Venous circulation

Thrombosis is the development of a thrombus , consisting of platelets, fibrin, red and white blood cells in the arterial or venous circulation. Platelet-rich white thrombi are found in the arterial system and can be prevented by antiplatelet drugs. 

The direct injection of potent vasodilatory agents such as papaverine or isosor-bide dinitrate, into the ventricles of the heart reverses the action of palytoxin in approximately one-half of the animals. These extreme measures are required because palytoxin kills quickly. Antidotes injected into the venous circulation were not able to reach the heart because the stagnation of venous blood occurs so rapidly that antidotes are simply pooled on the venous side of the circulation and never reach the heart. In these studies isosorbide dinitrate appeared to be approximately twice as effective as papaverine in reversing the toxic effects of palytoxin. 

Venous thromboembolism (VTE) is one of the most common cardiovascular disorders in the United States. VTE is manifested as deep vein thrombosis (DVT) and pulmonary embolism (PE) resulting from thrombus formation in the venous circulation (Fig. 7-1).1 It is often provoked by prolonged immobility and vascular injury and is most frequently seen in patients who have been hospitalized for a serious medical illness, trauma, or major surgery. VTE can also occur with little or no provocation in patients who have an underlying hypercoagulable disorder. 

FIGURE 7-1. Venous circulation. (Reproduced from Haines ST, Zeolla M, Witt DM. Venous thromboembolism. In DiPiro JT, Talbert RL, Yee GC, et al, (eds.) Pharmacotherapy A Pathophysiologic Approach. 6th ed. New York McGraw-Hill 2005 374, with permission.)... 

Venous thromboembolism (VTE) results from clot formation in the venous circulation and is manifested as deep vein thrombosis (DVT) and pulmonary embolism (PE). A DVT is a thrombus composed of cellular material (red and white blood cells, platelets) bound together with fibrin strands. A PE is a thrombus that arises from the systemic circulation and lodges in the pulmonary artery or one of its branches, causing complete or partial obstruction of pulmonary blood flow. 

Hypercoagulable states include malignancy activated protein C resistance deficiency of protein C, protein S, or antithrombin factor VIII or XI excess antiphospholipid antibodies and other situations. Estrogens and selective estrogen receptor modulators have been linked to venous thrombosis, perhaps due in part to increased serum clotting factor concentrations. Although a thrombus can form in any part of the venous circulation, the majority of thrombi begin in the lower extremities. Once formed, a venous... 

The fluid in the lung tissues may drain either toward the pleura or toward the center of the thoracic cavity (Fig. 3.5). The pleural fluid returns through the lymphatics to the central thoracic cavity, where a duct opens into the venous circulation. Thus, the fluid transported from lung tissue is returned to the systemic blood circulation by the lymphatics. Alternatively, fluid can be transported directly by and into the central lymphatic system. 

The flow of cerebrospinal fluid is essentially unidirectional that is, it flows from its site of formation in the choroid plexus through the ventricles to its site of exit at the arachnoid villi. Drugs in this fluid can either enter the brain tissue or be returned to the venous circulation in the bulk flow of cerebrospinal fluid carried through the arachnoid villi. Some drugs, such as penicillin, wUl not leave the cerebrospinal fluid compartment by bulk flow but will be actively transported by the choroid plexus out of the fluid and back into the blood. Finally, drugs may diffuse from brain tissue directly into blood capUlaries. 

Arteriolar and venous tone (smooth muscle tension) both play a role in determining myocardial wall stress (Table 12-1). Arteriolar tone directly controls peripheral vascular resistance and thus arterial blood pressure. In systole, intraventricular pressure must exceed aortic pressure to eject blood arterial blood pressure thus determines the systolic wall stress in an important way. Venous tone determines the capacity of the venous circulation and controls the amount of blood sequestered in the venous system versus the amount returned to the heart. Venous tone thereby determines the diastolic wall stress. 

PBPK models rely on a series of simultaneous differential equations that simulate chemical delivery to tissues via the arterial circulation and removal via the venous circulation. The models are run in time steps such that the entire course of chemical disposition can be presented for calculation of the area-under-the-curve (AUC) dose, often a key metric for chronic risk assessment. The physiologic parameters can be adapted for different species, sexes, age groups, and genetic variants to facilitate extrapolation from one type of receptor to another. 

Portal The term applied to the venous circulation draining the tissues of the gastrointestinal tract into the liver. 

Functional disturbances within the spinal cord are caused by the venous congestion due to arteriali-zation and elevated pressure of the medullary veins (Aminoff et al. 1974). In addition, venous outlets are insufficient and thus reinforcing impairment of the venous circulation and chronic spinal hypoxia. In particular, Merland and coworkers (1980) introduced the concept of blocked venous drainage into the epidural space. 

Return of gas-depleted blood to the lung As the venous circulation returns blood depleted of anesthetic to the lung, more gas moves into the blood from the lung according to the partial pressure difference. Over time, the partial pressure in the alveolar space closely approximates the partial pressure in the inspired mixture that is, there is no further anesthetic uptake from the lung. 

The ciliary epithelium in the posterior chamber secretes aqueous humour. The aqueous humour flows in between the cornea and iris. After filtration through the trabecular meshwork, it returns to the venous circulation via the canal of Schlemm. 

Some of the ftee fatty acids (the small and medium chain fiitty acids) are not incorporated into chylomicrons but enter the venous circulation directly. They are not too soluble, how ever, and travel attached to albumin. 

The Cruveilhier-von Baumgarten disease develops when postnatal occlusion of the umbilical vein is absent (P. VON Baumgarten, 1907). An open, dilated umbilical vein connects the left portal vein with the systemic venous circulation in the navel area. This collateral vein has a diameter of >3 mm and a hepatofugal flow of >5 cm/... 

Splenoportographic procedures allow an accurate depiction of the portal vein and its afferent flow areas. Despite the development of new techniques, these methods are of importance in clarifying the cause of portal hypertension, and they are (still) deemed to be a prerequisite for operations aimed at reducing the pressure and vol-mne in the portal venous circulation. Vessels with a diameter of <1 cm are unsuitable for long-term patency of a shunt. Direct and indirect procedures are available. (16, 35)... 

Ghio, S., de Servi, S, Perotti, R, Eleuteri, E, Montemartini, C, and Specchia, G. 1992. Different susceptibility to the development of nitroglycerin tolerance in the arterial and venous circulation in humans. Effects of N-acetylcysteine administration. Circulation 86 798-802. 

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