Preparation for Lecture
Pumping Blood – Pages 679-715.
1. What area of the ventricles initiates contraction? Why is blood pumped most
efficiently when contraction begins there?
Based on the anatomy of the action potential conducting pathways, the area of
the ventricles nearest the apex contract first. Remember, of course, that atrial
contraction precedes ventricular contraction.
The fibers of the ventricular myocardium are oriented in a spiral fashion.
Contraction at the apex forces blood upwards in the ventricles toward the base
where the aortic and pulmonary semilunar valves are located. Initiation of
contraction anywhere else could lead to blood flowing in an inefficient direction,
that is, not toward the semilunar valves.
2. Predict the consequences of a heart attack that damages the tissue of the SA
node.
The SA node sets the normal pace of the heart. This pace is modified by
parasympathetic (slows pace) and sympathetic (increases pace) nervous control
and adrenal hormone. Other areas of the myocardium are autorhythmic, but the
SA dominates in normal hearts.
A damaged SA will lead to irregular pace or a slower pace set by the AV node or
bundle tissue. Consequences include inability of the heart to generate a normal
pace, reduced heart rate, and inefficient pumping.
3. Relate your knowledge of the structure and function of intercalated disks to
the rhythmic contraction of the myocardium. Tell me how these two concepts are
related.
Intercalated disks connect cardiac muscle fibers. They consist of desmosomes
for strong ‘connectability’ and gap junctions for propagation of action potential
cell to cell. Nodal tissue sets the pace of the heart with spontaneous action
potential that spreads across the atria, and then the ventricles, cell to cell (noting
the involvement of the bundle tissue and Purkinje fibers). Action potential is slow
in cardiac muscle and the result is contraction spreads across the myocardium
with its progression.
4. In one type of interatrial septal defect there is a failure of the fetal foramen
ovale (or as we called it in class, oval window) to close after birth. What
complications can we expect in the circulation of blood in this individual? Tell me
about the ability of this patient to maintain optimal blood oxygen levels.
