Page 221 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

Endothelial Exchange Processes sults in only transient reabsorption. After several
minutes it stops because the interstitial oncotic pres-
Nutrients and waste products are exchanged sure rises due to “self-regulation”. Thus, a major part
across the walls of the capillaries and post- of the 18 L/d expected to be reabsorbed from the ex-
papillary venules (exchange vessels; ! p. 188). change vessels (see above) might actually be reab-
sorbed in the lymph nodes. Rhythmic contraction of
Their endothelia contain small (ca. 2–5 nm) or the arterioles (vasomotion) may also play a role by
large (20–80 nm, especially in the kidneys and decreasing P eff and thus by allowing intermittent
liver) functional pores: permeable, intercellu- capillary reabsorption.
lar fissures or endothelial fenestrae, respec- In parts of the body below the heart, the effects of
tively. The degree of endothelial permeability hydrostatic pressure from the blood column in-
varies greatly from one organ to another. Vir- crease the pressure in the capillary lumen (in the feet
tually all endothelia allow water and inorganic ! 90 mmHg). The filtration rate in these regions
therefore rise, especially when standing still. This is
ions to pass, but most are largely impermeable counteracted by two “self-regulatory” mechanisms:
Cardiovascular System passage of certain larger molecules. the capillaries (normally the case in glomerular capil-
to blood cells and large protein molecules.
(1) the outflow of water results in an increase in the
Transcytosis and carriers (! p. 26f.) allow for
luminal protein concentration (and thus ∆π) along
laries, ! p. 152); (2) increased filtration results in an
Filtration and reabsorption. About 20 L/day
increase in P int and a consequent decrease in ∆P.
of fluid is filtered (excluding the kidneys) into
Edema. Fluid will accumulate in the interstitial
the interstitium from the body’s exchange ves-
space (extracellular edema), portal venous system
sels. About 18 L/day of this fluid is thought to be
edema) if the volume of filtered fluid is higher than
The remaining 2 L/day or so make up the lymph
the amount returned to the blood.
8 reabsorbed by the venous limb of these vessels. (ascites), and pulmonary interstice (pulmonary
flow and thereby return to the bloodstream
(! A). The filtration or reabsorption rate Q f is a Causes of edema (! B):
! Increased capillary pressure (! B1) due to precapil-
factor of the endothelial filtration coefficient K f lary vasodilatation (P cap"), especially when the capil-
(= water permeability k · exchange area A) and lary permeability to proteins also increases (σ prot #
the effective filtration pressure P eff (Q f = K f · P eff). and ∆π #) due, for example, to infection or anaphy-
P eff is calculated as the hydrostatic pressure laxis (histamine etc.). Hypertension in the portal vein
difference ∆P minus the oncotic pressure leads to ascites.
difference ∆π across the capillary wall (Star- ! Increased venous pressure (P cap ", ! B2) due, for
ling’s relationship; ! A), where ∆P = capillary example, to venous thrombosis or cardiac insuffi-
ciency (cardiac edema).
pressure (P cap) minus interstitial pressure (P int, ! Decreased concentration of plasma proteins, es-
normally ! 0 mmHg). At the level of the heart, pecially albumin, leading to a drop in ∆π (! B3 and
∆P at the arterial end of the systemic capillar- p. 379 A) due, for example, to loss of proteins (pro-
ies is about 30 mmHg and decreases to about teinuria), decreased hepatic protein synthesis (e.g.,
22 mmHg at the venous end. Since ∆π (ca. in liver cirrhosis), or to increased breakdown of
24 mmHg; ! A) counteracts ∆P, the initially plasma proteins to meet energy requirements
high filtration rate (P eff = + 6 mmHg) is thought (hunger edema).
! Decreased lymph drainage due, e.g., to lymph tract
to change into reabsorption whenever P eff be- compression (tumors), severance (surgery), oblitera-
comes negative. (Since ∆P is only 10 mmHg in tion (radiation therapy) or obstruction (bilharziosis)
the lungs, the pulmonary P eff is very low). ∆π can lead to localized edema (! B4).
occurs because the concentration of proteins ! Increased hydrostatic pressure promotes edema
(especially albumin) in the plasma is much formation in lower regions of the body (e.g., in the
higher than their interstitial concentration. ankles; ! B).
The closer the reflection coefficient of the Diffusion. Although dissolved particles are
plasma proteins (σ prot) to 1.0, the higher ∆π dragged through capillary walls along with fil-
and, consequently, the lower the permeability tered and reabsorbed water (solvent drag;
of the membrane to these proteins (! p. 377). ! p. 24), diffusion plays a much greater role in
According to Starling’s relationship, water reab- the exchange of solutes. Net diffusion of a sub-
208 sorption should occur as long as P eff is negative. stance (e.g., O 2, CO 2) occurs if its plasma and in-
However, recent data suggest that a negative P eff re- terstitial conc. are different.
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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