Anatomy & Physiology I and II

Anatomy & Physiology Study Guide descend within the transverse foramina of the cervical vertebrae, in company with the vertebral arteries. The vertebral veins empty into the brachiocephalic veins of the chest. The Inferior Vena Cava The inferior vena cava (IVC) collects most of the venous blood from organs inferior to the diaphragm. A small amount reaches the superior vena cava via the azygos and hemiazygos veins . 20.13 The Hepatic Portal System The hepatic portal system begins in the capillaries of the digestive organs and ends in the liver sinusoids. Blood flowing in the hepatic portal system is quite different from that in other systemic veins because hepatic portal vessels contain substances absorbed by the stomach and intestines. The hepatic portal system delivers absorbed compounds directly to the liver for storage, metabolic conversion, or excretion. The largest vessel of the hepatic portal system is the hepatic portal vein, which delivers venous blood to the liver. The hepatic portal vein receives blood from three large veins draining organs within the peritoneal cavity: • The inferior mesenteric vein: This vein collects blood from capillaries along the inferior portion of the large intestine. Its tributaries include the left colic vein and the superior rectal veins, which drain the descending colon, sigmoid colon, and rectum. • The splenic vein: This is formed by the union of the inferior mesenteric vein and veins from the spleen, the lateral border of the stomach (left gastroepiploic vein), and the pancreas (pancreatic veins). • The superior mesenteric vein: This vein collects blood from veins draining the stomach (right gastroepiploic vein), the small intestine (intestinal and pancreaticoduodenal veins), and two- thirds of the large intestine (ileocolic, right colic, and middle colic veins). The hepatic portal vein forms through the fusion of the superior mesenteric and splenic veins. The superior mesenteric vein normally contributes the greater volume of blood and most of the nutrients. As it proceeds, the hepatic portal vein receives blood from the left and right gastric veins, which drain the medial border of the stomach, and from the cystic vein, emanating from the gallbladder. After passing through liver sinusoids, blood collects in the hepatic veins, which empty into the inferior vena cava. Since blood from the intestines goes to the liver first, with the liver regulating the nutrient content of the blood before it enters the inferior vena cava, the composition of the blood in the systemic circuit is relatively stable despite changes in diet and digestive activity. 20.14 Placental Blood Supply A pair of umbilical arteries, which arise from the internal iliac arteries and travel through the umbilical cord, provide blood flow to the placenta. Blood returns from the placenta in the single umbilical vein, carrying oxygen and nutrients to the developing fetus. The umbilical vein drains into the ductus venosus, a vascular connection to an intricate network of veins within the developing liver. The ductus venosus collects blood from the veins of the liver and from the umbilical vein and empties into the inferior vena cava. When the placental connection is broken at birth, blood flow ceases along the umbilical vessels, and they soon degenerate. However, remnants of these vessels persist throughout life as fibrous cords. ©2018 Achieve Test Prep Page 272 of 367