Non-Respiratory Functions of the Lungs

1. Left Ventricular Reservoir

2. Filtration of Small Emboli

• The lungs have a high content of heparin and plasminogen activator, which facilitate breakdown of entrapped fibrin debris.
• Average pulmonary capillary diameter is ~7 µm, though larger particles including fat macroglobules can pass through.
• A downside: cancer cells may find the pulmonary bed a favorable site for metastatic seeding. - Morgan & Mikhail's Clinical Anesthesiology, p. 977; Medical Physiology, p. 878

3. Biochemical / Metabolic Functions

Agents and their handling:

• Angiotensin conversion: ACE (angiotensin-converting enzyme) is bound to the surface of pulmonary endothelial cells and converts angiotensin I (decapeptide) to the active angiotensin II (octapeptide). This is a major step in the renin-angiotensin-aldosterone system.
• Serotonin & bradykinin are largely removed from circulation by the pulmonary endothelium.
• Histamine and epinephrine are NOT metabolized in the lungs; in fact, the lung can be a major site of histamine synthesis and release during allergic reactions.
• Morgan & Mikhail's, p. 977-978; Medical Physiology, p. 879

4. Surfactant Synthesis

• Reduces alveolar surface tension
• Prevents alveolar collapse (atelectasis) at end-expiration
• Contributes to lung compliance

5. Extrahepatic Drug Metabolism

6. Immune / Defense Functions

• Alveolar macrophages (dust cells): phagocytose inhaled particles and pathogens
• Mucociliary escalator: ciliated epithelium propels mucus and trapped particles up to the oropharynx
• Neutrophils and macrophages produce O₂-derived free radicals (reactive oxygen species) in response to infection
• Secretory IgA in airway secretions provides humoral defense
• Coughing and sneezing reflexes accelerate removal of particulates from conducting airways - Medical Physiology, p. 878; Morgan & Mikhail's, p. 977

7. Fluid Balance

Summary Table

Role of Lungs in Drug Metabolism

Important Note About the Library

What Crofton & Douglas States

"In addition to this primary function, the lungs have other functions, for example water balance, the maintenance of pH, elimination of inhaled particles and organisms, filtration of particulate matter from the circulation, and metabolism of certain drugs and enzymes. They also serve as a vehicle for the administration of anaesthetic and other drugs."

Mechanisms of Pulmonary Drug Metabolism

1. Cytochrome P450 (CYP) Enzyme System

• Drug metabolism occurs primarily in the liver via the cytochrome P450 family of enzymes.
• However, CYP enzyme levels in lung tissue are estimated to be 10-15% of those in the liver.
• Crucially, lung-specific CYP450 isoenzymes exist, implying lung-specific metabolism that differs qualitatively from hepatic metabolism - not just a smaller version of it.
• Host-specific nucleotide polymorphisms affecting these enzymes may confer increased risk of pulmonary drug toxicity in certain individuals.
• Fishman's Pulmonary Diseases and Disorders, p. 1131

2. Pulmonary Endothelial Metabolism (Vasoactive Substances)

3. Mixed-Function Oxidation (Extrahepatic)

• Pneumocytes (particularly Type II) account for a significant portion of extrahepatic mixed-function oxidation.
• The lung is second only to the liver as a site of P450-mediated biotransformation.
• This is relevant for inhaled drugs (e.g., inhaled corticosteroids, anesthetics) that avoid hepatic first-pass metabolism - the lung itself can metabolize the absorbed portion.

4. First-Pass Pulmonary Metabolism of Inhaled Drugs

• The swallowed fraction of inhaled drugs undergoes hepatic first-pass metabolism (up to 99% for fluticasone).
• The fraction absorbed directly through the lung into the systemic circulation bypasses hepatic metabolism.
• The lung itself can metabolize part of this absorbed fraction, affecting bioavailability.
• This principle governs the design of inhaled therapies - high local lung concentrations with minimal systemic effect.

5. Effect of Pulmonary Disease on Drug Metabolism

"Pulmonary disease may also affect drug metabolism, as indicated by the impaired hydrolysis of procainamide and procaine in patients with chronic respiratory insufficiency and the increased half-life of antipyrine (a P450 functional probe) in patients with lung cancer."

• Chronic respiratory insufficiency impairs hydrolysis reactions in the lung.
• Lung cancer disrupts lung-specific P450 activity.
• These effects are clinically relevant when dosing drugs with significant pulmonary metabolism components.

Summary

Platelet Formation in the Lungs

What Crofton & Douglas Says

Dickinson CJ, Martin JF. "Megakaryocytes and platelet clumps as the cause of finger clubbing." Lancet 1987.

The Broader Science: Lungs as a Site of Platelet Biogenesis

The Landmark 2017 Study (Lefrancais et al., Nature)

Lefrancais E, et al. "The lung is a site of platelet biogenesis and a reservoir for haematopoietic progenitors." Nature 2017;544:105-9.

• Megakaryocytes resident in lung capillaries shed platelets directly into the pulmonary circulation
• The lungs produced >50% of total platelet production in mice under steady-state conditions
• The lungs also harbor a reservoir of haematopoietic progenitor cells capable of reconstituting platelet production after bone marrow injury
• Megakaryocytes migrated from bone marrow to the lung, where the unique shear forces and endothelial interactions (particularly via vWF-GPIb interactions) drove proplatelet extension and fragmentation

Why the Lung is Suited for Platelet Release

Recent Challenge (2024)

"Analysis of murine and human scRNA-seq data sets revealed that only a marginal fraction of MK-like cells can be found within the lung and most likely only marginally contribute to platelet production in the steady state."

Connection to Clubbing - The Crofton & Douglas Perspective

• Normally, large megakaryocytes arriving in the pulmonary capillaries are fragmented into platelets and do not pass further.
• In conditions like bronchial carcinoma, bronchiectasis, lung abscess, cystic fibrosis, and idiopathic pulmonary fibrosis, arteriovenous shunts or vascular bypass allow intact megakaryocytes and platelet clumps to escape into systemic circulation.
• These release platelet-derived growth factor (PDGF) and other growth factors in peripheral tissues (nail beds, periosteum), causing the vascular changes and new bone formation characteristic of clubbing and HPOA.

Summary

Role of the Lungs in Clotting

From Crofton & Douglas's Respiratory Diseases

1. The Lung Endothelium as a Natural Anticoagulant Surface

"The normal venous endothelium possesses natural antithrombotic molecules, including heparan sulphate (which neutralizes thrombin), thrombomodulin (which inhibits thrombin) and plasminogen activator (which promotes local fibrinolysis). These, and the vasodilatory factors prostacyclin and nitric oxide, act to keep the [vessels patent]."

2. Heparin - Sourced From Lung Tissue

"Heparin is a sulphated mucopolysaccharide obtained from animal lung and gut mucosa. It is available as a sodium or calcium salt and combines with the naturally occurring clotting factor activator antithrombin III, improving the inhibitory effect of this substance on factors IIa (thrombin) and Xa."

• Commercial heparin is extracted from porcine lung and bovine gut mucosa - the lung is literally the biological source of the most widely used anticoagulant drug.
• The lung's own mast cells synthesize and store heparin endogenously; this contributes to local anticoagulation within the pulmonary vasculature.

3. Filtration of Thromboemboli and Fibrinolysis

• The pulmonary vasculature acts as a mechanical filter for thromboemboli, fat emboli, and air bubbles arising from the systemic venous circulation.
• The lung has a high content of plasminogen activator in pulmonary endothelium, which facilitates rapid dissolution of trapped fibrin clots.
• Small emboli are silently resolved by this local fibrinolytic activity without clinical consequence.

"This approach [anticoagulation] prevents the formation of new clot or the extension of existing thrombus and depends upon natural fibrinolysis to remove emboli from the lungs."

"Thrombolytic drugs stimulate the natural process by activating the plasminogen-plasmin system, which acts on preformed thrombus..."

4. Role in ARDS - Activation of Clotting Cascades

"There has been much less study of the role of the contact system that activates bradykinin and the clotting and fibrinolytic cascades, although these are likely to be important in the pathogenesis of ARDS."

• In ARDS, the normally anti-coagulant pulmonary endothelium becomes pro-coagulant due to endothelial injury.
• This results in intra-alveolar fibrin deposition, microthrombi, and hyaline membrane formation.
• The balance between the lung's pro- and anti-coagulant functions is thus central to the pathophysiology of acute lung injury.

5. Pro-coagulant Roles of the Lung

Summary