318 The Relation between Secretory and Capillary Pressure 



veins. The quality of the blood influences the secretory activity ; this, in its 

 turn, influences the blood flow within the kidney. When the ureter is 

 obstructed and the kidney secretes against pressure, the tubules must swell 

 until the membranae propriae are drawn taut. In the case of the salivary 

 gland, the swelling of the alveoli is limited by the niembranse proprise, so that 

 the blood-vessels remain patent in spite of the higher secretory pressure. In 

 the kidney, however, the secretion ceases before the secretory pressure rises 

 to the arterial pressure. Our next point of enquiry must be to determine 

 whether the cells of the tubules are able to swell to such an extent as 

 to impede the circulation. Further experiments will show us what 

 happens to the flow in the renal vein at the moment when the secretory 

 pressure of the urine reaches its highest point. 



In states of inflammation, as their osmotic pressure increases, the poisoned 

 tissues swell, the arteries dilate, the blood-pressure rises as the swollen 

 tissues press upon and narrow the veins. The vessels approximate to a 

 rigid system, hence the high tension and the pulsatile throb of the inflamed 

 part. As the tissue pressure rises in the part, so does the circulatory 

 pressure, since the tension depends on the arterial pressure ; its height, 

 however, being regulated by the metabolism of the tissues and their 

 imbibition pressure. Finally, stasis is produced in the swollen tissues. 

 The surgeon's knife by cutting, or the hot fomentation by softening, relieve 

 the tension and allow an ample flow of blood to and the escape of immunising 

 plasma in the part. It is a question here whether stasis results from 

 inflammatory changes in the blood or from strangulation of the circulation 

 by the increased imbibition and swelling of the tissues. The tissue cells in 

 the skin are confined by strong frameworks of connective tissue, which limit 

 expansion and act in a similar manner as does the capsule to a gland, the 

 sclerotic to the eye, or the skull wall to the brain. 



Summary. 



(1) When the pressure of the salivary secretion is raised above the arterial 

 pressure the venous flow of blood from the gland continues. 



(2) Under these conditions the gland feels very tense ; by squeezing it the 

 secretory pressure is raised but the flow of blood from the vein is stopped. 



(3) The view is advanced that the membranse propria? (aided possibly by 

 the strands of connective tissue which surround the alveoli) limit the 

 expansion of the alveoli and thus protect the circulation from strangulation 

 by the high secretory pressure. It is suggested that limiting membranes in 

 other parts of the body, e.g. the kidney, have a similar function. 



(4) By the expansion of the alveoli up to the limitation point set by the 



