First 369 words of the document:
Control of the Cardiac Cycle
The start of a Heartbeat
At the top of the right atrium, there is a
small patch of tissue known as the Sino
Atrial node (the SAN). At regular intervals,
the SAN generates a wave of excitation. In
humans this occurs around 55-80 times a
The Contraction of the Atria
The wave of excitation produced by the SAN travels down the walls of the atria,
moving through the membranes of the muscle tissue. As this wave of excitation
passes, it causes the cardiac muscle cells to contract. This is called Atrial Systole.
At the base of the atria, there is a disc of tissue separating them from the ventricles.
This disc of tissue does not conduct the electrical excitation caused by the SAN. To
conduct the wave of excitation to the ventricles, there is another node at the top of the
inter-ventricular septum. This is called the atrioventricular node (AVN) and is the
only way to conduct the wave of excitation through the disc of tissue. This node first
delays the wave of excitation, allowing the ventricles to fill with blood and the atria to
The Contraction of the Ventricles
After the delay caused by the AVN, the wave of excitation is carried from the AVN, via
special conducting tissue, which runs down the inter-ventricular septum. This tissue
is called the Purkyne Tissue. This wave travels through the Purkyne tissue down the
septum to the apex (the base of the heart) and up along the walls of the ventricles. As
the wave of excitation travels up the walls of the ventricles, it causes the muscle to
contract. This means that the ventricles contract from the base upwards, pushing the
blood out into the major arteries at the top of the heart.
HOW TO REMEMBER
An easy way to remember the coordination of the heartbeat is using the acronym SAP.
Just remember that the electrical impulse travels from the SAN, down to the AVN, and
down through the Purkyne Tissue.