Venous pooling, or the distention of veins, produces hemodynamic effects similar to the loss of an equivalent volume of blood. When an adult goes from a supine to a relaxed standing position, approximately 300 to 800 mL of blood accumulates in the legs, resulting in a reduction of cardiac output by about 2 L/min. The compensatory adjustments made to assume a standing position resemble those made in response to blood loss. This includes reflex increases in heart rate and cardiac contractility, as well as constriction of both arterioles and veins. However, arterioles are more affected than veins.

When a person lying down stands up but remains at rest, the pressure in the veins gradually rises in the lower parts of the body. It takes almost a minute for the venous pressure (Pv) in the legs to reach an equilibrium value. This gradual rise in Pv is due to the presence of venous valves that only allow blood flow towards the heart. These valves prevent blood from flowing towards the feet, supporting the venous column at various levels. As blood continues to enter the column from venules and small tributary veins, the pressure gradually increases. Once the pressure in one segment of the venous column exceeds that in the segment above it, the valve between them opens. Eventually, all the valves open, resulting in a continuous column of blood.

Certain drugs used to treat chronic hypertension can interfere with the reflex adaptation to standing. Similarly, astronauts exposed to weightlessness lose their adaptations to gravity within a few days in space and experience significant difficulties upon their return to Earth. When individuals with impaired reflex adaptations, such as astronauts, stand up, their blood pressure may significantly drop, leading to orthostatic hypotension. This response can cause lightheadedness or fainting.

Normally, the superficial veins in the neck are partially collapsed when a person is sitting or standing. Venous return from the head primarily occurs through the deeper cervical veins, which are protected from collapse due to their connection to surrounding structures. However, when central venous pressure is abnormally elevated, the superficial neck veins become distended and do not collapse even when a person sits or stands. This cervical venous distention is an important clinical sign of congestive heart failure.

People with varicose veins in their legs have a less effective auxiliary pumping mechanism generated by skeletal muscle contractions. The valves in these defective veins do not function properly, causing blood to be forced in both the retrograde and antegrade directions when the leg muscles contract. As a result, when individuals with varicose veins stand or walk, the pressure in the ankles and feet becomes excessively high (Pv). The high capillary pressure that ensues leads to the accumulation of edematous fluid in the ankles and feet.
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.