The pulmonary system
Lungs exchange O2 and CO2 between the blood and air, and participate in regulation of the pH in the body.
Air comes to the lungs via trachea, which is divided into bronchi, and further into smaller bronchioles (diameter 0.5-1 mm), ending up in alveoli (diameter 0.2-0.3 mm), where the gases are exchanged. Bronchioles are supported by smooth muscle which can contract or relax as needed. Alveolus is composed of epithelial tissue, formed by type I (squamous) and type II (cuboidal or great) alveolar cells, and macrophages. Squamous cells enable the gas exchange via diffusion, and cuboidal cells secrete surfactant into the lumen to lower the surface tension of water. Alveolus contains collagen and elastic fibres, allowing alveoli to stretch and spring back during inhalation/exhalation cycle. Alveoli are surrounded by dense mesh of blood capillaries. The support for capillaries is weak, causing those to collapse if intraluminal pressure is low or alveolar pressure is increased.
At rest, lungs provide about 1 L O2 per min to the tissues via the vascular system, of which only about 1/4 is used. O2 is poorly soluble in water, and therefore about 98% of oxygen is transported as bound to haemoglobin in red blood cells.
Carbon dioxide transport
CO2 is soluble in water, and about 7% of it in the blood is directly dissolved in the water, 70% is dissolved in water as bicarbonate ion, HCO3-, and the rest is bound to haemoglobin in red blood cells.
Pulmonary circulation is part of the vascular system. Pulmonary circulation receives all of the cardiac output; all venous blood is pumped from the right side of the heart via pulmonary arteries to the lungs for the gas exchange before returning via pulmonary veins into the left side of the heart.
Pulmonary capillaries have diameters between 2 and 13 µm, enabling effective exchange of molecules between blood and extravascular volume. Red blood cells are small and highly deformable and can transit rapidly, while more rigid and large white blood cells travel slowly (Doerschuk et al., 1993). Rigid particles with a diameter >10 µm may be permanently trapped in the lung capillaries.
Endothelial walls are normally tight, but inflammation leads to increased permeability.
Bronchial circulation provides oxygenated blood to the walls of pulmonary arteries and veins, bronchi and bronchioles, nerves, lymph nodes, and visceral pleura. It is part of the systemic circulation, but can also contribute to the gas exchange when pulmonary circulation is compromised. On the other hand, pulmonary circulation participates in supplying bronchi and bronchioles with blood flow. Bronchial blood flow is low, only 1-5% of pulmonary circulation. Bronchial arteries are less than 1.5 mm in diameter.
Bronchial smooth muscle cells contain NMDA receptors (subtype of iGluRs in glutamatergic system), regulating smooth muscle contraction.
Immune cells in the lung tissue and airway mucosa mainly enter via the bronchial circulation.
Pulmonary function tests (PFTs), including spirometry, provide noninvasively information about global lung function, and can be used to assess the severity of pulmonary impairment for example in asthma, chronic obstructive pulmonary disease (COPD), and cystic fibrosis. Static lung imaging is already a standard clinical tool enabling the assessment of regional lung function. Improvements in spatial and temporal resolution of imaging systems will allow development of new methods for quantitation of lung physiology (Robertson & Buxton, 2012).
Ventilation imaging can be performed using SPECT or PET with 99mTc (Technegas®) or 68Ga (Galligas) labelled nanoparticles (Nozaki et al., 1995; Kotzerke et al., 2010a and 2010b; Borges et al., 2011). For pulmonary perfusion imaging, macroaggregated albumin (MAA) can be labelled with the same isotopes. Pulmonary ventilation and perfusion imaging can provide functional lung volumes that correlate well with PFT parameters (Le Roux et al., 2015 and 2017). PET-Galligas has also been used to develop and validate 4D-CT methods for assessing pulmonary ventilation (Kipritidis et al., 2014; Eslick et al., 2016).
Perfusion images in lung cancer can be calculated from [15O]H2O PET data (van der Veldt et al., 2010). Lung water content can be estimated from [15O]H2 PET 3.5-5.5 min p.i., when equilibrium is reached (Velasquez et al., 1991).
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Created at: 2016-05-14
Updated at: 2018-09-15
Written by: Vesa Oikonen