Veins
•
Venules
(have little resistance or tone)
communicate
chemically with nearby arterioles to control capillary flow
•
Veins serve as a
blood reservoir
containing 60% of blood
volume in body
•
Veins return blood back to the heart
Large
vein
Capillaries → venules → small veins → large veins → Heart
Endothelial
cells
Smooth
Muscle
(& elastin)
valve
Systemic
veins
64%
Heart
7%
Pulmonary
vessels
9%
Systemic
arteries
13%
Systemic
arterioles
2%
Systemic
capillaries
5%
Percentage of total blood volume in different parts
of circulatory system
Venous Return
•
Venous return
:
volume of blood per min entering each
atrium from the veins
•
Venous return is enhanced by:
•
venous vasoconstriction
by sympathetic system
•
external compression of the veins
from contraction of
surrounding skeletal muscles
•
one-way venous valves
(spaced at 2-4 cm intervals)
•
These actions help counter the effects of gravity on the
venous system
Venous return
Cardiac suction effect
(
pressure in heart
pressure gradient)
Venous valves
(mechanically prevent
backflow of blood)
Pressure imparted to blood
by cardiac contraction
(
venous pressure
pressure gradient)
Salt and
water
retention
Skeletal muscle pump
(
venous pressure-
pressure gradient)
Respiratory pump
(
pressure in chest veins
pressure gradient)
Blood volume
(
venous pressure
pressure gradient)
Passive bulk-flow
shift of fluid
from interstitial
fluid into plasma
KEY
= Short-term control measures
= Long-term control measures
Factors that facilitate venous return
(by increasing the pressure gradient)
Calf
Foot vein supporting
column of blood
34 cm in height
Foot vein supporting
column of blood 1.5 m
(150 cm) in height
Walking
Standing
Foot
27
mm Hg
Venous
pressure
in foot
100
mm Hg
150 cm
Heart
Thigh
34 cm
Effect of contraction
of skeletal muscles of
the legs in
counteracting gravity
Contraction
empties a given
vein segment,
reducing amount
of blood the lower
veins must support
Constriction forces fluid
flow in both directions
Fluid-filled
tube
Point of
constriction
Action of venous valves
(permits flow of blood toward
heart & prevents backflow)
Closed venous valve
prevents backflow
of blood
Contracted
skeletal muscle
Vein
Open venous valve
permits flow of
blood toward heart
Venous valves help counteract gravity forces
Varicose veins
Deep vein thrombosis (DVT)
•
When a blood clot (thrombus) forms in a vein deep in the body
•
“Economy class syndrome”
•
most common in legs - lower leg & thigh
•
More common >60 yrs old, but can occur at any age
•
Blood clots can break loose & travel through bloodstream eventually
blocking blood flow to tissues (lungs, brain, heart, etc.)
Compression stockings
Compression stockings/socks
“flight socks”
Medical Compression Stockings
