The human heart and its nodal tissue

The size and the place of the heart

Make a fist. Your heart is about the same size. It lies in the chest, in the thoracic cavity, between the two lungs. It is tilted a little to the left.

The heart is covered by a double walled bag, the pericardium. The space between the two walls holds the pericardial fluid. The fluid lets the heart move without friction. The heart is made of cardiac muscle.

The four chambers

• The two upper chambers are the atria. They are small and have thin walls.
• The two lower chambers are the ventricles. They are larger and have thick walls. The wall of the left ventricle is thicker than the wall of the right ventricle, because it pumps blood to the whole body.
• A thin muscular wall, the inter-atrial septum, separates the two atria.
• A thick wall, the inter-ventricular septum, separates the two ventricles.
• The atrium and the ventricle of the same side are separated by the atrio-ventricular septum. A door in it lets the blood pass.

The vessels that join the heart

The valves

A valve lets the blood pass in one direction only.

• The door between the right atrium and the right ventricle has the tricuspid valve, with three flaps. NCERT calls them muscular flaps. In fact the flaps are made of tough connective tissue, and cords from small muscles of the ventricle hold them.
• The door between the left atrium and the left ventricle has the bicuspid valve, also called the mitral valve, with two flaps.
• The mouths of the pulmonary artery and of the aorta have semilunar valves.

So the blood goes from atrium to ventricle, and from ventricle to artery, and never back.

You should find that when the ventricle is relaxed, the pressure in the atrium is higher, and the valve between them is open. When the ventricle pumps hard, the pressure rises above that of the atrium and the artery. The first valve then shuts and the second one opens.

The nodal tissue

The heart beats without any order from the brain. It has special cardiac muscle, the nodal tissue, that can produce an action potential by itself. This is called being autoexcitable.

• The sino-atrial node (SAN) is in the upper right corner of the right atrium.
• The atrio-ventricular node (AVN) is in the lower left corner of the right atrium, near the atrio-ventricular septum.
• The AV bundle starts at the AVN, passes through the atrio-ventricular septum and comes out at the top of the inter-ventricular septum. There it divides into a right bundle and a left bundle.
• The bundles give rise to very fine Purkinje fibres in the walls of the ventricles.
• The bundles together with the Purkinje fibres are called the bundle of His.

The SAN makes the largest number of action potentials, 70 to 80 in a minute. So it sets the pace of the heart, and it is called the pacemaker. The heart beats about 72 times a minute.

If the SAN fails, the next part takes over at its own, slower rate. The AVN sets about 40 to 60 beats a minute, and the Purkinje fibres about 15 to 40.

Try it in Python

The program shows which valves are open for the pressures in a chamber, in mm Hg.

Practise with new numbers

Common mistakes

• Putting the tricuspid valve on the left. It is on the right. The bicuspid valve is on the left.
• Saying that the wall of the right ventricle is the thicker one. The left ventricle has the thicker wall.
• Placing the SAN in the ventricle. It is in the right atrium, at the upper right corner.
• Saying that the brain starts every beat. The nodal tissue starts the beat by itself.
• Mixing up the bundle of His. It is the right and left bundles with the Purkinje fibres.

Remember these

• The heart: in the chest between the lungs, tilted to the left, in a pericardium with pericardial fluid. Four chambers.
• Tricuspid valve on the right, bicuspid (mitral) valve on the left, semilunar valves at the pulmonary artery and the aorta.
• SAN: upper right corner of the right atrium, the pacemaker, 70 to 80 a minute. AVN: lower left corner of the right atrium. Then the AV bundle, the right and left bundles and the Purkinje fibres.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Name the valves between the atria and the ventricles, and say which side each is on.

2. Why is the wall of the left ventricle thicker than that of the right ventricle?

3. Where are the SAN and the AVN placed?

4. Why is the SAN called the pacemaker?

5. Write the parts of the nodal tissue in the order in which the signal passes through them.

References

• Hall J. E. and Hall M. E. (2021). Guyton and Hall Textbook of Medical Physiology, 14th edition, Elsevier, chapter 10, Rhythmical Excitation of the Heart.
• Betts J. G. et al. (2022). Anatomy and Physiology 2e, OpenStax, chapter 19, The Cardiovascular System: The Heart.
• NCERT (2023). Biology, Textbook for Class XI, chapter 15.