Chapter 38: Pulmonary Circulation, Pulmonary Edema, Pleural Fluid

Written By: Razaullah Khattak (DPT Medical Professional)

Student Source Reference: Guyton and Hall Textbook of Medical Physiology

Chapter: 38

To read more check out Chapter 21:

https://mednotesbyrazaullah.blogspot.com/2026/08/chapter-21-muscle-blood-flow-and.html

Pulmonary Circulation, Pulmonary Edema, and Pleural Fluid

The pulmonary circulatory system is specially designed to facilitate efficient gas exchange within the lungs. It operates under unique hemodynamic conditions compared to the systemic circulation, allowing the entire cardiac output to pass through the alveolar capillaries continuously.

Understanding these specialized blood flow patterns, pressure dynamics, and fluid balances is essential for evaluating lung function. This summary breaks down the anatomy, pressure curves, blood volume shifts, flow regulation, capillary fluid dynamics, pulmonary edema, and pleural fluid balance into concise key points.

Dual Circulation of the Lungs

The lungs receive blood from two distinct vascular systems to balance tissue nutrition and gas exchange. One system delivers oxygenated blood under high pressure, while the other transports deoxygenated blood under low pressure.

• The lungs have two separate circulatory systems.
• High-pressure, low-flow system supplies systemic blood.
• Bronchial arteries branch directly from thoracic aorta.
• Bronchial blood supplies trachea and bronchial tree.
• It supplies lung supporting tissues and adventitia.
• Low-pressure, high-flow system handles gas exchange.
• Right ventricle pumps venous blood to lungs.
• Pulmonary capillaries add oxygen and remove CO2.
• Pulmonary veins return blood to left atrium.

Physiologic Anatomy of Pulmonary Vessels

The structure of the pulmonary blood vessels is adapted for high compliance and distensibility. This structural design allows the pulmonary tree to accommodate large volume changes without significant pressure increases.

• Pulmonary artery extends 5 centimeters from right ventricle.
• It divides into right and left main branches.
• Pulmonary arterial walls are thin and distensible.
• Wall thickness is one-third that of aorta.
• Pulmonary arteries have larger diameters than systemic counterparts.
• High compliance averages about 7 mL/mm Hg.
• High compliance accommodates right ventricular stroke volume.
• Pulmonary veins are short and drain left atrium.

Bronchial Vessels and Lymphatics

Bronchial vessels deliver oxygenated blood to the non-respiratory structures of the lungs. Meanwhile, a dense lymphatic network continuously clears excess fluids, proteins, and foreign particles.

• Bronchial arteries carry 1 to 2 percent cardiac output.
• Bronchial arterial blood is fully oxygenated blood.
• It supplies connective tissue, septa, and bronchi.
• Bronchial blood empties into pulmonary veins directly.
• Left ventricular output exceeds right by 1–2%.
• Lymph vessels surround all terminal lung bronchioles.
• Lymph flows toward hilum and right thoracic duct.
• Lymphatics remove leaked plasma proteins and particles.
• Lymphatic drainage helps prevent acute pulmonary edema.

Pressures in the Pulmonary System

Pressures within the pulmonary circuit are significantly lower than those in the systemic circulation. This lower pressure profile protects the thin alveolar membranes while facilitating adequate perfusion.

• Right ventricular systolic pressure averages 25 mm Hg.
• Right ventricular diastolic pressure averages 0–1 mm Hg.
• Pulmonary arterial systolic pressure averages 25 mm Hg.
• Pulmonary arterial diastolic pressure averages 8 mm Hg.
• Mean pulmonary arterial pressure is 15 mm Hg.
• Mean pulmonary capillary pressure is 7 mm Hg.
• Left atrial pressure averages about 2 mm Hg.
• Left atrial pressure ranges from 1–5 mm Hg.

Pulmonary Wedge Pressure Measurement

Direct measurement of left atrial pressure is clinically difficult and invasive. Pulmonary wedge pressure provides an accurate indirect assessment of left atrial and capillary pressures.

• Direct left atrial catheterization is rarely feasible.
• Wedge pressure estimates left atrial pressure indirectly.
• Catheter passes through right heart into pulmonary artery.
• Catheter wedges tightly into small pulmonary branch.
• Normal pulmonary wedge pressure averages 5 mm Hg.
• Wedge pressure is 2–3 mm Hg above left atrial pressure.
• It monitors left heart failure and capillary pressure.

Blood Volume of the Lungs and Reservoir Function

The pulmonary circulation holds a substantial amount of blood that can shift dynamically during physical demands or illness. The lungs thus act as an adjustable blood reservoir for the systemic circulation.

• Total lung blood volume is 450 milliliters.
• Lung volume is 9 percent total blood volume.
• Pulmonary capillaries contain 70 milliliters of blood.
• Remaining blood divides equally in arteries and veins.
• High intra-alveolar pressure expels blood from lungs.
• Blowing a trumpet expels 250 mL of blood.
• Hemorrhage triggers blood shift to systemic vessels.
• Left heart failure doubles pulmonary blood volume.
• Mitral stenosis dams blood in pulmonary circuit.

Control of Regional Pulmonary Blood Flow

Pulmonary blood flow equals total cardiac output and is regulated by local alveolar conditions. A unique hypoxic vasoconstriction mechanism diverts blood away from poorly ventilated regions.

• Pulmonary blood flow equals total cardiac output.
• Peripheral factors controlling cardiac output regulate lung flow.
• Pulmonary vessels act as passive, distensible tubes.
• Low alveolar oxygen causes local pulmonary vasoconstriction.
• Oxygen drop below 70 percent triggers vessel constriction.
• Severe hypoxia increases vascular resistance over fivefold.
• Hypoxia causes local vasoconstrictor release from tissue.
• Vasoconstriction shifts blood to better aerated alveoli.
• Mechanism automatically matches blood flow to ventilation.

Hydrostatic Pressure Gradients and Regional Flow

Gravity creates a vertical hydrostatic pressure gradient from the top to the bottom of the upright lung. This gradient significantly impacts regional blood perfusion across different vertical zones.

• Standing position creates a 23 mm Hg gradient.
• Distance spans 30 cm from apex to base.
• Apical pressure is 15 mm Hg below heart level.
• Basal pressure is 8 mm Hg above heart level.
• Lower lung regions receive significantly higher blood flow.
• Lung bases receive five times more flow than apices.

Zones 1, 2, and 3 of Pulmonary Blood Flow

Capillary blood flow depends on the balance between internal capillary pressure and external alveolar air pressure. The lung is divided into three functional zones based on these pressure relationships.

• Alveolar air pressure compresses wall capillaries externally.
• Zone 1 means zero flow throughout cardiac cycle.
• Zone 1 occurs when alveolar pressure exceeds capillary pressure.
• Zone 2 means intermittent flow during cardiac systole.
• Zone 2 occurs at apices in upright position.
• Zone 2 begins 10 cm above heart level.
• Zone 3 means continuous flow throughout cardiac cycle.
• Zone 3 occurs in lower two-thirds of lungs.
• Supine position converts all lung areas to Zone 3.
• Severe blood loss can produce abnormal Zone 1 flow.
• Positive pressure breathing can cause Zone 1 flow.

Pulmonary Blood Flow During Heavy Exercise

During heavy exercise, the pulmonary circulation must accommodate massive increases in cardiac output. It accomplishes this through capillary recruitment and distension without dangerous pressure spikes.

• Exercise increases lung blood flow up to sevenfold.
• Apical blood flow increases by 700–800 percent.
• Basal blood flow increases by 200–300 percent.
• Exercise converts lung apices from Zone 2 to Zone 3.
• Closed capillaries open to increase blood flow capacity.
• Capillaries distend to increase flow velocity twofold.
• Pulmonary vascular resistance decreases significantly during exercise.
• Pulmonary arterial pressure increases very little in exercise.
• Energy is conserved for the right side heart.
• Low capillary pressure prevents acute pulmonary edema.

Effects of Left Atrial Pressure Elevation

Left-sided heart failure causes blood to back up into the pulmonary circulation. Significant rises in left atrial pressure directly increase pulmonary capillary pressure, predisposing tissues to fluid accumulation.

• Normal left atrial pressure stays below +6 mm Hg.
• Strenuous exercise rarely raises normal left atrial pressure.
• Left heart failure raises pressure to 40–50 mm Hg.
• Pressure increases above 7–8 mm Hg elevate arterial pressure.
• Elevated left atrial pressure increases right heart workload.
• Capillary pressure rises equally with left atrial pressure.
• Left atrial pressure exceeding 30 mm Hg causes edema.

Pulmonary Capillary Dynamics and Transit Time

Capillaries in the alveolar walls form an extensive interconnected meshwork often described as a sheet of flow. Rapid gas exchange occurs as blood passes through these microvessels in under a second.

• Alveolar capillaries form an interconnected "sheet of flow."
• Isogravimetric capillary pressure measures about 7 mm Hg.
• Normal capillary transit time is 0.8 seconds.
• Heavy exercise shortens transit time to 0.3 seconds.
• Capillary recruitment prevents extreme shortening of transit time.
• Gas exchange completes in a fraction of a second.

Fluid Exchange Across Pulmonary Capillaries

Fluid movement across pulmonary capillary membranes follows the same physical principles as peripheral tissues. However, lower capillary hydrostatic pressure and higher protein permeability create distinct quantitative differences.

• Pulmonary capillary pressure is low at 7 mm Hg.
• Peripheral functional capillary pressure is higher at 17 mm Hg.
• Pulmonary interstitial fluid pressure is negative 5 to 8 mm Hg.
• Pulmonary capillaries are relatively leaky to plasma proteins.
• Interstitial fluid colloid osmotic pressure is 14 mm Hg.
• Thin alveolar epithelium ruptures if interstitial pressure becomes positive.
• Positive interstitial pressure dumps fluid directly into alveoli.

Starling Forces and Fluid Balance in the Lungs

A subtle balance of hydrostatic and osmotic forces governs fluid filtration in the pulmonary interstitium. Under normal conditions, net forces favor slight continual fluid filtration.

• Capillary hydrostatic pressure outward force is 7 mm Hg.
• Interstitial colloid osmotic pressure outward force is 14 mm Hg.
• Negative interstitial fluid pressure outward force is 8 mm Hg.
• Total outward force equals 29 mm Hg.
• Plasma colloid osmotic pressure inward force is 28 mm Hg.
• Total inward force equals 28 mm Hg.
• Mean net filtration pressure equals +1 mm Hg.
• Continual filtered fluid is removed by pulmonary lymphatics.
• Evaporation removes a small fraction of alveolar fluid.

Mechanism for Keeping Alveoli Dry

Alveolar spaces must remain free of fluid excess to allow efficient gas diffusion. The negative interstitial pressure created by the lymphatic pump acts as a mechanical vacuum for liquid entering the alveoli.

• Openings exist between adjacent alveolar epithelial cells.
• Water, electrolytes, and proteins cross alveolar epithelial cells.
• Negative interstitial pressure sucks fluid from alveolar spaces.
• Excess alveolar fluid enters the lung interstitium mechanically.
• Lymphatics pump excess fluid away from interstitium continuously.
• Capillaries absorb excess fluid from interstitial space.
• Small fluid amounts remain to keep alveolar surfaces moist.

Causes and Dynamics of Pulmonary Edema

Pulmonary edema develops when fluid filtration exceeds the clearance capacity of the lymphatics. This shift fills the interstitial spaces and alveolar sacs with fluid, severely compromising respiration.

• Any factor increasing filtration causes pulmonary edema.
• Impaired lymphatic drainage causes fluid buildup in tissues.
• Interstitial pressure rising to positive range fills alveoli.
• Left-sided heart failure causes high pulmonary venous pressure.
• Mitral valve disease floods interstitial spaces and alveoli.
• Pneumonia damages pulmonary blood capillary membranes directly.
• Chlorine and sulfur dioxide gases damage capillary membranes.
• Membrane damage causes rapid protein and fluid leakage.

Pulmonary Edema Safety Factors

The body possesses protective safety mechanisms that prevent fluid accumulation under moderate pressure elevations. Chronic pressure increases trigger adaptations that enhance fluid clearance.

• Capillary pressure must exceed plasma colloid osmotic pressure.
• Critical capillary pressure threshold in humans is 28 mm Hg.
• Normal capillary pressure is 7 mm Hg.
• Acute safety factor against pulmonary edema is 21 mm Hg.
• Left atrial pressure above 23 mm Hg causes edema.
• Chronic capillary elevation expands lymphatic vessels up to 10-fold.
• Chronic stenosis permits 40–45 mm Hg without lethal edema.
• Massive acute pressure spikes cause death in 30 minutes.

Fluid Dynamics in the Pleural Cavity

The pleural cavity contains a lubricating fluid layer that allows smooth sliding of the lungs against the chest wall. Continuous lymphatic pumping maintains a negative pressure that keeps the lungs expanded.

• Mucoid fluid layer lubricates parietal and visceral pleurae.
• Mesenchymal pleural membrane continually transudes tissue proteins.
• Lymphatics pump excess fluid from the pleural space.
• Lymphatic drainage enters mediastinum, diaphragm, and parietal pleura.
• Pleural space functions normally as a potential space.
• Negative pleural fluid pressure keeps lungs pulled expanded.
• Normal lung collapse tendency equals −4 mm Hg.
• Measured pleural fluid pressure averages about −7 mm Hg.

Causes of Pleural Effusion

Pleural effusion occurs when excessive fluid accumulates within the pleural space. This condition parallels interstitial edema and is driven by similar mechanical disturbances.

• Pleural effusion represents edema of the pleural cavity.
• Blockage of lymphatic drainage causes pleural effusion buildup.
• Left heart failure raises pulmonary capillary pressures significantly.
• Right heart failure increases systemic capillary hydrostatic pressure.
• Excessive capillary pressure causes high transudation into pleura.

Disclaimer: This content is for educational purposes only and not a substitute for professional medical advice.