functions including hundreds of different kinds of

functions in the rapid internal transport of substancesYour body’s differentiated cells, which performspecialized tasks, cannot fend for themselves.

Differenttypes must interact in coordinated ways to maintain thecomposition, volume, and temperature of a tissue fluidsurrounding them, the interstitial fluid. A circulatingconnective tissue-blood-interacts with tissue fluid,making continual deliveries and pickups that help keepconditions tolerable for enzymes and other moleculesheart, generates pressure that keeps blood flowing. Likemany anirnals, you have a closed circulatory system-blood is confined within the continuously connectedwalls of the heart and blood vessels.capillary beds. At such beds, the flow fans out throughvast numbers of senall-diameter blood vessels calledheart’s right half pumps oxygen-poor blood to thelungs, where blood picks up oxygen and gives up car-bon dioxide. The freshly oxygenated blood then flowsto the heart’s left half. This is the pulmonary circuit.

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Bycontrast, in a systemic circuit, the Itft half of the heartpumps oxygenated blood to all tissues where oxygen isan elaborate network of drainage vessels picks upexcess interstitial fluid and reclaimable solutes, thenreturns them to the circulatory system. This network isBlood, a connective tissue, has multiple functions. Ittransports oxygen, nutrients, and other solutes to cells.

The volume of blood depends on body sizeaverage-size adult humans is about 6 to 8 percentbody weight. That amounts to about four or five blood cells, vehite blood cells, and platelets are itsaccounts for 50 to 60 percent of the total blood volume.Plasena Plasma, which is mostly water, functions as atransport medium for blood cells and platelets.

It alsoserves as a solvent for ions and molecules, includinghundreds of different kinds of plasma proteins. Some ofthe plasma proteins transport lipids and fat-solubleErythrocytes, or red blood ceils, arebiconcave disks, like doughnixts with a squashed-incount is the number of cells of a given type in a micro-I liter of blood. The average number of red blood cells is5.4 million in males and 4.8 million in females.( arise from stem cetls in bone marrow.

Each platelet only lasts five to nine days, buthundreds of thousands are always circulating in blood.if the volume of blood were to decrease by more than 30 percent, then circulatory shock would follow defensive response called agglutination, proteins calledantibodies that are circulating in plasma act against theforeign cells and cause them to clump.Molecular variations in one kind of self marker on redblood cells are analyzed in ABO blood typing.

cular system, a heart pumps blood into large-diameterarteries. From there, blood flows into small, musculararterioies, which branch into the even smaller diametercapillaries introduced earlier. Blood llows contintrouslyfrom capiliaries into sma!! venules, then into large-diameter veins that return blood to the heart.The pulmonary circuit, a short loop, rapidlyoxygenates blood. It leads from the heart’s right half tocapillary beds in both lungs, then returns to the heart’sleft half. The systemic circuit is a longer loop.

It starts atthe heart’s left half and the aorta (the main artery carry-ing oxygenated blood away from the heart), branches toall organs and tissues with metabolicaily active cells,then converges into major veins that deliver oxygen-poor blood to the heart’s right half.2.5 billion times during a seventy-year life span, andyou know it must be a truly durable pump.Each half of the heart has two chambers-an atriunThe sequence of contraction and relaxation is a cardiacAbout 1 percent of the cardiac musclecells don’t contract. Instead, they function as a cardiacconduction system. These specialized cells initiate andPropagate waves of excitation abottl seventy-five timesanother eell body cluster, the AV node. This is the onlyelectrical bridge between atria and ventricies (whichconnective tissue insuiates everys.vhere else).

After theAV node, conducting cells are arranged as a bundle i=the partition between the heart’s two halves. The cellsthen branch, and the branches detiver the excitatorywave up the ventricle walls. The ventricles contract inresponse with a forvisting movement, upward from theheart’s apex, that ejects blood into the great arteries.The SA node fires action potentials faster than therest of the system and serves as the cardiac pacemaker.

Blood pressure, the fluid pressure generated by heartcontractions, is highest in contracting ventricles. Duringthe time it takes for a given volume of blood to leaveand reenter the heart, pressure is still high in arteries,then drops along the cirettit, and is lowest tn the relaxedthe average restinn value stars fairly constant o-few weeks, even months about 120/80 mm Hg. ‘an increase in blood


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