Lung Metabolic Functions

The lung is not only a gas-exchange organ but also a metabolically active one. Because the entire cardiac output passes through the pulmonary circulation, the lung is uniquely positioned to synthesise, activate and inactivate circulating biologically active substances, modifying the chemical composition of blood as it traverses the pulmonary capillaries.

Definition

Lung metabolic functions are the non-respiratory biochemical activities of the lung, including the synthesis, activation and inactivation of circulating substances by pulmonary endothelial and epithelial cells as blood passes through the pulmonary circulation.

Scope

This topic covers the non-respiratory metabolic roles of the lung: processing of vasoactive and other mediators at the pulmonary endothelium, synthesis of substances such as surfactant lipids by the lung epithelium, and the lung's position as a filter and first-pass site in the circulation. It is reference-educational physiology and does not address drug dosing or treatment.

Core questions

• Why is the lung well placed to metabolise circulating substances?
• Which vasoactive substances are characteristically activated or inactivated in the pulmonary circulation?
• What does the lung itself synthesise, and where?
• How does first-pass pulmonary metabolism influence the composition of arterial blood?

Key concepts

• First-pass pulmonary metabolism
• Pulmonary endothelial processing of mediators
• Activation and inactivation of vasoactive substances
• Synthesis of surfactant lipids by the lung
• The lung as a circulatory filter

Mechanisms

Because virtually the whole cardiac output flows through the pulmonary capillary bed, and the bed presents an enormous endothelial surface, the lung is positioned to act on substances dissolved in the blood. Pulmonary endothelial cells carry enzymes that convert or remove circulating mediators, so some vasoactive substances are activated and others are inactivated during a single passage through the lung; classic physiology texts describe this selective handling of vasoactive agents at the pulmonary endothelium. The lung epithelium also synthesises substances of its own, notably the lipids of pulmonary surfactant produced by alveolar type II cells. Through these activities the lung modifies the chemical content of blood entering the systemic arterial circulation and contributes to whole-body mediator balance.

Clinical relevance

Recognising the lung's metabolic role explains why the pulmonary circulation can influence systemic levels of circulating mediators and why some substances given intravenously are altered on first pass through the lung. The topic is descriptive physiology and is not a basis for individual prescribing or treatment decisions.

Evidence & guidelines

The lung's metabolic functions are described in standard respiratory and medical physiology textbooks; this entry summarises that consensus rather than a clinical guideline.

History

The understanding of the lung as a metabolic organ developed alongside respiratory physiology in the twentieth century, as it became clear that the pulmonary endothelium processes circulating vasoactive substances and that the lung synthesises its own constituents such as surfactant; this body of work is consolidated in standard physiology textbooks.

Key figures

• John B. West
• Michael Levitzky

Seminal works

• west-2012
• boron-boulpaep-2017

Frequently asked questions

Why is the lung considered a metabolic organ?

Because the whole cardiac output passes through its large endothelial surface, the lung can synthesise, activate and inactivate circulating substances, changing the chemical composition of blood as it flows through.

Does the lung make anything itself?

Yes. Among other products, alveolar type II cells synthesise the lipids of pulmonary surfactant, illustrating the lung's synthetic, non-gas-exchange activity.