Circulatory System.

« C Additional Functions In addition to oxygen, the circulatory system also transports nutrients derived from digested food to the body.

These nutrients enter the bloodstream by passingthrough the walls of the intestine.

The nutrients are absorbed through a network of capillaries and veins that drain the intestines, called the hepatic portal circulation.The hepatic portal circulation carries the nutrients to the liver for further metabolic processing.

The liver stores a variety of substances, such as sugars, fats, andvitamins, and releases these to the blood as needed.

The liver also cleans the blood by removing waste products and toxins.

After hepatic portal blood has crossed theliver cells, veins converge to form the large hepatic vein that joins the vena cava near the right atrium. The circulatory system plays an important role in regulating body temperature.

During exercise, working muscles generate heat.

The blood supplying the muscles withoxygen and nutrients absorbs much of this heat and carries it away to other parts of the body.

If the body gets too warm, blood vessels near the skin enlarge todisperse excess heat outward through the skin.

In cold environments, these blood vessels constrict to retain heat. The circulatory system works in tandem with the endocrine system, a collection of hormone-producing glands.

These glands release chemical messengers, calledhormones, directly into the bloodstream to be transported to specific organs and tissues.

Once they reach their target destination, hormones regulate the body’s rate ofmetabolism, growth, sexual development, and other functions. The circulatory system also works with the immune system and the coagulation system.

The immune system is a complex system of many types of cells that worktogether to combat diseases and infections.

Disease-fighting white blood cells and antibodies circulate in the blood and are transported to sites of infection by thecirculatory system.

The coagulation system is composed of special blood cells, called platelets, and special proteins, called clotting factors, that circulate in the blood.Whenever blood vessels are cut or torn, the coagulation system works rapidly to stop the bleeding by forming clots. Other organs support the circulatory system.

The brain and other parts of the nervous system constantly monitor blood circulation, sending signals to the heart or bloodvessels to maintain constant blood pressure.

New blood cells are manufactured in the bone marrow.

Old blood cells are broken down in the spleen, where valuableconstituents, such as iron, are recycled.

Metabolic waste products are removed from the blood by the kidneys, which also screen the blood for excess salt and maintainblood pressure and the body’s balance of minerals and fluids. D Blood Pressure The pressure generated by the pumping action of the heart propels the blood to the arteries.

In order to maintain an adequate flow of blood to all parts of the body, acertain level of blood pressure is needed.

Blood pressure, for instance, enables a person to rise quickly from a horizontal position without blood pooling in the legs, whichwould cause fainting from deprivation of blood to the brain.

Normal blood pressure is regulated by a number of factors, such as the contraction of the heart, theelasticity of arterial walls, blood volume, and resistance of blood vessels to the passage of blood. Blood pressure is measured using an inflatable device with a gauge called a that is wrapped around the upper arm.

Blood pressure is measured during systole, theactive pumping phase of the heart, and diastole, the resting phase between heartbeats.

Systolic and diastolic pressures are measured in units of millimeters of mercury(abbreviated mm Hg) and displayed as a ratio.

Blood pressure varies between individuals and even during the normal course of a day in response to emotion, exertion,sleep, and other physical and mental changes.

Normal blood pressure is less than 120/80 mm Hg, in which 120 describes systolic pressure and 80 describes diastolicpressure.

Higher blood pressures that are sustained over a long period of time may indicate hypertension, a damaging circulatory condition.

Lower blood pressurescould signal shock from heart failure, dehydration, internal bleeding, or blood loss. IV CIRCULATORY SYSTEM DISORDERS Disorders of the circulatory system include any injury or disease that damages the heart, the blood, or the blood vessels.

The three most important circulatory diseasesare hypertension, arteriosclerosis, and atherosclerosis. Hypertension, or elevated blood pressure, develops when the body’s blood vessels narrow, causing the heart to pump harder than normal to push blood through thenarrowed openings.

Hypertension that remains untreated may cause heart enlargement and thickening of the heart muscle.

Eventually the heart needs more oxygen tofunction, which can lead to heart failure, brain stroke, or kidney impairment.

Some cases of hypertension can be treated by lifestyle changes such as a low-salt diet,maintenance of ideal weight, aerobic exercise, and a diet rich in fruits, vegetables, plant fiber, and the mineral potassium.

If blood pressure remains high despite theselifestyle adjustments, medications may be effective in lowering the pressure by relaxing blood vessels and reducing the output of blood. In arteriosclerosis, commonly known as hardening of the arteries, the walls of the arteries thicken, harden, and lose their elasticity.

The heart must work harder thannormal to deliver blood, and in advanced cases, it becomes impossible for the heart to supply sufficient blood to all parts of the body.

Nobody knows what causesarteriosclerosis, but heredity, obesity, smoking, and a high-fat diet all appear to play roles. Atherosclerosis, a form of arteriosclerosis, is the reduction in blood flow through the arteries caused by greasy deposits called plaque that form on the insides of arteriesand partially restrict the flow of blood.

Plaque deposits are associated with high concentrations of cholesterol in the blood.

Recent studies have also shown an associationbetween inflammation and plaque deposits.

Blood flow is often further reduced by the formation of blood clots ( see Thrombosis), which are most likely to form where the artery walls have been roughened by plaque.

These blood clots can also break free and travel through the circulatory system until they become lodged somewhereelse and reduce blood flow there ( see Embolism).

Reduction in blood flow can cause organ damage.

When brain arteries become blocked and brain function is impaired, the result is a stroke.

A heart attack occurs when a coronary artery becomes blocked and heart muscle is destroyed. Risk factors that contribute to atherosclerosis include physical inactivity, smoking, a diet high in fat, high blood pressure, and diabetes.

Some cases of atherosclerosiscan be corrected with healthy lifestyle changes, aspirin to reduce blood clotting or inflammation, or drugs to lower the blood cholesterol concentration.

For more seriouscases, surgery to dilate narrowed blood vessels with a balloon, known as angioplasty, or to remove plaque with a high-speed cutting drill, known as atherectomy, maybe effective.

Surgical bypass, in which spare arteries are used to construct a new path for blood flow, is also an option. V CIRCULATORY SYSTEMS IN NONHUMANS One-celled organisms and many simple multicelled animals, such as sponges, jellyfishes, sea anemones, flatworms, and roundworms, do not have a circulatory system.All of their cells are able to absorb nutrients, exchange gases, and expel wastes through direct contact with either the outside or with a central cavity that serves as adigestive tract. More complex invertebrates have a wide range of circulatory system designs.

These invertebrate circulatory systems are classified as either open or closed.

Opensystems—found in starfishes, clams, oysters, snails, crabs, insects, spiders, and centipedes—lack capillaries, and the blood bathes the tissues directly.

In closedsystems, the blood is confined to a system of blood vessels.

Invertebrates with closed systems include segmented worms, squids, and octopuses.. »