6.1: Background Information

Circulation Through the Heart and Body

The human heart consists of four chambers: The left side and the right side each have one atrium and one ventricle. Each of the upper chambers, the right atrium (plural = atria) and the left atrium, act as receiving chambers and the combination of gravity and atrial contraction move blood into the lower chambers, the right ventricle and the left ventricle. The ventricles serve as the primary pumping chambers of the heart, propelling blood to the lungs or to the rest of the body.

There are two distinct but linked circuits in the human circulation called the pulmonary and systemic circuits. Although both circuits transport blood and everything it carries, we can initially view the circuits from the point of view of gases. The pulmonary circuit transports blood to and from the lungs, where it picks up oxygen and delivers carbon dioxide for exhalation. The systemic circuit transports oxygenated blood to virtually all of the tissues of the body and returns relatively deoxygenated blood and carbon dioxide to the heart to be sent back to the pulmonary circulation. These two circulations function simultaneously and thus the heart functions as a dual pump. The right ventricle pumps deoxygenated blood into the pulmonary trunk, which leads toward the lungs and bifurcates into the left and right pulmonary arteries. Arteries carry blood away from the heart. These vessels in turn branch many times before reaching the pulmonary capillaries, where gas exchange occurs, carbon dioxide diffuses out of the blood and oxygen diffuses into the blood.

The pulmonary trunk arteries and their branches are the only arteries in the post-natal body that carry relatively deoxygenated blood. Highly oxygenated blood returning from the pulmonary capillaries in the lungs passes through a series of vessels that join together to form the pulmonary veins—the only post-natal veins in the body that carry highly oxygenated blood. Veins carry blood toward the heart. The pulmonary veins conduct blood into the left atrium, which pumps the blood into the left ventricle, which in turn pumps oxygenated blood into the aorta and on to the many branches of the systemic circuit. Eventually, these vessels will lead to the systemic capillaries, where exchange with the tissue fluid and cells of the body occurs. In this case, oxygen and nutrients out of the systemic capillaries to be used by the cells in their metabolic processes, and carbon dioxide and waste products diffuse into the blood.

The blood exiting the systemic capillaries is lower in oxygen concentration than when it entered. The capillaries will ultimately unite to form venules, joining to form ever-larger veins, eventually flowing into the two major systemic veins, the superior vena cava and the inferior vena cava, which return blood to the right atrium. The blood in the superior and inferior venae cavae flows into the right atrium, which pumps blood into the right ventricle. This process of blood circulation continues as long as the individual remains alive. Understanding the flow of blood through the pulmonary and systemic circuits is critical to all health professions.

Figure \(\PageIndex{1}\) provides a visual representation of how blood moves through the pulmonary circuit (between the heart and lungs) and the systemic circuit (between the heart and the rest of the body), which was explained earlier. The diagram includes a close-up of the heart and a whole-body circulation overview showing where pulmonary and systemic blood vessels and capillaries are located.

The heart close-up labels these major features:

• Superior vena cava and inferior vena cava (large veins bringing blood into the heart)
• Right atrium and right ventricle (right-side chambers)
• Tricuspid valve (between right atrium and right ventricle)
• Pulmonary semilunar valve (outflow valve from the right ventricle)
• Pulmonary trunk, right pulmonary arteries, and left pulmonary arteries (vessels carrying blood from the heart toward the lungs)
• Right pulmonary veins and left pulmonary veins (vessels returning blood from the lungs to the heart)
• Left atrium and left ventricle (left-side chambers)
• Mitral valve (between left atrium and left ventricle)
• Aortic semilunar valve (outflow valve from the left ventricle)
• Aorta (large artery carrying blood from the heart to the body)

The body overview shows how blood travels within the systemic and pulmonary circuits. In the pulmonary circuit, oxygen-poor blood travels from the right atrium in the heart to the lungs, moving through the pulmonary capillaries, then returning as oxygen-rich blood to the left atrium in the heart.

The systemic circuit shows oxygen-rich blood pumped from the left ventricle in the heart, moving via systemic arteries to the lower body to systemic capillaries and through the body’s tissues, then returning as oxygen-poor blood through the systemic veins from the lower body to the right ventricle in the heart.

Location, Size, and Shape of the Heart

The heart is a cone shaped muscular organ resides within the pericardial sac and is located in the mediastinal space within the thoracic cavity, between the lungs, and behind the sternum. The base of the heart is located at the level of the third costal cartilage and the apex, or inferior tip, is just to the left of the sternum between the fourth and fifth ribs. The heart is surrounded by a double layered membrane called the pericardium, which protects and lubricates the heart. The pericardium is attached to the mediastinum and ligaments in the spinal column and diaphragm.

Figure \(\PageIndex{2}\) contains two anatomical diagrams of the chest that show the location and orientation of the heart within the thorax and how it relates to nearby structures. In the left panel, the heart is shown slightly to the left of the midline, within the rib cage. The ribs are numbered from one to ten, beginning in the cervical region. The base of the heart - the broader portion - lies at an angle, spanning ribs 3 to 2. The apex - the pointed tip - lies between ribs 4 and 5. The right panel is described in the figure caption. The major vessels carrying blood away from the heart are the aorta and the pulmonary trunk, which splits into the right and left pulmonary arteries. The aorta has different regions:

• ascending aorta
• aortic arch
• descending aorta

The descending aorta is shown in the figures as being cut, but would continue to descend along the posterior aspect of the heart through the thoracic and abdominal cavities. The right and left pulmonary arteries travel to the right and left lungs, respectively.

The major vessels carrying blood away from the heart are the aorta and the pulmonary trunk which splits into the right and left pulmonary arteries. The aorta has different regions (ascending aorta, aortic arch, and descending aorta). The descending aorta is shown in the figures as being cut, but would continue to descend along the posterior aspect of the heart through the thoracic and abdominal cavities. The right and left pulmonary arteries travel to the right and left lungs, respectively.

Figure \(\PageIndex{3}\) shows an anatomical view of a heart specimen against a dark background. Multiple labels point to major blood vessels connected to the heart and to part of the left atrium. The purpose of the image is to identify the names and positions of these vessels as they enter or leave the heart.

The diagram labels the following structures:

• Pulmonary trunk: a large vessel leaving the heart that leads toward the pulmonary arteries.
• Ascending aorta: a large vessel leaving the heart that carries blood to the systemic circulation.
• Superior vena cava: a large vein entering the heart from the upper body region.
• Inferior vena cava: a large vein entering the heart from the lower body region.
• Left pulmonary artery (L. pulmonary a.): labeled as a branch associated with the pulmonary trunk.
• Right pulmonary artery (R. pulmonary a.): labeled as another branch associated with the pulmonary trunk.
• Left pulmonary veins (L. pulmonary vv.): labeled as vessels returning to the heart from the left lung side.
• Right pulmonary veins (R. pulmonary vv.): labeled as vessels returning to the heart from the right lung side.
• Left atrium (wall): labeled as the wall portion of the left atrium near where pulmonary veins connect.

Surface Features of the Heart

The heart is composed of four hollow chambers: two atria and two ventricles. The thin walled right and left atria are the two superior chambers. While both atria are visible from the anterior, the majority of the left atrium is seen on the posterior aspect. Each atrium has a superficial flap like extension called an auricle because its shape resembles the external ear of a human. You may also hear them referred to as atrial appendages. The auricles can increase the atrial capacity at times of exertion by filling with blood and, thus, relieving high atrial pressure. The two ventricles are inferior to the atria and they form the largest portion of the heart. Their walls are significantly thicker than those of the atria as a large amount of muscle mass is needed to push the blood into the arteries. The right ventricle makes up the right side and much of the anterior portion of the heart, while the left ventricle makes up the left side and much of the posterior portion of the heart.

Also prominent along the superficial surfaces of the heart is a series of fat-filled grooves, each of which is known as a sulcus (plural = sulci), containing major coronary blood vessels. The deepest is the coronary sulcus, which encircles the heart between the atria and ventricles. Located between the left and right ventricles are two shallower sulci: the anterior interventricular sulcus is visible on the anterior surface of the heart, whereas the posterior interventricular sulcus is visible on the posterior surface of the heart. 

Figure \(\PageIndex{4}\) shows an anatomical diagram of the heart in an anterior (front) view. Labels identify the four chambers and major surface vessels.

• The right atrium is labeled on the upper left side of the image and receives blood returning from the body.
• The left atrium is labeled on the upper right side and receives oxygen-rich blood returning from the lungs.
• The right ventricle is labeled below the right atrium in the lower center-left area and pumps blood toward the lungs.
• The left ventricle is labeled on the lower right side and pumps oxygen-rich blood to the rest of the body.
• Blood vessels on the surface represent the coronary arteries and veins, which supply the heart muscle. A large vessel at the top is labeled as the aorta, which carries blood away from the heart. 

Figure \(\PageIndex{5}\) shows a diagram of the human heart in a posterior (back) view, meaning the heart is seen from behind. Labels identify the four chambers and surface blood vessels.

• The left atrium is labeled near the upper left of the image and receives oxygen-rich blood from the lungs.
• The right atrium is labeled on the right side and receives oxygen-poor blood returning from the body.
• The left ventricle is labeled on the lower left portion of the heart and pumps oxygen-rich blood to the body. 
• The right ventricle is labeled near the lower center, below the right atrium, and pumps blood to the lungs.
• Blood vessels on the heart’s surface represent coronary arteries and coronary veins, which supply the heart muscle. Large veins connected at the top and bottom represent major vessels returning blood to the heart. 

Vessels of the Heart

The heart is divided into 4 chambers: two superior atria (receiving chambers) and two inferiorly positioned ventricles (discharging chambers). At least one blood vessel attaches to each of the chambers. The major vessels carrying blood toward the heart are the superior vena cava (vena=vein), inferior vena cava, left pulmonary veins, and right pulmonary veins. The superior vena cava and inferior vena cava carry blood to the heart from the superior and inferior regions of the body, respectively. The right and left pulmonary veins carry blood back to the heart from the right and left lungs, respectively. Coronary arteries are found along the surface of the heart. These arteries provide nutrients to the heart tissue. 

Figure \(\PageIndex{6}\) is a labeled anterior (front) view of the heart showing major blood vessels connected to the heart and several surface vessels. The purpose of the diagram is to identify the names and relative positions of the large vessels that carry blood to and from the heart and lungs. Labeled vessels and where they appear in the diagram:

• Aorta: A large vessel labeled near the top of the heart. It arches upward from the heart and is shown as the main outflow vessel to the body.
• Superior vena cava: A large vein labeled on the upper left side of the image, shown entering the heart from above.
• Inferior vena cava: A large vein labeled on the lower left side of the image, shown entering the heart from below.
• Pulmonary trunk: A large vessel labeled on the left side of the heart near the top; it emerges from the heart and leads toward the pulmonary arteries.
• Right pulmonary artery: Labeled on the left side of the image; it branches from the pulmonary trunk and travels toward the right lung side.
• Left pulmonary artery: Labeled on the right side of the image; it branches from the pulmonary trunk and travels toward the left lung side.
• Left pulmonary veins: Labeled on the right side of the image; multiple veins are shown returning toward the heart from the left lung side.

Figure \(\PageIndex{7}\) shows a labeled posterior (back) view of the heart. The purpose of the diagram is to identify major vessels connected to the heart as seen from behind, including vessels that carry blood between the heart and lungs and the large veins returning blood to the heart. Labeled structures in the diagram:

• Aorta: Labeled near the top of the image as a large vessel leaving the heart.
• Superior vena cava: Labeled on the upper right side as a large vein entering the heart from above.
• Inferior vena cava: Labeled on the lower right side as a large vein entering the heart from below.
• Left pulmonary artery: Labeled on the left side near the top; a branch carrying blood from the heart toward the left lung side.
• Right pulmonary artery: Labeled on the right side near the top; a branch carrying blood from the heart toward the right lung side.
• Left pulmonary veins: Labeled on the left side; multiple veins returning blood from the left lung side to the heart.
• Right pulmonary veins: Labeled on the right side; multiple veins returning blood from the right lung side to the heart.
• Coronary sinus: Labeled near the lower middle portion of the heart surface; a large venous channel that collects blood from veins of the heart wall and returns it to the heart.

Layers of the Heart Muscle

The heart is located in the pericardial cavity formed by the serous membranes of the pericardium. There are two layers of serous membranes (parietal and visceral) with the outer layer being the parietal pericardium which attaches the heart to the mediastinum and has fibrous tissue so the heart does not over-expand. The inner visceral layer is also called the epicardium and it lines the surface of the heart. Between these two membranes is a space, the pericardial cavity, containing a small amount of fluid to reduce friction as the heart moves. Both layers have mesothelium which produces serous fluid. The heart wall is composed of three layers (from outside to inside):

1. Epicardium - the same tissue as the visceral pericardium.
2. Myocardium - makes up most of heart wall, composed of cardiac muscle tissue which is made of uninucleated cardiac muscle cells.
3. Endocardium - a thin layer of endothelial tissue that lines the heart chambers.

Figure \(\PageIndex{8}\) is a simplified anatomical diagram that explains the layered structure of the heart wall and the pericardium (the surrounding sac). A small heart icon in the upper left includes a boxed region, and an arrow points from that box to a magnified cross-sectional view. Reading the magnified cross-section from left (heart interior side) to right (outer covering side), the diagram labels:

1. Endocardium

• The thin inner lining of the heart wall.

2. Myocardium

• The thick muscular middle layer of the heart wall.

3. Epicardium (visceral layer of serous pericardium)

• The outer surface layer of the heart wall; it is also identified as the visceral layer of the serous pericardium.

4. Pericardial cavity

• A thin fluid-filled space between the visceral and parietal serous pericardium layers. The diagram highlights this as a distinct “gap” or space.

5. Parietal layer of serous pericardium

• The serous membrane layer that lines the inside of the fibrous pericardium.

6. Fibrous pericardium

• The tough, outermost protective layer surrounding the heart.

Internal Anatomy of the Heart

The heart is divided into four chambers: right atrium, right ventricle, left atrium, and left ventricle. The atria are the two superior chambers of the heart and the ventricles are the two inferior chambers of the heart. The right side of the heart and the left side of the heart are isolated from each other. The right and left sides are separated by septa. Between the right and left atria is tissue called the interatrial septum and between right and left ventricles is tissue called the interventricular septum.

Normally in an adult heart, the interatrial septum bears an oval-shaped depression known as the fossa ovalis, a remnant of an opening in the fetal heart known as the foramen ovale. The foramen ovale allowed blood in the fetal heart to pass directly from the right atrium to the left atrium, allowing some blood to bypass the pulmonary circuit. Within seconds after birth, a flap of tissue that previously acted as a valve closes the foramen ovale and establishes the typical cardiac circulation pattern. Between the two ventricles is a second septum known as the interventricular septum. Unlike the interatrial septum, the interventricular septum is normally intact after its formation during fetal development. It is substantially thicker than the interatrial septum, since the ventricles generate far greater pressure when they contract.

The septum between the atria and ventricles is known as the atrioventricular septum. It is marked by the presence of four openings that allow blood to move from the atria into the ventricles and from the ventricles into the pulmonary trunk and aorta. Located in each of these openings between the atria and ventricles is a valve, a specialized structure that ensures one-way flow of blood. The valves between the atria and ventricles are known generically as atrioventricular valves. Each flap of an atrioventricular valve is attached to strong strands of connective tissue, the chordae tendineae, literally “tendinous cords,” or sometimes more poetically referred to as “heart strings”. There are several chordae tendineae associated with each of the valve flaps that are composed of approximately 80 percent collagenous fibers with the remainder consisting of elastic fibers and endothelium. They connect each of the flaps to a papillary muscle. The valves at the openings that lead to the pulmonary trunk and aorta are known generically as semilunar valves. The semilunar valves are both comprised of three small flaps of endothelium reinforced with dense connective tissue.

Figure \(\PageIndex{9}\) is a multi-panel instructional figure about the valves of the heart and related internal structures. It includes: 

1. A large cutaway diagram of the heart with labels
2. A simplified top view showing the four valves together
3. A photo of a heart specimen showing the valves. 

Panel 1 (left) - Cutaway heart diagram with labeled internal structures

• The main panel shows a cutaway view of the heart interior with many labels pointing to structures in the atria and ventricles. Labeled items include:

Atrial landmarks

• Interatrial septum: the wall separating the right and left atria.
• Fossa ovalis: a feature labeled on the interatrial septum.

Atrioventricular (AV) valves (between atria and ventricles)

• Tricuspid valve: labeled on the right side AV opening.
• Bicuspid valve (mitral valve): labeled on the left side AV opening.

Semilunar valves (outflow valves from the ventricles)

• Pulmonary valve: labeled at the ventricular outflow leading toward the pulmonary artery.
• Aortic valve: labeled at the ventricular outflow leading toward the aorta.

Supporting valve structures inside the ventricles

• Chordae tendineae: labeled as cord-like structures attached to the AV valves.
• Papillary muscles: labeled as muscle projections in the ventricles that anchor the chordae tendineae.
• Trabeculae carneae: labeled as muscular ridges lining the ventricular walls.
• Interventricular septum: labeled as the wall separating the right and left ventricles.

Panel 2 (upper right): Simplified view of the four valves together

This panel shows a top-down style view of the valve openings and labels all four:

• Pulmonary valve
• Aortic valve
• Bicuspid (mitral) valve
• Tricuspid valve

Panel 3 (lower right): Photo of a heart specimen with valve labels

This panel is a photograph of a heart specimen with labels pointing to:

• Pulmonary valve
• Aortic valve
• Bicuspid (mitral) valve
• Tricuspid valve