Which of the following ions is NOT involved in heart contraction?

In the context of heart contraction, calcium ions play a crucial role in initiating and regulating the contraction of cardiac muscle fibers. When the heart is stimulated by an electrical impulse, calcium ions are released from the sarcoplasmic reticulum into the cytoplasm of the heart cells. This influx of calcium is essential for the interaction between actin and myosin, which leads to muscle contraction.

Sodium ions also play a significant role in electrical signaling within the heart, as they are involved in the depolarization phase of action potentials. Potassium ions are important for repolarization and maintaining the resting membrane potential after the action potential.

Iron, while essential for various bodily functions, including oxygen transport as part of hemoglobin in red blood cells, does not have a direct role in the contraction of cardiac muscle. Therefore, it is appropriate to identify iron as the ion that is not involved in heart contraction. This underscores the specificity of the ions that participate directly in the electrochemical processes facilitating cardiac muscle contraction.