Blood Flow in Veins

Q6 A thrombus is a blood clot which is fixed to the blood vessel wall. When it detaches and is carried in the blood, it is known as an embolus. Both thrombi and emboli can block blood vessels and deprive tissues of oxygen. In arteries blood clots usually form because the inner surface has been altered by deposition of atheroma. In contrast venous thrombosis results from slow or stagnant blood flow in veins, or defects in mechanisms which normally oppose inappropriate coagulation. Three major risk factors for pulmonary embolism are (i) venous stasis, (ii) hypercoagulability ofblood and (iii) injury to vascular endothelium following trauma or plaque rupture. 

Intermittent pneumatic compression (IPC) is used widely for persons who have varicose veins of lower extremities [1], This method helps the blood flow in vein by compressing intermittently lower extremities with a pressure-controlled cuff [1] [2],... 

The answer is a. (Hardman, pp 762-764.) Experimentally, nitrates dilate coronary vessels. This occurs in normal subjects, resulting in an overall increase in coronary blood flow. In arteriosclerotic coronaries, the ability to dilate is lost, and the ischemic area may actually have less blood flow under the influence of nitrates. Improvement in the ischemic conditions is the result of decreased myocardial oxygen demand because of a reduction of preload and afterload. Nitrates dilate both arteries and veins and thereby reduce the work of the heart. Should systemic blood pressure fall, a reflex tachycardia will occur. In pure coronary spasm, such as Prinzmetal s angina, the effect of increased coronary blood flow is relevant, while in severe left ventricular hypertrophy with minimal obstruction, the effect on preload and afterload becomes important. 

Venous stasis is slowed blood flow in the deep veins of the legs resulting from damage to venous valves, vessel obstruction, prolonged periods of immobility, or increased blood viscosity. Conditions associated with venous stasis include major medical illness (e.g., heart failure, myocardial infarction), major surgery, paralysis (e.g., stroke, spinal cord injury), polycythemia vera, obesity, or varicose veins. 

Nitroglycerine reduces the load on the heart by dilating peripheral veins, reducing the myocardial need for oxygen, and facilitating redistribution of coronary blood flow in the region of the myocardium with reduced blood flow. 

The behavior of liquids in narrow tubes is one of the most common examples in which capillary forces are involved. It will be shown later how important this phenomenon is in many different parts of everyday life and technology. In fact, liquid curvature is one of the most important physical surface properties that requires attention in most of the application areas of this science. The range of these applications is from blood flow in the veins to oil recovery in the reservoir. Properties of fabrics are also governed by capillary forces (i.e., wetting, etc.). The sponge absorbs water or other fluids where the capillary forces push the fluid into the many pores of the sponge. This is also called wicking process (as in candlewicks). 

Deep vein thrombosis refers to clot formation within the vascular system, usually in the lower extremities. It requires prompt medical attention, as it not only disrupts blood flow in the affected area but can also lead to the formation of emboli, with serious, often fatal, medical consequences (Chapter 9). Such thrombi may be pinpointed by immunoscintigraphy, utilizing a radio-labelled antibody directed against platelets or, more often, flbrin. 

One jugular vein and one carotid artery are exposed on opposite sides. Small branches of the vein are clamped to avoid blood flow in spite of vessel occlusion. 

Diagnosis of portal hypertension (with detection of hepatofugal flow as well as the slowing down of blood flow in the portal vein)... 

Fig. 28.12 Alcoholic cirrhosis with portal hypertension stenosis of the portal vein (see arrow) with stagnation of blood flow in the portal vein (VP) and portal flow reversal (blue = hepatofugal) as well as enhanced arterial flow (red). Arterial signals are visible in the flow profile. Inhomogeneous liver structure...

Fig. 39.7 I Re-opened umbilical vein (VU) due to pronounced portal hypertension in haemochromatotic cirrhosis (VP = umbilical branch of the left portal vein) II Hepatofugal blood flow in the umbilical vein (VU) depicted by colour-encoded duplex sonography...

Venous bypass Insertion of a combined venovenous and portoven-ous bypass reduces the negative effects of the anhepatic operative phase (lasting 30-120 minutes) by interrupting the blood flow in the portal vein and inferior vena cava. This measure allows greater haemodynamic stability during the anhepatic phase. 

Ota K, Shijo H, Kokawa H, Kubara K, Kim T, Akiyoshi N, Yokoyama M, Okumura M. Effects of nifedipine on hepatic venous pressure gradient and portal vein blood flow in patients with cirrhosis. J Gastroenterol Hepatol 1995 10(2) 198-204. 

In this procedure, which is primarily for use with experimental animals, hepatic blood flow is calculated from the dye dilution curves recorded from the hepatic vein after injection of dye into the portal vein near its entrance to the liver. Hepatic blood flow in dogs has been measured with this technique using indocyanine green and correcting for uptake of dye by the liver (M34). It would seem advantageous in this procedure to use a dye that was not taken up by the liver. 

The existing methods to evaluate retinal blood flow are limited to large vessels or are subjective. The application of LTA to the measurement of blood flow was demonstrated in rabbits and nonhuman primates (2,3,27). A number of parameters relevant to hemodynamics were evaluated from the progression of the dye front in the arteries, through the capillary bed and into the veins. Blood vessel diameters were measured, a map of the blood flow in the macula was drawn, and the relationship between flow and diameter in mother-daughter branches of blood vessels was found to be consistent with Murray s law (which predicts the optimum branching pattern for a vascular bed) (3,28). 

Tanaka and Benedek (1974), in a novel and important study, have measured the velocity of blood flow in the femoral vein of rabbit by detecting (in the heterodyne mode) the Doppler shift of laser light introduced into the vein by means of a fiber optic catheter. The light is scattered from the moving erythrocytes in the blood. It is important to recognize that the blood-flow velocity is not uniform across a vein but varies from zero near the vein wall to a maximum in the center. Thus the spectrum observed should be an average over the distribution of velocities of the illuminated erythrocytes, approximately... 

Fig. 5.8.3. The heterodyne correlation function of the scattered light from the blood flow in the femoral vein of an albino rabbit. The lower curve was taken prior to killing the rabbit, and the upper curve was measured just after its death. (From Tanaka and Benedek, (1974.)...

---