These vessels in turn branch many times before reaching the pulmonary capillaries , where gas exchange occurs: Carbon dioxide exits the blood and oxygen enters. 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.

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 exit the systemic capillaries to be used by the cells in their metabolic processes, and carbon dioxide and waste products will enter 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 3. Blood flows from the right atrium to the right ventricle, where it is pumped into the pulmonary circuit.

The blood in the pulmonary artery branches is low in oxygen but relatively high in carbon dioxide. Gas exchange occurs in the pulmonary capillaries oxygen into the blood, carbon dioxide out , and blood high in oxygen and low in carbon dioxide is returned to the left atrium.

From here, blood enters the left ventricle, which pumps it into the systemic circuit. Following exchange in the systemic capillaries oxygen and nutrients out of the capillaries and carbon dioxide and wastes in , blood returns to the right atrium and the cycle is repeated.

Our exploration of more in-depth heart structures begins by examining the membrane that surrounds the heart, the prominent surface features of the heart, and the layers that form the wall of the heart. Each of these components plays its own unique role in terms of function. Figure 4. The pericardial membrane that surrounds the heart consists of three layers and the pericardial cavity.

The heart wall also consists of three layers. The pericardial membrane and the heart wall share the epicardium. The membrane that directly surrounds the heart and defines the pericardial cavity is called the pericardium or pericardial sac.

The fibrous pericardium is made of tough, dense connective tissue that protects the heart and maintains its position in the thorax. The more delicate serous pericardium consists of two layers: the parietal pericardium, which is fused to the fibrous pericardium, and an inner visceral pericardium, or epicardium , which is fused to the heart and is part of the heart wall.

The pericardial cavity, filled with lubricating serous fluid, lies between the epicardium and the pericardium. In most organs within the body, visceral serous membranes such as the epicardium are microscopic.

However, in the case of the heart, it is not a microscopic layer but rather a macroscopic layer, consisting of a simple squamous epithelium called a mesothelium , reinforced with loose, irregular, or areolar connective tissue that attaches to the pericardium. This mesothelium secretes the lubricating serous fluid that fills the pericardial cavity and reduces friction as the heart contracts.

If excess fluid builds within the pericardial space, it can lead to a condition called cardiac tamponade, or pericardial tamponade. With each contraction of the heart, more fluid—in most instances, blood—accumulates within the pericardial cavity. In order to fill with blood for the next contraction, the heart must relax. However, the excess fluid in the pericardial cavity puts pressure on the heart and prevents full relaxation, so the chambers within the heart contain slightly less blood as they begin each heart cycle.

Over time, less and less blood is ejected from the heart. If the fluid builds up slowly, as in hypothyroidism, the pericardial cavity may be able to expand gradually to accommodate this extra volume.

Some cases of fluid in excess of one liter within the pericardial cavity have been reported. Rapid accumulation of as little as mL of fluid following trauma may trigger cardiac tamponade.

Other common causes include myocardial rupture, pericarditis, cancer, or even cardiac surgery. Removal of this excess fluid requires insertion of drainage tubes into the pericardial cavity. Premature removal of these drainage tubes, for example, following cardiac surgery, or clot formation within these tubes are causes of this condition.

Untreated, cardiac tamponade can lead to death. Inside the pericardium, the surface features of the heart are visible, including the four chambers. Auricles are relatively thin-walled structures that can fill with blood and empty into the atria or upper chambers of the heart. You may also hear them referred to as atrial appendages.

Major coronary blood vessels are located in these sulci. The deep coronary sulcus is located between the atria and ventricles. Located between the left and right ventricles are two additional sulci that are not as deep as the coronary sulcus. 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 5 illustrates anterior and posterior views of the surface of the heart. Figure 5. Inside the pericardium, the surface features of the heart are visible.

The wall of the heart is composed of three layers of unequal thickness. From superficial to deep, these are the epicardium, the myocardium, and the endocardium. The outermost layer of the wall of the heart is also the innermost layer of the pericardium, the epicardium, or the visceral pericardium discussed earlier.

Figure 6. The middle and thickest layer is the myocardium , made largely of cardiac muscle cells. It is built upon a framework of collagenous fibers, plus the blood vessels that supply the myocardium and the nerve fibers that help regulate the heart. It is the contraction of the myocardium that pumps blood through the heart and into the major arteries.

The muscle pattern is elegant and complex, as the muscle cells swirl and spiral around the chambers of the heart. They form a figure 8 pattern around the atria and around the bases of the great vessels. Deeper ventricular muscles also form a figure 8 around the two ventricles and proceed toward the apex.

More superficial layers of ventricular muscle wrap around both ventricles. This complex swirling pattern allows the heart to pump blood more effectively than a simple linear pattern would. Figure 6 illustrates the arrangement of muscle cells. Although the ventricles on the right and left sides pump the same amount of blood per contraction, the muscle of the left ventricle is much thicker and better developed than that of the right ventricle.

In order to overcome the high resistance required to pump blood into the long systemic circuit, the left ventricle must generate a great amount of pressure. The right ventricle does not need to generate as much pressure, since the pulmonary circuit is shorter and provides less resistance. The image below illustrates the differences in muscular thickness needed for each of the ventricles. Figure 7. The myocardium in the left ventricle is significantly thicker than that of the right ventricle.

Both ventricles pump the same amount of blood, but the left ventricle must generate a much greater pressure to overcome greater resistance in the systemic circuit. The ventricles are shown in both relaxed and contracting states. Note the differences in the relative size of the lumens, the region inside each ventricle where the blood is contained. The innermost layer of the heart wall, the endocardium , is joined to the myocardium with a thin layer of connective tissue.

The endocardium lines the chambers where the blood circulates and covers the heart valves. It is made of simple squamous epithelium called endothelium , which is continuous with the endothelial lining of the blood vessels.

Once regarded as a simple lining layer, recent evidence indicates that the endothelium of the endocardium and the coronary capillaries may play active roles in regulating the contraction of the muscle within the myocardium. The endothelium may also regulate the growth patterns of the cardiac muscle cells throughout life, and the endothelins it secretes create an environment in the surrounding tissue fluids that regulates ionic concentrations and states of contractility.

Endothelins are potent vasoconstrictors and, in a normal individual, establish a homeostatic balance with other vasoconstrictors and vasodilators. In order to develop a more precise understanding of cardiac function, it is first necessary to explore the internal anatomical structures in more detail. The septa are physical extensions of the myocardium lined with endocardium. Located between the two atria is the interatrial 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 known as the septum primum 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. The valves at the openings that lead to the pulmonary trunk and aorta are known generically as semilunar valves. The interventricular septum is visible in the image below. In this figure, the atrioventricular septum has been removed to better show the bicupid and tricuspid valves; the interatrial septum is not visible, since its location is covered by the aorta and pulmonary trunk.

Since these openings and valves structurally weaken the atrioventricular septum, the remaining tissue is heavily reinforced with dense connective tissue called the cardiac skeleton , or skeleton of the heart. It includes four rings that surround the openings between the atria and ventricles, and the openings to the pulmonary trunk and aorta, and serve as the point of attachment for the heart valves.

The cardiac skeleton also provides an important boundary in the heart electrical conduction system. Figure 8. This anterior view of the heart shows the four chambers, the major vessels and their early branches, as well as the valves.

The presence of the pulmonary trunk and aorta covers the interatrial septum, and the atrioventricular septum is cut away to show the atrioventricular valves. One very common form of interatrial septum pathology is patent foramen ovale, which occurs when the septum primum does not close at birth, and the fossa ovalis is unable to fuse.

As much as 20—25 percent of the general population may have a patent foramen ovale, but fortunately, most have the benign, asymptomatic version.

Patent foramen ovale is normally detected by auscultation of a heart murmur an abnormal heart sound and confirmed by imaging with an echocardiogram. Despite its prevalence in the general population, the causes of patent ovale are unknown, and there are no known risk factors. The human circulatory system is really a two-part system whose purpose is to bring oxygen-bearing blood to all the tissues of the body.

When the heart contracts it pushes the blood out into two major loops or cycles. In the pulmonary loop , the blood circulates to and from the lungs, to release the carbon dioxide and pick up new oxygen.

The systemic cycle is controlled by the left side of the heart, the pulmonary cycle by the right side of the heart. The systemic loop begins when the oxygen-rich blood coming from the lungs enters the upper left chamber of the heart, the left atrium. As the chamber fills, it presses open the mitral valve and the blood flows down into the left ventricle.

When the ventricles contract during a heartbeat, the blood on the left side is forced into the aorta. This largest artery of the body is an inch wide. The used blood from the body returns to the heart through the network of veins. All of the blood from the body is eventually collected into the two largest veins: the superior vena cava , which receives blood from the upper body, and the inferior vena cava , which receives blood from the lower body region.

Both venae cavae empty the blood into the right atrium of the heart. From here the blood begins its journey through the pulmonary cycle. From the right atrium the blood descends into the right ventricle through the tricuspid valve. When the ventricle contracts, the blood is pushed into the pulmonary artery that branches into two main parts: one going to the left lung, one to the right lung.

The fresh, oxygen-rich blood returns to the left atrium of the heart through the pulmonary veins. Although the circulatory system is made up of two cycles, both happen at the same time. The contraction of the heart muscle starts in the two atria, which push the blood into the ventricles. Aortic valve: Allows blood to pass from the left ventricle to the aorta; prevents backflow of blood into the left ventricle.

Breadcrumb Home Medical services Heart and vascular care Your cardiovascular health How the heart works How the heart works.

Valves maintain direction of blood flow As the heart pumps blood, a series of valves open and close tightly. The tricuspid valve is situated between the right atrium and right ventricle. The pulmonary valve is between the right ventricle and the pulmonary artery. The mitral valve is between the left atrium and left ventricle. The aortic valve is between the left ventricle and the aorta. The circulatory system While the heart and lungs are the largest organs of the circulatory system, the blood vessels are the longest.

Electrical impulses keep the beat The heart's four chambers pump in an organized manner with the help of electrical impulses that originate in the sinoatrial node also called the "SA node". Heart anatomy: By the numbers 1. Pulmonary arteries: Carry oxygen-depleted blood from the heart to the lungs. Left atrium: Receives blood returning to the heart from the pulmonary veins. They are called vasodilators. Image 1: Blood Vessels. Arteries in red carry oxygen rich blood from the left side of the heart to the tissues and organs.

After oxygen leaves the blood and moves into the tissues, the level of oxygen in the blood becomes low. The veins in blue carry blood that has a low level of oxygen back to the right side of the heart. Blood from the veins is pumped from the right side of the heart through the blood vessels of the lungs, where new oxygen is picked up. This oxygen rich blood flows from the lungs to the left side of the heart.

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