials, things that the cells need, and then waste products that the body's trying to get rid of. After the blood flows through the capillaries, it's going to be collected in veins, and veins always carry blood back to the heart. All three of these vessels have an inner endothelium.
It's a really thin squamous type endothelium. It's going to reduce friction so the blood can flow through. moves efficiently.
So in the arteries they actually have three walls for support of the pressure that's in them. First is the endothelium. Again this is simple squamous epithelium.
Now the middle layer smooth muscle in it. It's going to be the structure for the arteries. It's going to help contain that blood.
It's also going to be involved in blood pressure regulation. The outer layers of fibers and loose connective tissue that's going to help the arteries stay in place and it's going to give it a little extra stability. So three layers again endo for inside, middle of course in the middle which is made of smooth muscle and then the outer layers on the outside of the artery of course.
So here's a cross-section from slide showing an artery and a vein. First those layers so the inner layer it's going to have that simple squamous, the middle layer by far is the thickest it's going to be the smooth muscle and then the that outer layer is going to be connective tissue. So first thing I want you to notice is that the walls of the artery are much thicker than the walls of the vein. The vein has a relatively thin wall. Arteries are high pressure.
They need that structure. Veins are low pressure. Arteries tend to be much smaller. Veins tend to be much larger in size.
With the artery, that's going to help the blood move forward in that small... diameter. With the vein it's going to reduce the friction of the blood as it's making its way back to the heart.
So the largest artery in the body is the aorta. This is what comes directly off the top of the heart out of the left ventricle. It's about an inch wide when it comes out. It's about the size of a garden hose and its job is to take the oxygenated blood that has just come back from the lungs and then distribute it all through all parts of the body.
Now as the aorta leaves the heart it's going to start to branch and go into different portions of the body. And so eventually what we have is blood supply literally from the head all the way down to the tips of your toes. Arteries as they branch become smaller and smaller until the last branch of an artery is an arterial.
Now an arterial is what's going to feed into those capillary beds and these are pretty small. Half of a millimeter, that's pretty small. And we see that. started up here at a complete inch this is 25 millimeters that's going to be right about an inch now arteries can be dilated or constricted they could be opened up or they could be made smaller and this is where blood pressure regulation is going to happen is in the arteries now the arterioles that I just mentioned they're going to connect eventually to the venules which is going to be the smallest vein in the system but first we're going to move through the cap beds the whole point of moving the blood through the body is to have exchange with every cell in the body and so the capillaries are really super thin because we want to have diffusion happen pretty rapidly So capillaries are everywhere.
Everywhere where you have healthy tissue, you're going to find capillaries. So this is going to give a surface area of about 6,000 square meters. And so a meter, a square meter, is roughly a little bit bigger than a 3 foot by 3 foot square.
And now you have 6,000 of those, so that is a tremendous amount of surface area for things to diffuse in and out. Capillaries are usually organized and they are called... capillary beds at this point and those capillary beds are going to serve a particular region and of course they're going to be located literally again from head to toe.
Capillaries, like most things in the human body, are going to play an important role in homeostasis by allowing nutrients to come in and waste products to come right back out into that bloodstream. And so, again, some examples is oxygen, and then CO2 is going to come back out. waste and then water every cell in your body has to have water so this is the delivery system for that but excess water especially as it's going into your digestive system can be picked up and then eventually can be excreted from the body Blood flow to capillary beds varies so that only certain ones are completely open at any given time.
And what that means is not all of your tissues are active at any one time, so you don't need a full blood supply then. For example... After you eat, all of the blood vessels, all the capillary beds that are around the digestive system are going to be open and their job now is to pick up all the nutrients that are going to go through your digestive system and then into the bloodstream.
so it can be transported through the body. Generally, when you're eating, you're pretty stationary, and so the muscles aren't very active. And so the capillary beds in the muscles are going to close down a little bit.
They're going to kind of reduce the amount of blood flow. And so the blood can actually go into the digestive system. Now, just the opposite happens if you're really active. If you're running or you're doing heavy exercise, now the muscles are going to be fully open. and the ones in your digestive system are actually going to be mostly closed.
And so the blood is shunted or moved from side to side depending on what is active at the time. Now capillary beds are organized into capillaries that branch off something called a shunt. Now a shunt, atrial venous shunt, is a bypass, if necessary, through those capillary beds.
And so again, if it's not active, the blood can flow right through and go to where it's needed. So here's an example of an artery. Remember arteries are taking blood away from the heart. It's just come from the lungs and then its job is to deliver it into every branch eventually into an arterial.
The capillaries are going to branch off. of that of the atrial venous shunt the atrial venous shunt would go straight through if this capillary bed was not active and it does that because around each of the capillaries this little ring of muscle smooth muscle can actually clamp down and it can close off the flow into the capillaries if this is not active and then the blood goes right back into the venial and then right back out to where it's needed. This one is open and so the blood is now flowing through here and then again collected in the venial.
So we go arterial, capillary beds, venial and then blood is headed back to the heart. So again, arterial blood is under a little bit of pressure as it gets into the arterial system, and then even when it enters the capillary system. And that's important.
So what that's going to do is facilitate... things to move out a lot more quickly. For example, water delivery, oxygen delivery, nutrient delivery is going to happen at a little bit greater rate. And remember, these capillary beds are really thin, and so diffusion is going to happen pretty quickly. But at we're going to start to allow carbon dioxide to flow, excess salt if it's found in the tissues, any waste products in this capillary bed, and then water if there's excess as well.
And this is going to happen at the same time. Remember when arterial blood is coming in, it's high in oxygen and so the diffusion gradient is oxygen into the tissue that is relatively poor. The tissue is also going to have a pretty high high carbon dioxide level compared to what's in the bloodstream so it's going to diffuse down its gradient back into the blood supply so it can be transferred back to the heart and then eventually into the lungs so we can have oxygen co2 exchange Now back in the veins, off of the venules, and venules are going to collect until they make larger structures, and then eventually we're going to get veins, and eventually we're going to get right back to the heart.
Venules drain blood from the capillaries and then form to join a vein, and then each time...