What is the primary factor that increases cardiac contractility?

The primary factor that increases cardiac contractility is increased extracellular calcium. Cardiac muscle cells, or myocytes, rely on calcium ions for contraction. When calcium is present in higher concentrations outside the cell, it enters the myocytes during the action potential through calcium channels. This influx of calcium not only promotes the release of more calcium from the sarcoplasmic reticulum but also directly interacts with the contractile proteins, enhancing the force of contraction.

Calcium plays a critical role in the excitation-contraction coupling process. This begins when an action potential triggers the influx of calcium into the myocyte. The increased calcium concentration inside the cell activates the proteins responsible for muscle contraction, ultimately leading to a more forceful contraction of the heart muscle.

In contrast, increased extracellular sodium primarily affects the resting membrane potential and can influence the action potential; however, it does not directly enhance contractility to the same extent as calcium. Increased extracellular potassium can lead to depolarization of the cell membrane, which can impair contractility by affecting action potential propagation. Decreased intracellular calcium would lead to weakened contractions, as the availability of calcium for muscle contraction would be reduced. Thus, increased extracellular calcium is the most significant factor driving enhanced cardiac contractility.