Why characteristics of veins enable blood to pool

A thoroughfare channel connects the metarteriole to a venule. An arteriovenous anastomosis, which directly connects the arteriole with the venule, is shown at the bottom.

A venule is an extremely small vein, generally 8— micrometers in diameter. Postcapillary venules join multiple capillaries exiting from a capillary bed.

Multiple venules join to form veins. The walls of venules consist of endothelium, a thin middle layer with a few muscle cells and elastic fibers, plus an outer layer of connective tissue fibers that constitute a very thin tunica externa. Venules as well as capillaries are the primary sites of emigration or diapedesis, in which the white blood cells adhere to the endothelial lining of the vessels and then squeeze through adjacent cells to enter the tissue fluid.

A vein is a blood vessel that conducts blood toward the heart. Compared to arteries, veins are thin-walled vessels with large and irregular lumens see Figure 6. Figure 6. Many veins have valves to prevent back flow of blood, whereas venules do not. In terms of scale, the diameter of a venule is measured in micrometers compared to millimeters for veins. Because they are low-pressure vessels, larger veins are commonly equipped with valves that promote the unidirectional flow of blood toward the heart and prevent backflow toward the capillaries caused by the inherent low blood pressure in veins as well as the pull of gravity.

Table 2 compares the features of arteries and veins. Higher in pulmonary veins Valves Not present Present most commonly in limbs and in veins inferior to the heart Disorders of the Cardiovascular System: Edema and Varicose Veins Despite the presence of valves and the contributions of other anatomical and physiological adaptations we will cover shortly, over the course of a day, some blood will inevitably pool, especially in the lower limbs, due to the pull of gravity.

Any blood that accumulates in a vein will increase the pressure within it, which can then be reflected back into the smaller veins, venules, and eventually even the capillaries. Increased pressure will promote the flow of fluids out of the capillaries and into the interstitial fluid. The presence of excess tissue fluid around the cells leads to a condition called edema. Most people experience a daily accumulation of tissue fluid, especially if they spend much of their work life on their feet like most health professionals.

However, clinical edema goes beyond normal swelling and requires medical treatment. Edema has many potential causes, including hypertension and heart failure, severe protein deficiency, renal failure, and many others. In order to treat edema, which is a sign rather than a discrete disorder, the underlying cause must be diagnosed and alleviated.

Figure 7. Varicose veins are commonly found in the lower limbs. Edema may be accompanied by varicose veins, especially in the superficial veins of the legs. This disorder arises when defective valves allow blood to accumulate within the veins, causing them to distend, twist, and become visible on the surface of the integument. Varicose veins may occur in both sexes, but are more common in women and are often related to pregnancy.

More than simple cosmetic blemishes, varicose veins are often painful and sometimes itchy or throbbing. Without treatment, they tend to grow worse over time. The use of support hose, as well as elevating the feet and legs whenever possible, may be helpful in alleviating this condition. Laser surgery and interventional radiologic procedures can reduce the size and severity of varicose veins. Severe cases may require conventional surgery to remove the damaged vessels.

As there are typically redundant circulation patterns, that is, anastomoses, for the smaller and more superficial veins, removal does not typically impair the circulation. There is evidence that patients with varicose veins suffer a greater risk of developing a thrombus or clot. In addition to their primary function of returning blood to the heart, veins may be considered blood reservoirs, since systemic veins contain approximately 64 percent of the blood volume at any given time.

Their ability to hold this much blood is due to their high capacitance , that is, their capacity to distend expand readily to store a high volume of blood, even at a low pressure.

The large lumens and relatively thin walls of veins make them far more distensible than arteries; thus, they are said to be capacitance vessels. When blood flow needs to be redistributed to other portions of the body, the vasomotor center located in the medulla oblongata sends sympathetic stimulation to the smooth muscles in the walls of the veins, causing constriction—or in this case, venoconstriction.

This increases pressure on the blood within the veins, speeding its return to the heart. As you will note in the image above, approximately 21 percent of the venous blood is located in venous networks within the liver, bone marrow, and integument. This volume of blood is referred to as venous reserve. Vascular surgery is a specialty in which the physician deals primarily with diseases of the vascular portion of the cardiovascular system.

This includes repair and replacement of diseased or damaged vessels, removal of plaque from vessels, minimally invasive procedures including the insertion of venous catheters, and traditional surgery. Following completion of medical school, the physician generally completes a 5-year surgical residency followed by an additional 1 to 2 years of vascular specialty training. Vascular technicians are specialists in imaging technologies that provide information on the health of the vascular system.

They may also assist physicians in treating disorders involving the arteries and veins. This profession often overlaps with cardiovascular technology, which would also include treatments involving the heart. Although recognized by the American Medical Association, there are currently no licensing requirements for vascular technicians, and licensing is voluntary.

The United States Bureau of Labor projects this profession to grow by 29 percent from to Blood pumped by the heart flows through a series of vessels known as arteries, arterioles, capillaries, venules, and veins before returning to the heart. Arteries transport blood away from the heart and branch into smaller vessels, forming arterioles. Arterioles distribute blood to capillary beds, the sites of exchange with the body tissues.

Capillaries lead back to small vessels known as venules that flow into the larger veins and eventually back to the heart. The arterial system is a relatively high-pressure system, so arteries have thick walls that appear round in cross section.

The venous system is a lower-pressure system, containing veins that have larger lumens and thinner walls. They often appear flattened. Arteries, arterioles, venules, and veins are composed of three tunics known as the tunica intima, tunica media, and tunica externa. Capillaries have only a tunica intima layer. The tunica intima is a thin layer composed of a simple squamous epithelium known as endothelium and a small amount of connective tissue.

The tunica media is a thicker area composed of variable amounts of smooth muscle and connective tissue. In pulmonary circulation the veins return oxygenated blood from the lungs to the left atrium, which empties into the left ventricle, completing the cycle of blood circulation.

The return of blood to the heart is assisted by the action of the skeletal-muscle pump and by the thoracic pump action of breathing during respiration. As muscles move, they squeeze the veins that run through them. Veins contain a series of one-way valves. As the vein is squeezed, it pushes blood through the valves, which then close to prevent backflow. Standing or sitting for prolonged periods can cause low venous return from venous pooling. In venous pooling, the smooth muscles surrounding the veins become slack and the veins fill with the majority of the blood in the body, keeping blood away from the brain, which can cause unconsciousness.

Venous valve : Venous valves prevent back flow and ensure that blood flows in one direction. Although most veins take blood back to the heart, portal veins carry blood between capillary beds.

For example, the hepatic portal vein takes blood from the capillary beds in the digestive tract and transports it to the capillary beds in the liver. The blood is then drained in the gastrointestinal tract and spleen, where it is taken up by the hepatic veins and blood is taken back into the heart.

Since this is an important function in mammals, damage to the hepatic portal vein can be dangerous. Blood clotting in the hepatic portal vein can cause portal hypertension, which results in a decrease of blood fluid to the liver. One-way valves consist of two flaps cusps or leaflets with edges that meet.

These valves help veins return blood to the heart. As blood moves toward the heart, it pushes the cusps open like a pair of one-way swinging doors shown on the left. If gravity momentarily pulls the blood backward or if blood begins to back up in a vein, the cusps are immediately pushed closed, preventing backward flow shown on the right. Superficial veins have the same type of valves as deep veins, but they are not surrounded by muscle. Thus, blood in the superficial veins is not forced toward the heart by the squeezing action of muscles, and it flows more slowly than blood in the deep veins.

Much of the blood that flows through the superficial veins is diverted into the deep veins through the many connecting veins between the deep and superficial veins. Valves in the connecting veins allow blood to flow from the superficial veins into the deep veins but not vice versa.

Abnormal connections between the arteries and veins called arteriovenous fistulas Arteriovenous Fistula An arteriovenous fistula is an abnormal channel between an artery and a vein. Rarely, a large fistula may divert enough blood to cause symptoms of reduced blood flow in the affected arm or leg Inflammation of a superficial vein due to a blood clot thrombophlebitis Superficial Venous Thrombosis Superficial venous thrombosis is inflammation and clotting in a superficial vein, usually in the arms or legs.

The skin over the vein becomes red, swollen, and painful. Doctors examine the area Blood clotting within a deep vein thrombosis Deep Vein Thrombosis DVT Deep vein thrombosis is the formation of blood clots thrombi in the deep veins, usually in the legs.

That is not the color of the actual blood. So so far I have not given you a clear definition of what arteries versus veins are. A better definition, so let me cross these two out, are that arteries carry blood away from the heart. Away from the heart.

And veins carry blood towards the heart. Towards the heart. And I can get a zoomed in image of the heart right here and that will make it a little bit clearer. And you can also see, or we're about to see, why this first definition, or this first distinction between arteries and veins does not always hold. So let's just imagine some blood that is being pumped away from the heart.

So right when it gets pumped away from the heart, it'll be right over here. It gets pumped through the aorta, and you can see the aorta branches, so some blood can go up towards your head, and if it didn't, you would pass out and die.

And then a lot of the blood goes down towards the rest of your body. And that, indeed, is the most oxygenated blood. And so it'll flow through your body.

And these arteries will keep branching and branching into smaller vessels, all the way until they form these very small branches. And it's that place, especially, where they will lose a lot of their oxygen to the fluid and the cells around them. And then the blood is less oxygenated.

And then even though deoxygenated blood is not blue, it often gets depicted as blue in a lot of diagrams. So I will do the same. And these vessels start building into your veins. And these really small vessels that really bridge between arteries and veins, where a lot of the gas and nutrient exchange occurs, these are called capillaries. And so after going through the capillaries, the blood will then come back to the heart and now it's coming towards the heart through the veins.