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58 PA R T I / Anatomy and Physiology

which is equal to the product of hydraulic conductivity and the Pulmonary blood volume decreases or is diverted to the sys-
available area (L p A), is expressed as milliliters of net filtrate formed temic circulation in conditions such as generalized systemic va-
p p
in 100 g of tissue per minute for each milliliter increase in mean sodilation, the standing position, positive end-expiratory pressure,
–1
capillary filtration pressure (ml min –1 mm Hgg –1 100 gg ). or circulatory shock. Conditions that increase pulmonary blood
The capillary filtration coefficient is a useful indicator of capillary volume include generalized systemic vasoconstriction, the supine
permeability. 147 A decrease in the capillary filtration coefficient, position, mitral stenosis, and left heart failure. 156
for example by a decrease in the area available for exchange, re- The pulmonary circulation originates from the base of the
duces the rate of net capillary filtration for any given net filtration right ventricle, extends 5 cm, and divides into the right and left
pressure. The second factor, the reflection coefficient ( ), repre- pulmonary arteries. As the pulmonary artery rises, the right pul-
sents the osmotic pressure exerted by a difference in the concen- monary artery is positioned posterior to the aorta and superior
tration gradient of a substance across a membrane (oncotic effect vena cava and anterior to the right mainstem bronchus. The left
of the concentration gradient) and the greater the ratio of the pulmonary artery extends over the left main bronchus and di-
solute size to pore size, the greater the ratio. 155 vides into lobar branches. The pulmonary arteries and segmen-
The reflection coefficient is close to 1 for tight-junction en- tal and lobar branches are composed of elastic arteries to main-
dothelium, which is completely impermeable to protein. In nor- tain low vascular resistance. These arteries contain smooth
mal systemic exchange vessels in the skin and skeletal muscle, with muscle with the capability of vasoconstriction and vasodilata-
continuous or fenestrated endothelium, the reflection coefficient tion. The muscular arteries have internal and external elastic
ranges from 0.8 to 0.95 for albumin and total protein, 146,147 laminae with a layer of smooth muscle cells. The acinour and su-
which indicates that these vessels are not completely impermeable pernumerary arteries (precapillary arteries) are muscular. In-
to proteins. In the lungs, the reflection coefficients are, in general, creases in pulmonary vascular resistance come from the precap-
lower for albumin (0.5 to 0.6) and protein (0.5 to 0.7). 147 In cases illary arteries. Arterioles are vessels with a thin intima and a
of injury to the endothelium, the reflection coefficient is markedly single elastic lamina. These vessels make up the accessory
reduced, allowing increased movement of large molecules (e.g., branches of the respiratory tree and end at the alveolar capillary
protein) out of the exchange vessels. network (see Table 2-3 for abbreviations used in this section).

Cellular and Hormonal Effects
THE LYMPHATIC SYSTEM
The pulmonary vascular bed is lined with endothelium. In the
Removal of fluid and plasma proteins from the interstitium by the pulmonary vasculature, the primary endothelium relaxing fac-
terminal lymphatics is essential in the maintenance of equilibrium tors released by the endothelial cells are NO and prostacyclin
in microvascular-interstitial exchange. 155 Depending on the pro-
tein concentration in the lymph, 8 to 12 L/day of lymph, which
reflects net filtration caused by movement of fluid out of the vas-
cular bed, is removed from the interstitium by the lymphatic sys-
tem 144,155 (see Fig. 2-3). Approximately 4 to 8 L of the ultrafil- Table 2-3 ■ ABBREVIATIONS
trate is directly reabsorbed from the lymphatic vessels back into V V Tidal volume
V T
the blood vessels, and the remaining 4 L of efferent lymph, which V V Expired volume
V E
V V Dead space volume
includes all of the filtered protein, is delivered back to the central V D Alveolar volume
V V
circulation. 144,147 This high level of lymphatic flow supports the V A Alveolar pressure
P
P A
idea that filtration (return of lymph to the systemic vasculature) P a P P Arterial pressure
occurs along the entire length of lymphatic bed and not just in the P v P P Venous pressure
central circulation. PA O2 Alveolar partial pressure of oxygen
PA CO2 Alveolar partial pressure of carbon dioxide
Arterial partial pressure of oxygen
PA O2
Arterial partial pressure of carbon dioxide
Pa CO2
PULMONARY CIRCULATION PvO 2 Mixed venous partial pressure of oxygen
PCO 2 Partial pressure of carbon dioxide
Partial pressure of oxygen
Gross Anatomy
PO 2
FI O2 Fraction of inspired oxygen
Pressure of inspired oxygen
PI O2
The primary function of the pulmonary circulation is to expose P P P 50 Partial pressure of oxygen at which Hgb is 50% saturated
the blood to alveolar air so that oxygen can be taken up by the Sa O2 Arterial blood saturation
blood and carbon dioxide can be excreted. The pulmonary circu- Ca O2 Oxygen content of arterial blood
lation is in series with the systemic circulation and receives the Cv O2 Oxygen content of mixed venous blood
same cardiac output, approximately 5 to 6 L/min at rest for an C(a v) O2 Difference between arterial and venous oxygen content
SvO 2 Mixed venous oxygen saturation
adult weighing 70 kg. The pulmonary circulation has only 10% O 2 ER Oxygen extraction ratio
the capacity of the systemic circulation, yet it must accommodate Sp O2 Pulse oximetry oxygen saturation

the same ejected volume. V Q Ventilation–perfusion ratio

Although pulmonary blood flow is equal to that of the systemic VO 2 Oxygen consumption

Oxygen delivery

system, its vascular resistance is seven to eight times lower than sys- DO 2 Oxygen transport
Q Q
QO 2
temic resistance. The pulmonary vascular bed is regulated by pas- WOB Work of breathing
sive factors, such as lung volume, and active factors, such as alveo- Hgb Hemoglobin
lar gas. These mechanisms alter pulmonary vascular resistance.