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| 91. | preload | (also, end diastolic volume) amount of blood in the ventricles at the end of atrial systole just prior to ventricular contraction |
| 92. | prepotential depolarization | (also, spontaneous depolarization) mechanism that accounts for the autorhythmic property of cardiac muscle; the membrane potential increases as sodium ions diffuse through the always-open sodium ion channels and causes the electrical potential to rise |
| 93. | primitive atrium | portion of the primitive heart tube that eventually becomes the anterior portions of both the right and left atria, and the two auricles |
| 94. | primitive heart tube | singular tubular structure that forms from the fusion of the two endocardial tubes |
| 95. | primitive ventricle | portion of the primitive heart tube that eventually forms the left ventricle |
| 96. | pulmonary arteries | left and right branches of the pulmonary trunk that carry deoxygenated blood from the heart to each of the lungs |
| 97. | pulmonary capillaries | capillaries surrounding the alveoli of the lungs where gas exchange occurs: carbon dioxide exits the blood and oxygen enters |
| 98. | pulmonary circuit | blood flow to and from the lungs |
| 99. | pulmonary trunk | large arterial vessel that carries blood ejected from the right ventricle; divides into the left and right pulmonary arteries |
| 100. | pulmonary valve | (also, pulmonary semilunar valve, the pulmonic valve, or the right semilunar valve) valve at the base of the pulmonary trunk that prevents backflow of blood into the right ventricle; consists of three flaps |
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