In the Anrep effect, demonstrated in isolated hearts, an increase in contractility results acutely from increased afterload. It may result from intra-myocardial catecholamine release; however, it is probably not important in the in situ heart.
The force-frequency relationship referred to as the Bowditch effect, Staircase, or Treppe, discussed earlier, involves increases in contractility as contraction (heart) rate rises. The Anrep and Bowditch effects are referred to as Homeometric Autoregulation. The internal environment (PO2, PCO2, etc.) may also influence stroke volume.
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