Chapter 32. Cardiovascular Regulatory Mechanisms
A 49-year-old woman was admitted to the hospital after experiencing recurrent episodes of shortness of breath, severe headache, sweating, and abdominal pain. These episodes occurred primarily during physical exertion. Vitals were taken and included a BP recording of 160/95 mm Hg. She underwent a series of exams including imaging studies that showed a tumor on her adrenal medulla (pheochromocytoma). When this tumor suddenly releases a large amount of epinephrine into the circulation, the patient’s heart rate would be expected to
A. increase because the increase in BP stimulates the carotid and aortic baroreceptors.
B. increase because epinephrine has a direct chronotropic effect on the heart.
C. increase because of increased tonic vagal nerve activity to the heart.
D. decrease because the increase in BP reduces activity in the carotid and aortic chemoreceptors.
E. decrease because of increased vagal activity to the AV node.
The correct answer is B. The binding of epinephrine to the β1-adrenoceptors in the SA node will increase heart rate. The increase in BP would lead to an increase in baroreceptor nerve activity that would activate vagal nerve activity to the heart and potentially decrease heart rate, but the direct action of epinephrine on the heart would likely overcome the reflex-induced change (rules out options A and C). The increase in BP would not activate the carotid and aortic chemoreceptors; they are activated by reduction in the partial pressure of oxygen in the arterial circulation (rules out option D). An increase in vagal nerve activity to the AV node would reduce conduction velocity through the AV node but would not reduce heart rate (rules out option E).
A 69-year-old man made an appointment with his primary care physician after he began to experience syncope when he got out of bed in the mornings. Vitals at rest were normal for a man at this age and included a BP reading of 135/85 mm Hg. The physician asked him to lie down for a few minutes and then to stand up quickly. Upon standing, his BP was 80/50 mm Hg. The physician said he likely was experiencing episodes of orthostatic hypotension which is not uncommon in individuals over the age of 65 due to dysfunction of the baroreceptor reflex. Where are the arterial baroreceptors located, what are the afferent fibers, and where do the afferent fibers terminate?
A. Atria and pulmonary veins, vagus nerve, and nucleus ambiguus
B. Carotid body and aortic body, glossopharyngeal nerve and vagus nerve, and NTS