Chapter 19 · The cardiovascular system · Topic 96

Heart valves, septa and internal structure

A&P IIanatomyRead the notes

1Why this matters

Mrs. Okafor, 71, suddenly becomes short of breath two days after chest pain. Her doctor hears a new whooshing sound with every beat. An ultrasound shows why: one of the small muscles that holds the cords of her mitral valve has torn, and the valve now flips backward into the left atrium each time the left ventricle squeezes.

2What this builds on

3Quick check before you start

1. Which layer of the heart wall lines the inside of the heart and covers its valves?

  1. Epicardium
  2. Myocardium
  3. Endocardium
Show the answer

The endocardium is the endothelial lining of every space inside the heart, and it covers the valve cusps. The epicardium covers the outside, and the myocardium is the muscle in between.

  • Epicardium:
  • Myocardium:
  • Correct: Endocardium:

2. Which chamber do the pulmonary veins empty into?

  1. The right atrium
  2. The left atrium
  3. The left ventricle
Show the answer

The pulmonary veins bring blood back from the lungs into the left atrium. The venae cavae empty into the right atrium, and the left ventricle receives blood from the left atrium and sends it out through the aorta.

  • The right atrium:
  • Correct: The left atrium:
  • The left ventricle:

3. Blood flows through a tube from region A to region B. What must be true?

  1. Pressure is higher at B than at A
  2. Pressure is higher at A than at B
  3. Pressure is equal at A and B
Show the answer

Fluid flows down a pressure gradient, from higher pressure to lower pressure. With no pressure difference, there is no flow.

  • Pressure is higher at B than at A:
  • Correct: Pressure is higher at A than at B:
  • Pressure is equal at A and B:

4Anatomy

A front view of the heart cut open in a frontal plane. It shows the two upper and two lower chambers, the wall between the lower chambers, the valves between upper and lower chambers held by thin cords to small muscles on the lower chamber walls, the valves at the exits to the two large arteries, the large veins entering, and the thick muscular wall of the lower left chamber.
The heart opened in a frontal plane. Hide the labels and name each valve and septum first, then the chambers and great vessels around them. OpenStax Anatomy and Physiology 2e, Figure 19.9, openstax.org, CC BY 4.0.

With labels hidden, select a box to reveal its label.

5How it works, step by step

  1. The ventricle muscle contracts, raising the pressure of the blood inside the ventricle.Blood pushes up on the cusps of the atrioventricular valve, and the cusps meet and seal the opening.
  2. The papillary muscles, part of the same ventricle wall, contract along with it.They pull the chordae tendineae tight.
  3. The taut chordae tendineae hold the cusp edges down.The cusps cannot flip up into the atrium, so blood does not flow backward into the atrium.
  4. With the atrioventricular valve sealed, ventricular pressure keeps rising until it is higher than the pressure in the artery.Blood presses the semilunar valve's cusps flat against the artery wall and flows out into the artery.
  5. When the ventricle relaxes and blood starts to slip back toward it, blood fills the semilunar cusps' pockets.The three pockets bulge together and seal, so blood does not flow back into the ventricle.

6Core concepts

Structure and function

7A common mistake

The wrong idea: The left ventricle has a thicker wall because it pumps more blood than the right ventricle.

What actually happens: On average, both ventricles pump the same volume of blood each beat. The left wall is thicker because the vessels of the whole body resist flow far more than the vessels of the lungs, so the left ventricle has to generate about five times the pressure (about 120 versus 25 mm Hg). Heart muscle thickens when it works against a heavier load.

8Check yourself

Anything you miss goes into your review queue.

1. Name the pinned valve.

  1. Tricuspid valve
  2. Aortic valve
  3. Pulmonary valve
  4. Mitral valve
Show the answer

The valve between the left atrium and the left ventricle is the mitral valve (also called the bicuspid valve). It has two cusps.

  • Tricuspid valve: The tricuspid valve sits between the right atrium and the right ventricle, on the other side of the heart.
  • Aortic valve: The aortic valve sits at the exit from the left ventricle into the aorta, not between the left atrium and ventricle.
  • Pulmonary valve: The pulmonary valve sits at the exit from the right ventricle into the pulmonary trunk.
  • Correct: Mitral valve: Correct. This is the mitral valve, the left atrioventricular valve.

2. Name the pinned structure.

  1. Interatrial septum
  2. Atrioventricular septum
  3. Interventricular septum
  4. Trabeculae carneae
Show the answer

The interventricular septum is the thick, mostly muscular wall between the right and left ventricles. It runs from the valves down toward the apex.

  • Interatrial septum: The interatrial septum is the thin wall between the two atria, higher up in the heart.
  • Atrioventricular septum: The atrioventricular septum is the region between the atria and the ventricles, where the valve openings pass through. It runs across the heart, not down between the ventricles.
  • Correct: Interventricular septum: Correct. The thick wall between the two ventricles is the interventricular septum.
  • Trabeculae carneae: Trabeculae carneae are ridges of muscle on the inner walls of the ventricles, not the wall that divides them.

3. Order the parts of the mitral valve apparatus, starting at the opening between the left atrium and left ventricle and ending in the ventricle wall.

  1. Valve cusps
  2. Chordae tendineae
  3. Papillary muscle
  4. Myocardium of the ventricle wall
Show the answer

The cusps hang into the ventricle, the chordae tendineae run from the cusp edges down to the papillary muscles, and the papillary muscles rise from the ventricle's myocardium.

  • Correct order: 1. Valve cusps 2. Chordae tendineae 3. Papillary muscle 4. Myocardium of the ventricle wall

4. Mrs. Okafor, 71, tears one of the papillary muscles in her left ventricle. What happens each time her left ventricle contracts?

  1. The mitral valve cannot open, so blood cannot enter the ventricle
  2. Mitral cusps flip into the left atrium and blood leaks backward
  3. The aortic valve flips back into the ventricle and blood leaks in
  4. The tricuspid valve closes too early, so the right atrium overfills
Show the answer

Papillary muscles contract with the ventricle and pull the chordae tendineae tight, which stops the mitral cusps flipping up into the atrium. With one torn, the cusps it held swing up when the ventricle squeezes, and blood leaks backward into the left atrium.

  • The mitral valve cannot open, so blood cannot enter the ventricle: Papillary muscles do not open the valve. Blood pushes the cusps open. Their job is to keep the closed cusps from flipping up, so losing one causes backward leak, not a stuck valve.
  • Correct: Mitral cusps flip into the left atrium and blood leaks backward: Correct. Without the anchor, the cusps flip into the left atrium and blood flows backward with every contraction.
  • The aortic valve flips back into the ventricle and blood leaks in: The aortic valve is a semilunar valve with no papillary muscles, so a torn papillary muscle does not affect it directly.
  • The tricuspid valve closes too early, so the right atrium overfills: The tricuspid valve is on the right side of the heart. Mrs. Okafor's torn muscle is in the left ventricle.

5. The left ventricle wall is about three times as thick as the right ventricle wall. What best explains this?

  1. The left ventricle pumps a larger volume of blood with each beat
  2. Blood leaving the left ventricle has to travel a longer distance
  3. The blood in the left ventricle is thicker than the blood in the right
  4. The left ventricle has to generate about five times higher pressure
Show the answer

The vessels of the whole body resist flow far more than those of the lungs. To push the same volume through them, the left ventricle must raise pressure to about 120 mm Hg, versus about 25 mm Hg on the right. Muscle that works against a heavier load thickens.

  • The left ventricle pumps a larger volume of blood with each beat: On average, both ventricles pump the same volume each beat. If they did not, blood would pile up on one side within minutes.
  • Blood leaving the left ventricle has to travel a longer distance: Distance alone is not the cause. What sets the work is the pressure needed to overcome the resistance of the vessels.
  • The blood in the left ventricle is thicker than the blood in the right: Blood on both sides is the same blood, with the same thickness. It passes through the right side first and the left side next.
  • Correct: The left ventricle has to generate about five times higher pressure: Correct. The higher pressure load on the left ventricle is what thickens its wall.

6. Fill the gap in the right side of the heart: right atrium → ____ → right ventricle → pulmonary valve → pulmonary trunk.

  1. Tricuspid valve
  2. Mitral valve
  3. Aortic valve
  4. Interatrial septum
Show the answer

The tricuspid valve is the right atrioventricular valve. It lets blood pass from the right atrium down into the right ventricle, which then pumps out through the pulmonary valve into the pulmonary trunk.

  • Correct: Tricuspid valve: Correct. The tricuspid valve sits between the right atrium and the right ventricle.
  • Mitral valve: The mitral valve is the left atrioventricular valve, between the left atrium and the left ventricle.
  • Aortic valve: The aortic valve guards the exit from the left ventricle into the aorta, on the other side of the heart.
  • Interatrial septum: The interatrial septum is the wall between the two atria. Blood does not pass through it on its way to the right ventricle.

7. The electrical signal that makes the atria contract cannot spread directly into the ventricle muscle. Which structure is responsible?

  1. The interventricular septum
  2. The cardiac skeleton
  3. The pericardial sac
  4. The chordae tendineae
Show the answer

The cardiac skeleton is dense collagen lying between the atria and the ventricles. Collagen does not conduct electrical signals like muscle does, so the signal can cross only at one specialized pathway.

  • The interventricular septum: The interventricular septum separates the right and left ventricles, so it lies between the two pumping chambers rather than between the atria and the ventricles.
  • Correct: The cardiac skeleton: Correct. The dense connective tissue of the cardiac skeleton insulates the atria from the ventricles.
  • The pericardial sac: The pericardial sac surrounds the whole heart on the outside. It does not sit between the atria and the ventricles.
  • The chordae tendineae: The chordae tendineae are cords inside the ventricles that hold valve cusps. They do not block electrical signals between chambers.

9Summary

Building on the four chambers and great vessels: three septa divide the chambers, and the fossa ovalis marks the right atrial side of the interatrial septum. The tricuspid and mitral valves sit between atria and ventricles and are held by chordae tendineae and papillary muscles; the pulmonary and aortic semilunar valves guard the exits into the great arteries and close by filling like pockets. Pectinate muscles line the auricles and trabeculae carneae ridge the ventricles. The left ventricle wall is thickest because it works against the highest pressure. The cardiac skeleton holds the valve openings in shape, anchors the muscle and stops electrical signals crossing straight from atria to ventricles.

10What comes next

11Connections