The right atrium of the heart serves as the starting point for the pulmonary circulation. Deoxygenated blood enters the right ventricle through the tricuspid valve and is then pumped into the pulmonary artery (pulmonary trunk) at a low pressure of 9 to 24 mm Hg. The pulmonary artery branches into the right and left main pulmonary arteries, which supply blood to the respective lungs. The arteries of the pulmonary circulation are unique in that they carry deoxygenated blood. These arteries further divide into a series of branching vessels, including larger arteries (diameter >500 µm), arterioles (diameter 10 to 200 µm), and capillaries (diameter <10 µm). The capillaries form a complex network and surround the alveoli, where gas exchange takes place. The sequential branching pattern of the pulmonary arteries mirrors that of the airway branching. The pulmonary circulatory system has three main functions: reoxygenating the blood and releasing carbon dioxide, maintaining fluid balance in the lung, and facilitating the distribution of metabolic products to and from the lung. The optimal configuration of the alveolar wall allows for efficient gas exchange through the alveolar-capillary network.
The pulmonary circulation begins in the right atrium of the heart. Deoxygenated blood from the right atrium enters the right ventricle via the tricuspid valve, and it is then pumped under low pressure (9 to 24 mm Hg) into the pulmonary artery (pulmonary trunk), which is approximately 3 cm 200 µmPVCTB PARB AD A PV PV A I A A C C C II M10 µm 200 µm PAPA A A A I I I I EL C R COL E * E Nu 1.0 µmB •Fig. 20.6 Alveoli. A. Theterminalrespiratoryunitconsistsofthealveoli(A)andthealveolarducts(AD)arisingfromarespiratorybronchiole(RB).Eachunitisapproximatelyspherical,assuggestedbythedashed outline. Pulmonaryvenousvessels(PV)haveaperipherallocation.PA,pulmonaryartery;TB,terminalbronchiole.Inset, typeIandtypeIIalveolarepithelialcells.Alargefractionofthealveolarwallconsistsofcapillaries(C)andtheircontents.B.Illustrationofacross-sectionofanalveolarwall,showingthepathofdiffusionofO2andCO2.Thethinsideofthealveolarwallbarrier(short double arrow) consistsoftypeIepithelium(I),interstitium(asterisk) formedbythefusedbasallaminaeoftheepithelialandendothelialcells,capillaryendothelium(E),plasmainthealveolarcapillary(C),andthecytoplasmoftheredbloodcell(R).Thethicksideofthegas-exchangebarrier(long double arrow) hasanaccumulationofelastin(EL),collagen(COL),andmatrixthatjointlyseparatethealveolarepitheliumfromthealveolarcapillaryendothelium.Nu,nucleusofthecapillaryendothelialcell.
in diameter, and branches quickly (5 cm from the right progressively smaller series of branching vessels—arteries ventricle) into the right and left main pulmonary arteries, (diameter, >500 µm); arterioles (diameter, 10 to 200 µm); which supply blood to the right and left lungs, respectively. and capillaries (diameter, <10 µm)—that end in a complex The arteries of the pulmonary circulation are the only arter-mesh-like network of capillaries. The sequential branching ies in the body that carry deoxygenated blood. The deoxy-pattern of the pulmonary arteries follows the pattern of genated blood in the pulmonary arteries passes through a airway branching.
•Fig. 20.7 Illustrationoftheanatomicalrelationshipofthepulmonaryartery,thebronchialartery,theairways,andthelymphaticvessels.A,alveoli;AD,alveolarducts;RB,respiratorybronchioles;TB,terminalbronchioles.AD AD AD AD RB TB Lymphatics Lymphatics Vasomotor nerves Pulmonary vein Bronchomotor nerve Pulmonary artery Bronchial artery Bronchus A A A A A
The functions of the pulmonary circulatory system are (1) to reoxygenate the blood and release CO2, (2) to aid in fluid balance in the lung, and (3) to distribute metabolic products to and from the lung. Red blood cells are oxygenated in the capillaries that surround the alveoli, where the pulmonary capillary bed and the alveoli come together in the alveolar wall in a unique configuration for optimal gas exchange. Gas exchange occurs through this alveolar-capillary network (see