The Cardiovascular System and a Pacemaker | Virtual Body
by Robert Tallitsch, PhD | February 9, 2022
Short video with 3D human anatomy of the heart with an explanation, patient case, and example of a pacemaker!
Written by: Robert Tallitsch, PhD
The cardiovascular system is composed of the heart and a network of blood vessels. The heart is a relatively small, four-chambered organ — approximately the size of your closed fist. The network of vessels is divided into two circuits: the pulmonary circuit and the systemic circuit. The pulmonary circuit carries carbon dioxide-rich blood from the heart to the lungs, where gas exchange with the blood occurs. From the lungs the pulmonary circuit returns oxygen-rich blood back to the heart. The systemic circuit carries oxygen-rich blood from the heart to all of the organs of the body, and returns carbon dioxide-rich blood back to the heart. We will concentrate on the anatomy of the heart in this Brain Builder Activity.
The heart is located within the mediastinum of the thoracic cavity, and 5/6 of the entire heart lies to the left of the median plane. It is surrounded by the pericardium, which is composed of two layers: the outer fibrous pericardium and an inner serous pericardium.
Surface Anatomy of the Heart
The surface of the heart has several prominent grooves and sulci. The coronary sulcus (atrioventricular groove) is a superficial groove that encircles the base of the heart. This groove serves as an approximate dividing line between the atria above and the ventricles below. The shallow interatrial groove superficially separates the right and left atria. Finally, the interventricular grove extends from the coronary sulcus to the apex of the heart, serving as an external landmark separating the right and left ventricles.
Internal Anatomy of the Chambers of the Heart
The right atrium is separated from the left atrium by a muscular septum termed the interatrial septum. The right atrium receives carbon dioxide-rich blood from the periphery via the superior and inferior vena cavae, as well as carbon dioxide-rich blood from the coronary circulation of the heart. The anterior walls of the right atrium are thin and possess muscular ridges on their internal surface. The largest of these ridges are formed by the crista terminalis, while the smaller ridges are formed by the pectinate muscles. The opening between the right atrium and the right ventricle is the location of the right atrioventricular valve (right AV valve), which controls the direction of blood flow between the right atrium and the right ventricle.
Blood flows from the right atrium, through the right atrioventricular valve, into the right ventricle. The posterior portion of the right ventricle forms the interventricular septum, which separates the right and left ventricles.
The right ventricle is divided into two segments: the crista supraventricularis and the conus arteriosus. The crista supraventricularis is the “inflow” portion of the right ventricle, and is marked by prominent muscular ridges and columns termed trabeculae carneae. The walls of the conus arteriosus (the “outflow” portion) are smooth and lead into the pulmonary trunk. The junction between the conus arteriosus and the pulmonary trunk is the location of the pulmonary (pulmonary semilunar) valve.
The left atrium forms most of the base of the heart. Typically 3 to 4 pulmonary veins, which carry oxygen-rich blood from the lungs, empty into the left atrium. The walls of the left atrium are slightly thicker than those of the right atrium, and lack pectinate muscles. The opening between the left atrium and the left ventricle is the location of the left atrioventricular valve (left AV valve), which controls the direction of blood flow between the left atrium and the left ventricle.
The left ventricle receives blood from the left atrium and pumps it out, via the aorta, to all of the organs of the body. This chamber has the thickest walls of the heart and is divided into the crista supraventricularis (the “inflow” segment) and the aortic vestibule (the “outflow” segment). The trabeculae carneae within the supraventricularis are more delicate than those found within the right ventricle. The aortic vestibule has smooth walls, and marks the junction between the left ventricle and the aorta. This junction is marked by the fourth valve of the heart, the aortic (aortic semilunar) valve.
The Heart’s Conducting System
The contraction and relaxation of the chambers of the heart are highly coordinated during the cardiac cycle. This coordination is accomplished by specialized cardiac muscle cells that comprise the conducting system of the heart. Three parts of this conducting system are within the atria of the heart: the sinoatrial node (SA node), the internodal tracts, and the atrioventricular node (AV node). The remaining three parts, the atrioventricular bundle (AV bundle, or bundle of His), right and left bundle branches, and the Purkinje fibers, are within the ventricles.
The sinoatrial node is located near the opening of the superior vena cava into the right atrium. The internodal tracts extend from the SA node throughout the wall of the right atrium, terminating at the AV node in the posterior, inferior portion of the interatrial septum. The AV node communicates with the atrioventricular bundle, which extends inferiorly in the interventricular septum. The AV bundle divides into the right and left bundle branches. The bundle branches continue inferiorly in the interventricular septum and then into the ventricles, terminating within various locations of the ventricles. The bundle branches communicate with small, specialized cardiac muscle cells termed Purkinje fibers, which are the terminal portions of the conducting system within the ventricles of the heart.
An abnormal heartbeat or cardiac cycle is often due to dysfunction with one or more components of the conducting pathway system. If such a condition becomes chronic a physician may recommend the implantation of a cardiac pacemaker to control the heartbeat or cardiac cycle.
During the surgical procedure one or more wires are inserted into a major vein under or near the clavicle. Each of the wires are secured into position at the appropriate position(s) within your heart. The pulse generator is then implanted deep to the skin in close proximity to the clavicle. The pulse generator either provides constant stimulation to the heart to increase the heart rate or will only electrically stimulate the heart when needed to correct an abnormal heartbeat.
Having a pacemaker should improve overall symptoms caused by an abnormally slow or irregular heart rate, allowing the patient to resume a more active lifestyle.
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Pulmonary circulatory circuit - Carries carbon dioxide-rich blood from the heart to the lungs and returns oxygen-rich blood back from the lungs back to the heart.
Systemic circulatory circuit - Carries oxygen-rich blood from the heart to all of the organs of the body, and returns carbon dioxide-rich blood back to the heart.
Atrioventricular bundle - One of the ventricular portions of the heart’s conducting system
Pericardiac sac - Two-layered structure that surrounds the heart
Crista supraventricularis - “Inflow” portion of the right and left ventricles
Aortic vestibule - “Outflow” portion of the left ventricle
Conus arteriosus - “Outflow” portion of the right ventricle
Atrioventricular groove - Also termed the coronary sulcus. This external structure circles the base of the heart, externally separating the atria from the ventricles.
Sinoatrial node - A part of the conducting system of the heart located near the opening of the superior vena cava into the right atrium.
Purkinje fibers - The terminal portion of the conducting system of the heart. These structures are found within the ventricular musculature of the heart.
- What is the anatomical name of the valve found between the right ventricle and the pulmonary artery?
A: Pulmonary valve (pulmonary semilunar valve)
- Which structure of the heart’s conducting system is located near the entrance of the superior vena cava into the right atrium?
A: Sinoatrial (SA) node
- The trabeculae carneae of the supraventricularis are more delicate within which ventricle of the heart?
A: Left ventricle
- What is the name of the “outflow” portion of the left ventricle?
A: Conus arteriosus
- Which atrium of the heart lacks pectinate muscles?
A: Left atrium
- How many pulmonary veins typically enter the right atrium of the heart?
A: 3 to 4
- The interventricular groove extends from the coronary sulcus to what external structure of the heart?
A: Apex of the heart
- Which circulatory circuit returns oxygen-rich blood back to the heart?
A: The pulmonary circuit
- What is the average resting heart rate range for an adult?
A: 60-100 bpm
- What is bradycardia?
A: Bradycardia is a slower than normal heart rate.
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