9.1: The Pulmonary Vasculature
- Page ID
- 34515
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)The primary role of the pulmonary circulation is to achieve gas exchange with the airspaces. Sufficient perfusion of this circulation is as essential to gas exchange as sufficient ventilation of the alveoli. Because of this role, and the fact that pulmonary vessels are exposed to airway and alveolar pressures the pulmonary circulation has some unique characteristics that are covered in this chapter.
Functional anatomy
The pulmonary circulation takes all cardiac output from the right heart via the pulmonary arteries. Thus, even at rest it has a tremendous blood flow – about 5 liters per minute, just the same as the systemic circulation. This volume enters a vast array of vessels that penetrate all the lung structure – so much so that the complete lung structure is visible from the cast of the pulmonary vasculature in figure 9.1.
Main arteries follow a similar branching pattern to the bronchial tree until the terminal bronchioles are reached. This anatomical arrangement allows perfusion to follow the ventilation. Upon reaching the terminal bronchioles the vessels divide into a vast array of capillaries that wrap around the respiratory ducts and alveoli to form the respiratory zone of the lungs.
The density of the capillary beds is so great that individual capillaries can loose their distinct anatomy as can be seen in this electron micrograph where the capillaries are seen to form more sheet-like structures around where the alveoli would be. A common analogy for this is the capillaries look more like a floor of a parking garage with pillars for support but mainly open space – rather (figure 9.3) than the distinct tubes seen in other circulations.
The capillary beds converge into small veins after traveling over the alveolar surfaces, and these small veins then collect into four pulmonary veins that lead back to the left heart. This is an unusual example of veins carrying blood with arterial gas pressures.
Despite receiving the same blood volume per minute as the systemic circulation the pulmonary circulation is a low-pressure system. Systolic pressure is normally only 25 mmHg, compared to 120 in the systemic circulation, diastolic is 8, compared to 80 and mean pulmonary artery pressure is only 15. These numbers are well worth remembering.
So how can this one circulation receive so much volume (the complete cardiac output) and yet remain at such low pressure? The first reason is the vast size of the capillary beds. As figure 9.4 suggests, the much higher density of pulmonary capillary beds than that seen in the systemic circulation allows pressure to dissipate much more quickly.
Figure 9.3: Schematic of the pulmonary and systemic circulations – compare capillary densities and pressures.
The pulmonary arteries show different characteristic to their systemic counterparts as well. The walls of a pulmonary arterioles are thin compared to systemic arterioles. They also lack the smooth muscle layer seen in the systemic arteriole. In fact pulmonary arterioles look much more like systemic veins and they are often mistaken for such in biopsy or dissection. With little smooth muscle it’s clear that these vessels have little role in controlling the distribution of blood flow – a vital role of their systemic counterparts. As the pulmonary circulation receives all cardiac output, all the time, such precise control isn’t required.
The thin walls and lack of smooth muscle also make the pulmonary arterioles highly compliant and so they behave much more like veins in their pressure response – extending when pressure increases. This gives the pulmonary arteriole system a rather unique pressure-resistance relationship that we’ll look at in a moment.
This low pressure and compliant system also means that the right heart has much less work to perform to generate its output. In fact the right ventricle has about a tenth of the work of the left heart to move exactly the same blood volume. Hence the structure and work capacity of the right heart is so much smaller than the left – something worth bearing in mind if disease causes changes in the pulmonary vasculature that in turn causes the less substantial right heart to work harder and undergo hypertrophy