Pada fisiologi jantung, kekuatan yang melawan kontraksi otot jantung; pada otot yang terisolasi, kekuatan yang menahan kontraksi otot setelah otot itu dirangsang untuk berkontraksi; pada jantung yang utuh, tekanan yang menahan ejeksi darah dari rongga ventrikel.
Afterload is the pressure in the wall of the left ventricle during ejection. In other words, it is the end load against which the heart contracts to eject blood. Afterload is readily broken into components: one factor is the aortic pressure the left ventricular muscle must overcome to eject blood. The greater the aortic/pulmonary pressure, the greater the after load on the left/right ventricle, respectively. Following Laplace's law, the tension upon the muscle fibers in the heart wall is the pressure within the ventricle multiplied by the volume within the ventricle divided by the wall thickness (this ratio is the other factor in setting the afterload). Therefore, when comparing a normal heart to a heart with a dilated left ventricle, if the aortic pressure is the same in both hearts, the dilated heart must create a greater tension to overcome the same aortic pressure to eject blood because it has a larger internal radius and volume. Thus, the dilated heart has a greater total load (tension) on the myocytes, i.e., has a higher afterload. This is also true in the eccentric hypertrophy consequent to high intensity aerobic training. Conversely, a concentrically hypertrophied left ventricle may have a lower afterload for a given aortic pressure. When contractility becomes impaired and the ventricle dilates, the afterload rises and limits output. This may start a vicious circle, in which cardiac output is reduced as oxygen requirements are increased.
Afterload can also be described as the pressure that the chambers of the heart must generate in order to eject blood out of the heart and thus is a consequence of the aortic pressure (for the left ventricle) and pulmonic pressure or pulmonary artery pressure (for the right ventricle). The pressure in the ventricles must be greater than the systemic and pulmonary pressure to open the aortic and pulmonic valves, respectively. As afterload increases, cardiac output decreases. Cardiac imaging is a somewhat limited modality in defining afterload because it depends on the interpretation of volumetric data.