Identifying external heart structures in mammals Steps: - Examine the front view: the heart's apex points down-left, with atria superior and ventricles inferior. - Locate P as the upper left bulge, corresponding to the left atrium receiving oxygenated blood. - Identify Q as the thick-walled lower left chamber, the left ventricle pumping to the body. - Spot R as a surface vessel supplying heart muscle, the coronary artery; S as a large vein entering the top right, the vena cava. Why B is correct: - Matches standard anatomy: left atrium (P) is posterior-superior left, left ventricle (Q) forms the apex, coronary artery (R) branches on epicardium, vena cava (S) drains systemic blood to right atrium. Why the others are wrong: - A: Mislabels Q as cardiac muscle (tissue, not chamber) and S as pulmonary vein (enters left atrium, not visible front-right). - C: Wrong on P (right atrium is right side) and R/S (Purkinje fibers internal, not external; pulmonary artery anterior but not vena cava position). - D: Errors in Q (right ventricle thinner, right side) and R/S (Purkinje internal; …