Heart anatomy and blood flow

1. 1

   Step 1: Superior and inferior venae cavaeRight side

   Oxygen-poor blood comes back from the body: the superior vena cava brings it from everything above the diaphragm (head, neck, arms and chest), the inferior vena cava from everything below it. A third vessel, the coronary sinus, adds the blood that has just fed the heart wall. All three open into the right atrium.

2. 2

   Step 2: Right atriumRight side

   The thin-walled receiving chamber. While the ventricles are relaxed the tricuspid valve below it stands open, so most of the blood (about 70 to 80 percent of what the ventricle will hold) flows straight through without any pumping.

3. 3

   Step 3: Tricuspid valve (right atrioventricular valve)Right side

   Three cusps between the right atrium and right ventricle. The right atrium then contracts (atrial systole) and pushes the last 20 to 30 percent of the blood through it to finish filling the ventricle.

4. 4

   Step 4: Right ventricleRight side

   The ventricle contracts. Its rising pressure pushes the tricuspid cusps shut; the chordae tendineae, tightened by the papillary muscles, keep the cusps from flipping back into the atrium. The two atrioventricular valves close at the same moment, and that is the first heart sound, S1, the "lub".

5. 5

   Step 5: Pulmonary semilunar valveRight side

   Once the pressure in the right ventricle is higher than the pressure in the pulmonary trunk, the three cusps of the pulmonary valve open and blood is ejected. Semilunar valves have no chordae tendineae or papillary muscles.

6. 6

   Step 6: Pulmonary trunk and pulmonary arteriesRight side

   The pulmonary trunk divides into the right and left pulmonary arteries, one to each lung. These are the exception to the rule that arteries carry oxygen-rich blood: they carry oxygen-poor blood away from the heart.

7. 7

   Step 7: Pulmonary capillaries (the lungs)Lungs

   Gas exchange: carbon dioxide leaves the blood, oxygen enters it. Back at the heart, as the right ventricle relaxes and its pressure drops below the pressure in the pulmonary trunk, the pulmonary valve closes; with the aortic valve closing at the same time, that is the second heart sound, S2, the "dub".

8. 8

   Step 8: Pulmonary veinsLungs

   Four pulmonary veins carry the oxygen-rich blood from the lungs to the left atrium. They are the other exception: veins that carry oxygen-rich blood toward the heart.

9. 9

   Step 9: Left atriumLeft side

   The left receiving chamber. As on the right, the mitral valve below it is open while the ventricle relaxes, and most of the filling is passive.

10. 10

    Step 10: Mitral valve (bicuspid, left atrioventricular valve)Left side

    Two cusps between the left atrium and left ventricle. The left atrium contracts with the right one and tops up the left ventricle through it.

11. 11

    Step 11: Left ventricleLeft side

    The thickest-walled chamber contracts. The mitral cusps are pushed shut (S1 again, together with the tricuspid), held by their own chordae tendineae and two papillary muscles. The left ventricle pumps the same volume per beat as the right one, but at a much higher pressure, because the systemic circuit is longer and offers more resistance.

12. 12

    Step 12: Aortic semilunar valveLeft side

    When the pressure in the left ventricle rises above the pressure in the aorta, the three cusps of the aortic valve open and blood is ejected into the ascending aorta. It closes as the ventricle relaxes (S2, with the pulmonary valve).

13. 13

    Step 13: Aorta and the coronary arteriesLeft side

    Just above the aortic valve, at the aortic sinuses, the right and left coronary arteries leave the aorta: the heart wall is the first tissue the freshly oxygenated blood reaches. The rest continues up the ascending aorta, around the aortic arch and down the descending aorta.

14. 14

    Step 14: Systemic arteries and arteriolesBody

    The aorta branches again and again into the arteries and arterioles that reach every organ. This is the systemic circuit, driven by the left side of the heart.

15. 15

    Step 15: Systemic capillariesBody

    Oxygen and nutrients leave the blood for the tissues; carbon dioxide and wastes come in. The blood is now oxygen-poor again.

16. 16

    Step 16: Venules and systemic veins, back to the venae cavaeBody

    The capillaries join into venules, then veins, and finally the superior and inferior venae cavae, which return the blood to the right atrium. The loop closes: in each beat the right side sends the same volume to the lungs as the left side sends to the body.

Short form to memorize: venae cavae → right atrium → tricuspid valve → right ventricle → pulmonary valve → pulmonary trunk and arteries → lungs → pulmonary veins → left atrium → mitral valve → left ventricle → aortic valve → aorta → body → venae cavae.

Location and size

Where it sits

In the mediastinum, the middle of the thoracic cavity between the lungs, inside the pericardium.

Base and apex

The broad base (superior end) lies at the level of the third costal cartilage. The apex (inferior tip) points down and to the left, just left of the sternum at the level where the fourth and fifth ribs meet their costal cartilages.

Size

About the size of a fist: roughly 12 cm long, 8 cm wide and 6 cm thick. About 250 to 300 g in an adult female, 300 to 350 g in an adult male.

The three layers of the heart wall

1. 1 · Outer layer

   Epicardium

   Simple squamous mesothelium over areolar connective tissue

   The visceral pericardium, the surface of the heart. The major coronary vessels run on that surface, in the grooves (sulci) between the chambers.

2. 2 · Middle layer, by far the thickest

   Myocardium

   Cardiac muscle cells on a framework of collagen

   Does the pumping. The fibers swirl around the chambers in a figure-8 pattern, which pumps blood more effectively than a simple straight-line arrangement would.

3. 3 · Inner lining

   Endocardium

   Simple squamous endothelium over connective tissue

   Lines the chambers and covers the valves; continuous with the endothelium of the vessels that enter and leave the heart.

The pericardium

• Fibrous pericardiumThe outer sac of tough dense connective tissue that holds the heart in place.
• Parietal pericardiumThe outer layer of the serous pericardium, fused to the inside of the fibrous sac.
• Visceral pericardium (the epicardium)The inner layer of the serous pericardium, fused to the heart itself. It is the same structure as the outer layer of the heart wall.
• Pericardial cavityThe space between the parietal and visceral layers, holding a thin film of serous fluid so the beating heart slides without friction.

The four chambers

The four valves

• The two atrioventricular valves (tricuspid and mitral) are tethered: chordae tendineae run from the free edge of each cusp to papillary muscles in the ventricle wall. When the ventricle contracts, the papillary muscles tighten the cords so the cusps meet in the middle instead of flipping back into the atrium.
• The two semilunar valves (pulmonary and aortic) have three pocket-shaped cusps and no cords or papillary muscles. Blood flowing backward fills the pockets and pushes them together, which is what closes the valve.
• A cusp is a flap of endocardium reinforced with connective tissue, with no muscle of its own: the pressure on its two sides decides which way it swings.

1. 1

   1. Sinoatrial (SA) node

   Superior and posterior wall of the right atrium, near the opening of the superior vena cava

   The pacemaker. It reaches threshold fastest, so its rhythm wins. The impulse spreads across both atria through the atrial muscle and the internodal pathways (and to the left atrium through Bachmann’s bundle), and the atria contract from top to bottom.

   Own rate:

   About 80 to 100 a minute (the resting rate is lower because the vagus nerve slows it)

   Timing:

   About 50 ms to reach the AV node

2. 2

   2. Atrioventricular (AV) node

   Lower part of the right atrium, in the atrioventricular septum

   The only normal electrical doorway from the atria to the ventricles: the atrioventricular septum keeps the impulse from spreading to the ventricles any other way. The node slows the impulse on purpose, so the atria finish pushing blood into the ventricles before the ventricles contract.

   Own rate:

   40 to 60 a minute

   Timing:

   About 100 ms to pass through the node

3. 3

   3. Atrioventricular bundle (bundle of His)

   Top of the interventricular septum

   Carries the impulse out of the AV node and into the septum, then splits in two.

   Own rate:

   About 30 to 40 a minute

4. 4

   4. Right and left bundle branches

   Down each side of the interventricular septum toward the apex

   Each branch serves its own ventricle, and both run down to the apex before handing the impulse to the Purkinje fibers, so the bottom of the heart is reached first.

   Own rate:

   20 to 30 a minute

   Timing:

   About 25 ms from the AV node, through the bundle and both branches, to the Purkinje fibers at the apex

5. 5

   5. Purkinje fibers

   Spread through the ventricular myocardium from the apex upward

   Fast fibers that reach every ventricular muscle cell. Because they start at the apex, the ventricles contract from the apex toward the base, squeezing blood up and out through the semilunar valves.

   Own rate:

   15 to 20 a minute

   Timing:

   About 75 ms to reach all the ventricular muscle; the textbook puts the whole trip from the SA node at about 225 ms

Short form: SA node → AV node → atrioventricular bundle (bundle of His) → right and left bundle branches → Purkinje fibers.

• Cardiac muscle cells are joined end to end by intercalated discs: desmosomes hold the cells together under the strain of contraction, and gap junctions let ions pass straight from cell to cell, so a whole chamber contracts almost as one unit.
• On an ECG the P wave is the atria depolarizing, the QRS complex is the ventricles depolarizing (the atria repolarize at the same time, hidden inside it), and the T wave is the ventricles repolarizing. The atria begin contracting about 25 ms after the P wave starts; the ventricles begin contracting at about the peak of the R wave.

1. 1 Atrial systole

   About 100 ms

   AV valves open, semilunar valves closed

   Both atria contract and push the last 20 to 30 percent of the blood into the ventricles (the ventricles are already 70 to 80 percent full from passive flow). The ventricles now hold their end-diastolic volume, about 130 mL in a resting adult.

2. 2 Ventricular systole · isovolumic contraction

   First part of the 270 ms of ventricular systole

   All four valves closed

   The ventricles start to contract. Pressure rises, the AV valves slam shut (S1), but the semilunar valves are not open yet, so for a moment the ventricles squeeze a fixed volume.

3. 3 Ventricular systole · ejection

   Rest of the 270 ms

   Semilunar valves open, AV valves closed

   Ventricular pressure passes the pressure in the pulmonary trunk and the aorta, the semilunar valves open and the ventricles eject the stroke volume, normally 70 to 80 mL each. About 50 to 60 mL stays behind (the end-systolic volume).

4. 4 Ventricular diastole · isovolumic relaxation

   First part of the 430 ms of ventricular diastole

   All four valves closed

   The ventricles relax and their pressure falls. Blood in the great vessels starts back toward the heart and fills the semilunar pockets, closing them (S2).

5. 5 Ventricular diastole · filling

   Rest of the 430 ms

   AV valves open, semilunar valves closed

   Ventricular pressure drops below atrial pressure, the AV valves open and blood that has been pooling in the atria flows passively into the ventricles. Then the atria contract and the cycle starts over.

Add the three up and you get about 800 ms (100 + 270 + 430), which works out to 75 beats a minute; that sum is our arithmetic, not a figure the textbook states. Both sides of the heart run the same cycle at the same time: the right and left atria contract together, then the right and left ventricles contract together.

The heart sounds

Listening on the chest (auscultation) uses four spots, and a different valve is heard best at each one. The textbook also describes a third and a fourth sound; this page covers the two that every cardiac cycle makes.

• Left alone, the SA node would run at about 100 beats a minute (section 19.4; section 19.2 gives 80 to 100); the resting rate is lower because the cardioinhibitory center in the medulla oblongata keeps a steady parasympathetic brake on it through the vagus nerve (acetylcholine).
• The cardioaccelerator center raises the rate through sympathetic nerves (norepinephrine), which speed the SA node’s depolarization and strengthen contraction.

For study only. These are the textbook’s resting values, written to explain the formulas, not a measure of anyone’s heart.

8 · Label the heart

Label the schematic

The same schematic as part 2, with the names taken off. Match each number to a structure from the list, then check. Every row is marked, and the labeled version in part 2 is the answer key.

9 · Practice questions

20 practice questions

The flow path, the chambers and valves, the wall layers, the coronary vessels, the conduction order, the heart sounds and the cardiac output formula, multiple choice, one at a time. Every answer is explained, and your score is free: no sign-up needed.