Shape and Size of the Heart

The heart is shaped like a cone or pinecone, wide at the top and narrow at the bottom. It is about the size of a fist, measuring roughly 12 cm long, 8 cm wide, and 6 cm thick. On average, a female heart weighs 250–300 grams, while a male heart weighs 300–350 grams. 

Athletes often have larger hearts due to exercise-induced hypertrophy, which increases muscle size and pumping efficiency. However, heart enlargement can also result from disease, such as hypertrophic cardiomyopathy, which may lead to sudden death.

Heart Membranes and Wall Layers

The heart is enclosed by a protective membrane called the pericardium, also known as the pericardial sac. 

This membrane surrounds the heart and the bases, or “roots,” of the major blood vessels connected to it. The pericardium is made up of two main layers: an outer fibrous pericardium and an inner serous pericardium.

• The fibrous pericardium is a tough layer of dense connective tissue. Its primary functions are to protect the heart from physical damage and to hold it in place within the chest cavity. Beneath this layer is the serous pericardium, which is thinner and more delicate. 

• The serous pericardium has two parts: the parietal pericardium, which lines the inside of the fibrous pericardium, and the visceral pericardium, also called the epicardium, which is tightly attached to the outer surface of the heart and forms part of the heart wall.

In addition, two shallower grooves lie between the left and right ventricles. The anterior interventricular sulcus is found on the front (anterior) surface of the heart, while the posterior interventricular sulcus is located on the back (posterior) surface. 

These sulci mark the division between the ventricles and provide pathways for coronary vessels. Together, these surface features help define the external structure of the heart and support its blood supply.

The right ventricle pumps deoxygenated blood into the pulmonary trunk, which quickly divides into the left and right pulmonary arteries. 

These arteries branch repeatedly as they travel toward the lungs, eventually forming pulmonary capillaries. It is within these capillaries that gas exchange occurs: carbon dioxide diffuses out of the blood to be exhaled, and oxygen diffuses into the blood. The pulmonary arteries and their branches are unique in that they are the only postnatal arteries that carry deoxygenated blood.

After gas exchange in the lungs, oxygen-rich blood flows through vessels that converge to form the pulmonary veins. These veins are also unique, as they are the only postnatal veins that carry highly oxygenated blood. The pulmonary veins return this blood to the left atrium, which contracts and pushes it into the left ventricle. 

The left ventricle then pumps the oxygenated blood into the aorta, distributing it through the many branches of the systemic circuit.

As blood travels through the systemic capillaries, oxygen and nutrients diffuse from the blood into surrounding tissues to support cellular metabolism. At the same time, carbon dioxide and other metabolic waste products enter the blood. As a result, the blood leaving the systemic capillaries has a lower oxygen concentration.

This blood flows from capillaries into venules, which merge into progressively larger veins. 

Eventually, blood returns to the heart through the two largest systemic veins: the superior vena cava, which drains regions superior to the diaphragm, and the inferior vena cava, which drains regions inferior to the diaphragm. Both veins empty into the right atrium, completing the cycle. From there, the blood is pumped into the right ventricle, and the process of circulation continues uninterrupted throughout life.

The myocardium is responsible for the heart’s pumping action, as its contraction moves blood through the heart and into the major arteries. The muscle fibers of the myocardium are arranged in a complex, spiral pattern rather than in straight lines. These fibers wrap around the heart chambers in figure-eight formations, allowing the heart to contract in a twisting motion. Musculature The swirling pattern of cardiac muscle tissue contributes to the heart’s ability to pump blood effectively.

This arrangement increases the efficiency of blood ejection compared to a simple squeezing motion.

Although both ventricles pump the same amount of blood with each heartbeat, the left ventricle has a much thicker myocardium than the right ventricle. This is because the left ventricle must generate enough force to push blood through the long and resistant systemic circulation. In contrast, the right ventricle only pumps blood to the nearby lungs, which requires much less pressure.

Septa of the Heart

In the heart, a septum (plural: septa) is a wall that divides the heart into separate chambers. These septa are made of cardiac muscle (myocardium) and are lined on their inner surfaces by endocardium. 

The septa are essential because they keep oxygen-rich blood and oxygen-poor blood from mixing and help direct blood flow through the heart.

• The interatrial septum separates the atria and contains a depression called the fossa ovalis, which is a remnant of a fetal opening that allowed blood to bypass the lungs before birth. 

• The interventricular septum separates the ventricles and is thicker because the ventricles generate high pressure during contraction.

• The atrioventricular septum separates the atria from the ventricles and contains openings that allow blood to flow through the heart.

Because the atrioventricular septum contains multiple openings and valves, it is structurally weaker than other regions of the heart. To provide strength and support, this area is reinforced by dense connective tissue known as the cardiac skeleton. The cardiac skeleton forms four strong rings around the valve openings and serves as an attachment point for the heart valves. In addition to its structural role, the cardiac skeleton also helps regulate the heart’s electrical conduction by separating the electrical activity of the atria from that of the ventricles.

The inner surface of the right atrium is mostly smooth; however, the medial wall contains a noticeable depression called the fossa ovalis, which is a remnant of a fetal opening. In contrast, the anterior wall of the right atrium contains raised muscular ridges known as pectinate muscles. These ridges are also present in the right auricle, a small flap-like extension of the atrium. The left atrium, in comparison, lacks pectinate muscles except within its auricle.

The atria receive venous blood almost continuously, ensuring that blood flow back to the heart does not stop while the ventricles are contracting.

• Although most blood flows passively from the atria into the ventricles while the heart is relaxed, the atria also undergo a brief contraction. This contraction occurs just before ventricular contraction and actively pushes additional blood into the ventricles. 

The opening between the right atrium and the right ventricle is controlled by the tricuspid valve, which prevents blood from flowing backward during ventricular contraction. 

When the ventricular myocardium contracts, pressure inside the right ventricle increases. Blood naturally moves from areas of higher pressure to lower pressure, which directs it toward the pulmonary trunk and also back toward the atrium. 

• To prevent blood from flowing backward into the right atrium, the papillary muscles contract at the same time as the ventricular wall. This contraction tightens the chordae tendineae, holding the valve leaflets firmly in place. 

As a result, the tricuspid valve remains closed during ventricular contraction, preventing regurgitation of blood into the atrium and ensuring that blood flows forward into the pulmonary circulation.

These muscles contract during ventricular contraction, tightening the chordae tendineae and preventing the valve from opening backward into the atrium.

The left ventricle serves as the primary pumping chamber for the systemic circuit. When it contracts, it forces oxygen-rich blood into the aorta through the aortic semilunar valve, sending blood to the rest of the body.

At the base of the aorta is the aortic semilunar valve, or aortic valve, which prevents blood from flowing back into the left ventricle. This valve normally has three cusps. When the left ventricle relaxes, blood attempting to flow backward from the aorta fills the cusps, causing the valve to close and producing an audible sound.

During ventricular contraction, pressure inside the ventricles increases and blood initially moves toward the atria, where pressure is lower. This causes the tricuspid and mitral valves to close. As the ventricles contract, the papillary muscles also contract, increasing tension on the chordae tendineae. This tension holds the valve cusps in place and prevents them from being pushed back into the atria.

In contrast, the semilunar valves (the aortic and pulmonary valves) do not use chordae tendineae or papillary muscles. Instead, they rely on their pocket-like structure. When the ventricles relax and blood flows back toward the heart, pressure fills the cusps, sealing the valve openings and preventing backflow into the ventricles.

Coronary Circulation

The heart is an exceptionally active pump composed primarily of cardiac muscle cells (cardiomyocytes) that must function continuously throughout life. Like all living cells, cardiomyocytes require a constant supply of oxygen and nutrients and an efficient means of waste removal. To meet these demands, the heart possesses its own specialized and highly developed blood supply known as the coronary circulation.

Because the heart’s workload is constant and intense, its metabolic requirements are greater than those of most other tissues. As a result, the coronary circulation is extensive and finely regulated. 

• However, unlike blood flow to many other organs, coronary blood flow is not continuous. 

Instead, it is closely linked to the cardiac cycle. Coronary circulation reaches its peak during ventricular relaxation (diastole), when the heart muscle is not contracting and the coronary vessels are open. During ventricular contraction (systole), the powerful contraction of the myocardium compresses the coronary vessels, significantly reducing blood flow to the heart muscle.

This cyclical pattern ensures that the heart receives adequate oxygen and nutrients despite its constant activity, and it highlights the critical dependence of cardiac function on efficient coronary perfusion.

An anastomosis is a junction where blood vessels interconnect, providing alternate routes for blood flow if one vessel becomes partially obstructed. In the heart, however, these anastomoses are relatively small and often insufficient to fully compensate for a blocked coronary artery. As a result, obstruction of a coronary artery frequently leads to myocardial infarction, or death of cardiac muscle tissue supplied by that vessel.

The right coronary artery (RCA) travels along the coronary sulcus and supplies blood to:

• the right atrium

• portions of both ventricles

• much of the cardiac conduction system

Inferior to the right atrium, one or more marginal arteries typically branch from the RCA and supply the superficial regions of the right ventricle. On the posterior surface of the heart, the RCA gives rise to the posterior interventricular artery, also known as the posterior descending artery (PDA). This artery runs along the posterior interventricular sulcus toward the apex of the heart and supplies branches to the interventricular septum and adjacent regions of both ventricles.

Coronary Veins

The coronary veins are responsible for draining deoxygenated blood from the myocardium and generally run parallel to the major coronary arteries on the surface of the heart. These veins collect blood from the heart muscle and return it to the right atrium.

The largest of these vessels is the great cardiac vein, which is initially visible on the anterior surface of the heart as it follows the anterior interventricular sulcus. It runs alongside the anterior interventricular artery and drains the regions supplied by that artery. As it continues, the great cardiac vein curves into the coronary sulcus and empties into the coronary sinus on the posterior surface of the heart.

Along its course, the great cardiac vein receives several major tributaries. These include:

• the posterior cardiac vein, which parallels the marginal branch of the circumflex artery and drains the regions supplied by that vessel

• the middle cardiac vein, which follows the posterior interventricular sulcus and drains areas supplied by the posterior interventricular artery. 

The small cardiac vein runs alongside the right coronary artery and drains blood from the posterior surfaces of the right atrium and right ventricle.

The coronary sinus is a large, thin-walled venous structure located on the posterior surface of the heart within the atrioventricular (coronary) sulcus. It serves as the main venous collection point for the heart and empties directly into the right atrium.

In addition to these vessels, the anterior cardiac veins drain the anterior surface of the right ventricle. These veins typically parallel small cardiac arteries. Unlike most coronary veins, the anterior cardiac veins do not drain into the coronary sinus; instead, they empty directly into the right atrium.

References

1. Gray, H. (1918). Anatomy of the human body (W. H. Lewis, Ed.; 20th ed.). Lea & Febiger.

2. J Gordon Betts, Desaix, P., Johnson, E., Johnson, J. E., Korol, O., Kruse, D., Poe, B., Wise, J., Womble, M. D., & Young, K. A. (2013). Anatomy & physiology. Openstax College, Rice University. https://openstax.org/details/books/anatomy-and-physiology

Based on OpenStax, Anatomy and Physiology (2013), licensed under CC BY 4.0.

Access for free at https://openstax.org/books/anatomy-and-physiology/pages/1-introduction.

Content paraphrased; adaptations were made.