Muzio 1 Mammalian Circulation: A Critical Understanding of How the Heart Pumps Blood Anna Muzio The heart, composed of cardiac muscle, works under unconscious control. Like respiration, the body automatically sends signals for the heart to beat on demand without thought. The heart acts as a mechanism for the body to deliver nutrient oxygen from the lungs to the body. BASIC GROSS ANATOMY The heart, comprised of a dual circuit, pumps blood through the simultaneous squeezing and ventilating of the four chambers. The atria, or collecting ducts, consist of the upper chambers of the heart, while the ventricles make up the lower chambers. The hearts’ atria and ventricles are described by their anatomical position; for example, the term “right ventricle” refers to a cadavers’ right ventricle. The opening and closing of valves catalyzes blood flow. The heart contains four valves: the mitral valve, bicuspid valve, pulmonary semilunar valve (pulmonic valve), and atrioventricular valve (aortic valve). Valves operate like control mechanisms to flow; their subsequent opening and closing relate to heartbeat. Valves prevent deleterious backflow and mixing of oxygenated blood with deoxygenated blood. Valves are analogous to checkpoints; in order to enter the next phase of the cycle, the valve must open to liberate blood. Property of Dr. John Waters; Penn State Biology Lecture “Blood and Circuitry” Muzio 2 FUNCTIONING AND CIRCUITRY The main function of the heart consists of moving blood for oxygen transport. Travelling through two circuited systems, blood returns to heart deoxygenated from the body and, conversely, oxygenated from the lungs. The systemic circuit refers to bloods flow from the heart to the body. The pulmonary circuit entails blood flow from the lungs back to the body. The systemic circuit dictates delivery of oxygenated blood, while the pulmonary circuit controls the re-oxygenation of blood. Blood travels along the tracks of arteries and veins. Arteries transport blood away from the heart, while veins carry blood towards the heart. Capillaries, or small vesicles, promote gas exchange between tissues. In the systemic circuit, oxygenated blood moves from the heart, into the body, and through the capillaries, where the body emits oxygen to the muscles, and thus becoming deoxygenated. This blood progresses back to the heart and travels to the lungs, where it can aptly re-oxygenate. Keep in mind, the heart is a cycle! Property of Dr. John Waters; Penn State Biology Lecture “Blood and Circuitry” Muzio 3 PROGRESSION OF BLOOD FLOW Again, the cycle of blood flow is circular without a finite entry point or ending point. Analogous to a toll road on a turnpike, blood can enter the cycle of blood flow at any given point, just as a car can enter the turnpike at several locations. For purposes of this discussion, I will reference the beginning of blood flow from the right ventricle onward. The progression follows: 1. 2. 3. 4. 5. 6. Right Ventricle o Blood begins its journey by moving out of the right ventricle (1) and into the pulmonary artery (2). This flow, categorized by the closing of the pulmonary semilunar valve, prepares blood to flood into the pulmonary circuit to become re-oxygenated. Pulmonary Artery o The pulmonary artery (2) continues moving blood away from the heart and into the capillaries of the lungs for gas exchange (3). Gas Exchange in Lungs o Blood begins travelling through the one-cell thick, thin walled capillaries to participate in gas exchange (3). The carbon dioxide that the body produces from inhalation diffuses out of the capillaries, while oxygen diffuses in. This oxygenated blood continues flowing towards the body. Pulmonary Veins to Left Atrium o Blood starts returning back to the heart through transport in the pulmonary vein (4). This richly oxygenated blood begins entering the left atrium (5). Left Ventricle o Catalyzed by the contraction of the Atrioventricular Valve, or AV valve, blood continues progressing into the left ventricle (5) to prepare for transport out of the body. Aorta o The aorta (6) functions by pushing blood out of the heart and into the systemic circuit, initiating deliverance of oxygenated blood to nourish the extremities (7 & 8). Property of Dr. John Waters; Penn State Biology Lecture “Blood and Circuitry” Muzio 4 7. Gas Exchange in Extremities of Upper Body o Blood continues flowing to the upper extremities, consisting of the head, arms, and upper trunk (7). Blood, squeezing through the capillaries, then deoxygenates. 8. Gas Exchange in Extremities of Lower Body o Comprised of the lower trunk, legs, and feet, the lower body extremities receive nourishment by deoxygenating in the capillaries. (8) 9. Superior Vena Cava o The superior vena cava (9) functions by moving blood from the upper extremities and back into the heart to prepare for entrance into the pulmonary circuit. 10. Inferior Vena Cava o The inferior vena cava (10) works by transporting blood from the lower extremities to return to the heart. Remember, blood is now deoxygenated! 11. Right Atrium o The right atrium (11), acting like a collecting pool for the deoxygenated blood returning from the heart, starts distributing blood back into the right ventricle (1). After contraction of the Bicuspid valve, the cycle starts again. PROGRESSION OF BLOOD FLOW Characterized by the continuous flow between the systemic and pulmonary circuit, blood flow delivers oxygen to the body. Blood travels through the upper atria and lower ventricles of the heart; arteries transport blood away from the heart, while veins return blood. Blood flow can be traced in the heart beginning at any point, as it is a circuit. Starting with the right ventricle, blood moves through the pulmonary artery and into the pulmonary circuit. Gas exchange occurs, and the newly oxygenated blood returns back to the heart through the pulmonary vein, and into the left atrium. Blood now progresses into the left ventricle and aorta to distribute oxygen rich blood to the extremities. Gas exchange occurs in the extremities’ tissues, the deoxygenated blood returns to the heart by the inferior and superior vena cava, and into the right atrium.