Lesson 7 of 12 · 7 min
The human heart
NCERT §15.3.1
At the finish line Meher puts two fingers on her wrist and counts her pulse. Something in her chest is setting that beat without her having to think about it. What is it, and where?
The lesson in notes
In short
The human circulatory (blood vascular) system has a muscular chambered heart, a closed network of branching blood vessels, and blood, the fluid that circulates.
The heart develops from mesoderm and sits in the thoracic cavity between the two lungs, tilted slightly to the left. It is about the size of a clenched fist and is enclosed by a double-walled membranous bag, the pericardium, containing pericardial fluid.
It has four chambers: two smaller upper atria and two larger lower ventricles. A thin muscular inter-atrial septum separates the atria; a thick-walled inter-ventricular septum separates the ventricles; a thick fibrous atrio-ventricular septum separates each atrium from the ventricle below it, with an opening connecting the two on each side.
The right atrio-ventricular opening is guarded by the tricuspid valve (three muscular flaps or cusps); the left by the bicuspid or mitral valve. The openings of the right and left ventricles into the pulmonary artery and aorta carry semilunar valves.
The valves let blood flow only one way, atria to ventricles and ventricles to the pulmonary artery or aorta, and prevent backflow.
The whole heart is cardiac muscle, and the ventricle walls are much thicker than the atrial walls.
Nodal tissue, a specialised cardiac musculature, is spread through the heart. The sino-atrial node (SAN) lies in the right upper corner of the right atrium; the atrio-ventricular node (AVN) lies in the lower left corner of the right atrium, near the atrio-ventricular septum.
From the AVN, the atrio-ventricular bundle (AV bundle) passes through the atrio-ventricular septa, emerges on top of the inter-ventricular septum and divides into right and left bundles, which give rise to fine Purkinje fibres through the ventricular muscle of each side.
Nodal musculature is autoexcitable: it produces action potentials without any outside stimulus. The SAN produces the most, 70–75 per minute, so it starts and maintains the heart's rhythm and is called the pacemaker.
The heart normally beats 70–75 times a minute, averaging 72 beats per minute.