Question: Endothelin is a vasoactive peptide involved in a variety of processes, including the control of blood flow. Endothelin is capable of binding to two different receptors. There are receptors for endothelin located on smooth muscle (ETA) and receptors for endothelin located on vascular endothelium (ETB). When endothelin binds to ETA, it initiates vasoconstriction by narrowing blood vessels. However, when endothelin binds to ETB, vasodilation occurs due to the widening of blood vessels. The activity of the ETB receptor seems to counterregulate the ETA effects and prevents excessive vasoconstriction.
Drug $X$ is a selective receptor antagonist for endothelin. It is designed to bind preferentially to certain endothelin receptors. Drug $\mathrm{Y}$ is another selective receptor antagonist for endothelin. It is also designed to preferentially bind to one of the endothelin receptors. Researchers are hopeful that drugs $\mathrm{X}$ and $\mathrm{Y}$ will be of value in the treatment of conditions such as pulmonary arterial hypertension (PAH). In PAH, excess endothelin is produced, which causes an increase in blood pressure within the pulmonary arteries. Normal pressure in the pulmonary arteries is expected to be about $14 \mathrm{~mm} \mathrm{Hg}$, and pressure at or above $25 \mathrm{~mm}$ Hg constitutes PAH. Both drugs have been tested in patients with PAH. There were five patients in each group. At the end of the study, the percent decrease in mean pulmonary artery pressure was measured for each patient and reported in the table.
\begin{tabular}{cccc}
\hline Patient & Drug X & DrugY & Placebo \\
\hline 1 & $+10 \%$ & $-10 \%$ & $+5 \%$ \\
2 & $+3 \%$ & $-5 \%$ & $0 \%$ \\
3 & $-5 \%$ & $+2 \%$ & $+13 \%$ \\
4 & $+2 \%$ & $0 \%$ & $-4 \%$ \\
5 & $0 \%$ & $-7 \%$ & $-2 \%$ \\
Average decrease & $+2 \%$ & $-4 \%$ & $+2.4 \%$ \\
\hline
\end{tabular}
If a patient has PAH, the heart can become overly stressed which can ultimately lead to heart failure. Which area of the heart is MOST likely to initially be affected by an increased blood pressure in the pulmonary arteries?

Options:
A. the right and left atria
B. the right and left ventricles
C. the right ventricle
D. the left ventricle

Answer: To answer this question, you must be familiar with the anatomy of the heart. The right side of the heart is the pulmonary circuit while the left side is the systemic circuit. Deoxygenated blood enters the right atrium of the heart via the superior and inferior vena cava. Blood from the right atrium passes through the tricuspid valve to the right ventricle and then leaves the heart through the pulmonary semilunar valve. Blood is carried via the pulmonary arteries to the lungs where gas exchange occurs. After gas exchange occurs, oxygenated blood returns to the heart via pulmonary veins to enter the left atrium. From there blood passes through the bicuspid valve into the left ventricle. Blood leaves the left side of the heart by passing through the aortic valve into the aorta. If there was a pressure increase in the pulmonary arteries, the right ventricle of the heart would be most likely affected as it is the last chamber that blood is located in before it moves toward the lungs in the pulmonary arteries. The correct answer is C.