Heart Anatomy and Blood Flow: How Blood Moves Through the Heart, for EMS

The heart is two pumps side by side. The right side collects blood that has given up its oxygen to the body and pushes it through the lungs; the left side collects that blood once the lungs have reloaded it with oxygen and pushes it out to the rest of the body. This guide covers the anatomy an EMS student needs: the chambers, valves and great vessels, the order blood flows through them, the coronary arteries that feed the heart muscle itself, the electrical conduction system, and why each of those matters on a cardiac call.

The chambers

The heart has four chambers. The two upper chambers, the atria, receive blood returning to the heart. The two lower chambers, the ventricles, pump blood out of it. A muscular wall, the septum, divides the right side from the left, so in a normal heart the blood on the two sides does not mix.

The left ventricle has the thickest wall of the four, because it pumps against the pressure of the entire body's circulation. The right ventricle only has to push blood through the lungs, a much lower-pressure circuit, and its wall is thinner. That difference matters when either one is damaged.

The heart wall has three layers: the endocardium lining the chambers, the myocardium, which is the heart muscle that does the pumping, and the epicardium on the outside. The whole heart sits in the pericardium, a two-layered sac with a small amount of fluid between the layers. When blood or fluid fills that sac under pressure, the heart cannot fill, which is cardiac tamponade.

The valves

Four one-way valves keep blood moving forward. Two sit between the atria and the ventricles, and two sit where blood leaves the ventricles.

The tricuspid and mitral valves are the atrioventricular valves. Thin cords called chordae tendineae tie their leaflets to papillary muscles on the ventricle wall, which stops the leaflets from flipping backward when the ventricle squeezes. The pulmonary and aortic valves are the semilunar valves, named for their half-moon-shaped cusps. The two heart sounds you hear with a stethoscope are valves closing: the first sound is the atrioventricular valves closing as the ventricles start to contract, and the second is the semilunar valves closing as the ventricles relax.

The great vessels

• Superior vena cava: returns blood from the head, neck and arms to the right atrium.
• Inferior vena cava: returns blood from the rest of the body below the heart to the right atrium.
• Pulmonary trunk and the right and left pulmonary arteries: carry blood from the right ventricle to the lungs. After birth, they are the only arteries that carry deoxygenated blood.
• Pulmonary veins: carry oxygenated blood from the lungs back to the left atrium. After birth, they are the only veins that carry oxygenated blood.
• Aorta: carries oxygenated blood from the left ventricle to the body. Its first branches are the coronary arteries.

Arteries carry blood away from the heart and veins carry it toward the heart, whatever its oxygen content. That definition is why the pulmonary arteries and veins look like exceptions.

Blood flow through the heart, step by step

1. Deoxygenated blood from the body returns through the superior and inferior vena cava into the right atrium.
2. It passes through the tricuspid valve into the right ventricle.
3. The right ventricle contracts and pushes it through the pulmonary valve into the pulmonary trunk and the pulmonary arteries.
4. In the capillaries of the lungs, the blood gives off carbon dioxide and picks up oxygen.
5. Oxygenated blood returns through the pulmonary veins into the left atrium.
6. It passes through the mitral valve into the left ventricle.
7. The left ventricle contracts and pushes it through the aortic valve into the aorta.
8. The aorta and its branches carry it to the body's tissues, including the heart muscle itself through the coronary arteries, and the cycle begins again.

The two sides work at the same time, not one after the other. Both atria contract together to top up the ventricles, and then both ventricles contract together, the right sending blood to the lungs while the left sends it to the body. The contraction phase of the ventricles is systole and the relaxation and filling phase is diastole, which is where the two numbers of a blood pressure get their names.

Calculate a mean arterial pressure from a systolic and diastolic pressure

The coronary arteries

The heart muscle cannot rely on the blood passing through its chambers for its oxygen. It is fed by its own arteries, the coronary arteries, which branch off the aorta just above the aortic valve. They fill mostly during diastole, when the heart muscle relaxes and stops squeezing them, so a very fast heart rate, which shortens diastole, also shortens the time the heart has to feed itself.

• The left main coronary artery is short and divides into two large branches.
• The left anterior descending artery runs down the front of the heart. It supplies the front of the left ventricle, most of the septum and usually the apex. Diagonal branches come off it toward the lateral wall.
• The circumflex artery wraps around the left side of the heart in the groove between the left atrium and ventricle. It supplies the lateral wall of the left ventricle and, in some people, part of the back.
• The right coronary artery runs around the right side. It supplies the right ventricle and, in most people, the inferior wall of the left ventricle. In most people it also supplies the SA node and the AV node.

The conduction system

Heart muscle contracts when an electrical impulse spreads through it, and the conduction system is the specialized tissue that starts the impulse and carries it in the right order. Several parts of it can start a beat on their own, each at its own commonly taught intrinsic rate, so if a higher pacemaker fails, a lower one can take over at a slower rate.

1. The sinoatrial (SA) node, in the wall of the right atrium near where the superior vena cava enters, is the heart's normal pacemaker. Its commonly taught intrinsic rate is 60 to 100 beats per minute.
2. The impulse spreads across both atria, which contract. On the ECG this is the P wave.
3. The atrioventricular (AV) node, low in the right atrium near the septum, receives the impulse and holds it briefly. That delay gives the atria time to finish emptying into the ventricles before the ventricles contract, and it is most of the PR interval. If the SA node fails, the area around the AV node, the junction, can take over at about 40 to 60 beats per minute.
4. The bundle of His carries the impulse from the AV node into the septum.
5. The right and left bundle branches carry it down each side of the septum. The left bundle branch divides further to reach the larger left ventricle.
6. The Purkinje fibers spread the impulse through the ventricular muscle so the ventricles contract from the apex upward, squeezing blood toward the valves. On the ECG this is the QRS complex. The ventricles can pace themselves at about 20 to 40 beats per minute if nothing above them does.
7. The ventricles then recover electrically, which on the ECG is the T wave.

Electrical activity and mechanical pumping are not the same thing. The monitor shows the electricity; only a pulse check tells you whether the heart is moving blood. A rhythm on the screen can come with no pulse at all.

Practice naming rhythms on a lead II strip

Why this matters in the field

Which part of the heart an infarct affects

A heart attack is heart muscle dying, usually because the artery that feeds it is blocked. Which artery is blocked decides which muscle is at risk, and that predicts what else may go wrong. The 12-lead shows the region; the anatomy tells you what that region does.

• An anterior or septal infarct, usually the left anterior descending, puts a large part of the left ventricle at risk. The bigger the area of the pump that stops working, the more the output can fall, toward cardiogenic shock.
• An inferior infarct, usually the right coronary, raises two questions: whether the right ventricle is involved too, and whether the SA or AV node is affected. Because the right coronary supplies both nodes in most people, slow rhythms and AV blocks are a known complication of inferior infarcts.
• A posterior infarct is easy to miss because no standard lead looks directly at the back of the heart; it shows up in reverse, as ST depression in V1 to V3, often with tall R waves and upright T waves; posterior leads V7 to V9 show it directly.

Right ventricular infarction and preload

When the right ventricle is infarcted it cannot push enough blood through the lungs, so the left ventricle is underfilled and the cardiac output falls even though the left ventricle itself may be working. The patient's output then depends heavily on preload, the volume of blood returning to the heart and stretching the ventricle before it contracts. The commonly taught picture is low blood pressure with distended neck veins and clear lungs, although not every patient shows all three. Right-sided leads such as V4R look for it directly.

The link to cardiac output and shock

Cardiac output is the heart rate multiplied by the stroke volume, and stroke volume depends on preload, afterload and contractility. The anatomy maps onto each of those. A damaged left ventricle loses contractility. A right ventricular infarct cuts the filling of the left ventricle; a tension pneumothorax or cardiac tamponade blocks blood returning to and filling the heart, which the Perfusion and Shock guide classes as obstructive shock. A rhythm that is too slow lowers the rate, and one that is too fast shortens the filling time and the time the coronary arteries have to fill. Our Perfusion and Shock guide covers cardiac output and the types of shock in detail.

Which side fails also decides where the blood backs up. When the left ventricle fails, blood backs up into the lungs, and the patient becomes short of breath with crackles. That is the fluid backing up that the Perfusion and Shock guide describes in cardiogenic shock, but not every patient with it is in shock, and a patient in shock from a right ventricular infarct can have clear lungs. When the right ventricle fails, blood backs up into the body's veins instead, which shows as distended neck veins and, over time, swelling of the legs.

Calculate the shock index from a heart rate and a systolic pressure

The short version

• Four chambers: two atria receive blood, two ventricles pump it out. The left ventricle has the thickest wall.
• Four valves: tricuspid and pulmonary on the right, mitral and aortic on the left.
• Blood flow: vena cava, right atrium, tricuspid valve, right ventricle, pulmonary valve, pulmonary arteries, lungs, pulmonary veins, left atrium, mitral valve, left ventricle, aortic valve, aorta.
• The coronary arteries come off the aorta and fill mostly in diastole. The left anterior descending feeds the front and septum, the circumflex the lateral wall, and the right coronary the right ventricle and, in most people, the inferior wall and both nodes.
• Conduction: SA node, atria, AV node, bundle of His, bundle branches, Purkinje fibers. Commonly taught intrinsic rates are 60 to 100, 40 to 60 and 20 to 40.
• An inferior infarct raises the questions of right ventricular involvement and slow rhythms. A right ventricular infarct makes the patient preload dependent.