Circulatory System, or cardiovascular system, inhumans, the combined function of the heart, blood, andblood vessels to transport oxygen and nutrients toorgans and tissues throughout the body and carry awaywaste products. Among its vital functions, the circulatorysystem increases the flow of blood to meet increasedenergy demands during exercise and regulates bodytemperature. In addition, when foreign substances ororganisms invade the body, the circulatory systemswiftly conveys disease-fighting elements of the immunesystem, such as white blood cells and antibodies, toregions under attack. Also, in the case of injury orbleeding, the circulatory system sends clotting cells andproteins to the affected site, which quickly stop bleedingand promote healing.
The heart, blood, and blood vessels are the three structuralelements that make up the circulatory system. The heart is theengine of the circulatory system. It is divided into four chambers: theright atrium, the right ventricle, the left atrium, and the left ventricle.The walls of these chambers are made of a special muscle calledmyocardium, which contracts continuously and rhythmically to pumpblood. The pumping action of the heart occurs in two stages for eachheart beat: diastole, when the heart is at rest; and systole, when theheart contracts to pump deoxygenated blood toward the lungs andoxygenated blood to the body. During each heartbeat, typically about60 to 90 ml (about 2 to 3 oz) of blood are pumped out of the heart. Ifthe heart stops pumping, death usually occurs within four to fiveminutes. Blood consists of three types of cells: oxygen-bearing red bloodcells, disease-fighting white blood cells, and blood-clottingplatelets, all of which are carried through blood vessels in a liquidcalled plasma. Plasma is yellowish and consists ofwater, salts, proteins, vitamins, minerals, hormones, dissolvedgases, and fats.
Three types of blood vessels form a complexnetwork of tubes throughout the body. Arteries carryblood away from the heart, and veins carry it towardthe heart. Capillaries are the tiny links between thearteries and the veins where oxygen and nutrientsdiffuse to body tissues. The inner layer of bloodvessels is lined with endothelial cells that create asmooth passage for the transit of blood. This innerlayer is surrounded by connective tissue and smoothmuscle that enable the blood vessel to expand orcontract. Blood vessels expand during exercise tomeet the increased demand for blood and to cool thebody. Blood vessels contract after an injury to reducebleeding and also to conserve body heat.
Arteries have thicker walls than veins towithstand the pressure of blood being pumpedfrom the heart. Blood in the veins is at a lowerpressure, so veins have one-way valves toprevent blood from flowing backwards awayfrom the heart. Capillaries, the smallest of bloodvessels, are only visible by microscope—tencapillaries lying side by side are barely as thick asa human hair. If all the arteries, veins, andcapillaries in the human body were placed end toend, the total length would equal more than100,000 km (more than 60,000 mi)—they couldstretch around the earth nearly two and a halftimes.
The arteries, veins, and capillaries aredivided into two systems of circulation:systemic and pulmonary. The systemiccirculation carries oxygenated blood from theheart to all the tissues in the body except thelungs and returns deoxygenated bloodcarrying waste products, such as carbondioxide, back to the heart. The pulmonarycirculation carries this spent blood from theheart to the lungs. In the lungs, the bloodreleases its carbon dioxide and absorbsoxygen. The oxygenated blood then returns tothe heart before transferring to the systemiccirculation.
Only in the past 400 years have scientists recognizedthat blood moves in a cycle through the heart and body.Before the 17th century, scientists believed that the livercreates new blood, and then the blood passes through theheart to gain warmth and finally is soaked up and consumedin the tissues. In 1628 English physician William Harvey firstproposed that blood circulates continuously. Using modernmethods of observation and experimentation, Harvey notedthat veins have one-way valves that lead blood back to theheart from all parts of the body. He noted that the heartworks as a pump, and he estimated correctly that the dailyoutput of fresh blood is more than seven tons. He pointedout the absurdity of the old doctrine, which would requirethe liver to produce this much fresh blood daily. Harvey’stheory was soon proven correct and became the cornerstoneof modern medical science.
The heart ejects oxygen-rich blood under highpressure out of the heart’s main pumping chamber, theleft ventricle, through the largest artery, the aorta.Smaller arteries branch off from the aorta, leading tovarious parts of the body. These smaller arteries in turnbranch out into even smaller arteries, called arterioles.Branches of arterioles become progressively smaller indiameter, eventually forming the capillaries. Once bloodreaches the capillary level, blood pressure is greatlyreduced.
Capillaries have extremely thin walls thatpermit dissolved oxygen and nutrients from theblood to diffuse across to a fluid, known asinterstitial fluid, that fills the gaps between the cellsof tissues or organs. The dissolved oxygen andnutrients then enter the cells from the interstitialfluid by diffusion across the cell membranes.Meanwhile, carbon dioxide and other wastes leavethe cell, diffuse through the interstitial fluid, crossthe capillary walls, and enter the blood. In thisway, the blood delivers nutrients and removeswastes without leaving the capillary tube.
After delivering oxygen to tissues andabsorbing wastes, the deoxygenated blood inthe capillaries then starts the return trip to theheart. The capillaries merge to form tinyveins, called venules. These veins in turn jointogether to form progressively larger veins.Ultimately, the veins converge into two largeveins: the inferior vena cava, bringing blood fromthe lower half of the body; and the superior venacava, bringing blood from the upper half. Both ofthese two large veins join at the right atrium ofthe heart.
Because the pressure is dissipated in thearterioles and capillaries, blood in veins flowsback to the heart at very low pressure, oftenrunning uphill when a person is standing. Flowagainst gravity is made possible by the one-way valves, located several centimetersapart, in the veins. When surrounding musclescontract, for example in the calf or arm, themuscles squeeze blood back toward the heart.If the one-way valves work properly, bloodtravels only toward the heart and cannot lapsebackward. Veins with defective valves, whichallow the blood to flow backward, becomeenlarged or dilated to form varicose veins.
C-2 . PULMONARY CIRCULATION In pulmonary circulation, deoxygenated blood returning fromthe organs and tissues of the body travels from the right atrium of theheart to the right ventricle. From there it is pushed through thepulmonary artery to the lung. In the lung, the pulmonary arterydivides, forming the pulmonary capillary region of the lung. At thissite, microscopic vessels pass adjacent to the alveoli, or air sacs ofthe lung, and gases are exchanged across a thin membrane: oxygencrosses the membrane into the blood while carbon dioxide leavesthe blood through this same membrane. Newly oxygenated bloodthen flows into the pulmonary veins, where it is collected by the leftatrium of the heart, a chamber that serves as collecting pool for theleft ventricle. The contraction of the left ventricle sends blood into theaorta, completing the circulatory loop. On average, a single bloodcell takes roughly 30 seconds to complete a full circuit through boththe pulmonary and systemic circulation.
In addition to oxygen, the circulatory system alsotransports nutrients derived from digested food to thebody. These nutrients enter the bloodstream bypassing through the walls of the intestine. Thenutrients are absorbed through a network ofcapillaries and veins that drain the intestines, calledthe hepatic portal circulation. The hepatic portalcirculation carries the nutrients to the liver for furthermetabolic processing. The liver stores a variety ofsubstances, such as sugars, fats, and vitamins, andreleases these to the blood as needed. The liver alsocleans the blood by removing waste products andtoxins. After hepatic portal blood has crossed the livercells, veins converge to form the large hepatic veinthat joins the vena cava near the right atrium.
The circulatory system plays an important role inregulating body temperature. During exercise, workingmuscles generate heat. The blood supplying the muscleswith oxygen and nutrients absorbs much of this heat andcarries it away to other parts of the body. If the body gets toowarm, blood vessels near the skin enlarge to disperse excessheat outward through the skin. In cold environments, theseblood vessels constrict to retain heat. The circulatory system works in tandem with theendocrine system, a collection of hormone-producingglands. These glands release chemical messengers, calledhormones, directly into the bloodstream to be transported tospecific organs and tissues. Once they reach their targetdestination, hormones regulate the body’s rate ofmetabolism, growth, sexual development, and otherfunctions.
The circulatory system also works with the immunesystem and the coagulation system. The immune systemis a complex system of many types of cells that worktogether to combat diseases and infections. Disease-fighting white blood cells and antibodies circulate in theblood and are transported to sites of infection by thecirculatory system. The coagulation system is composedof special blood cells, called platelets, and specialproteins, called clotting factors, that circulate in theblood. Whenever blood vessels are cut or torn, thecoagulation system works rapidly to stop the bleeding byforming clots.
Other organs support the circulatory system. Thebrain and other parts of the nervous system constantlymonitor blood circulation, sending signals to the heartor blood vessels to maintain constant blood pressure.New blood cells are manufactured in the bone marrow.Old blood cells are broken down in the spleen, wherevaluable constituents, such as iron, are recycled.Metabolic waste products are removed from the bloodby the kidneys, which also screen the blood for excesssalt and maintain blood pressure and the body’sbalance of minerals and fluids.
The pressure generated by the pumping actionof the heart propels the blood to the arteries. Inorder to maintain an adequate flow of blood to allparts of the body, a certain level of blood pressureis needed. Blood pressure, for instance, enables aperson to rise quickly from a horizontal positionwithout blood pooling in the legs, which wouldcause fainting from deprivation of blood to thebrain. Normal blood pressure is regulated by anumber of factors, such as the contraction of theheart, the elasticity of arterial walls, bloodvolume, and resistance of blood vessels to thepassage of blood.
Blood pressure is measured using an inflatable devicewith a gauge called a that is wrapped around the upper arm.Blood pressure is measured during systole, the activepumping phase of the heart, and diastole, the resting phasebetween heartbeats. Systolic and diastolic pressures aremeasured in units of millimeters of mercury (abbreviated mmHg) and displayed as a ratio. Blood pressure varies betweenindividuals and even during the normal course of a day inresponse to emotion, exertion, sleep, and other physical andmental changes. Normal blood pressure is less than 120/80mm Hg, in which 120 describes systolic pressure and 80describes diastolic pressure. Higher blood pressures that aresustained over a long period of time may indicatehypertension, a damaging circulatory condition. Lower bloodpressures could signal shock from heartfailure, dehydration, internal bleeding, or blood loss.
Disorders of the circulatory system include any injury ordisease that damages the heart, the blood, or the blood vessels.The three most important circulatory diseases arehypertension, arteriosclerosis, and atherosclerosis. Hypertension, or elevated blood pressure, develops when thebody’s blood vessels narrow, causing the heart to pump harderthan normal to push blood through the narrowed openings.Hypertension that remains untreated may cause heartenlargement and thickening of the heart muscle. Eventually theheart needs more oxygen to function, which can lead to heartfailure, brain stroke, or kidney impairment. Some cases ofhypertension can be treated by lifestyle changes such as a low-saltdiet, maintenance of ideal weight, aerobic exercise, and a diet richin fruits, vegetables, plant fiber, and the mineral potassium. Ifblood pressure remains high despite these lifestyleadjustments, medications may be effective in lowering thepressure by relaxing blood vessels and reducing the output ofblood.
In arteriosclerosis, commonly known as hardening of thearteries, the walls of the arteries thicken, harden, and lose theirelasticity. The heart must work harder than normal to deliverblood, and in advanced cases, it becomes impossible for the heart tosupply sufficient blood to all parts of the body. Nobody knows whatcauses arteriosclerosis, but heredity, obesity, smoking, and a high-fatdiet all appear to play roles. Atherosclerosis, a form of arteriosclerosis, is the reduction inblood flow through the arteries caused by greasy deposits calledplaque that form on the insides of arteries and partially restrict theflow of blood. Plaque deposits are associated with highconcentrations of cholesterol in the blood. Recent studies have alsoshown an association between 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 arterywalls have been roughened by plaque. These blood clots can alsobreak free and travel through the circulatory system until theybecome lodged somewhere else and reduce blood flow there (seeEmbolism). Reduction in blood flow can cause organ damage. Whenbrain arteries become blocked and brain function is impaired, theresult is a stroke. A heart attack occurs when a coronary arterybecomes blocked and heart muscle is destroyed.
Risk factors that contribute toatherosclerosis include physicalinactivity, smoking, a diet high in fat, highblood pressure, and diabetes. Some cases ofatherosclerosis can be corrected with healthylifestyle changes, aspirin to reduce bloodclotting or inflammation, or drugs to lower theblood cholesterol concentration. For moreserious cases, surgery to dilate narrowedblood vessels with a balloon, known asangioplasty, or to remove plaque with a high-speed cutting drill, known asatherectomy, may be effective. Surgicalbypass, in which spare arteries are used toconstruct a new path for blood flow, is also anoption.
One-celled organisms and many simple multicelledanimals, such as sponges, jellyfishes, seaanemones, flatworms, and roundworms, do not have acirculatory system. All of their cells are able to absorbnutrients, exchange gases, and expel wastes through directcontact with either the outside or with a central cavity thatserves as a digestive tract. More complex invertebrates have a wide range ofcirculatory system designs. These invertebrate circulatorysystems are classified as either open or closed. Open systems—found instarfishes, clams, oysters, snails, crabs, insects, spiders, andcentipedes—lack capillaries, and the blood bathes the tissuesdirectly. In closed systems, the blood is confined to a system ofblood vessels. Invertebrates with closed systems includesegmented worms, squids, and octopuses.
All vertebrate animals have closed circulatorysystems. These systems are classified by thenumber of chambers in the heart, which determinesthe basic configuration of blood flow. Fish have two-chambered hearts with one atrium and oneventricle. Blood pumped from the ventricle travelsthrough arteries to the gills, where it diverges intocapillaries and exchanges gases. Leaving thegills, the capillaries reconvene into blood vesselsthat carry the oxygenated blood to the rest of thebody, where the vessels again diverge intocapillaries before reconvening into veins that returnto the heart. In this way, the blood passes throughfirst the respiratory organs (the gills) and then thesystemic circulation between each pass through theheart.
Frogs and amphibians have three-chamberedhearts, with two atriums and one ventricle. Bloodpumped from the ventricle enters a forked artery.One fork, the pulmonary circulation, leads to thelung. The other fork, the systemiccirculation, leads to the rest of the body. Bloodreturning from the pulmonary circulation entersthe left atrium, while blood from the systemiccirculation enters the right atrium. Although thereis some mixing of oxygenated and deoxygenatedblood in the ventricle, a ridge within the ventricleassures that most of the oxygenated blood isdiverted to the systemic circulation and most ofthe deoxygenated blood goes to the pulmonarycirculation. In reptiles, this ridge is moredeveloped, forming a partial wall. Incrocodiles, the wall is complete, forming a four-chambered heart like that found in mammals andbirds.