WBR0865

Author	[[PageAuthor::Serge Korjian M.D. (Reviewed by Serge Korjian)]]
Exam Type	USMLE Step 1
Main Category	Physiology
Sub Category	Pulmonology
Prompt	[[Prompt::An important characteristic of the pulmonary circulation is its ability to adapt to different oxygen concentrations to maintain good exchange and oxygen delivery to tissues. Below is a graph showing the arterial responses of various tissue beds as oxygen concentrations are modified. Which of the presented curves closely resembles the response of the pulmonary vasculature? ]]
Answer A	A
Answer A Explanation	This response represents an exxagerated normal response in tissues. It is usually not seen in physiologic situations.
Answer B	B
Answer B Explanation	This response represents the normal tissue response seen in most vascular beds except in the lung.
Answer C	C
Answer C Explanation	This is not a physiologic response pattern to oxygen variation.
Answer D	D
Answer D Explanation	This is not a physiologic response pattern to oxygen variation.
Answer E	E
Answer E Explanation	This represents the typical response of the pulmonary vascular bed to oxygen variation.
Right Answer	E
Explanation	The pulmonary vasculature is peculiar in its response to hypoxia. Where hypoxia typically causes vasodialtion to increase tissue perfusion, alveolar hypoxia in the lungs causes local vasoconstriction. This physiologic mechanism is thought to be an adaptive process that decreases perfusion of the hypoventilated areas and shunts blood towards better perfused alveoli. This mechanism is the basis behind several disease processes including RVH in the context of chronic hypoxia where the latter leads to vasoconstriction and pulmonary hypertension. Educational Objective: The pulmonary vasculature reacts to hypoxia by vasoconstriction to shunt blood to better perfused areas on the lung. References: Moudgil R, Michelakis ED, Archer SL. Hypoxic pulmonary vasoconstriction. J Appl Physiol. 2005;98(1):390-403.
Approved	Yes
Keyword	Pulmonary vasculature, Hypoxia, Vasoconstriction, Vasodilation, Perfusion, Ventilated., Ventilation/perfusion ratio
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