9.4: Pulmonary Vasculature’s Response to Hypoxia
- Page ID
- 34532
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)If systemic tissue becomes hypoxic then local arterioles open to allow more blood flow and increase oxygen delivery. The opposite is true for the pulmonary circulation where the response to local hypoxia is vasoconstriction.
First, remember that the bronchial circulation provides oxygen and nutrients to the pulmonary itself, and this behaves as all other systemic circulations. But the pulmonary circulation is for gas exchange. So if an area of the lung has become hypoxic (i.e., has a low oxygen partial pressure), this is indicative of that area having insufficient ventilation.
If there is little ventilation going to that area then there is little point sending perfusion to it. So the little smooth muscle there is in the pulmonary vasculature contracts to constrict the vessel when hypoxia is present. The blood follows the path of least resistance and thereby goes to vessels that are open (i.e., to areas where ventilation is maintaining a higher PO2).
Figure 9.9 shows that blood flow increases with increasing alveolar PO2, or more pertinently it decreases with decreasing alveolar PO2. This shunting of pulmonary blood away from unventilated (or hypoxic) areas helps maintain matching of ventilation and perfusion (or V/Q) and efficient gas exchange.
Figure 9.9: Changes in blood flow caused by vasoconstriction in response to pulmonary hypoxia.
Summary
Because of its unique role in gas exchange, rather than supplying local tissue, and the pressures that are present in the lung beyond vasculature pressure, plus the different vasculature structure, the pulmonary circulation has some unusual characteristics. These produce unique blood flow patterns in response to lung volume, gravity, and the need to match ventilated areas with adequate perfusion.
References, Resources, and Further Reading
Text
Levitsky, Michael G. "Chapter 4: Blood Flow to the Lung." In Pulmonary Physiology, 9th ed. New York: McGraw Hill Education, 2018.
West, John B. "Chapter 4: Blood Flow and Metabolism—How the Pulmonary Circulation Removes Gas from the Lung and Alters Some Metabolites." In Respiratory Physiology: The Essentials, 9th ed. Philadelphia: Wolters Kluwer Health/Lippincott Williams and Wilkins, 2012.
Widdicombe, John G., and Andrew S. Davis. "Chapter 5." In Respiratory Physiology. Baltimore: University Park Press, 1983.
Figures
Figure 9.1: The pulmonary circulation. Image courtesy of Dr Wei Liao, Imaging Solutions, Bayer AG Engineering & Technology, Computer Vision Innovation. 2020. Fair use. https://www.mdpi.com/2075-4418/10/12/1004/htm
Figure 9.2: A parking lot—a bit like pulmonary capillary structure. Leung, Enoch. 2018. CC BY-SA 2.0. https://flic.kr/p/28ci9r1
Figure 9.3: Schematic of the pulmonary and systemic circulations, comparing capillary densities and pressures. Grey, Kindred. 2022. CC BY 4.0. https://archive.org/details/9.4_20220125
Figure 9.4: Pulmonary vascular resistance decreases as pressure increases. Grey, Kindred. 2022. CC BY 4.0. https://archive.org/details/9.6_20220125
Figure 9.5: Pulmonary vessels can be categorized as alveolar or extra-alveolar. Grey, Kindred. 2022. CC BY 4.0. https://archive.org/details/9.7_20220125
Figure 9.6: The relationship between lung volume and pulmonary vascular resistance. Grey, Kindred. 2022. CC BY 4.0. https://archive.org/details/9.8_20220125
Figure 9.7: Perfusion distribution up the lung. Grey, Kindred. 2022. CC BY 4.0. https://archive.org/details/9.9_20220125
Figure 9.8: Relationships of alveolar, arterial, and venous pressures down the lung. Grey, Kindred. 2022. CC BY 4.0. https://archive.org/details/9.10_20220125
Figure 9.9: Changes in blood flow caused by vasoconstriction in response to pulmonary hypoxia. Grey, Kindred. 2022. CC BY 4.0. https://archive.org/details/9.11_20220125