Lungs

The gas exchangers — and minor metabolic organs besides. The lungs' first job is oxygen and carbon dioxide; quietly, they also clear or modify a whole pharmacopoeia of circulating compounds.

Primary Role

Gas exchange for the blood stream

Metabolic Pathways

Surfactant synthesis, angiotensin conversion, mediator clearance

Energy Budget

Modest: the work of breathing is a few percent of rest at baseline

Key Cell Types

Type II pneumocytes, Endothelium, Alveolar macrophages

Structure & Metabolism

Three hundred million alveoli present 70 square meters of membrane to the air — a surface the size of a tennis court in a volume of a few liters. Type II cells renew the surfactant that keeps those alveoli open; alveolar macrophages keep them clean; the endothelium lining every capillary is metabolically active tissue in its own right.

Metabolic Pathways

• Surfactant: dipalmitoylphosphatidylcholine made and recycled by type II cells — the molecule that lets alveoli exist.
• ACE on pulmonary endothelium converts angiotensin I to II — a blood-pressure enzyme living in lung.
• Clearance of serotonin, bradykinin, and many drugs on first pass through the pulmonary circulation.

Regulation

• Ventilation matches metabolism: chemoreceptors tune depth and rate to CO2.
• Hypoxic pulmonary vasoconstriction routes blood to where air actually is.
• Surfactant turnover is continuous — the fetal lung's last preparation for birth.

Interactions

• Right-heart output in, oxygenated blood out — the heart's pulmonary partner.
• CO2 clearance serves the excretory system's acid-base mission.
• Its structure and mechanics are covered at the respiratory system.

Clinical Relevance

• ARDS: surfactant failure and alveolar flooding — the ICU's lung catastrophe.
• Pulmonary fibrosis stiffens the exchange membrane; emphysema deletes it.
• ACE inhibitors — the lung's own enzyme — are blood-pressure therapy by accident of anatomy.
• The lung also traps circulating microemboli as a serial filter, a duty that costs it in systemic disease.
• Lung volume halves from youth to old age as elastic recoil fades — the physiology behind the aging chest.

Sources

• Wikipedia: Lung
• Wikipedia: Pulmonary alveolus