Blood Vessels - Vascular System
BLOOD VESSELS; AORTA; ARTERY; ARTERIOLE; CAPILLARY; VENULE; VEIN ; VENA CAVA A blood vessel is a tube that carries blood. Arteries, arterioles, capillaries, venules, and veins differ in size, structure, and function. Kelsee, Aimee. How it works. Blood leaves heart through pulmonary. The blood vessels consist of. Arteries. Arterioles. Capillaries. Venules. Veins Arteries and arterioles have relatively thick muscular walls because blood.
Uncompensated overproduction of endothelins may contribute to hypertension high blood pressure and cardiovascular disease. Next to the endothelium is the basement membrane, or basal lamina, that effectively binds the endothelium to the connective tissue. The basement membrane provides strength while maintaining flexibility, and it is permeable, allowing materials to pass through it. The thin outer layer of the tunica intima contains a small amount of areolar connective tissue that consists primarily of elastic fibers to provide the vessel with additional flexibility; it also contains some collagenous fibers to provide additional strength.
In larger arteries, there is also a thick, distinct layer of elastic fibers known as the internal elastic membrane also called the internal elastic lamina at the boundary with the tunica media. Like the other components of the tunica intima, the internal elastic membrane provides structure while allowing the vessel to stretch. It is permeated with small openings that allow exchange of materials between the tunics.
The internal elastic membrane is not apparent in veins. In addition, many veins, particularly in the lower limbs, contain valves formed by sections of thickened endothelium that are reinforced with connective tissue, extending into the lumen.
Under the microscope, the lumen and the entire tunica intima of a vein will appear smooth, whereas those of an artery will normally appear wavy because of the partial constriction of the smooth muscle in the tunica media, the next layer of blood vessel walls. Tunica Media The tunica media is the substantial middle layer of the vessel wall see Figure It is generally the thickest layer in arteries, and it is much thicker in arteries than it is in veins.
The tunica media consists of layers of smooth muscle supported by connective tissue that is primarily made up of elastic fibers, most of which are arranged in circular sheets. Toward the outer portion of the tunic, there are also layers of longitudinal muscle.
Contraction and relaxation of the circular muscles decrease and increase the diameter of the vessel lumen, respectively.
Specifically in arteries, vasoconstriction decreases blood flow as the smooth muscle in the walls of the tunica media contracts, making the lumen narrower and increasing blood pressure. Similarly, vasodilation increases blood flow as the smooth muscle relaxes, allowing the lumen to widen and blood pressure to drop. These are generally all sympathetic fibers, although some trigger vasodilation and others induce vasoconstriction, depending upon the nature of the neurotransmitter and receptors located on the target cell.
- Shared Structures
Parasympathetic stimulation does trigger vasodilation as well in erection during sexual arousal in the external genitalia of both sexes. Nervous control over vessels tends to be more generalized than the specific targeting of individual blood vessels. Local controls, discussed later, account for this type of specific regulation.
Arteries, arterioles, venules, and veins
Hormones and local chemicals also control blood vessels. Together, these neural and chemical mechanisms reduce or increase blood flow in response to changing body conditions, from exercise to hydration.
Regulation of both blood flow and blood pressure is discussed in detail later in this chapter. The smooth muscle layers of the tunica media are supported by a framework of collagenous fibers that also binds the tunica media to the inner and outer tunics.
Along with the collagenous fibers are large numbers of elastic fibers that appear as wavy lines in prepared slides. Separating the tunica media from the outer tunica externa in larger arteries is the external elastic membrane also called the external elastic laminawhich also appears wavy in slides.
This structure is not usually seen in smaller arteries, nor is it seen in veins. Tunica Externa The outer tunic, the tunica externa also called the tunica adventitiais a substantial sheath of connective tissue composed primarily of collagenous fibers. Some bands of elastic fibers are found here as well. The tunica externa in veins also contains groups of smooth muscle fibers.
This is normally the thickest tunic in veins and may be thicker than the tunica media in some larger arteries. The outer layers of the tunica externa are not distinct but rather blend with the surrounding connective tissue outside the vessel, helping to hold the vessel in relative position.
If you are able to palpate some of the superficial veins on your upper limbs and try to move them, you will find that the tunica externa prevents this. If the tunica externa did not hold the vessel in place, any movement would likely result in disruption of blood flow. Arteries An artery is a blood vessel that conducts blood away from the heart. All arteries have relatively thick walls that can withstand the high pressure of blood ejected from the heart.
However, those close to the heart have the thickest walls, containing a high percentage of elastic fibers in all three of their tunics. This type of artery is known as an elastic artery Figure Vessels larger than 10 mm in diameter, such as the aorta, pulmonary trunk, common carotid, common iliac and subclavian arteries are typically elastic.
Their abundant elastic fibers allow them to expand, as blood pumped from the ventricles passes through them, and then to recoil after the surge has passed. If artery walls were rigid and unable to expand and recoil, their resistance to blood flow would greatly increase and blood pressure would rise to even higher levels, which would in turn require the heart to pump harder to increase the volume of blood expelled by each pump the stroke volume and maintain adequate pressure and flow.
Artery walls would have to become even thicker in response to this increased pressure. The elastic recoil of the vascular wall helps to maintain the pressure gradient that drives the blood through the arterial system.
Between beats, when the heart is relaxed, diastolic pressure is provided by this elastic recoil. An elastic artery is also known as a conducting artery, because the large diameter of the lumen enables it to accept a large volume of blood from the heart and conduct it to smaller branches. Comparison of the walls of an elastic artery, a muscular artery, and an arteriole is shown.Blood Vessels, part 1 - Form and Function: Crash Course A&P #27
In terms of scale, the diameter of an arteriole is measured in micrometers compared to millimeters for elastic and muscular arteries. The artery at this point is described as a muscular artery also called a distributing artery because the relatively thick tunica media allows precise control of blood vessel diameter to control blood flow to different areas or organs.
The diameter of muscular arteries typically ranges from 0. Their thick tunica media allows muscular arteries to play a leading role in vasoconstriction. In contrast, their decreased quantity of elastic fibers limits their ability to expand. Fortunately, because the blood pressure has eased by the time it reaches these more distant vessels, elasticity has become less important. Rather, there is a gradual transition as the vascular tree repeatedly branches.
In turn, muscular arteries branch to distribute blood to the vast network of arterioles. Arterioles An arteriole is a very small artery that leads to a capillary. Larger arterioles have the same three tunics as the larger vessels, but the thickness of each is greatly diminished.
The critical endothelial lining of the tunica intima is intact. The tunica media is restricted to one or two smooth muscle cell layers in thickness. The tunica externa remains but is very thin see Figure The smallest arterioles do not have a tunica external and the tunica media is limited to a single incomplete layer of smooth cells. With a lumen averaging 30 micrometers or less in diameter, arterioles are critical in slowing down—or resisting—blood flow and, thus, causing a substantial drop in blood pressure.
Because of this, you may see them referred to as resistance vessels. The muscle fibers in arterioles are normally slightly contracted, causing arterioles to maintain a consistent muscle tone—in this case referred to as vascular tone—in a similar manner to the muscular tone of skeletal muscle. In reality, all blood vessels exhibit vascular tone due to the partial contraction of smooth muscle. The importance of the arterioles is that they will be the primary site of both resistance and regulation of blood pressure.
And the aorta is very, very wide across.
And that's why I say it's a large artery. And from the aorta-- I'm actually not drawing all the branches of the aorta. But from the aorta, it's going to go down into my belly.
And it's going to branch towards my left leg and my right leg. So let's say we follow just the left leg. So this artery over here on the top, it's going to get a little bit smaller.
And maybe I'd call this a medium-sized artery by this point. This is actually now getting down towards my ankle. Let's say we've gone quite a distance down in my ankle. And then there are, of course, little branches. And let's just follow the branch that goes towards my foot, which is this top one.
Let's say this one goes towards my foot, and this is going to be now an even smaller artery. Let's call it small artery. From there, we're actually going to get into what we call arterioles, so it's going to get even tinier. It's going to branch. Now, these are very, very tiny branches coming off my small artery.
And let's follow this one right here, and this one is my arteriole. So these are all the different branches I have to go through.
And finally, I'm going to get into tiny little branches. I'm going to have to draw them very, very skinny just to convince you that we're getting smaller and smaller. Let me draw three of them. Let's draw four just for fun. And this is actually going to now get towards my little toe cells. So let me draw some toes cells in here to convince you that I actually have gotten there. Let's say one, two over here, and maybe one over here.
These are my toes cells. And after the toe cells have kind of taken out whatever they need-- maybe they need glucose or maybe they need some oxygen. Whatever they've taken out, they're also going to put in their waste. So they have, of course, some carbon dioxide waste that we need to drag back. This is now going to dump into what we call a venule. And this venule is going to basically then feed into many, many other venules.
Maybe there's a venule down here coming in, and maybe a venule up here coming in maybe from the second toe. And it's going to basically all kind of gather together, and again, to a giant, giant set of veins. Maybe veins are dumping in here now, maybe another vein dumping in here. And these veins are all going to dump into an enormous vein that we call the inferior vena cava.
Biology of the Blood Vessels - Heart and Blood Vessel Disorders - MSD Manual Consumer Version
I'll write that right here, inferior vena cava. And this is the large vein that brings back all the blood from the bottom half of the body. There's also another one over here called the superior vena cava, and this is bringing back blood from the arms and head. So these two veins, the superior vena cava and the inferior vena cava, are dragging the blood back to the heart.
And generally speaking, these are all considered, of course, veins. Let's back up now and start with the large and medium arteries. These guys together are sometimes referred to as elastic arteries. And the reason they're called elastic arteries, one of the good reasons why they're called that is that they have a protein in the walls of the blood vessel called elastin.
They have a lot of this elastin protein. And if you think about the word elastin or elastic-- obviously very similar words-- you might think of something like a rubber band or a balloon.