24.1 Cardiovascular System

Heart Chambers and Valves

The heart is an intricately designed pump composed of four chambers and four valve structures. Understanding the anatomy and function of these chambers and valves is paramount for nursing professionals because they play a pivotal role in assessing and intervening when necessary to help the heart maintain the continuous circulation of blood throughout the body. Having a thorough understanding of the complexity of the chambers and valves will allow the nurse to create nursing care plans that provide comprehensive care to patients with disorders such as atrial fibrillation, ventricular tachycardia, and mitral stenosis.

The heart has four primary chambers, two on each side—the atria (upper chambers) and the ventricles (lower chambers) (Figure 24.2). Deoxygenated blood from the body flows into the right atrium through the superior and inferior vena cavae. The left atrium receives oxygenated blood from the lungs through the pulmonary veins. The atria serve as reservoirs, collecting blood and then contracting to propel it into the ventricles.

Figure 24.2 The heart pumps blood through the chambers and valves to ensure that blood reaches all areas of the body. The four chambers work in tandem to create a cardiac cycle cadence that works smoothly, pumping blood in and out of the heart. (credit: modification of work from Anatomy and Physiology. attribution: Copyright Rice University, OpenStax, under CC BY 4.0 license)

The ventricles, on the other hand, are responsible for pumping blood out of the heart. The right ventricle propels deoxygenated blood to the lungs through the pulmonary artery, where it undergoes gas exchange of carbon dioxide with oxygen. Simultaneously, the left ventricle pumps oxygenated blood into the systemic circulation, delivering it to the rest of the body. This rhythmic sequence ensures a continuous flow of oxygenated blood to meet the metabolic demands of the tissues.

The heart is equipped with four vital valves that regulate blood flow and prevent backflow, ensuring unidirectional circulation. The atrioventricular (AV) valves, located between the atria and ventricles, include the tricuspid valve on the right side and the bicuspid (mitral) valve on the left. These valves open to allow blood to flow from the atria to the ventricles and close to prevent regurgitation (backward leakage) during ventricular contraction.

The semilunar valves (pulmonary valve on the right, aortic valve on the left) protect the exits of the ventricles. These valves open during ventricular contraction, enabling blood to be ejected into the pulmonary and systemic circulations, respectively. They then promptly close to prevent blood from regurgitating back into the ventricles during relaxation.

Understanding the dynamics of these chambers and valves is fundamental for nurses in assessing cardiac function, detecting murmurs or irregularities, and interpreting diagnostic findings found on echocardiograms and electrocardiograms. In summary, blood flowing through the heart can be described as deoxygenated blood from the body entering the right atrium which contracts to send the blood through the tricuspid valve to the right ventricle. From here, the blood must enter the lungs as it passes through the pulmonary valve. Once it is oxygenated, it returns to the left atrium of the heart where it prepares for its journey throughout the body. From the left atrium, it enters the left ventricle, passing through the bicuspid valve. From the left ventricle, it passes through the aortic valve and enters the aorta to perfuse the body. In clinical practice, nurses collaborate with healthcare team members to monitor and intervene effectively in various cardiac conditions, emphasizing the importance of maintaining the integrity of the heart’s chambers and valves for optimal cardiovascular health.

Walls of the Heart

Our exploration of more in-depth heart structures begins by examining the membrane that surrounds 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.

The membrane that directly surrounds the heart and defines the pericardial cavity is called the pericardium (also known as the pericardial sac). It also surrounds the “roots” of the major vessels—the areas of closest proximity to the heart. The pericardium, which literally translates as “around the heart,” consists of two distinct sublayers: the sturdy outer fibrous pericardium and the inner serous pericardium (Figure 24.3). 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 (also called the epicardium). The pericardial cavity, filled with lubricating serous fluid, lies between the epicardium and the pericardium.

Figure 24.3 The pericardium that surrounds the heart consists of two layers: fibrous pericardium and serous pericardium. The heart wall consists of three layers: epicardium, myocardium, and endocardium. The pericardial cavity filled with lubricating serous fluid lies between the pericardium and epicardium. (credit: modification of work from Anatomy and Physiology. attribution: Copyright Rice University, OpenStax, under CC BY 4.0 license)

The wall of the heart is composed of three layers of unequal thickness. From superficial to deep, these are the epicardium, myocardium, and endocardium. The outermost layer of the heart is the epicardium, which is fused to the heart and is part of the heart wall. The middle and thickest layer is the myocardium, made largely of cardiac muscle cells. It is built on 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 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.

The muscle pattern is elegant and complex, as the muscle cells swirl and spiral around the chambers of the heart (Figure 24.4). They form a figure-eight pattern around the atria and around the bases of the great vessels. Deeper ventricular muscles also form a figure eight 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 24.4 The swirling pattern of cardiac muscle tissue contributes significantly to the heart’s ability to pump blood effectively. (credit: modification of work from Anatomy and Physiology. attribution: Copyright Rice University, OpenStax, under CC BY 4.0 license)

Although the right and left ventricles 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. 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 because the pulmonary circuit is shorter and provides less resistance. As a result, the muscle in the left ventricle is thicker than in the right ventricle (Figure 24.5).

Figure 24.5 The myocardium in the left ventricle is significantly thicker than that of the right ventricle. The ventricles are shown in both relaxed and contracted states. Note the differences in the relative size of the lumens (the region inside each ventricle) where the blood is contained. (credit: modification of work from Anatomy and Physiology. attribution: Copyright Rice University, OpenStax, under CC BY 4.0 license)

Blood

As you recall from anatomy and physiology, blood is a connective tissue made up of cellular elements and an extracellular matrix. The cellular elements, or formed elements, include red blood cells (RBCs), white blood cells (WBCs), and cell fragments called platelets. The extracellular matrix, called plasma, makes blood unique among connective tissues because it is fluid. This fluid is primarily water and continuously suspends the formed elements, enabling them to circulate throughout the body within the cardiovascular system. Fluids, whether gases or liquids, are materials that flow according to pressure gradients; they move from regions of higher pressure to regions of lower pressure.

Blood pumped by the heart flows through a series of vessels known as arteries, arterioles, capillaries, venules, and veins before returning to the heart. Arteries transport blood away from the heart and branch into smaller vessels, forming arterioles. Arterioles distribute blood to capillary beds, the sites of exchange with the body tissues. Capillaries lead back to small vessels known as venules that flow into the larger veins and eventually back to the heart.

Based on this concept, when the heart chambers are relaxed (diastole), blood flows into the atria from the veins, which are higher in pressure. As blood flows into the atria, the pressure rises, causing the blood to initially move passively from the atria into the ventricles. An action potential triggers the muscles in the atria to contract (atrial systole), causing the pressure within the atria to rise even further, pumping blood into the ventricles. During ventricular systole, pressure rises in the ventricles, pumping blood into the pulmonary trunk from the right ventricle and into the aorta from the left ventricle.

Electrical Conduction of the Heart

If embryonic heart cells are separated into a Petri dish and kept alive, each is capable of generating its own electrical impulse followed by contraction. When two independently beating embryonic cardiac muscle cells are placed together, the cell with the higher inherent rate sets the pace, and the impulse spreads from the faster cell to the slower cell to trigger a contraction. As more cells are joined together, the fastest cell continues to assume control of the rate. A fully developed adult heart maintains the capability of generating its own electrical impulse, triggered by the fastest cells, as part of the cardiac conduction system. The components of the cardiac conduction system include the sinoatrial node, the atrioventricular node, the atrioventricular bundle, the atrioventricular bundle branches, and the Purkinje fibers (Figure 24.6).

Figure 24.6 Specialized conduction components of the heart include the sinoatrial node, atrioventricular node, atrioventricular bundle, right and left bundle branches, and Purkinje fibers. (credit: modification of work from Anatomy and Physiology. attribution: Copyright Rice University, OpenStax, under CC BY 4.0 license)

Electrical Activity

Action potentials are considerably different between cardiac conductive cells and cardiac contractile cells. Although Na⁺ and K⁺ play essential roles, Ca²⁺ is also critical for both types of cells. Unlike skeletal muscles and neurons that have a steady, full reserve of energy that is ready to send off a signal when needed (called the resting potential), cardiac conductive cells do not have a strong or stable resting potential. Conductive cells contain a series of sodium ion channels that allow a normal and slow influx of sodium ions that causes the membrane potential to rise slowly from an initial value of −60 mV up to about –40 mV. The resulting movement of sodium ions creates spontaneous depolarization (also, prepotential depolarization). At this point, calcium ion channels open and Ca²⁺ enters the cell, further depolarizing it at a more rapid rate until it reaches a value of approximately +15 mV. At this point, the calcium ion channels close and K⁺ channels open, allowing outflux of K⁺ and resulting in repolarization. When the membrane potential reaches approximately −60 mV, the K⁺ channels close and Na⁺ channels open, and the prepotential phase begins again. This phenomenon explains the auto-rhythmicity properties of cardiac muscle (Figure 24.7).

Figure 24.7 The prepotential phase occurs as the influx of sodium ions slowly begins until the threshold is reached followed by a rapid depolarization and repolarization. Prepotential accounts for the membrane reaching threshold and initiates spontaneous depolarization and contraction of the cell. The image helps visualize the weaker resting potential, which is what allows the buildup in the prepotential phase. (credit: modification of work from Anatomy and Physiology. attribution: Copyright Rice University, OpenStax, under CC BY 4.0 license)

Electrical activity in the heart’s contractile cells follows a specific pattern. First, there’s a quick depolarization, then a plateau phase, and finally repolarization. This process ensures that the heart muscle cells have enough time to effectively pump blood before they can contract again.

Unlike some other cells, cardiac myocytes (cells in the heart) do not start their own electrical signals. Instead, they wait for an impulse to reach them. These cells have a more stable resting phase compared to other types of cells, typically at around −80 mV for cells in the atria and −90 mV for cells in the ventricles. Despite this initial difference, the rest of their action potentials are very similar.

When these cells are stimulated by an action potential, specific channels quickly open, starting the process of depolarization. This quick influx of positively charged ions raises the cell’s potential, reaching approximately +30 mV. At this point, the sodium channels close and the rapid depolarization phase lasts for about 3 to 5 ms. After depolarization, the plateau phase occurs, where the membrane potential decreases relatively slowly. This is because slow calcium channels open, letting calcium enter the cell, while only a few potassium channels are open, allowing potassium to exit. The relatively long plateau phase lasts about 175 ms.

When the membrane potential reaches approximately zero, calcium channels close, and potassium channels open again, letting potassium exit the cell. This repolarization lasts about 75 ms. The membrane potential then drops until it reaches resting levels, and the cycle repeats. This whole event takes between 250 and 300 ms.

The absolute refractory period for these heart muscle cells (a time when the cell cannot respond to another stimulus) is about 200 ms. The relative refractory period, a time when the cell can respond but needs a stronger stimulus, is about 50 ms. This extended period is crucial because the heart must contract effectively to pump blood, and this contraction must follow the electrical events. Without these longer refractory periods, the heart might contract prematurely, which would not be compatible with life (Figure 24.8).

Figure 24.8 (a) This illustration shows the long plateau phase caused by the influx of calcium ions. The extended refractory period allows the cell to fully contract before another electrical event can occur. (b) This illustration shows the action potential of heart muscle compared to that of skeletal muscle. (credit a and b: modification of work from Anatomy and Physiology. attribution: Copyright Rice University, OpenStax, under CC BY 4.0 license)

There are two important roles that calcium ions play in how the heart muscles work. First, calcium ions come in through special channels and help create a steady phase and a time when the heart cannot respond to signals. This helps the heart muscle work correctly. Second, calcium ions join in the troponin complex with another protein called tropomyosin. This complex removes a block that stops myosin (a type of protein) from connecting with actin, which is needed for the heart muscles to contract and pump blood. This process is quite similar to how muscles work in our body. About 20 percent of the calcium needed for the heart’s contraction comes from the influx of calcium during the steady phase. The rest of the calcium comes from storage in a part of the cell called the sarcoplasmic reticulum.

The heart’s pattern of prepotential, quick depolarization, and repolarization is controlled by the SA node and some other heart cells. The SA node is like the boss, and it becomes ready to send signals faster than any other part of the heart’s system. It starts the signals that go to the other cells.

The SA node can send signals on its own about 80 to 100 times in a minute, even without any control from nerves or hormones. Each part of the heart system can initiate its own signals, but the rate slows down as the signal moves from the SA node to the Purkinje fibers. Without the SA node, the AV node would initiate signals at a rate of 40 to 60 times per minute. If the AV node is blocked, the atrioventricular bundle takes over and initiates signals about 30 to 40 times per minute. The bundle branches have a rate of 20 to 30 signals per minute, and the Purkinje fibers have a rate of 15 to 20 signals per minute.

Some very well-trained athletes might have a resting heart rate of 30 to 40 beats per minute. However, a heart rate lower than 60 beats per minute could mean the patient is experiencing bradycardia. When rates fall below this level, the heart might not pump enough blood to important tissues, leading to problems such as loss of function, passing out, and, eventually, death. The immediate intervention for a patient with sudden bradycardia is stabilization with respiratory and cardiovascular support, such as initiating oxygen, addressing any bleeding, and monitoring the patient with telemetry. After the patient is stabilized, the underlying cause should be addressed, and the patient’s provider notified.

Systole

As the heart pumps, ventricular systole occurs and is split into two phases. First, the heart’s electrical activity occurs as represented by the QRS complex. This is followed by the ventricles contracting. At this point, the blood pressure in the ventricles rises, although it is not high enough to push blood out yet. Instead, the blood flows back toward the atria, closing the valves between the atria and ventricles. At this phase, no blood has left the heart yet so volume in the heart does not change, which is referred to as isometric contraction.

In the second phase, ventricular ejection, the pressure in the ventricles is high enough to open valves by pushing blood out. This is called isovolumetric contraction. Ventricular ejection, which is the main measure of ventricular function, is measured as ejection fraction, the percentage of blood that the left ventricle pumps out with each contraction. Even though the left ventricle has to push against higher pressure in the aorta, both sides of the heart pump the same amount of blood. The amount of blood pumped out of the left ventricle is called stroke volume (SV), or afterload. Normally, this is about 70 to 80 mL of blood. After this contraction, there’s still about 50 to 60 mL of blood left in the ventricle, known as end-systolic volume.

Diastole

After the heart pumps, it takes a break during a phase called ventricular relaxation (diastole). This relaxation is represented by the T wave of the ECG and has two parts, lasting about 430 ms.

In the first phase, as the heart muscles relax, the pressure on the remaining blood in the heart decreases. When this pressure becomes lower than the pressure in the pulmonary trunk and aorta, blood flows back into the heart, causing a small dip in blood pressure called the dicrotic notch in an arterial waveform. Some valves close to prevent the blood from going back into the heart (regurgitation). The isovolumetric relaxation is the time interval between aortic valve closure and mitral valve opening when the ventricular pressures falls below the aortic and pulmonary pressures. During this time, cardiac circulation itself happens.

In the second phase, as the heart muscles keep relaxing, the pressure on the blood in the heart decreases even more. Eventually, it becomes lower than the pressure in the atria. When this happens, blood flows from the atria into the ventricles, opening the tricuspid and mitral valves. As the pressure decreases further, blood comes from the veins into the relaxed atria and stimulates another round of the cardiac cycle.