During birth, an infant is normally stimulated to breathe by_________________.
surfactant
CO2 accumulating in the infant's blood
prostaglandins
an increased O2 level in the infant's new environment
The Correct Answer is B
A. Surfactant reduces surface tension in the alveoli, aiding in lung expansion, but it does not directly stimulate breathing.
B. CO2 accumulating in the infant's blood: The accumulation of CO2 in the infant's blood is a significant stimulus for the first breath after birth. High levels of CO2 trigger the infant's respiratory centers to start breathing.
C. Prostaglandins are involved in labor and delivery but do not directly stimulate breathing in the newborn.
D. An increased O2 level in the infant's new environment: Increased O2 levels do not directly stimulate breathing; rather, the accumulation of CO2 is the primary stimulus.
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Correct Answer is B
Explanation
A. the embryo; part of the placenta. The embryoblast (inner cell mass) will become the embryo, but the trophoblast will become part of the placenta, not the embryo.
B. part of the placenta; the embryo. The trophoblast forms part of the placenta and helps in its development, while the embryoblast (inner cell mass) will develop into the embryo.
C. part of the placenta; the yolk sac. While the trophoblast does contribute to the formation of the placenta, the embryoblast (inner cell mass) develops into the embryo, not the yolk sac. The yolk sac is derived from extraembryonic mesoderm and endoderm, not the inner cell mass directly.
D. the embryo; the yolk sac. The embryoblast (inner cell mass) develops into the embryo, but the trophoblast does not become the yolk sac. The yolk sac is formed from structures surrounding the embryo, not from the trophoblast itself.
Correct Answer is C
Explanation
A. Esophageal glands secrete mucus to lubricate the esophagus, but they do not cause acid reflux.
B. Pharyngeal constrictors are muscles that help in the swallowing process, not in preventing acid reflux.
C. The lower esophageal sphincter (LES) is a ring of muscle that controls the passage of food from the esophagus to the stomach. If the LES is weakened or relaxes inappropriately, stomach acid can flow back into the esophagus, causing acid reflux or "heartburn."
D. The upper esophageal sphincter controls the passage of food from the pharynx into the esophagus, but it is not involved in preventing acid reflux.