This section focuses on the anatomy and physiology of the cardiovascular system, which primarily includes the heart. The heart functions as a pump that is mainly responsible for moving oxygenated blood from the left ventricle of the heart (through the systemic circulation) and for pumping deoxygenated blood from the right ventricle to the lungs where the blood is oxygenated (through the pulmonary circulation).
We will also talk about the electrical and circulatory system of the heart as well as heart health and the various things that prevent heart disease.
Anatomy of the Heart
The heart is a muscular pump consisting of four chambers, the right atrium, the right ventricle, the left atrium, and the left ventricle. The right atrium receives deoxygenated blood from the venous system in the peripheral circulation and pumps the blood through the tricuspid valve to the right ventricle, where it is forced out of the pulmonic valve into the pulmonary circulation (through the pulmonary artery) under low pressure. While in the pulmonary circulation, the carbon dioxide in the blood is exchanged with oxygen in the alveoli of the lungs and sent back to the heart via the pulmonary vein.
The blood in the left side of the heart is fully oxygenated. The left atrium receives the blood and passes it through the mitral valve into the left ventricle. From there it exits the aortic valve, travels into the aorta (the largest blood vessel in the body) and begins its journey through the peripheral circulation—first through large arteries, then through small arteries, and finally through arterioles and capillaries, which are the interface between the blood and the tissues.
The blood pressure created in the arterial system by the contraction of the heart is high and is known as the “systolic blood pressure.” This is the pressure in the arterial system during the contraction phase of the left ventricle. When the ventricle has completed its contraction, it starts to relax and dilate, filling with blood from the left atrium. The pressure in the arteries at this time is the much lower “diastolic pressure.”
Functionally, the four chambers work in synchronicity with one another. The two atria contract together and the two ventricles contract together.
The Myocardium and Electrical Activity of the Heart
The myocardium or heart muscle is a specialized type of electrically-active muscle, which consists of millions of individual cells that are electrically connected to one another. It is a rise in intracellular calcium concentration that ultimately leads to the depolarization of these cells, with each cell passing the electrical message onto its neighboring cell. These lead to what’s known as a “wave of depolarization” that travels along the myocardium, resulting from the contraction of the myocardium and the heart itself.
The heart has a specific electrical system with cells along the system that have different rates of depolarization. The primary pacemaker cells of the heart are localized in the sinoatrial node or SA node, found in the right atrium. The electrical signal begins in the SA node and travels across the entire atria until it reaches the heart’s second principal pacemaker, the atrioventricular node or AV node.
After passing through the AV node, the electrical signal travels to the bundle of His, located in the interventricular septum, sending the signal to the muscle cells of the left and right ventricle themselves. The signal must divide into two parts (the left bundle and the right bundle) to send signals to both ventricles. The upside-down innervation sends the signal from the bottom of the heart to the top of the heart, spreading out to become the Purkinje fibers that cause a bottom-up contraction of both ventricles at the same time.
The systolic contraction or squeezing of the heart lasts for about 250 milliseconds after which there is the diastolic or relaxation phase. The actual time spent in diastole is variable depending on what the heart rate is.
While the SA node is the natural pacemaker of the heart, it sometimes fails, leading to parts of the rest of the atrium becoming the heart’s pacemaker or to the AV node becoming the pacemaker, leading to what’s known as a “junctional rhythm.” If this rate is too slow, patients may have a permanent pacemaker implanted, which supersedes any other pacemaker in the heart and leads to a more normal heart rate. In some cases, the patient may require temporary pacing wired (TPWs), which will help the heart beat at a regular rate.
There are about 500 million cells or more that make up the totality of the muscular parts of the heart. Most of the cells are in the ventricle with 100 million cells located in the atrium. The atria cells can contract over a third of a second, allowing for contraction of the two atria at the same time.
There is a slight delay when the electrical signal in the heart reaches the atrioventricular node, which allows the contractions of the atria to have enough time to send most of the blood in the atria to the ventricles. The atria then begin the filling process, with the electrical signal passing through the AV node, up the Bundle of His and into both bundle branches—finally spreading across the ventricles through the Purkinje fibers. It is only then that the ventricles contract, sending the blood out into the circulation.
About 400 million electrically-active cells in the ventricles will contract in a wave of forceful contractions that last less than a third of a second. The right ventricle contracts along with the left ventricle, with the former sending, deoxygenated blood to the lungs and the later sending oxygenated blood through the aorta (and into the coronary circulation and the arterial circulation).
After the ventricles contract, they are relatively empty of blood, while the atria are full of blood. The valves between the atria and ventricles are closed, and the system is primed to get another electrical stimulus from the SA node. There is only one electrical stimulus coming from the SA node and the AV node at any given period. These two nodes need recharging before they can send a new signal out to the rest of the heart.
With the heart, the sinoatrial node must recharge during the time of refilling of the atria. The atrioventricular node or AV node must recharge while the ventricles are being refilled. This is a short recharging period, lasting less than a third of a second. Based on a heart rate of sixty beats per minute, this accounts for the totality of one beat per second.
The release of the electrical stimulus of the heart muscle and the nodes is called depolarization, while the recharging period is called repolarization. This leads to the three stages of the heartbeat, including 1) atrial depolarization, 2) ventricular depolarization, and 3) the combined repolarization of the atria and ventricles. On an ECG, each of these phases can be seen on the rhythm strip except for the repolarization of the atria, which is not seen on an ECG because the activity is buried in the QRS complex, which represents the depolarization of the ventricles.
The electrocardiograph or ECG is a useful clinical tool used to identify aspects of the physiology of the heart. Its function is to define the electrical activity of the heart by placing ten electrodes on the limbs and chest, forming 12 leads that collectively measure the electrical activity of the various parts of the heart, including the inferior and lateral aspects. Damage to the myocardium (ischemia and necrosis) will alter the electrical activity of the heart so that changes can be seen on the ECG.
Any upward deflection on the electrocardiogram is representative of a depolarization episode, showing the passage of electrical activity toward the receiving electrode. Any downward deflection on the electrocardiogram is representative of repolarization in the heart, with an electrical signal passing away from the receiving electrode. There are several different waves on the standard ECG, each of which represents a different cardiac activity.
The P wave is first and represents the repolarization of the atria. This is a small (usually upward) deflection because the atria are much smaller in size when compared to the ventricles. The Q wave is next, reflecting the depolarization of the bundle of His, which is also a slight deflection. The R wave is much larger, representing the main body of the depolarization of the main part of the ventricles. It represents the contractility of the bulk of the ventricles. The S wave is the depolarization of the ventricles from the bottom-up (from the base upwards). The T wave is representative of the repolarization of the ventricles after the completion of systole. Because it is a slow process, the wave shows a gradual upward and then downward deflection.
The Coronary Arteries
The coronary circulation provides the heart muscle with the necessary blood supply for it to function. There are two main coronary arteries that branch off as the descend from the aorta. These include the left coronary artery (supplying the left side of the heart) and the right coronary artery (supplying the right side of the heart). They lie in grooves running over the myocardial surface and are covered for protection by the epicardium. The coronary arteries are relatively unique in that they don’t have a lot of overlap is circulation, so a blockage in one major artery in the heart often leads to myocardial ischemia and infarction.
The right coronary artery has a major split to form the right marginal artery, the sinoatrial nodal artery, and the atrioventricular nodal artery. The left coronary artery has several splits, ending in the anterior interventricular artery, the left marginal artery, and the diagonal artery. Each of these serves a different portion of the heart.
Preventing Heart Disease
Patients with heart disease often don’t go into having the disease without risk factors. Typical risk factors for coronary artery disease include having diabetes, hypertension, hyperlipidemia, and obesity. People with a family history of heart disease will have an increased risk for coronary artery disease. The modifiable risk factors mainly involve things like hypertension, hyperlipidemia, and obesity. There can be antihypertensive medication to reduce hypertension, cholesterol-reducing drugs to lower blood cholesterol levels, and methods of losing weight that can reverse obesity.
The main strategies for weight loss that can reduce coronary artery disease risk include exercise and reducing the total daily calorie intake to achieve a weight reduction of 5-10 percent of initial body weight. Evidence suggests that a simple weight reduction of this magnitude is sufficient to cause a significant reduction in heart disease risk factors.