Blood Supply to the Brain The brain is supplied with oxygenated blood in arteries (vertebral and internal carotids) that give off branches. These branches join to form the circle of Willis, a circle in the region of the pituitary gland. The value of having the blood vessels join in this way is that, if one becomes blocked, the brain still can receive blood via three other routes.
Circulation to the brain includes the circle of Willis, vvhich protects the brain from reduced blood supply.
Blood Supply to the Heart The coronary arteries), which are a part of the systemic circulation, are extremely important because they serve the myocardium. (The heart is not nourished by the blood in its chambers.) The right and left coronary arteries arise from the aorta, just beyond the aortic semilunar valve. They lie on the exterior surface of the heart, where they branch off in various directions into smaller arteries and arterioles. The coronary capillary beds join to form venules, which converge into the cardiac veins. The cardiac veins follow the path of the coronary arteries and finally empty into the coronary sinus, an enlarged vein on the posterior surface of the heart. The coronary sinus enters the right atrium. Although the coronary arteries receive blood under high pressure, they have a very small diameter and can become occluded, or blocked.
Circulation to the myocardium is dependent upon the proper functioning of the coronary arteries.
Blood Supply to the Liver The hepatic portal system carries blood from the stomach, intestines, and other organs to the liver. A portal system is one that begins and ends in capillaries; thus, there are two sets of capillaries between an artery and a final vein. The superior mesenteric artery brings blood to the small intestine, where the first set of capillaries occurs. Various veins join to form the hepatic portal vein, which takes blood to the liver, where the second set of capillaries occurs. Then, the hepatic veins leave the liver to enter the inferior vena cava.
The hepatic portal system carries blood from the stomach end intestines to the liver.
Fetal Circulation
The fetus has four circulatory features that are not present in adult circulation:
Foramen ovale, or oval window, an opening'between the two atria. This window is covered by a flap of tissue that acts as a valve.
Ductus arteriosus, or arterial duct, a connection between the pulmonary artery and the aorta.
Umbilical arteries and vein, vessels that travel to and from the placenta, leaving waste and receiving nutrients.
Ductus venosus, or venous duct, a connection between the umbilical vein and the inferior vena cava.
All of these features can be related to the fact that the fetus does not use its lungs for gas exchange, since it receives oxygen and nutrients from the mother's blood at the placenta.
The path of blood in the fetus can be traced, beginning from the right atrium. Most of the blood that enters the right atrium passes directly into the left atrium by way of the foramen ovale because the blood pressure in the right atrium is somewhat greater than that in the left atrium . The rest of the fetal blood entering the right atrium passes into the right ventricle and out through the pulmonary trunk but because of the ductus arteriosus, most blood then passes into the aorta. Notice that by whatever route blood takes, most of it reaches the aorta instead of the lungs.
Blood within the aorta travels to the various branches, including the iliac arteries, which connect to the umbilical arteries leading to the placenta. Exchange between maternal and fetal blood takes place at the placenta. Blood in the umbilical arteries is oxygen-poor, but blood in the umbilical vein, which travels from the placenta, is oxygen-rich. The umbilical vein enters the ductus venosus, which passes directly through the liver. The ductus venosus then joins with the inferior vena cava, a vessel that contains oxygen-poor blood. The vena cava returns this mixture to the right atrium.
The most common of all cardiac defects in the newborn is the persistence of the foramen ovale. With the tying of the umbilical cord and the expansion of the lungs, blood enters the lungs in quantity. Return of this blood to the left side of the heart usually causes a small valve located on the left side of the interatrial septum to close the foramen ovale. Incomplete closure occurs in nearly one out of four individuals, but even so, blood rarely passes from the right atrium to the left atrium because either the opening is small or it closes when the atria contract. In a small number of cases, the passage of oxygen-poor blood from the right side to the left side of the heart is sufficient to cause cyanosis, a bluish cast to the skin. This condition can now be corrected by open-heart surgery.
The ductus arteriosus closes because endothelial cells divide and block the duct. Remains of the arterial duct and parts of the umbilical arteries and vein are later transformed into connective tissue.
Fetal circulation includes four unique features: (1) the foramen ovale, (2) the ductus arteriosus, (3) the umbilical arteries and vein, and (4) the ductus venosus. These features are necessary because the fetus does not use its lungs for gas exchange.
