The hemodynamic effects of such venous distention (venous pooling) resemble those caused by the hemorrhage of an equivalent volume of blood from the body. When an adult shifts from a supine position to a relaxed standing position, 300 to 800 mL of blood pools in the legs. This pooling may reduce cardiac output by approximately 2 L/min. The compensatory adjustments made to assume a standing position are similar to the adjustments to blood loss (see also ): There are reflex increases in heart rate and cardiac contractility. In addition, both arterioles and veins constrict; the arterioles are affected to a greater extent than are the veins.
When a recumbent person stands but remains at rest, the pressure in the veins rises in the dependent regions of the body (
Fig. 17.21 ). The Pv in the legs increases gradually and does not reach an equilibrium value until almost 1 minute after the person begins standing. The slowness of this rise in Pv is attributable to the venous valves, which allow flow only toward the heart. When a person stands, the valves prevent blood in the veins from falling toward the feet. Hence, the column of venous blood is supported at numerous levels by these valves. Because of these valves, the venous column can be thought of as consisting of many discontinuous segments. However, blood continues to enter the column from many venules and small tributary veins, and the pressure continues to rise. As soon as the pressure in one segment exceeds that in the segment just above it, the intervening valve is forced open. Ultimately, all the valves are open, and the column is continuous.
•Fig. 17.21 Meanpressures(±95%confidenceintervals)inthefootveinsofhumansubjectsduringquietstanding,duringwalking,andduringrunning.(FromStickC,etal.J Appl Physiol. 1992;72:2063.)100 80 60 40 20 0 Venous pressure (mm Hg) Walking (3 km/hr) Walking (6 km/hr) Running (10 km/hr) Standing
Someofthedrugsusedtotreatchronichypertensioninterferewiththereflexadaptationtostanding.Similarly,astronautsexposedtoweightlessnesslosetheiradaptationstogravityafterafewdaysinspace,andtheyexperiencepronounceddifficultieswhentheyfirstreturntoearth.Whensuchastronautsandotherindividualswithimpairedreflexadaptationsstand,theirbloodpressuremaydropsubstantially.Thisresponseiscalledorthostatic hypotension, whichmaycauselightheadednessorfainting.
Thesuperficialveinsintheneckordinarilyarepartiallycollapsedwhenanormalindividualissittingorstanding.Venousreturnfromtheheadisconductedlargelythroughthedeepercervicalveins,whichareprotectedfromcollapsebecausetheyaretetheredtosurroundingstructures.Whencentralvenouspressureisabnormallyelevated,thesuperficialneckveinsaredistended,andtheydonotcollapseevenwhenthepersonsitsorstands.Suchcervicalvenousdistentionisanimportantclinicalsignofcongestiveheartfailure.
Theauxiliarypumpingmechanismgeneratedbyskeletalmusclecontractionsismuchlesseffectiveinpeoplewithvaricoseveinsintheirlegs.Thevalvesinthesedefectiveveinsdonotfunctionproperly,andthereforewhenthelegmusclescontract,thebloodinthelegveinsisforcedinboththeretrogradeandantegradedirections.Thuswhenanindividualwithvaricoseveinsstandsorwalks,Pvintheanklesandfeetisexcessivelyhigh.Theconsequenthighcapillarypressureleadstotheaccumulationofedematousfluidintheanklesandfeet.