Pulmonary capillary circulation

How many cell layers continuously separate alveolar air from pulmonary capillary circulation ... 

Two different circulatory systems, the bronchial and the pulmonary, supply the lungs with blood [133], The bronchial circulation is a part of the systemic circulation and is under high pressure. It receives about 1% of the cardiac output and supplies the conducting airways, pulmonary blood vessels and lymph nodes [133], It is important for the distribution of systemically administered drugs to the airways and to the absorption of inhaled drugs from the airways [18]. The pulmonary circulation comprises an extensive low-pressure vascular bed, which receives the entire cardiac output. It perfuses the alveolar capillaries to secure efficient gas exchange and supplies nutrients to the alveolar walls. Anastomoses between bronchial and pulmonary arterial circulations have been found in the walls of medium-sized bronchi and bronchioles [18, 65, 67],... 

Kishi et al. (169)evaluated the acute toxicity of Lipiodol infusion into the hepatic arteries (HAD of beagles and found the influence of Lipiodol HAI to be dose dependent. The infused Lipiodol first passed through an arterioportal communication and distributed through the hepatic sinusoids to pulmonary capillaries and thence into systemic blood circulation. The circulation and embolization of oil droplets were found in the renal tubular cells of supracapsular cortex, the choroid plexus, the vascular endothelium, and the pancreatic duct epithelium, showing a process of intracellular collection of Lipiodol from the systemic blood circulation and of further metabolism-provoking cellular reactions. 

A small amount of Lewisite on the skin will cause local edema because of the effects of this agent on local capillaries. With a large amount of Lewisite, the pulmonary capillaries are also affected there is edema at the site of exposure and pulmonary edema. With even larger amounts of Lewisite, all capillaries are affected, and proteins and plasma leak from the circulation into the periphery. Even after small amounts of Lewisite, the fluid loss can be sufficient to cause diminution of renal function and hypotension (Goldman and Dacre, 1989). 

The safety factor in the pulmonary circulation is of the order of 21 mmHg, i.e. pulmonary capillary pressure has to be raised by 21 mmHg before fluid starts to accumulate in the interstitial spaces of the lung. One cause of confusion regarding the safety factor has been the confusion alluded to above between increase of fluid flow and increase of interstitial volume. Let us now examine the balance of forces across the pulmonary capillaries. 

In patients with impaired function of the left ventricle, NO has a potential to further impair left ventricular performance by dilating the pulmonary circulation and increasing the blood flow to the left ventricle, thereby increasing left atrial pressure and promoting pulmonary edema formation. Careful monitoring of cardiac output, left atrial pressure, or pulmonary capillary wedge pressure is important in this situation. 

There is no true pulmonary analogue to the systemic arterioles, since the pulmonary circulation occurs under relatively low pressure [West, 1977]. Pulmonary blood vessels, especially capiQaries and venules, are very thin walled and flexible. Unlike systemic capillaries, pulmonary capillaries increase in diameter, and pulmonary capillaries within alveolar walls separate adjacent alveoli with increases in blood pressure or decreases in alveolar pressure. Flow, therefore, is significantly influenced by elastic deformation. Although pulmonary circulation is largely unaffected by neural and chemical control, it does respond promptly to hypoxia. 

Atmospheric air is breathed in during inspiration. This inspired air mixes with the gas already in the respiratory tree and some is drawn into the alveoli where gas comes into intimate relation with the blood circulating in the pulmonary capillaries. The gas here is called alveolar gas. Blood entering the lungs from the pulmonary artery has a partial pressure of oxygen which is lower than that in the alveolar gas. Oxygen diffuses from the alveolar gas into the pulmonary blood. For carbon dioxide, the partial pressure gradient is from blood to alveolar gas and so this is the direction in which carbon dioxide diffuses. 

Capillary endothelial cells comprise 30-42% of cells in the alveolar region and comprise the walls of the extensive network of blood capillaries in the lung parenchyma. The endothelium forms a continuous, attenuated cell layer that transports respiratory gases, water, and solutes. However, it also forms a barrier to the leakage of excess water and macromolecules into the pulmonary interstitial space. Pulmonary endothelial cells, like type I cells, are vulnerable to injury from inhaled substances and substances in the systemic circulation. Injury to the endothelium results in fluid and protein leakage into the pulmonary interstitium and alveolar spaces, resulting in pulmonary edema. 

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