Page 244 - Color Atlas Of Pathophysiology (S Silbernagl Et Al, Thieme 2000)

Edemas
Functional pores in the capillary endothelium in the lungs (→ p. 80). Possible causes of ede-
allow largely protein-free plasma fluid to filter ma are (→ B):
into the interstitial spaces. About 20 L/d are fil- ! Blood pressure rise at the arterial end due to
tered through all capillaries of the body (ex- precapillary vasodilation (P cap ↑), especially
cluding the kidneys), of which 90% are imme- during a simultaneous increase in permeabil-
diately reabsorbed. The remaining 2 L/d reach ity to proteins (σ prot ↓ and thus ∆π ↓), for exam-
the blood compartment only via the lymph ple, in inflammation or anaphylaxis (hista-
(→ A). mine, bradykinin, etc.).
The filtration or reabsorption rate Q f is de- ! Rise in venous pressure (P cap ↑ at the capil-
termined by the filtration coefficient K f (= wa- lary end), which may be caused locally by ve-
ter permeability · exchange area) of the capil- nous thrombosis or systemically (cardiac ede-
lary wall, as well as by the effective filtration
ma), for example, by heart failure (→
Heart and Circulation between the hydrostatic pressure difference ∆P cites (→ p.170).
p. 224ff.). Portal vein congestion leads to as-
pressure P eff (Q f = P eff · K f ). P eff is the difference
! Reduced plasma concentration of proteins
and the oncotic (colloidal osmotic) pressure dif-
(especially albumin) causes ∆π to fall exces-
ference ∆π across the capillary wall (Starling’s
sively. This may be the result of renal loss of
law), where ∆P = blood pressure in the capil-
proteins (proteinuria; → p.104) or of too little
laries (P cap ) – interstitial pressure (P int , normal-
liver cirrhosis; → p.172ff.), or of an increased
centration being higher in plasma than in the
breakdown of plasma proteins to meet amino
interstitial space by ∆C prot (≈ 1 mmol/L), and it
7 ly ≈ 0 mmHg). ∆π arises due to the protein con- hepatic synthesis of plasma proteins (e.g., in
is the greater, the closer the reflexion coeffi- acid demand if there is a protein deficiency
cient for plasma proteins (σ prot ) is to 1.0, i.e., (hunger edema).
the smaller the endothelial permeability for ! Diminished lymphatic flow may also cause
plasma proteins (∆π = σ prot · R · T · ∆C prot ). At local edemas, either by compression (tumors),
heart level, ∆P at the arterial end of the capil- transection (operations), fibrosis (radiother-
laries is ca. 30 mmHg; at the venous end it falls apy), or occlusion (Bilharziasis) of the lym-
to ca. 22 mmHg. ∆π (ca. 24 mmHg; → A, right) phatic vessels.
counteracts these pressures so that the intially When edemas form, the interstitial space is
high filtration (P eff = + 6 mmHg) is turned into enlarged until a new equilibrium is estab-
reabsorption when P eff becomes negative. (In lished (filtration = absorption + lymphatic out-
the lungs ∆P is only 10 mmHg, so that P eff is flow). An increased compliance of the intersti-
very low.) tial space encourages edemas to form just as
Below the level of the heart the hydrostatic much as a raised hydrostatic pressure in the
pressure of the column of blood is added to dependent parts of the body (e.g., ankle ede-
the pressure in the capillary lumen (at foot lev- ma) does.
el ca. + 90 mmHg). It is especially on standing As edema fluid originates from blood, the
still that the filtration pressure is very high in consequence of systemic edema (→ B, bottom)
the legs. It is compensated by self-regulation will be a decrease in blood volume, and thus
in that because of the outflow of water, the cardiac output. Renal perfusion is reduced not
protein concentration and thus ∆π is increased only directly by the fall in CO, but also as a re-
along the capillaries. It is also part of self-regu- sult of sympathetic stimulation. The renal fil-
lation that P int rises when filtation is increased tration fraction is raised and the renin–angio-
(limited compliance of the interstitial space), tensin mechanism is initiated. The resulting
+
and as a result ∆P decreases. Na retention raises the extracellular fluid vol-
If the amount of filtrate exceeds the sum of ume which, while increasing the blood vol-
+
reabsorbed volume plus lymphatic outflow, ume, actually makes the edema worse. Na re-
edemas develop, ascites develop in the region tention in renal failure also results in edema
234 of portal vein supply, as do pulmonary edemas being formed.
Silbernagl/Lang, Color Atlas of Pathophysiology © 2000 Thieme
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