By pacing both sides of the left ventricle, the pacemaker can resynchronize the ventricular contractions. The mitral valve lies between the left atrium and left ventricle. Between the right atrium and the right ventricle is the tricuspid valve. As atrial pressure increases above that of the left ventricle, the mitral valve opens. The first region is the ventricular zone in the roof of the fourth ventricle. No decrease of the diastolic function of the left ventricle occurs. The main pulmonary artery begins at the base of the right ventricle. Hypertrophic cardiomyopathy is the thickening of the left ventricle, occasionally right ventricle. The lead usually lodges in the apex or septum of the right ventricle. The marginal arteries supply blood to the superficial portions of the right ventricle. The arterial bulb forms the trabecular portion of the right ventricle. The fourth ventricle, pons and medulla are in front of the cerebellum. They have a heart that consists of a single ventricle and two atria. Systemic arterial pressures are generated by the forceful contractions of the heart's left ventricle. This is in contrast with the sustained impulse of the hypertrophied right ventricle.