In class we discussed the functions of the atria as essentially blood reservoirs

Question

In class we discussed the functions of the atria as essentially blood reservoirs and primer pumps for the ventricles. Thus, the ventricles are "forcibly filled" slightly by mild atrial contractions. Given what you know about the molecular "micromechanics" of muscle cell contraction, comment on how this atrial contraction might facilitate the subsequent contraction of the ventricle. Try to consider both the passive effects (e.g. natural cell elasticity) as well as cellular and molecular level mechanisms involved in generating contractile force when answering this question.

Explanation / Answer

The atrial contraction causes atrial pressure to increase. This pushes blood into the ventricles, raising the ventricular pressure slightly. Then ventricular excitation triggers ventricular contraction.

Atrial contraction normally accounts for 10% of left ventricular filling when a person is at rest because most of the ventricular filling occurs prior to atrial contraction as blood flows passively from pulmonary veins into left atrium, then into the left ventricle through open mitral or bicuspid valve.

At high heart rates when there is less time for passive ventricular filling, the atrial contraction may account for up to 40%of ventricular filling. This is sometimes referred to as the "atrial kick". The atrial contribution to ventricular filling varies inversely with duration of ventricular diastole and directly with atrial contractility.

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