Cardiac Cycle: purpose

The cardiac cycle refers to the events which take place during one beat of the human heart. As the result of the cardiac cycle, the heart accomplishes three primary goals: 1) oxygen-poor blood is received from the peripheral venous system 2) blood is circulated through the lungs where it is reoxygenated. 3) the freshly oxygenated blood is circulated to all organs and tissues of the body by the arterial system.

Cardiac Anatomy

In order to better understand how the cardiac cycle works, let’s start by looking at some anatomy of the heart. The model I’m using today is that of an infant, but for purposes of this discussion, the anatomy and cardiac cycle are essentially the same as in an adult. The heart is enclosed by the lungs on either side. Looking at the heart in a cross-section view reveals four chambers; two smaller chambers on top, and two larger chambers below. The two smaller chambers are known as the right and left atrium. Just to avoid confusion, remember that right and left designations are always labeled from the viewpoint of the patient, rather than the viewer. The two larger chambers are known as the right and left ventricle. The atria and ventricles are essentially hollow chambers whose walls are composed of muscle. The heart also contains four valves: the tricuspid valve, which lies between the right atrium and ventricle; the mitral valve, which lies between the left atrium and ventricle (the mitral valve is less commonly referred to as the bicuspid valve; the pulmonary valve is situated between the right ventricle and the start of the main pulmonary artery. If we remove the pulmonary artery, we can see the aortic valve, situated between the left ventricle and the beginning of the aorta. The function of the cardiac valves is to ensure one-way flow from chamber to chamber, and from chamber to blood vessel.

Blood Flow Through the Heart

Now that we have a better understanding of the anatomical structures of the heart, let’s take a look at the flow of blood through it. Oxygen-poor blood flows from the venous system via the vena cava into the right atrium. Assisted by contractions of the right atrium, blood then passes through the tricuspid valve and into the right ventricle. Contractions of the right ventricle then pump the blood through the pulmonary valve and into the main pulmonary artery. From the main pulmonary artery the blood flows through the arterial system of the lungs, where it is oxygenated. We’ll look the oxygenation process in much more detail later in this review. The freshly oxygenated blood then returns to the heart, flowing through the pulmonary veins and into the left atrium. Assisted by contractions of the left atrium, the blood then flows through the mitral valve and into the left ventricle. Finally, the left ventricle pumps the oxygenated blood through the aortic valve and into the aorta, where it is distributed to all organs and tissues of the body.[heart beats]

Cardiac Cycle: Systole and Diastole

We’ve identified the key anatomical components of the cardiac cycle, as well as tracing the flow of blood from the time that it enters, and then exits, the heart. Now let’s study the timing and coordination of chamber contraction and valvular function. The cardiac cycle is divided into two main phases, systole and diastole.

Systole

Let’s begin by looking at the events which occur during systole. The first event that occurs during systole is the simultaneous contraction of both the right and left ventricles. The contraction of the right ventricle pushes blood against both the pulmonary and tricuspid valves. This pressure causes the tricuspid valve to close, preventing backflow of blood into the right atrium, while the pulmonary valve opens, allowing the blood to flow into the main pulmonary artery. At the same time, contraction of the left ventricle pushes blood against the aortic valve and the mitral valve. This pressure results in opening of the aortic valve, allowing flow of freshly oxygenated blood into the aorta, and closure of the mitral valve, preventing backflow of blood into the left atrium. To summarize then, during systole both ventricles contract. The mitral and tricuspid valves close to prevent backflow into the left and right atrium, respectively. The pulmonary and aortic valves open to permit flow of blood into the lungs and aorta, respectively.

Diastole

Turning now to diastole, the events which occur are almost a mirror image of systole. The left and right ventricles simultaneously relax and expand. This causes the mitral valve to open, allowing freshly oxygenated blood from the left atrium to fill the left ventricle. At the same time, negative pressure pulls on the aortic valve, causing it to close, preventing backflow of blood into the left ventricle. In similar fashion, when the right ventricle expands, negative pressure pulls on both the pulmonary and tricuspid valves. This results in closure of the pulmonary valve, preventing backflow of blood into the right ventricle, and opening of the tricuspid valve, which allows oxygen-poor blood to flow into the right ventricle from the right atrium. The final event during diastole, occurring just before the next systolic phase, is simultaneous contraction of the right and left atria. This last-second atrial contraction has the effect of topping up the volume of blood in both the right and left ventricles right before their next contraction. To summarize, in diastole, both the right and left ventricle simultaneously relax, expand and refill with blood. The aortic and pulmonary valves close to prevent backflow of blood into the left and right ventricles, respectively. The mitral and tricuspid valves open to allow the left and right ventricles to refill with blood, and the left and right atria contract to top up the blood volume in the ventricles just prior to their next contraction.[heart beats]

Oxygenation of the Blood

Now, as promised, let’s take a closer look at the process of oxygenation of blood in the lungs. As noted previously, oxygen-poor blood is pumped into the main pulmonary artery by the right ventricle. The main pulmonary artery then divides into a left and right branch to distribute the blood to both lungs. The pulmonary arterial system consists of an intricate web of blood vessels, simplified here for graphical presentation purposes. The bronchial tree is an equally intricate system of airways distributed throughout both lungs. There is a physical interface between the airway system and the pulmonary arterial system, which we’ll take a closer look at in just a moment. To begin the process of blood oxygenation, air is inhaled into the bronchial tree. Let’s zoom in for a closer look to see how the process of oxygen transfer occurs. The pulmonary arterial system and bronchial tree divide down into ever smaller branches. The smallest arterial branch is known as a pulmonary arteriole. Tiny airways known as terminal bronchioles give way to even smaller airways known as respiratory bronchioles. At the end of the respiratory bronchioles are structures known as alveolar sacs. Each alveolar sac consists of a number of individual alveoli, which we will take a closer look at in just a moment. The smallest blood vessels, known as capillaries, also pronounced as capillary in Canada and the United Kingdom, originate from the pulmonary arterials and interface directly with the alveolar sacs. Taken altogether, the capillaries and alveolar sacs are termed the alveolar capillary complex. And just to give you some sense of the size of these structures, here is a shaft of a typical paper clip, which is about one millimeter in diameter. Moving in closer now, we can see the intimate physical relationship between the capillaries and the individual alveoli. I have made the capillaries semi-transparent, exposing the individual red blood cells circulating through them. The capillary is in close contact with a single alveolus. As noted previously, a respiratory bronchial is attached to the alveolar sac. Examining a single alveolus and capillary allows a more complete view of their anatomical relationship. The diameter of a capillary is just greater than that of a red blood cell, which keeps the red blood cells in close proximity to the wall of the alveolus. Air enters the alveolus from the respiratory bronchial. The walls of both the alveolus and the capillary are permeable to oxygen molecules, and allow the movement of oxygen from one to the other. Oxygen molecules attach to red blood cells. Within each red blood cell is an iron containing protein structure known as hemoglobin, which binds the oxygen molecules. Each red blood cell is typically capable of carrying about 1 billion molecules of oxygen. As previously discussed, the freshly oxygenated blood is carried back to the heart through the pulmonary venous system, and then ejected into the aorta for distribution to all bodily organs and tissues.

Cal Shipley, M.D. copyright 2020