What term refers to a mass of cells that act as a unit?

A. Right atrium: Receives systemic venous blood (SVC/IVC), thin-walled chamber on the heart’s right side -not the thick, muscular chamber shown if the figure indicates the left ventricle.
B. Right ventricle: Pumps blood to the pulmonary trunk; its wall is thicker than an atrium but thinner than the left ventricle and sits more anteriorly -not the most muscular chamber.
C. Left atrium: Receives oxygenated blood from the pulmonary veins and sits posteriorly; it has thinner walls than the left ventricle.
D. Left ventricle: The left ventricle has the thickest muscular wall (high systemic pressure), forms the cardiac apex, and pumps blood into the aorta

A. Muscle contraction of upper airways: Upper airway muscle movement does not create the main pressure change driving inspiration
B. Partial pressure gradients for oxygen and carbon dioxide: Partial pressure gradients drive gas diffusion across membranes, not bulk airflow into lungs
C. Pressure gradient between atmosphere and alveoli: Contraction of the diaphragm/intercostals lowers intrapulmonary pressure relative to atmospheric pressure, causing air to flow in
D. Concentration gradients for oxygen and carbon dioxide: Concentration gradients drive diffusion at the alveolar–capillary level, not the gross movement of air into the lungs