A class of immunoglobulins found concentrated in breast milk, and thus serve to passively and naturally enhance the specific immune system of a newborn is:

Correct Answer: A

Rationale: Step 1: IgA is the correct answer. It is found in breast milk and provides passive immunity to newborns through breastfeeding. Step 2: IgD is mainly found on the surface of B cells and is involved in the activation of B cells. Step 3: IgE is involved in allergic responses and parasitic infections, not typically found in high concentrations in breast milk. Step 4: IgG is the most abundant immunoglobulin in the bloodstream but is not typically concentrated in breast milk like IgA. Summary: IgA is the correct answer because it is specifically designed to provide passive immunity to newborns through breast milk, while the other choices do not serve this function.

Correct Answer: B

Rationale: The correct answer is B: less than hemoglobin's affinity for oxygen when the BPG level is low. When the 2,3-bisphosphoglycerate (BPG) level is high, it binds to hemoglobin and decreases its affinity for oxygen. This results in easier unloading of oxygen to tissues. In contrast, when the BPG level is low, hemoglobin's affinity for oxygen is higher, making it harder to release oxygen to tissues. Choices A and C are incorrect because high BPG levels decrease hemoglobin's affinity for oxygen, making it less than when BPG levels are low. Choice D is incorrect because pH levels do not directly affect hemoglobin's affinity for oxygen in the context of this question.

Correct Answer: C

Rationale: The correct answer is C because vital capacity actually decreases with age due to changes in lung elasticity and muscle strength. A is correct as aging causes the lungs to lose elastic tissue. B is correct as lung compliance changes with age. D is correct as respiratory muscles weaken over time. Therefore, only option C is false in the context of age-related changes in lung function.

Correct Answer: C

Rationale: The correct answer is C: Left ventricle. The left ventricle is the chamber responsible for pumping oxygenated blood to the rest of the body, making it the most commonly affected in heart failure. When the left ventricle weakens or becomes stiff, it can lead to inefficient pumping, causing fluid buildup and symptoms of heart failure. The other choices, A: Left atrium, B: Right atrium, and D: Right ventricle, are less commonly affected in heart failure as they have different functions in the heart's circulation.

Correct Answer: B

Rationale: During expiration, the diaphragm relaxes, causing it to move upwards, which decreases the thoracic space. This leads to the expulsion of air from the lungs. In contrast, during inspiration, the diaphragm contracts and moves downwards, increasing the thoracic space to allow air to enter the lungs. Option C, Pause, does not involve any specific respiratory phase. Option D, both a and b, is incorrect because during inspiration, the diaphragm contracts and increases thoracic space. Therefore, the correct answer is B, expiration, as this phase specifically involves the relaxation of the diaphragm and the decrease in thoracic space.