Standing

Foot vein.supporting Foot vein supportingcolumn of blood 1.5 m column of blood(150 cm) in height 34 cm in height

f;lgilre 10-3'i Effect of contraction of the skeletal muscles of the legsin counteracting the effects of gravity. Contract on of skeletal musc es (as inwalking) comp etely empties gtver vetn segments, nterrupt ng ttre co umn ofb ood that the ower veins must support.isof rce: Ada pted f tatrt physralagy' af the H_oart an(j Circulatior, 4th ecl., by R C L tt e a nd V,/ C

L ttre. copyfight o 1989 yea- Book Niredica pLrb shers, rna., w th perm ssron from E sevierr

Iar system and restoring effective circulation. For this reasois counterproductive to try to hold upright someone whofainted. Fainting is a remedy to the problem, not the probitself.

Because the skeletal muscle pump facilitates venous retand helps counteract the detrimental ellects of gravity oncirculatory system, when you are l,vorking at a desk it,s a gidea to get up periodically and, when you are on your feet,should move around. The mild muscular activity ,gets

blood moving." It is further recommended that people rmust be on their feet for long periods wear elastic stockings I

apply a continuous gentle external compression, sirnilar toeffect of skeletal muscle contraction, to further counter thefect of gravitational pooling of blood in the 1eg veins.

nous vasoconstriction and external venous compression bdrive blood toward the heart. yet if you squeeze a fluid-filtube in the middle, fluid is pushed in both directions fron.tpoint of constriction ( Figure l0-32a). Then why isnt blcdriven backward, as well as forward, by venous vasoconstrtion and the skeietal muscle pump? Blood can only be clrirforr'vard because the large veins are equipped with one-r,valves spaced at 2- to 4-cm intervals; these valves let b1cmor.e forlard toward the heart but keep it from moving bitoward the tissues (' Figure 10-32b and c). These venous l,al.also play a role in counteracting the gravitational effects of iright posture by helping minimize the backflow of blood tltends to occur nhen a person stands up and by temporarsupporting portions of the column of blood rvhen the skelemuscles are relaxed.

$"]''*. Varicose veins occur when the venoLls .,.alves becorl: incompetent and can no longer support the column,: biood above them. People predisposed to this conditi

34 cm

pressure that occurs when a person moves from a lying_downto an upright position triggers sympathetically induced venousvasoconstriction, which drives some of the pooled bloodforward. Second, the skeletal muscle pump ..interrupts,,

thecolumn of blood by completelyemptying given vein segments in-termittently so that a particularportion of a vein is not subjectedto the weight of the entire venouscolumn from the heart to thatportion's level () Figure l0-31;also see ) Figure 10-29). Reflexvenous vasoconstriction cannotcompletely compensate for gravi-tational effects without skeletalmuscle activity. When a personstands still for a long time, there-fore, blood flow to the brain isreduced because of the declinein effective circulating volume,despite reflexes aimed at main-taining mean arterial pressure.Reduced flow of blood to thebrain, in turn, leads to fainting,which returns the person to a hor-izontal position, eliminating thegravitational effects on the vascu-

'Open venous valvepermits flow ofblood toward heart

Vein

Contractedskeletal muscle

Closed venous valveprevents backflowof blood

(a) Fluid moving (b) Action of venous valves.in both directions permitting flow of blood towardon squeezing a heart and preventing backflowfluid-filled tube of blood

) Figure 10-32 Function of venous valves.

(c) Photograph of a close:venous valve

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