AssessmentsRegulation of pulmonary blood flow
Regulation of pulmonary blood flow
USMLE® Step 1 style questions USMLE
A 5-year-old boy is brought to the emergency room after accidentally ingesting a small plastic bead. Chest radiography reveals a radiopaque object in the right lower lobe. The bead is suspected to be causing near complete occlusion of one of the right lower lobe bronchioles. Which of the following physiological changes will most likely take place in the affected portions of the lung?
Content Reviewers:Rishi Desai, MD, MPH
The pulmonary arteries divide into smaller arteries known as pulmonary arterioles and then eventually into pulmonary capillaries which surround the alveoli - which are the millions of tiny air sacs where gas exchange happens.
The pulmonary capillaries drain into small veins that join to form the two pulmonary veins exiting each lung, and these pulmonary veins complete the circuit by delivering oxygen-rich blood into the left atrium.
Now pulmonary blood flow is directly proportional to the difference in pressure between the pulmonary artery and the left atrium, or the delta P; and inversely proportional to the resistance of the pulmonary vasculature (R).
The normal pulmonary artery pressure is about 25/10 mmHg with a mean arterial pressure of 15 mmHg.
If pulmonary blood flow needs to change in response to a situation or vasoactive substance, it’s done by changing the resistance of the vasculature, particularly the arterioles, which is related to the diameter of the blood vessels.
Specifically, a decrease in the diameter of the arterioles causes an increase in resistance, and that leads to a decrease in blood flow.
On the other hand, an increase in the diameter of the arterioles causes a decrease in resistance, and that leads to an increase in blood flow.
In contrast, extra-alveolar blood vessel resistance depends on pressure in the pleural space which lies between the parietal pleura, which is stuck to the chest wall, and the visceral pleura, which is stuck to the lungs.
Pressure within the pleural space is established by two main opposing forces. One is the muscle tension of the diaphragm and chest wall which contract and expand the thoracic cavity outwards, and the other is the elastic recoil of the lungs, which try to pull the lungs inward.
The two forces pull on each other creating a slight vacuum in the pleural space - which results in a pressure of -5 centimeters of water relative to the pressure of 0 centimeters of water in both the thoracic cavity and within the alveoli of the lungs.
Based on this, one way to control pulmonary vasculature resistance is through changes in lung volume.
When lung volume is high, like at the end of maximum inspiration, alveoli are extended and the alveolar pressure increases, pressing down on and applying increased pressure on the alveolar blood vessels. The result is the alveolar vessels constrict and resistance increases.
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