Blood circulation/stream/vessel

The villi of the small intestine, the structures that participate in absorption, measure about 0.5 to 1.5 mm. in length, and contain blood and lymphatic vessels through which absorbed materials enter the body. Substances that enter the blood channels pass directly to the liver via the portal vein, whereas substances that ultimately enter the lymphatics traverse the thoracic duct and, at the junction of the duct with the neck veins, are poured into the systemic circulation. In an anatomical sense, the liver occupies a strategic position with respect to substances absorbed into the portal blood stream. 

Another aspect of blood flow in vivo, and one that plays a major role in such circulation complications such as atherosclerosis and stroke, is that cells will partition themselves into certain areas of the flowing stream. For example, RBCs will typically travel in the middle of the open tube where the convective forces are at a minimum. However, platelets, being smaller in size, then become forced to flow more toward the wall. In fact, a close examination of the components of blood at the wall of the vessel or tube will show that there is actually a cell-free layer (often called the skimming layer) along the wall. This phenomenon was first noticed in the 1920s by Fahraeus and Lindqvist today bears the name of their discoverers. ... 

The terms heart attack or myocardial infarction are more commonly used than thrombosis. The infarct-muscle destruction is simply the end result and thrombosis is the real cause of the heart attack. Thrombosis may be defined as the process of formation of a coalescent or agglutinated solid mass of blood components in the blood stream. Thrombi formed in either arteries or veins often cause occlusion in the vascular system and prevent blood flow. Obstruction to the blood vessel usually occurs at the site where the thrombi deposit. Furthermore, thrombi may break loose, travel through the circulating blood stream, and cause obstruction at some distal point of narrowing elsewhere. The mass or thrombus that moves is referred to as an embolus. The two phenomena are lumped together under the term thromboembolic disease. Thrombosis that reduces blood supply to the heart is the primary factor in heart attacks. 

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