Why is lumen large in veins

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 collagen fibers to provide additional strength. In larger arteries, there is also a thick, distinct layer of elastic connective tissue 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 one-way 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.

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 smooth 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. Neural and chemical mechanisms reduce or increase blood flow in response to changing body conditions, from exercise to hydration.

The smooth muscle layers of the tunica media are supported by a framework of collagen fibers that also binds the tunica media to the inner and outer tunics. Along with the collagen 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 dense irregular connective tissue composed primarily of collagen 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. 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 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 gives it a low resistance and enables it to accept a large volume of blood from the heart which is conducted to smaller branches within regions of the body.

The artery at this point is described as a muscular artery. The diameter of muscular arteries typically ranges from 0. Their thick tunica media allows muscular arteries to play a leading role in vasoconstriction which controls blood flow to individual organs. 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 that deliver blood to capillaries within specific organs and tissues. For this reason, a muscular artery is also known as a distributing artery. An arteriole is a very small artery that leads to a capillary. 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. 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.

The precise diameter of the lumen of an arteriole at any given moment is determined by neural and chemical controls, and vasoconstriction and vasodilation in the arterioles are the primary mechanisms for distribution of blood flow to capillary beds as well as regulation of systemic blood pressure. Compliance allows an artery to expand when blood is pumped through it from the heart, and then to recoil after the surge has passed. This helps promote blood flow. In arteriosclerosis, compliance is reduced, and pressure and resistance within the vessel increase.

This is a leading cause of hypertension and coronary heart disease, as it causes the heart to work harder to generate a pressure great enough to overcome the resistance. Arteriosclerosis begins with injury to the endothelium of an artery, which may be caused by irritation from high blood glucose, infection, tobacco use, excessive blood lipids, and other factors.

Artery walls that are constantly stressed by blood flowing at high pressure are also more likely to be injured—which means that hypertension can promote arteriosclerosis, as well as result from it.

Recall that tissue injury causes inflammation. As inflammation spreads into the artery wall, it weakens and scars it, leaving it stiff sclerotic.

As a result, compliance is reduced. Moreover, circulating triglycerides and cholesterol can seep between the damaged lining cells and become trapped within the artery wall, where they are frequently joined by leukocytes, calcium, and cellular debris. Eventually, this buildup, called plaque, can narrow arteries enough to impair blood flow. Sometimes a plaque can rupture, causing microscopic tears in the artery wall that allow blood to leak into the tissue on the other side.

When this happens, platelets rush to the site to clot the blood. This clot can further obstruct the artery and—if it occurs in a coronary or cerebral artery—cause a sudden heart attack or stroke.

Alternatively, plaque can break off and travel through the bloodstream as an embolus until it blocks a more distant, smaller artery. Ischemia in turn leads to hypoxia—decreased supply of oxygen to the tissues. Hypoxia involving cardiac muscle or brain tissue can lead to cell death and severe impairment of brain or heart function. A major risk factor for both arteriosclerosis and atherosclerosis is advanced age, as the conditions tend to progress over time. However, obesity, poor nutrition, lack of physical activity, and tobacco use all are major risk factors.

Treatment includes lifestyle changes, such as weight loss, smoking cessation, regular exercise, and adoption of a diet low in sodium and saturated fats. Medications to reduce cholesterol and blood pressure may be prescribed.

For blocked coronary arteries, surgery is warranted. In angioplasty, a catheter is inserted into the vessel at the point of narrowing, and a second catheter with a balloon-like tip is inflated to widen the opening. To prevent subsequent collapse of the vessel, a small mesh tube called a stent is often inserted. In an endarterectomy, plaque is surgically removed from the walls of a vessel.

This operation is typically performed on the carotid arteries of the neck, which are a prime source of oxygenated blood for the brain. In a coronary bypass procedure, a non-vital superficial vessel from another part of the body often the great saphenous vein or a synthetic vessel is inserted to create a path around the blocked area of a coronary artery.

A capillary is a microscopic channel that supplies blood to the tissues, through a process called perfusion. Exchange of gases and other substances occurs in the capillaries between the blood and the surrounding cells and their tissue fluid interstitial fluid.

The diameter of a capillary lumen ranges from 5—10 micrometers; the smallest are just barely wide enough for an erythrocyte to squeeze through. Flow through capillaries is often described as microcirculation. The wall of a capillary consists of the endothelial layer surrounded by a basement membrane with occasional smooth muscle fibers.

Some variation in wall structure is seen depending on the size of the capillary. In a large capillary, several endothelial cells bordering each other may line the lumen, while in a small capillary, there may be only a single cell layer that wraps around to contact itself. For capillaries to function, their walls must be leaky, or permeable , allowing some substances to pass through. The most common type of capillary, the continuous capillary , is found in almost all vascularized tissues.

Continuous capillaries are characterized by a complete endothelial lining with tight junctions between endothelial cells. Although a tight junction is usually impermeable and only allows for the passage of water and ions, they are often incomplete in capillaries, leaving intercellular clefts that allow for exchange of water and other very small molecules between the blood plasma and the interstitial fluid.

Substances that can pass between cells include metabolic products, such as glucose, water, and small hydrophobic molecules like gases and hormones, as well as various leukocytes.

An arteriole usually has only one layer of smooth muscle and not more than two. Capillaries are the easiest vessels to define but not to find. They consist of an endothelial layer and its underlying basal lamina.

There may also be an associated pericyte within the basal lamina of the endothelial cell. They are classified on the basis of their "leakiness" as continuous e. These are: Tunica intima. This consists of the endothelial lining and its basement membrane, and a delicate layer of loose subendothelial connective tissue.

The endothelial lining is diagnostic in distinguishing blood vessels. The nuclei of the simple squamous epithelial cells of the endothelium protrude into the lumen of the vessel.

In arteries and arterioles , an internal elastic lamina delimits the outer margin of the tunica intima. Tunica media. Small lumen relative to the large, muscular vessel ensure this pressure is maintained as the blood is transported around the body.

Veins carry unoxygenated blood towards the heart, away from tissues at low pressure so the lumen is large. Blood moves more slower and often against gravity so valves and a larger lumen ensure it is still transported efficiently.

Capillaries have the smallest lumen but relative to their size the lumen is quite large. This is because capillaries are where exchange of oxygen, nutrients and waste products occur between blood and tissues so they have evolved to have the greatest surface area to volume ratio to increase efficiency of the exchange.