Why is blood flow in capillaries slow

Courtesy of. Arteries and arterioles. Venules and veins. Biology of the Heart and Blood Vessels. Test your knowledge. Swelling is a condition caused by excess fluid in the tissues. The fluid involved in swelling is primarily water. Swelling may occur in one or several parts of the body, often the feet and lower legs.

Swelling that occurs throughout the body has different causes than swelling in a single limb or part of a limb. Which of the following is a frequent cause of swelling in a single limb or part of a limb? More Content. Blood Vessels: Circulating the Blood Blood travels from the heart in arteries, which branch into smaller and smaller vessels, eventually becoming arterioles.

For example, after a large meal, most of the blood is diverted to the stomach by vasodilation of vessels of the digestive system and vasoconstriction of other vessels.

During exercise, blood is diverted to the skeletal muscles through vasodilation while blood to the digestive system would be lessened through vasoconstriction. The blood entering some capillary beds is controlled by small muscles, called precapillary sphincters, illustrated in Figure If the sphincters are open, the blood will flow into the associated branches of the capillary blood.

If all of the sphincters are closed, then the blood will flow directly from the arteriole to the venule through the thoroughfare channel see Figure These muscles allow the body to precisely control when capillary beds receive blood flow. Varicose veins are veins that become enlarged because the valves no longer close properly, allowing blood to flow backward. Varicose veins are often most prominent on the legs. Why do you think this is the case?

Proteins and other large solutes cannot leave the capillaries. The loss of the watery plasma creates a hyperosmotic solution within the capillaries, especially near the venules. The fluid in the lymph is similar in composition to the interstitial fluid. The lymph fluid passes through lymph nodes before it returns to the heart via the vena cava.

Lymph nodes are specialized organs that filter the lymph by percolation through a maze of connective tissue filled with white blood cells. The white blood cells remove infectious agents, such as bacteria and viruses, to clean the lymph before it returns to the bloodstream.

After it is cleaned, the lymph returns to the heart by the action of smooth muscle pumping, skeletal muscle action, and one-way valves joining the returning blood near the junction of the venae cavae entering the right atrium of the heart. Blood circulation has evolved differently in vertebrates and may show variation in different animals for the required amount of pressure, organ and vessel location, and organ size.

Animals with longs necks and those that live in cold environments have distinct blood pressure adaptations. Long necked animals, such as giraffes, need to pump blood upward from the heart against gravity.

These checks and balances include valves and feedback mechanisms that reduce the rate of cardiac output. Long-necked dinosaurs such as the sauropods had to pump blood even higher, up to ten meters above the heart. The important thing to remember is this: Two of these variables, viscosity and vessel length, will change slowly in the body.

Only one of these factors, the radius, can be changed rapidly by vasoconstriction and vasodilation, thus dramatically impacting resistance and flow. Further, small changes in the radius will greatly affect flow, since it is raised to the fourth power in the equation. Recall that we classified arterioles as resistance vessels, because given their small lumen, they dramatically slow the flow of blood from arteries.

In fact, arterioles are the site of greatest resistance in the entire vascular network. This may seem surprising, given that capillaries have a smaller size.

How can this phenomenon be explained? Figure Although the diameter of an individual capillary is significantly smaller than the diameter of an arteriole, there are vastly more capillaries in the body than there are other types of blood vessels. Part c shows that blood pressure drops unevenly as blood travels from arteries to arterioles, capillaries, venules, and veins, and encounters greater resistance.

However, the site of the most precipitous drop, and the site of greatest resistance, is the arterioles. This explains why vasodilation and vasoconstriction of arterioles play more significant roles in regulating blood pressure than do the vasodilation and vasoconstriction of other vessels.

Part d shows that the velocity speed of blood flow decreases dramatically as the blood moves from arteries to arterioles to capillaries. This slow flow rate allows more time for exchange processes to occur. As blood flows through the veins, the rate of velocity increases, as blood is returned to the heart. Disorders of the…Cardiovascular System: Arteriosclerosis 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.

The pumping action of the heart propels the blood into the arteries, from an area of higher pressure toward an area of lower pressure. If blood is to flow from the veins back into the heart, the pressure in the veins must be greater than the pressure in the atria of the heart. Two factors help maintain this pressure gradient between the veins and the heart. First, the pressure in the atria during diastole is very low, often approaching zero when the atria are relaxed atrial diastole.

These physiological pumps are less obvious. In many body regions, the pressure within the veins can be increased by the contraction of the surrounding skeletal muscle. This mechanism, known as the skeletal muscle pump Figure As leg muscles contract, for example during walking or running, they exert pressure on nearby veins with their numerous one-way valves. This increased pressure causes blood to flow upward, opening valves superior to the contracting muscles so blood flows through.

Simultaneously, valves inferior to the contracting muscles close; thus, blood should not seep back downward toward the feet. Military recruits are trained to flex their legs slightly while standing at attention for prolonged periods. Failure to do so may allow blood to pool in the lower limbs rather than returning to the heart.

Consequently, the brain will not receive enough oxygenated blood, and the individual may lose consciousness. Venules and veins are also richly innervated by sympathetic nerves and smooth muscles within constrict when these nerves are activated. Thus, increased sympathetic nerve activity is associated with a decreased venous volume, which results in increased cardiac filling and therefore an increased cardiac output via Starling's Law of the Heart.

Many veins, especially those in the limbs, also feature abundant valves which are notably also found in the cardiac venous system which are thin folds of the intervessel lining that form flaplike cusps. The valves project into the vessel lumen and are directed towards the heart promoting unidirectional flow of blood.

Because blood pressure is normally low in veins, these valves are important in aiding in venous return by preventing the backflow of blood which is especially true in the upright individual. In addition, contractions of skeletal muscles e. The pulmonary circulation is composed of a similar circuit. Blood leaves the right ventricle in a single great vessel, the pulmonary artery trunk which, within a short distance centimeters , divides into the two main pulmonary arteries, one supplying the right lung and another the left.

Once within the lung proper, the arteries continue to branch down to arterioles and then ultimately form capillaries. From there, the blood flows into venules, eventually forming four main pulmonary veins which empty into the left atrium. As blood flows through the lung capillaries, it picks up oxygen supplied to the lungs by breathing air; hemoglobin within the red blood cells is loaded up with oxygen oxygenated blood.

Figure 3. While standing at rest, the relaxed vein acts as a reservoir for blood; contractions of limb muscles not only decrease this reservoir size venous diameter , but also actively force the return of more blood to the heart. Note that the resulting increase in blood flow due to the contractions is only towards the heart due to the valves in the veins. Go to the U of M home page.