Physiology Ch. 38 p477-483
Pulmonary Circulation, Pulmonary Edema, Pleural Fluid
-Lung has 2 circulations:
1. High-pressure, low-flow circulation supplies systemic arterial blood to trachea, bronchial tree,
supporting tissues of lung, and adventitia.
a. Bronchial arteries supply most of this blood at a slightly lower pressure than aortic
2. Low-Pressure, high-flow circulation that that supplies venous blood from all parts of body to
alveolar capillaries where oxygen is added and CO2 is removed
a. Pulmonary artery receives blood from right ventricle and carry it to alveolar capillaries
Physiologic Anatomy of Pulmonary Circulatory System
1. Pulmonary Vessels – pulmonary artery comes from right ventricle and divides into right and left
main branches
a. wall is 1/3rd the thickness of the aorta, and the pulmonary arteries and arterioles have
larger diameters than systemic arteries, and are thin and distensible
i. gives these vessels a large compliance (7ml/mm Hg) similar to entire systemic
arterial tree and allows pulmonary arteries to accommodate stroke volume of
right ventricle
b. pulmonary veins empty into the left atrium
2. Bronchial Vessels – blood flows to lungs through small arteries from systemic circulation (1-2%)
a. This is oxygenated blood as opposed to deoxygenated blood in pulmonary arteries
b. Empties into the pulmonary veins after oxygenating the lungs and enters left atrium
instead of right atrium
3. Lymphatics – present in all supportive tissues of the lung- beginning in connective tissue spaces
around bronchioles, coursing to hilum of lung and entering right thoracic lymph duct.
a. Particulate matter in alveoli is removed through these channels
Pressures in the Pulmonary system – pressure of the right ventricle and pulmonary arteries are
normally 1/5th of the pressure in the left ventricle and aorta
Pressures in the Pulmonary Artery – during SYSTOLE, pressure in pulm. Artery is equal to pressure in
right ventricle.
-after pulmonary valve closes, the ventricular pressure falls, while pulmonary artery pressure
falls more slowly as blood flows through capillaries of lungs
Pulmonary Capillary Pressure – capillary pressure is important in relation to fluid exchange (7mmHg)
Left Atrial and Pulmonary Venous Pressures – pressure in the left atrium is normally 2mmHg measured
by pulmonary wedge pressure – insert a catheter through peripheral veinto right atrium then through
right side of heartpulmonary artery and wedges into one of small branches of pulmonary artery.
Blood Volume of the Lungs – lung blood volume is 450mL (9% of total body blood)
-Lungs serve as blood reservoir- quantity of blood in lungs can vary from ½ to twice normal, such as
when person blows air really hard, or has a hemorrhage.
Cardiac Pathology can shift Blood from Systemic to Pulmonary Circulation – failure of left side of heart
or increased resistance to blood flow through mitral valve causes blood to dam up in pulmonary
circulation, increasing pulmonary blood volume by as much as 100%
Blood Flow through the Lungs and Its Distribution – blood flow through lungs equal to cardiac output.
Similar factors controlling heart output can also control pulmonary blood flow
-pulmonary vessels are distensible and get bigger with higher pressure and smaller with lower pressure
Decreased Alveolar Oxygen Reduces Local Alveolar Blood Flow and Regulates Pulmonary Blood Flow
Distribution - when O2 concentration decreased below a certain amount, vessels constrict in the lungs,
up to 5-fold at low oxygen levels, which is opposite to what happens in the systemic blood flow (dilate in
response to low O2).
-constriction in low O2 serves to redirect blood flow to where it is most important: functioning alveoli
Effect of Hydrostatic Pressure Gradients in Lungs on Pulmonary Blood Flow – hydrostatic pressure is
the weight of the blood itself in its vessels. This accounts for why pressure in the lungs varies on the
region.
-30cm difference between highest and lowest point in the lungs (difference of 23 mm Hg).
-15mmHg above the heart and 8mmHg below the heart
-pressure at lowest portion of lungs is 8mmHg greater.
-Differences can be explained by dividing the lung into Three Zones that describe pressure in capillaries
1. Zone 1: No blood flow during all portions of cardiac cycle because local alveolar capillary
pressure in that area of lung never rises higher than alveolar air pressure
2. Zone 2: Intermittent blood flow only during peaks of pulmonary arterial pressure because
systolic pressure is greater than alveolar air pressure but diastolic pressure is less than alveolar
air pressure. Flow only during systole and cessation of flow during diastole
a. Normally in the apices of lung (top), begins 10cm above midlevel of heart and goes to
top
3. Zone 3: Continuous blood flow because alveolar capillary pressure remains greater than alveolar
air pressure during the entire cardiac cycle
a. Found in lower areas of the lung, 10cm above the heart and below, pressures during
systole and diastole remain greater than zero alveolar pressure, therefore continuous
blood flow.
-lying down, no part of the lung is > a few cm above the heart, therefore Zone 3 flow through all
capillaries
-Normally, lung only has zones 2-3 blood flow
-Zone 1 blood flow occurs only under abnormal conditions, when pulmonary systolic pressure is too
low or alveolar pressure is too high to allow flow.
Effect of Exercise on Blood Flow Through Different Parts of Lungs – blood flow to lungs increases
everywhere during exercise. In top part of lung, can increase up to 700-800%, bottom part only 250%.
Pulmonary vascular pressures can rise enough during exercise such that the lung apices convert from a
zone 2 pattern to a zone 3 pattern.
Increased Cardiac Output During Heavy Exercise is Normally Accommodated by the Pulmonary
Circulation Without Large Increases in Pulmonary Artery Pressure
During heavy exercise, blood flow to lungs increases up to 7-fold. Extra flow is accommodated 3 ways
1. Increasing number of open capillaries up to 3x
2. Distending all capillaries and increasing rate of flow through each capillary 2x
3. Increasing pulmonary arterial pressure
-ability of lungs to accommodate increased blood flow during exercise without increasing pulmonary
arterial pressure conserves energy of right side of heart. Also prevents increase in pulmonary
capillary pressure, which would prevent pulmonary edema.
Function of Pulmonary Circulation When Left Atrial Pressure Rises as a Result of Left-Sided Heart
Failure – left atrial pressure is never > 6mmHg. When left side of heart fails, blood begins to dam up in
the left atrium. This causes left atrial pressure to rise from 1-5mmHg to 40-50mmHg.
-Can cause increases in pulmonary artery pressure when atrial pressure increases by >7mmHg.
-Increases capillary pressure equally
-Above 30mmHg, pulmonary edema is likely to develop
Pulmonary Capillary Dynamics – Alveolar walls are lined with so many capillaries that they can touch
one another, therefore it is often thought of as a sheet of flow rather than individual capillaries
Pulmonary Capillary Pressure – no direct measurements of pulmonary capillary pressure have been
made, but you can use isogravimetric measurement of pulmonary capillary pressure which gives it a
value of 7mmHg
Length of Time Blood Stays in Pulmonary Capillaries – when cardiac output is normal, blood passes
pulmonary capillaries in 0.8 seconds, increased cardiac output shortens time to 0.3s.
Capillary Exchange of Fluid in Lungs and Pulmonary Interstitial Fluid Dynamics – qualitatively the same
fluid exchange across lung capillaries as peripheral tissues, but quantitatively different
1. Low pulmonary capillary pressure compared to peripheral tissues
2. Interstitial fluid pressure in the lung is slightly more negative than that in peripheral
subcutaneous tissue
3. Pulmonary capillaries are leaky to proteins, so colloid osmotic pressure of pulmonary interstitial
fluid is about 14mmHg; less than half in peripheral tissues
4. Alveolar walls are extremely thin and alveolar epithelium covering surfaces is weak and can be
ruptured by positive pressure
Interrelations between Interstitial Fluid Pressure and Other Pressures in the Lung –
-normal outward forces are slightly greater than inward forces, providing a mean filtration pressure at
the pulmonary capillary membrane (difference between outward and inward force in mmHg).
-this filtration pressure causes slight continual flow of fluid from capillaries into interstitial spaces,
pumped back into circulation by lymphatics
Negative Pulmonary Interstitial Pressure and Mechanism for Keeping Alveoli Dry – alveoli are
prevented from being filled with fluid.
-negative pressure in between capillaries and pulmonary lymphatics cause fluid in alveoli to get sucked
into interstitium
Pulmonary Edema – fluid accumulation in lungs occurs same as anywhere else in the body. Any factor
that causes fluid filtration out of the pulmonary capillaries or that impedes lymphatic function causes
pulmonary interstitial fluid to rise from negative pressure to positive pressure and cause rapid filling of
pulmonary interstitial spaces and alveoli with large amounts of free fluid.
-Causes of pulmonary edema include
1. Left-sided heart failure or mitral valve disease with consequent increases in pulmonary venous
pressure and pulmonary capillary pressure
2. Damage to pulmonary blood capillary membranes caused by infections such as pneumonia or by
breathing noxious substances such as chlorine gas or sulfur dioxide to cause leakage of plasma fluid out
of capillaries and into both interstitial spaces and alveoli
Pulmonary Edema Safety Factor - pulmonary capillary pressure rises to a value equal to the colloid
osmotic pressure of plasma inside the capillaries before significant pulmonary edema will occur
Safety Factor in Chronic Conditions - when pulmonary capillary pressure remains elevated for 2 weeks,
lungs become more resistant to edema because lymph vessels expand
Rapidity of Death in Acute Pulmonary Edema – when pulmonary capillary pressure rises above safety
factor level, lethal pulmonary edema can occur within hours or minutes, thus in acute left-sided heart
failure, death frequently ensues in 30 minutes
Fluid in the Pleural Cavity – when lungs expand and contract during breathing, they slide back and forth
within pleural cavity.
-Pleural membrane is porous, mesenchymal, serous membrane through which small amounts of
interstitial fluid transude continuously into pleural space.
-Fluids carry tissue proteins to give pleural fluid a mucoid characteristic (easy slippage of lungs)
-Total amount of fluid is only a few mL, and excess is pumped away by lymphatics into the mediastinum,
superior surface of the diaphragm, and lateral surfaces of parietal pleura
-Pleural space between parietal and visceral pleurae is also called potential space because it is very
narrow
Negative Pressure in Pleural Fluid – negative force is always required on outside of the lungs to keep
lungs expanded, this is provided by negative pressure in the pleural space.
-caused by pumping of fluid from space by lymphatics
-pleural fluid must always maintain a pressure of about -4mmHg to keep lungs from collapsing
-negativity of pleural fluid keeps normal lungs pulled against parietal pleura in between thin layer of
mucoid fluid to act as lubricant
Pleural Effusion – Collection of large amounts of free fluid in pleural space – pleural effusion is
analogous to edema fluid in tissues. Causes of effusion include
1. Blockage of lymphatic drainage from pleural cavity
2. Cardiac failure (causes excessive high peripheral and pulmonary capillary pressures)
3. Greatly reduced plasma colloid osmotic pressure allowing excessive transudation of fluid
4. Infection or any other cause of inflammation of surfaces of pleural cavity to break down
capillary membranes and allows rapid dumping of plasma proteins and fluid into pleural cavity.