Venous flow

Field tourniquets must be capable of reliably applying enough pressure to occlude arterial bleeding. If a tourniquet applies enough pressure to stop only the venous flow, then the rate of blood loss from an arterial wound will most likely increase. It should be noted that some small blood seepage may continue in the case of traumatic amputation due to medullar blood flow even with a properly tightened tourniquet. 

In the Budd-Chiari syndrome, the central area of the liver shows a normal or even increased concentration of radioactivity, whereas the peripheral regions of both lobes of liver exhibit reduced or even no uptake ( hot spots and multiple focal storage defects). Only the caudate lobe shows increased activity due to its separate venous flow, it is not functionally affected by hepatic vein thrombosis. (26)... 

Block surgery This surgical measure stops the venous flow to the bleeding oesophageal collaterals. The disadvantage is that portal hypertension is not influenced, so that there is a possibility of... 

The transformation of the lobular architecture is initiated and maintained by at least two histomorphological processes (i.) piecemeal necrosis and (2.) bridging necrosis, which provides string-like links between the central veins and the portal fields. Portocentral shunts, which are of significance for the fate of cirrhosis, make use of these bridges as routes for their development. During the course of time, these channels, which acquire solid basal membranes like capillaries, carry the portal blood directly to the venous flow-off. As a result, blood is withdrawn from the respective acini these areas become more susceptible to disruption and damage and are forced to restructure anew, (see chapter 35)... 

Reroute the venous flow rate from Gut through Liver CV mixed = CV mixed + (CV BLOOD(ID.Liv) - CV BLOOD(ID.Gut)). . . ... 

High shear rate (arterial) flow conditions promote thrombi composed largely of platelets such deposits are called white thrombi. Low shear rate (venous) flow conditions promote thrombi composed of red cells and platelets entrapped in a fibrin mesh, referred to as red thrombi. Sometimes, a smooth layer of fibrin also may be deposited. Embolization of the white or red thrombi may produce ischemia and infarction in distal circulatory beds. 

Horton KK (2005) Pathophysiology and current management of necrotizing enterocolitis. Neonatal Netw 24 37-46 Hurth PJ, Meyer JS, Shaaban A (2002) Ultrasound of a torsed ovary characteristic gray-scale appearance despite normal arterial and venous flow on Doppler. Pediatr Radiol 32 586-588... 

In D.P.E Moller (Ed.), Advanced Simulation in Biomedicine, pp. 173-195, New York, Springer-Verlag. Thomas J.D., Zhou J., Greenberg N., Bibawy G., McCarthy P.M., and Vandervoort P.M. 1997. Physical and physiological determinants of pulmonary venous flow numerical analysis. Am. f. Physiol. 272 H2453-H2465. 

Venous resistance R) refers to the hindrance to blood flow through the venous vasculature caused by friction of the moving blood along the venous vascular wall. By definition it is the ratio of the pressure gradient between the entrance of the venous circulation, namely the capillaries, and the venous outflow divided by the venous flow rate. Thus... 

Venous flow is a complex interaction between the compliant structures (veins and surrounding tissues) and the flow of blood. Since venous blood pressure is low, transmural pressure can become negative, thereby resulting in blood flow through a partially collapsed tube. Early studies with a thin-walled elastic tube revealed the relevant experimental evidence (Conrad, 1969). The steady flow rate (Q) through a given length of a uniform collapsible tube depends on two pressure differences... 

The suggested mechanisms for venous stenosis in AVMs are varied endovascular proliferation in reaction to increased venous flow or pressure (Fry 1968), congenital extrinsic anatomic narrowing of the lumen as it traverses the dura mater or curves around bone (Crawford et al. 1986 Willinsky et al. 1988), kinking in ectatic veins (Nataf et al. 1997). 

Results of duplex and color Doppler sonography in the neck and arms are useful to demonstrate the presence or absence of venous flow,but overlying bone and lung severely hamper examination of mediastinal veins. The SVC can also be evaluated with transesophageal Doppler US, but the modality has a small field of view and is to some degree invasive and time-consuming (SvENSSON et al. 1991). Examination of the iliac veins and IVC is easier with duplex ultrasound, including demonstration of the hepatic and renal veins. 

Fig. 8.9. Plaque-related venous leakage. Annular plaque producing hourglass deformity and reduced turgidity in the distal portions of the corpora cavernosa and glans. Doppler interrogation shows venous flows at the level of penile deformation due to incomplete distension of the erectile tissue...

Archibald and Moody obtained a correlation coefficient of 0.98 when the plotted total venous flow against total flow determined with microspheres. The slope of the line was 0.95. The mean percentage difference between the two flows was 12.3 SEM 1.7%, and Archibald and Moody concluded that microspheres give a valid measure of total flow. When they plotted aminopyrine clearance against mucosal flow determined by microspheres, they obtained a correlation coefficient of 0.96, but the slope of the line was 0.83, significantly less than 1. The 95% confidence limits were 0.74 and 0.93, showing that when acid secretion is stimulated, aminopyrine clearance underestimates mucosal blood flow. 

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