The signal of the heart goes from the SAN to the ventricles…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
The signal of the heart goes from the SAN to the ventricles.
- In a patient the SAN does not work. Which part of the nodal tissue will set the pace, and will the rate be higher or lower than normal?
- The AV bundle is cut. What happens to the contraction of the atria, and what happens to that of the ventricles?
- Why is the atrio-ventricular node needed between the atria and the ventricles?
Need a reminder? The key ideaShow
The heart has four chambers, the tricuspid valve on the right, the bicuspid (mitral) valve on the left, and semilunar valves at the pulmonary artery and the aorta. A valve opens when the pressure behind it is higher than the pressure in front of it. The SAN is the pacemaker, at about beats a minute. The signal goes SAN, AVN, AV bundle, right and left bundles, Purkinje fibres.
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. (a) The AVN takes over. Its rate is lower than that of the SAN, so the heart beats more slowly.
Step 2. (b) The atria still contract, because the SAN works. The signal cannot reach the ventricles by the bundle. The ventricles then beat on their own, from the Purkinje fibres, at a much slower rate and not in step with the atria.
Step 3. (c) The AVN and the AV bundle pass the signal from the atria to the ventricles. The atria and the ventricles are separated by the atrio-ventricular septum, so this is the only link.
Answer: (a) the AVN, slower; (b) the atria keep their rhythm, the ventricles beat slowly and out of step; (c) it is the only link.
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