Cardiac Anatomy & Physiology: What the Medical Malpractice Attorney Needs to Know
This article discusses what the medical malpractice litigator needs to know about various medical topics. The purpose of this article is to impart information about fundamental concepts of medicine that the medical negligence litigator needs to effectively work in this area of practice. While we, as attorneys, will never be able to endorse our own opinions to support our clients’ claims, we can certainly learn enough to determine whether or not the potential clients who come to us have claims, what experts will be needed to support those claims, and how to have more productive conversations with experts.
As medical malpractice attorneys, we work in a world inundated with medical terms, images, records, facts, and theories. With a few exceptions, most of us did not go to medical school, and yet we are called upon every day in our practices to evaluate the care provided by physicians and others, to determine if it was adequate, and if not, whether some act or omission of the doctor caused or failed to prevent a bad outcome.
Without a basic understanding of certain core medical concepts, one could easily bankrupt his or her practice by depending on medical experts—charging hundreds of dollars per hour—to provide the most basic information. Fortunately, it is not necessary to do so. Even for those of you with no formal medical training or experience, the medicine you need to know is not out of your reach.
“Lasting knowledge results from understanding.”
—Dale Dubin, M.D.
Anatomy of the Heart (Structure)
The normal heart is roughly the size of its owner’s clenched fist. In adult males, the heart should weigh between 300-350 grams; in adult females, 250-300 grams.¹ It is a four-chambered, double, self-adjusting suction and pressure pump that works to move blood to all parts of the body. By ‘double’, what I mean is that the heart really consists of 2 separate pumps—the right heart and the left heart—that work in unison.
The right side of the heart receives poorly oxygenated blood from the body and moves it to the lungs for gas exchange. The left side of the heart receives the oxygenated blood from the lungs and moves it to the body for gas exchange.
Gas exchange occurs in both places—in the lungs, and in the body tissues. In the lungs, CO2 (carbon dioxide) is offloaded so that it can be exhaled and eliminated, while oxygen is loaded into the blood for transport to the tissues. In the body, oxygen is offloaded to the tissues for use, and is replaced by CO2, which is moved out of the tissues into the blood for transport to the lungs, where it can be eliminated.
The right and left sides of the heart, while part of the same organ, should be thought of as 2 separate systems because they can malfunction independently of one another.
4 Chambers, 4 valves. The normal heart has 4 chambers: two atria and two ventricles. The atria receive blood and the ventricles discharge blood. The heart also has 4 valves: Mitral, Tricuspid, Aortic, and Pulmonic (M-T-A-P).
The Mitral² and Tricuspid valves separate the atria from the ventricles—the Mitral on the left side and the Tricuspid on the right. These valves are therefore called atrioventricular valves. The Aortic and Pulmonic valves are located in the aortic and pulmonary arteries leaving the heart. These valves are called the semilunar valves.
Physiology of the Heart (Function)
I like to think of blood flow as being similar to traffic on a system of roadways, with lots of different cars driving around. Some roads are multi-lane highways with high speed limits, while other roads are alleys so narrow that two cars cannot pass, where speeds are much, much slower.
The cars in this analogy correspond to the different kinds of blood cells, with each red 4-passenger sedan corresponding to a single red blood cell.³ With that in mind, the circuit of one red car, assuming the driver never goes the wrong way on any one-way street, requires the car to pass through structures in a certain order, as follows:
Now that we understand that blood flows in an ordered pathway, let’s talk about how. The left atrium and ventricle pump in unison with the right atrium and ventricle, in a repeating cardiac cycle. The cycle runs from the beginning of one heartbeat to the beginning of the next heartbeat. Contraction of the ventricles is the most important part of the cardiac cycle—it is what creates the heartbeat and provides the main propulsive force for blood flow
Because this event is so important, we always want to know what the ventricles are doing. Thus, there is specific terminology used to describe when the ventricles are contracting and when they are at rest. Systole&sup4; is the name given to that portion of time in the cardiac cycle when the ventricles are contracting. Diastole&sup5; is the name given to that portion of time in the cardiac cycle when the ventricles are at rest, while they are filling with blood. These terms should sound familiar to you—they are also used to describe blood pressure. The systolic blood pressure is the pressure in the arteries when the heart is contracting, and the diastolic pressure is the pressure in the arteries when the ventricles are at rest.
These terms are also used to describe atrial contraction and rest, as you will see below. Note however, that when either term is used alone, it refers to ventricular action. If the terms are used to refer to atrial action, the term ‘atrial’ will precede them. So, the cardiac cycle, the time from the beginning of one heartbeat to the beginning of the next, includes one period of diastole and one period of systole, and works like this:
The valves, then, work like traffic control devices on the blood roadway system, and they open and close according to the pressures exerted on each side. A normal valve functions as a one-way portal to the next destination after it (see Figure 3). Most people are familiar with the existence of weight sensors underlying the roadway at some traffic intersections. When the sensors detect one or more cars sitting at the light, they trigger the lights to change so the waiting cars can move. Thus, the weight (pressure) of the cars waiting at the light can sometimes trigger the light to change. Similarly, normal valves open or close on the basis of the pressure gradients across them. When the pressure in the atria rises to a certain threshold, the atrioventricular (AV) valves open to let the waiting blood exit and move into the ventricles. The atria contract to help the blood along and to add a little extra kick (it’s actually called “atrial kick”), pushing a bit more blood volume into the ventricles. Similarly, when the pressure in the ventricles builds to a certain threshold, the semilunar (SL) valves open to allow the blood to move out into the aorta and pulmonary arteries. The ventricles also contract to help this along.
All 4 valves cannot be open at the same time, so they alternate. In order to ensure that blood from the ventricles does not move backwards into the atria when the ventricles contract, the AV valves must be closed and the SL valves are open. In order to ensure that blood is retained in the ventricles when the AV valves open to fill the ventricles, the SL valves must be closed when the atria contract and the AV valves are open.
So, when the atria are contracting, the AV valves are OPEN and the SL valves are CLOSED. When the ventricles are contracting, the SL valves are OPEN and the AV valves are CLOSED. When we talk about heart murmurs, this will be handy to know.
Mechanically, that’s about all there is to it. Easy, right?
Now for the interesting part. The heart is not just a mechanical pump—it’s also electric! Heart muscle is special—different than any other muscle tissue in the body. There are three types of muscle tissue found in our bodies: smooth muscle, skeletal muscle, and cardiac muscle. Skeletal muscle is what you work on in the gym. It’s your quads, hamstrings, biceps, etc. You can control the movement of your skeletal muscle, and it is referred to as being voluntary muscle, because of that control. Skeletal muscle, when viewed under a microscope, has striations (or stripes), and so is also referred to as striated muscle.
Smooth muscle is generally involuntary muscle, such as is found in the walls of the intestines and blood vessels—in other words, you cannot control the contraction and relaxation of smooth muscle, these things happen for you.
Cardiac muscle is different from regular smooth muscle and regular striated muscle. Cardiac muscle is involuntary, like smooth muscle, but striated, like skeletal muscle. Cardiac muscle is also special because it is highly resistant to fatigue. This comes with a trade-off however: skeletal muscle, although it fatigues easily, can continue to work in conditions of inadequate oxygen supply, without becoming damaged or dying. It does this by switching from aerobic metabolism (with oxygen) to anaerobic metabolism (without oxygen). When your skeletal muscles are forced to function without sufficient oxygen, lactic acid builds up, and causes muscle aches that typically resolve in about 48 hours. No harm, no foul. Anyone who works out or engages in physical activity has experienced this.
Unlike skeletal muscle, however, cardiac muscle always requires oxygen to continue functioning. There is no option for anaerobic metabolism in cardiac muscle. Without sufficient oxygen, bad things happen. When oxygen is cut off, muscle tissue progresses rapidly from insult to injury, to death. This is why time is so critical when dealing with a heart attack.
Fatigue-resistant muscle isn’t the only thing that makes the heart special. The heart has its own independent electrical conduction system, and it is the electrical current generated in this system that causes the heart muscle to contract. The nerve cells serving cardiac muscle have some similarities to those of skeletal muscle, but are different in a few important ways. Importantly, unlike the nerves innervating skeletal muscle, cardiac neurons are uniquely influenced by the sympathetic and parasympathetic influence of the autonomic nervous system. The sympathetic and parasympathetic influences are parts of the nervous system that function on autopilot. The sympathetic nervous system is what takes over when there is a tiger chasing you (fight or flight response), and the parasympathetic nervous system is like the maintenance crew that handles your ordinary bodily functions when there is no emergency threatening your life. For now that’s all we need to know about sympathetic/parasympathetic responses, though they will become very important again when we look at abnormal heart rhythms.
The heart’s electrical conduction system is very organized, and very predictable. Unlike any other nerves in the body, these nerve cells are able to generate their own impulses, without stimulation from the brain. Like any organized, dependable system, there is direction from a manager. In the heart’s conduction system, the senior manager is the sinoatrial (Sinus or “SA”) node.
The SA node functions as the heart’s dominant pacemaker. But, as is true of any organized, reliable system, there are mechanisms in place to ensure that if the manager cannot perform his or her duties, there is a chain of command that takes over to keep the operation running. If the SA node cannot function as the pacemaker, the next in the chain of command is the atrioventricular (“AV”) node. If the AV node also ceases to function as a pacemaker, then individual cells in the heart muscle have the ability to generate an impulse that can activate the heart muscle.
In the normal heart, the electrical impulses result in muscular activation and contraction, and this is what causes the atria and ventricles to contract, resulting in a heartbeat. Because electrical activity can be precisely measured, we are able to “see” what the heart is doing by using an electrocardiogram (ECG or EKG). One thing to keep in mind when thinking about the electrical system of the heart is that, while it is designed to stimulate muscular contraction (and therefore result in a heartbeat) it is a separate system than the mechanical pumping system. More to the point, electrical activity can happen without mechanical activity, resulting in an impressive, sometimes even normal-appearing EKG rhythm that does not create a pulse (pulseless electrical activity or PEA).
The interpretation of EKGs is a topic unto itself, but for purposes of discussing cardiac structure and function here, I should mention the basics of how the electrical impulses are conducted. The system involves precise time delays, which is fairly impressive when you think about it. Here’s how it works:
- First, the electrical impulse is generated by the SA node. The impulse spreads out in the atrial tissue, traveling across both atria (yellow arrows in Figure 7), activating muscle cells along the way and causing a synchronized atrial contraction.
- Next, the electrical impulse reconvenes at the junction between the atria and ventricles, where it collects in the AV node. There is an important PAUSE in the cardiac cycle (and EKG rhythm) as the AV node WAITS for all the electricity that has spread out over the atria to make its way in.
- Finally, when it has collected all the electricity again, the AV node re-releases the impulse again. But this time, because it has so much territory to cover, the electricity cannot be released right into the muscle tissue. Instead, it is delivered precisely to all areas in the ventricles at once, like an injection, by traveling on a conduction pathway (yellow structures in Figure 7). The conduction pathways provide a direct line of access designed to transport the impulse rapidly to all areas of the ventricles simultaneously. This serves to control the timing of the ventricular contraction, so that both ventricles contract at the same time, with enough force to push all the blood out.
You have just learned the basics of the structure and function of the heart. Keep in mind, there is a lot more to cardiology than this. But, the information presented here is enough to allow you to understand what should happen in a normal heart, and that the foundation for learning about all the abnormal things that can also happen. The basic concepts here are an important first step in understanding what you see in your clients’ medical records, and as Dale Dubin says, understanding is the foundation of lasting knowledge.
If this article was helpful to you, please let me know!
- Cotran, Ramzi S., M.D., Vinay Kumar, M.D., and Tucker Collins, M.D., Pathologic Basis of Disease, 6th edition, Philadelphia, PA: Harcourt Brace Publishing. 1999. P. 544.
- The Mitral valve is special for a couple reasons which will be discussed in a subsequent article. For now, just know that it is the only valve with only 2 leaflets—the others all have 3.
- To learn about the other kinds of cars on the highway, read my article What the Medical Malpractice Lawyer Needs to Know about Blood.
- Pronounced: SISS-toh-lee
- Pronounced: dye-ASS-toh-lee