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Selected Circulatory Pathways | See Also: [Upper Vessels] [Lower Vessels]

Coronary circulation:

Coronary arteries leave the aorta behind the semilunar valves. This means that they fill during ventricular diastole. The right coronary artery branches to smaller arteries including the marginal, which leads down the margin or edge of the right ventricle. The main portion of the right coronary artery proceeds to the back of the heart becoming the posterior interventricular. The left coronary artery divides to form the circumflex which curves to the back of the heart, and the anterior inter-ventricular which descends between the two ventricles. The arteries anastomose (See Figure 19.10a) to provide collateral circulation to the ventricular myocardium.

Coronary veins drain the myocardium from the anterior interventricular area through the great cardiac (or coronary) vein, from the right atrial area through the small cardiac vein, and from the posterior interventricular area through the middle cardiac vein. All of these come together to form the coronary sinus which drains directly into the right atrium, the only systemic venous drainage not through the vena cavae.

The Circle of Willis - provides collateral blood flow to the brain.

The Circle of Willis is supplied by:

the right vertebral artery, a branch off the right subclavian, and the left vertebral artery, a branch off the left subclavian. The vertebral arteries merge to become the basilar artery before entering the Circle of Willis. The left and right internal carotid arteries also lead to the Circle of Willis. (See Table 20.4). Communicating arteries complete the circle and the anastomosis which then leads to anterior, medial, and posterior cerebral arteries which carry blood into the brain.

The hepatic portal system carries the blood from the GI tract and spleen to the liver before it enters the inferior vena cava and the general circulation. This is needed because this blood has digestive end-products and absorbed toxins from the GI tract and bilirubin from hemoglobin destruction in the spleen. The liver is in charge of processing these substances. Blood in the splenic vein from the spleen receives blood from the inferior mesenteric vein draining the large intestine, and then combines with the superior mesenteric vein coming from the small intestine to form the portal vein (hepatic portal vein) which enters the liver. The liver also receives oxygenated blood through the hepatic artery and blood from these two sources mixes in liver sinusoids which are lined with the hepatocytes (liver cells). Once processed by these hepatocytes the blood returns to the circulation through the hepatic vein.
The Fetal Circulation (See Figure 29.13) differs from that after birth because the lungs are mostly bypassed. This is because the fetus receives oxygen and gets rid of carbon dioxide and wastes through the placenta. Umbilical arteries, branches from the internal iliacs, take blood from the fetal circulation to the placenta. Oxygenated blood returns from the placenta to the fetal circulation through the umbilical veins. These veins travel through the ductus venosus into the inferior vena cava. At this point the blood becomes mixed as it travels back to the right atrium.

From the right atrium there are two alternatives. Blood may travel in the usual route to the right ventricle and then to the pulmonary arteries to the lungs, and about half the blood goes toward this pathway. But most travels through a detour, the foramen ovale (literally oval hole) to the left atrium. What does pass to the right ventricle and then the pulmonary trunk is mostly diverted by a shunt, the ductus arteriosus, into the aorta. Only about 10% of the blood flow enters the pulmonary circuit.

The fetal structures close at or shortly after birth. The ductus venosus becomes part of the falciform ligament which supports the liver. The foramen ovale reflexively closes due to strong heart contractions after birth and normally grows completely closed within a couple of weeks. Failure of the foramen ovale to close produces atrial septal defect. The ductus arteriosus constricts at birth and later becomes the ligamentum arteriosum seen as a remnant attaching the aorta and pulmonary trunk. Failure of the ductus arteriosus to close is called a patent ductus. In both cases surgery is necessary.



Revised: May 03, 2001

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