Lesson 1, Topic 1
In Progress

Anatomy of the Blood Flow

April 11, 2024

Learning Objective: Differentiate among the pulmonary, systemic, coronary, hepatic portal, and fetal circulations.
        The body has several types of blood flow pathways or circulations. The heart, being a double pump, sends blood in two different directions, resulting in two major circulatory pathways:
            • Pulmonary circulation: Deoxygenated blood is pumped from the right side of the heart to the lungs, gas is exchanged, and oxygenated blood returns to the heart.
            • Systemic circulation: Oxygenated blood is pumped from the left side of the heart and moves through the body. Oxygen, nutrients, and other substances are brought to the cells while the blood picks up waste products. The deoxygenated blood returns to the heart.
Because the pulmonary and the systemic circulation are closely tied, they will be discussed together. The coronary circulation (blood flow through the heart tissues) and fetal circulation (blood flow of a baby in utero) will also be examined.

Pulmonary and Systemic Circulations
Learning Objective: Describe the pulmonary and systemic circulations.
      Pulmonary circulation begins as the blood returns from the body. The superior vena cava and the inferior vena cava bring deoxygenated blood from the body to the right atrium. As the atria contract, the blood from the right atrium passes the tricuspid valve and empties into the right ventricle. When the ventricles contract, the blood in the right ventricle is pushed out past the pulmonary valve and enters the pulmonary artery trunk. The pulmonary artery trunk splits into the right and left pulmonary arteries. From there, the blood moves into the arterioles and then into the pulmonary capillaries in the lungs. The gas exchange occurs. Carbon dioxide leaves the blood and enters the lungs to be expelled. Oxygen enters the blood from the lungs. The oxygenated blood leaves the pulmonary capillaries and enters the pulmonary veins. The right and left pulmonary veins bring the blood back to the left atrium. This is the start of the systemic circulation.
      When the atria are full of blood, they contract. The oxygenated blood in the left atrium moves past the bicuspid, or mitral, valve and empties into the left ventricle. When the ventricles contract, the blood in the left ventricle moves out of the heart, passing the aortic valve, and empties into the aorta. From here, the blood will move through the body before it returns to the heart. Blood from the head, neck, chest, and upper extremities empties into the superior vena cava before returning to the right atrium. Blood from the lower body empties into the inferior vena cava before returning to the right atrium.

25.2 Critical Thinking Application
Rebecca is helping Lizzy review the conduction system of the heart. To see what Lizzy can remember, Rebecca has her write down the five conduction system structures. What should Lizzy write down? What might be a creative way to remember these structures in order?

Coronary Circulation
Learning Objective: Discuss the coronary circulation.
The heart has its blood vessels that support the tissues. The right and left coronary arteries are the first branches of the ascending aorta (FIGURE 25.3). The coronary arteries bring nutrients and oxygen to the heart tissue. The blood moves from the arteries to the capillaries in the myocardium. From there, the blood moves into the coronary veins. The coronary veins remove the waste products from the heart tissue. Blood from the coronary veins drains into the coronary sinus, which opens into the right atrium.
      Coronary arteries have the important role of maintaining the myocardium. If a branch of the coronary artery becomes blocked, a person will experience a heart attack. The tissue supplied by the blocked artery will be deprived of oxygen and nutrients.

Hepatic Portal Circulation
Learning Objective: Describe the hepatic portal circulation.
In most cases, veins leaving an abdominal organ empty blood into the inferior vena cava as it heads to the heart. Veins from the spleen, gallbladder, pancreas, stomach, and intestines take an alternative route. Veins from these organs dump the blood into the hepatic portal vein, which takes the blood to the liver. In the liver, the blood moves through capillaries as it is filtered. Eventually, the blood drains into the hepatic veins before emptying into the inferior vena cava.
The liver has a special role in filtering the blood; it also metabolizes, or breaks down substances. The hepatic portal system has many advantages:
            • The glucose absorbed can be filtered and stored in the liver as glycogen. It will later be added back to the blood when the glucose levels are low.
            • Toxic substances, such as alcohol and medications, can be partially filtered before moving to the rest of the body.

Fetal Circulation
Learning Objective: Describe fetal circulation and the changes that occur after birth.
      Prior to birth, the baby is called a fetus. Fetal circulation differs from what we have discussed so far in this chapter. Before birth, the baby’s lungs, gastrointestinal tract, and kidneys are not functioning as they will after birth. Toward the end of pregnancy, babies practice breathing as they breathe in the amniotic fluid. No gas exchange occurs. Babies also urinate, but the waste products in their blood are removed by their mothers’ blood. Nutrients are passed through the blood from the mother.
      During the early weeks of pregnancy, the placenta (reproductive organ) begins to grow. It attaches to the mother’s uterus and connects to the growing baby via the umbilical cord. The umbilical cord contains two umbilical arteries and one umbilical vein. The arteries carry the fetal blood to the placenta. The umbilical vein carries oxygen and nutrient-rich blood to the baby. The waste, oxygen, and nutrient exchange occur in the placenta. There is a very thin wall separating the fetal blood from the mother’s blood. There is no mixing of the two different blood supplies, although substances can pass between the separating wall. Oxygen and nutrients move from the mother’s bloodstream to the fetal blood. Waste products (i.e., carbon dioxide) move from the fetal blood to the mother’s blood.
Other structures that are unique to the growing baby in utero include the following:
            • Ductus venosus: Shifts the majority of the blood from the umbilical vein and empties it into the inferior vena cava (FIGURE 25.4). This structure helps the blood bypass the immature liver. After birth, with the lack of blood flow from the umbilical vein, the ductus venosus constricts. Within 1 to 3 months after birth, it is permanently sealed.
            • Foramen ovale: A small flaplike opening in the interatrial septum that allows blood to move from the right atrium to the left atrium (see FIGURE 25.4). This allows most of the blood to bypass the immature lungs. After birth, the flap opening is forced closed by the pressure of the blood pumping in the heart. During infancy, the flap should seal permanently.
            • Ductus arteriosus: A short vessel that connects the pulmonary artery with the aorta. About 90% of the blood in the pulmonary artery is redirected to the aorta, bypassing the immature lungs. Usually, it closes at birth or shortly after.

FIGURE 25.3  Coronary arteries. From Frazier MS, et al: Essentials of human diseases and conditions, ed 5, St. Louis, 2013, Saunders.

FIGURE 25.4  Fetal circulation. From Patton KT, Thibodeau G: The human body in health and disease, ed 7, St. Louis, 2018, Mosby.

      At birth, when a baby takes the first breath and the umbilical cord is cut, these special structures are no longer needed. With the first breath, more pressure is created in the cardiovascular system. This helps with the closure of the foramen ovale. With the cutting of the umbilical cord, the remaining structures (ductus venosus, ductus arteriosus, and umbilical vessels) collapse.