What is the difference between veins and venules




















Since the pressure within arteries is relatively high, the vasa vasorum must function in the outer layers of the vessel or the pressure exerted by the blood passing through the vessel would collapse it, preventing any exchange from occurring.

The lower pressure within veins allows the vasa vasorum to be located closer to the lumen. The restriction of the vasa vasorum to the outer layers of arteries is thought to be one reason that arterial diseases are more common than venous diseases, since its location makes it more difficult to nourish the cells of the arteries and remove waste products.

There are also minute nerves within the walls of both types of vessels that control the contraction and dilation of smooth muscle. These minute nerves are known as the nervi vasorum. Both arteries and veins have the same three distinct tissue layers, called tunics from the Latin term tunica , for the garments first worn by ancient Romans; the term tunic is also used for some modern garments.

From the most interior layer to the outer, these tunics are the tunica intima, the tunica media, and the tunica externa. Table 1 compares and contrasts the tunics of the arteries and veins. The tunica intima also called the tunica interna is composed of epithelial and connective tissue layers.

Lining the tunica intima is the specialized simple squamous epithelium called the endothelium, which is continuous throughout the entire vascular system, including the lining of the chambers of the heart. Damage to this endothelial lining and exposure of blood to the collagenous fibers beneath is one of the primary causes of clot formation. Until recently, the endothelium was viewed simply as the boundary between the blood in the lumen and the walls of the vessels.

Recent studies, however, have shown that it is physiologically critical to such activities as helping to regulate capillary exchange and altering blood flow. The endothelium releases local chemicals called endothelins that can constrict the smooth muscle within the walls of the vessel to increase blood pressure. 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.

The tunica media is the substantial middle layer of the vessel wall see Figure 2. 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.

Parasympathetic stimulation does trigger vasodilation as well as 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 phenomenon. Seek additional content for more information on these dynamic aspects of the autonomic nervous system.

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 lamina , which also appears wavy in slides.

This structure is not usually seen in smaller arteries, nor is it seen in veins. The outer tunic, the tunica externa also called the tunica adventitia , is 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.

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 see Figure 3. Vessels larger than 10 mm in diameter 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. 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.

Look at this photograph of a venule, and identify the lumen containing red blood cells and endothelial cells. In a section that has both arteries and veins, the artery and veins are very easy to tell apart. The thickness of the walls of the veins is much less, compared to the lumen, and the lumen is often collapsed as shown here. In this higher power image of part of the vein shown above, can you identify the three layers of the vein:.

Tunica Intima: A thin endothelial lining, in some veins, you may be able to see the valves. Sonhood vs. Ricochet vs. Channel vs. Trending Comparisons. Mandate vs. Ivermectin vs. Skinwalker vs. Socialism vs. Man vs. Supersonic vs. Gazelle vs. Jem vs. Mouse vs. You vs. Lubuntu vs. Virtual vs. Featured Comparisons Guidence vs. Togather vs. Maintenance vs. Brachycardia vs.

Villainize vs. Catagory vs. Correspondance vs. Incentivise vs. Turnip vs. The thin walls of the capillaries allow oxygen and nutrients to pass from the blood into tissues and allow waste products to pass from tissues into the blood. Blood flows from the capillaries into very small veins called venules, then into the veins that lead back to the heart.

Veins have much thinner walls than do arteries, largely because the pressure in veins is so much lower. Veins can widen dilate as the amount of fluid in them increases. Some veins, particularly veins in the legs, have valves in them, to prevent blood from flowing backward. When these valves leak, the backflow of blood can cause the veins to stretch and become elongated and convoluted tortuous.

Stretched, tortuous veins near the body's surface are called varicose veins Varicose Veins Varicose veins are abnormally enlarged superficial veins in the legs. Varicose veins may ache or cause itching or a sensation of tiredness. Doctors can detect varicose veins by examining the Merck and Co. From developing new therapies that treat and prevent disease to helping people in need, we are committed to improving health and well-being around the world.

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