Endothelial cells possess surface receptors for various substances such as bradykinin, tumor necrosis factor, complement system components, immunoglobulin crystallizable fragments, and adhesion molecules. Moreover, they are involved in the synthesis and secretion of substances like prostacyclin, endothelin, clotting factors, nitric oxide, prostaglandins, and cytokines. However, these cells do not have the ability to synthesize leukotrienes, which are smooth muscle constrictors. Certain compounds like epinephrine, dopamine, histamine, isoproterenol, angiotensin II, and substance P are not metabolized by the pulmonary capillary bed.

The presence of angiotensin-converting enzyme (ACE) on the surface of pulmonary endothelial cells facilitates the conversion of angiotensin I to angiotensin II, a potent vasoconstrictor. This conversion plays a significant role in the regulation of blood pressure. ACE inhibitors are commonly used therapeutically in managing high blood pressure.

The lungs play a crucial role in the activation of prodrugs, which are inactive substances, into active drugs. By administering prodrugs, the amount of active drug delivered to its intended targets in the body is increased. In many cases, inactive precursor prodrugs are delivered to the lungs either systemically or through inhalation, where they undergo activation. An example of this is beclomethasone dipropionate, an asthma medication that is activated by esterases within the lung.

1. The respiratory system possesses unique structural and immunological features to counter the constant exposure to foreign substances, which help limit or inhibit inflammation.
2. The mucociliary transport system comprises the periciliary fluid (sol phase), mucus (gel phase), and cilia.
3. The maintenance of the depth of the periciliary fluid layer is crucial for normal ciliary beating and is achieved through a balance between Cl− secretion and Na+ absorption.
4. Mucus is a complex macromolecule consisting of glycoproteins, proteins, electrolytes, and water. It exhibits low viscosity and high elastic mechanical properties.
5. Mucus is produced by goblet cells, Clara cells, and the mucous and serous cells found in the tracheobronchial glands.
6. Particle deposition in the lungs depends on factors such as size, density, shape, distance traveled, airflow speed, and relative humidity. The primary mechanisms of particle deposition include impaction, sedimentation, interception, and brownian movement.
7. The respiratory system is part of the mucosal immune system, which also encompasses the intestinal (GALT) and urinary tract systems. These systems lack true lymph nodes with afferent and efferent lymph flow, consisting mainly of nonencapsulated lymph nodules without proper lymphatic drainage.
8.
Endothelial cells also have surface receptors for bradykinin, tumor necrosis factor, components of the complement system, immunoglobulin crystallizable fragments, and adhesion molecules. In addition, vascular endothelial cells synthesize and secrete prostacyclin, endothelin, clotting factors, nitric oxide, prostaglandins, and cytokines. Vascular endothelial cells, however, lack 5-lipoxygenase and are not able to synthesize leukotrienes (smooth muscle constrictors). Compounds not metabolized by the pulmonary capillary bed include epinephrine, dopamine, histamine, isoproterenol, angiotensin II, and substance P.
Angiotensin-convertingenzyme(ACE)ispresentinsmallindentations(caveolae)onthesurfaceofpulmonaryendothelialcellsandcatalyzestheconversionofthephysiologicalinactiveangiotensinItotheactiveangiotensinII,apotentvasoconstrictor.Thisisamajormechanisminthebody’sabilitytosupplysystemiclevelsofangiotensinIIandthusinfluencebloodpressure(see
Chapter36).
ThetherapeuticuseofACEinhibitorsisimportantinthemanagementofpatientswithhighbloodpressure.
Thelungsplayakeyroleinthemetabolismofmanyprodrugs,whichareinactive,intoactivedrugs.Administrationofprodrugsimprovestheamountofactivedrugdeliveredtotheirtargetsinthebody.Inmanycases,inactiveprecursorprodrugsaredeliveredsystemicallyorlocally(throughinhalation)tothelungs,wheretheyareactivatedinsitu.Anexampleofthisisbeclomethasonedipropionate(Qvar,BeconaseAQ),amedicationinhaledbypatientswithasthma,whichisactivatedbyesteraseswithinthelungtotheactiveform17-beclamethasonemonopropionate.
1.
The respiratory system has developed unique structural (mucociliary transport system) and immunological (mucosal immune system) features to cope with the constant environmental exposure to foreign substances; these features limit or inhibit inflammation.
2.
The three components of the mucociliary transport system are the sol phase (periciliary fluid), the gel phase (mucus), and cilia.
3.
The depth of the periciliary fluid layer is maintained by the balance between Cl− secretion and Na+ absorption and is essential to normal ciliary beating.
4.
Mucus is a complex macromolecule composed of glycoproteins, proteins, electrolytes, and water. It has low viscosity and high elastic mechanical properties.
5.
Goblet cells, Clara cells, and the mucous and serous cells residing in the tracheobronchial glands produce mucus.
6.
Particle deposition in the lung is dependent on their size, density, and shape; the distance traveled; airflow speed; and relative humidity. The major mechanisms for particle deposition are impaction (particles larger than 10 µm, in nasal passages, and particles 2 to 10 µm, in the nasopharynx, trachea, and bronchi), sedimentation (particles 0.2 to 2 µm in size, in distal airways), interception (particles with elongated shape, in the lower airways), and brownian movement (particles smaller than 0.2 µm, in the alveoli).
7.
The respiratory system is part of the mucosal immune system, which is composed of the intestinal (GALT), the respiratory (BALT), and urinary tract systems. These systems do not contain true lymph nodes with afferent and efferent lymph flow; they are composed mainly of nonencapsulated lymph nodules without true lymphatic drainage.
8.