Heart valves, septa and internal structure: Notes

Your heart is two pumps built side by side, each with a receiving chamber on top and a pumping chamber below. The heart valves are what make those pumps work: four one-way valves that keep blood moving forward. This page opens the heart to show the heart chambers and valves from inside: the walls between the chambers, the two kinds of valve and how each one seals, why the left ventricle has the thickest wall, and the fibrous skeleton that holds it all together. The next topic follows blood through these parts in order.

Recap: the chambers and great vessels

This topic builds on the chambers and great vessels from the previous topic. In brief:

The two atria sit on top and receive blood. The two ventricles sit below and pump it out.
The superior and inferior venae cavae empty into the right atrium, and the right ventricle sends blood out through the pulmonary trunk.
The pulmonary veins empty into the left atrium, and the left ventricle sends blood out through the aorta.
The auricles are ear-shaped pouches of the atria, and the coronary and interventricular sulci are the surface grooves that mark where the chambers meet.

Find each of these on the outside of the heart in Figure 1. Everything else on this page is about what lies between these chambers and vessels: the walls that divide them and the valves that connect them.

Two drawings of the outside of the heart. The front view shows the four chambers from the surface, the ear-shaped flaps of the upper chambers, the large vessels leaving the top, and surface vessels running in fat-filled grooves toward the tip. The back view shows the veins entering the upper chambers and the surface vessels on the back of the heart. — **Figure 1.** The outside of the heart from the front and the back. Find the auricles, the grooves between the chambers, and the great vessels at the base, then compare them with the inside view in Figure 2. OpenStax Anatomy and Physiology 2e, Figure 19.6, openstax.org, CC BY 4.0.

Inside the chambers

Cut the heart open in a frontal plane (Figure 2) and three things stand out. The chambers are separated by walls. Each atrium opens into the ventricle below it through a valve, and each ventricle opens into its great artery through another valve. And the inner walls look different in each chamber.

Pectinate muscles (pectin- = comb) are the comb-like ridges of muscle on the inner wall of the auricles and the front wall of the right atrium. The back wall of each atrium, where the great veins enter, is smooth.
Trabeculae carneae (trabecula = little beam; carne- = flesh) are the irregular ridges and bridges of muscle on the inner walls of both ventricles. A few larger cones of ventricle muscle rise out of them and hold the valve cords, which you will meet below.

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. — **Figure 2.** The heart cut open from the front. Trace each chamber, the walls between them, and the four valves. OpenStax Anatomy and Physiology 2e, Figure 19.9, openstax.org, CC BY 4.0.

The walls between the chambers

A septum (plural septa; saept- = fence) is a dividing wall. The heart has three septa of the heart.

The interatrial septum separates the right and left atria. It is thin.
The interventricular septum separates the right and left ventricles. It is thick and mostly muscle, with a small, thin fibrous part near the top.
The atrioventricular septum is the region between the atria and the ventricles. The openings from each atrium into its ventricle pass through it.

The fossa ovalis

On the right atrium's side of the interatrial septum you can see a shallow oval dip. This is the fossa ovalis (fossa = shallow pit, ovalis = oval). Its floor is thinner than the rest of the septum.

Two kinds of valves

A valve is a set of flaps, called cusps, that lets blood through in one direction only. Your heart has four. Two sit between each atrium and its ventricle. Two sit at the exits from the ventricles into the large arteries. Figure 3 shows all four from above.

The heart seen from above with the upper chambers and large vessels removed, so all four valves are visible in one plane. Two large valves sit at the back, one with three flaps on the right side and one with two flaps on the left. Two smaller round valves with three half-moon flaps each sit in front of them. — **Figure 3.** All four valves seen from above, with the atria and large vessels removed. The two atrioventricular valves lie behind; the two semilunar valves lie in front. OpenStax Anatomy and Physiology 2e, Figure 19.12, openstax.org, CC BY 4.0.

Atrioventricular valves

The atrioventricular valves (AV valves) sit between each atrium and the ventricle below it.

The tricuspid valve (tri- = three, cusp = point) is the right atrioventricular valve. It has three cusps and sits between the right atrium and the right ventricle.
The mitral valve (also called the bicuspid valve) is the left atrioventricular valve. It has two cusps (bi- = two), and its shape reminded early anatomists of a bishop's hat, a miter. It sits between the left atrium and the left ventricle. This course calls it the mitral valve from here on.

The cusps of each AV valve hang down into the ventricle. Their edges are tied to the ventricle wall by thin, strong cords of collagen called chordae tendineae (chord- = string, tendin- = tendon), often called "heart strings." Each cord runs from a cusp to a papillary muscle (papilla = nipple), a cone of muscle that rises from the ventricle wall.

Here is how they work together. When the ventricle contracts, blood pushes the cusps upward and they meet, sealing the opening. As the ventricle contracts, the papillary muscles, which are part of the ventricle wall, contract too. They pull the chordae tendineae tight. Taut cords stop the cusps from flipping up into the atrium, the way the lines of a parachute stop it from turning inside out.

Semilunar valves

The semilunar valves (semi- = half, lun- = moon) guard the exits from the ventricles.

The pulmonary valve sits between the right ventricle and the pulmonary trunk.
The aortic valve sits between the left ventricle and the aorta.

Each has three cusps shaped like half-moon pockets. When blood flows out of the ventricle, it presses the pockets flat against the artery wall. When blood starts to slip back toward the ventricle, it fills the pockets, and the three pockets bulge together and seal the opening. They need no cords and no muscles.

All four valves move only because blood pushes on them. No muscle opens or closes a valve. A later topic on the timing of each heartbeat explains exactly when each valve opens and closes.

	Atrioventricular (AV) valves	Semilunar valves
Names	Tricuspid (right) and mitral (left)	Pulmonary (right) and aortic (left)
Location	Between each atrium and its ventricle	Between each ventricle and its artery
Number of cusps	Three on the right, two on the left	Three each
Cusp shape	Flat flaps hanging into the ventricle	Half-moon pockets facing the artery
Held by cords and muscles?	Yes: chordae tendineae and papillary muscles	No
Stops blood flowing back into	The atrium	The ventricle

Why the left ventricle wall is thicker

Look at the two ventricles in Figure 4. The left ventricle wall is about three times as thick as the right, and its cavity is round. The right ventricle is a thin crescent wrapped around it. Yet on average both ventricles pump out the same volume of blood with each beat. So why the difference?

Figure 4. A slice across both ventricles. The left ventricle's thick wall surrounds a round cavity; the right ventricle is a thin crescent against the septum.

The answer is the pressure each ventricle has to produce.

The right ventricle pumps into the vessels of the lungs. They are short, wide and many, so they offer little resistance to flow. The right ventricle only has to push the pressure to about 25 mm Hg to move its blood.
The left ventricle pumps into the vessels of the entire rest of the body. Together they offer much more resistance. To push the same volume of blood through them, the left ventricle has to raise the pressure to about 120 mm Hg, roughly five times higher.
Heart muscle, like other muscle, grows thicker when it works against a heavier load for a long time. At birth the two ventricle walls are about equally thick. Over the first months of life, the left ventricle works against the higher pressure, and its wall thickens to match.

So the thick wall is not because blood travels farther or because the left ventricle moves more blood. It reflects the higher pressure the left ventricle works against, beat after beat. The atria have the thinnest walls of all, because they only have to raise the pressure a little to push blood into the relaxed ventricle right below them.

The cardiac skeleton

Between the atria and the ventricles lies a flat sheet of dense connective tissue called the cardiac skeleton (also called the fibrous skeleton of the heart). It is not bone. It is four rings of dense collagen, one around each valve, joined to each other and to the top of the interventricular septum.

It does three jobs, each following from its structure:

It holds the valves open to the right size. The firm rings keep each valve opening from stretching as pressure rises, so the cusps still meet.
It anchors the muscle. The muscle of the atria and of the ventricles attaches to it, giving the contracting muscle something firm to pull against.
It insulates the atria from the ventricles electrically. Collagen does not conduct electrical signals the way muscle does. So the signal that makes the atria contract cannot spread directly into the ventricles. It can only cross at one specialized pathway, which you will meet three topics from now, when you follow the heart's electrical signal from chamber to chamber. This forces the atria to contract first and the ventricles a moment later.

Summary

This topic builds on the four chambers and the great vessels: atria on top receive blood, ventricles below pump it out. Three septa divide the chambers: the thin interatrial septum, the thick interventricular septum and the atrioventricular septum between the atria and ventricles. The fossa ovalis is a shallow dip in the interatrial septum. The tricuspid and mitral valves, held by chordae tendineae and papillary muscles, stop backflow into the atria; the pulmonary and aortic semilunar valves close by filling like pockets and stop backflow into the ventricles. Pectinate muscles line the auricles, and trabeculae carneae ridge the ventricle walls. The left ventricle wall is thicker because it works against about five times the pressure of the right. The cardiac skeleton holds the valve openings in shape, anchors the muscle and insulates the atria from the ventricles.