A textbook of human physiology / translated from [the] 6th German edition by W. Stirling.

  • Landois, Leonard
Date:
1888
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Licence: Public Domain Mark

Credit: A textbook of human physiology / translated from [the] 6th German edition by W. Stirling. Source: Wellcome Collection.

Provider: This material has been provided by Royal College of Physicians, London. The original may be consulted at Royal College of Physicians, London.

    183/980 page 131
    (■5 00). The trunks of the puhiionary artery and veins are subjected to the same conditions of pressure. The elastic traction of the lungs is greater the more they are distended. The blood of the pulmonary capillaries will, therefore, tend to How towards the large blood-vessels. As the elastic traction of the lungs acts chiefly on the thin-walled pulmonary veins, while the semi-lunar valves of the pulmonary artery, as well as the systole of the right ventricle, prevent the blood from flowing backwards, it follows that the blood in the capillaries of the lesser ciradation mtist jioio towards the puhiionary veins. If tubes with thin walls be placed in the walls of an elastic distensible bag, the lumen of these tubes changes according to the manner in which the bag enclosing them is distended. If the bag be directly inflated so as to increase the pressure within it, the lumen of the tubes is diminished {Funke and Latscheidmyer). If the bag be placed within a closed space, and the tension within this space be diminished so that the bag thereby becomes distended, the tubes in its wall dilate. In the latter case—viz., by negative aspiration—the lungs are kept distended within the thorax, hence the blood-vessels of the lungs containing air are wider than those of collapsed lungs {Quincke and Ffeiffer, Boivditrh and Garland, De Ji'ujer). Hence also, more blood Mows through the lungs distended within the thorax than through collapsed lungs. The dilatation which takes place during inspiration acts in a similar manner. The negative pressure that obtains within the lungs during inspiration causes a considerable dilatation of the pulmonary veins, into which the blood of the lungs flows readily, whilst the blood under high pressure in the thick-walled pulmonary artery scarcely undergoes any alteration. The velocity of the blood-stream in the pulmonary vessels is accelerated during inspiration {De Jdger, Lalfsque). The blood-pressure in the pulmonary circuit is raised when the lungs are inflated. Contraction of small arteries, which causes an increase of the blood-pressure in the systemic circulation, also raises the pressure in the pulmonary circuit, because more blood flows to the right side of the heart. The vessels of the pulmonary circulation are very distensible and their tonus is slight. [Occlusion of one branch of the pulmonary artery does not raise the pressure within the aorta. Even when one pulmonary artery is plugged with an embolon of paraffin, the pressure within the aortic system is not raised {Lichtheim). When a large branch of the pulmonary artery becomes impervious, the obstruction is rapidly compensated for, and this is not due to the action of the nervous system. The vaso-motor system has much less effect upon the pulmonary blood-vessels than upon those of the systemic circulation. The compensation seems to be due chiefly to the great distensibility and dilatation of the pulmonary vessels {Lichtheim).^ We know little of the effect of physiological conditions upon the pulmonary artery. According to Lichtheim suspension of the respiration causes an increase of the pressure. [In one experiment he found that the pressure within the pulmonary artery was increased, while it was not increased in the carotid, and he regards this experiment as proving the existence of vaso-motor nerves in the lung.] Duiing the act of great straining, tlie blood at first flows rapidly out of the pulmonary veins, and afterwards ceases to flow, because the inflow of Ijlooil into the pulmonary vessels is inter- fered with. As soon as the straining ceases, blood Hows rapidly into the pulmonary vessels {Lalcsquc). Severini found that the blood-stream through the lungs is gi'eater and more rapid when the lungs are filled with air rich in C0._; than when the air within them is rich in 0. He supposes that these gases act upon the vascular ganglia within the lung, and thus affect the diameter of the vessels. Pathological.—Increase of the pressure within the area of the pulmonary arteiy occui s fre- quently in man, in certain cases of heart disease. In these cases the second pulmonary sound is always accentuated, while the elevation caused thereby in the cardiogram is always more marked and occurs earlier (§ 52). Electrical and mechanical stimulation of abilominal organs raises the blood-pressure in the pulmonary artery {Mord). [The action of drugs o)i the pulmonary circulation may be tested by Holmgren's apparatus
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