140 INTRODUCTION TO GENERAL PHYSIOLOGY 



trolling the rate of formation of urine, it is clear how this rate can 

 be modified apart from the effect of dilution of blood. If the small 

 arteries conveying blood to the filters are narrowed, by contraction 

 of their muscular coats, the pressure is reduced and the rate of 

 filtration decreased. And conversely, if they are dilated, the rate 

 goes up. Moreover, changes in the main arterial pressure will have 

 the same effects, independently of local changes in the renal cir- 

 culation. There are means of bringing about these various changes 

 by reflexes from the nervous system, as we shall see presently. 



Lymph. There is no reason to suppose that the blood vessels 

 of the glomeruli differ essentially in the nature of their permea- 

 bility from those of the rest of the body ; in fact, we have direct 

 evidence that the blood vessels generally are permeable to water and 

 crystalloids, impermeable to colloids. Hence, it may be asked, do 

 they not in other places than the glomeruli allow protein-free filtrate 

 to escape into the tissues, and, if so, what becomes of it? This, 

 indeed, is actually the case. As the blood flows from the arteries 

 through the capillaries to the veins by virtue of the greater pressure 

 in the former, in a part of its course the pressure is greater than the 

 osmotic pressure of the proteins. Liquid is filtered out here, and 

 is known as " lymph." It is the part of the blood with which the 

 tissue cells are in immediate relation. As the blood current passes 

 onwards to the veins, where the pressure is very low, at a certain 

 region the pressure has fallen to a value equal to that of the 

 osmotic pressure of the colloids, and beyond this point the internal 

 pressure in the capillaries is lower than the osmotic pressure of the 

 colloids. Accordingly, this osmotic pressure becomes active here 

 in attracting water, so that the lymph which was filtered off in the 

 previous part of the course is, to a large extent, reabsorbed. 

 Although the area in which reabsorption occurs is probably larger 

 than that in which filtration occurs, the rate of reabsorption is 

 insufficient to remove the whole of the filtrate, and what remains 

 passes away in channels which commence in the spaces between 

 the cells and gradually become definite vessels with thin walls, 

 finally joining together to form a large vessel, the " thoracic duct," 

 which opens into the veins at the root of the neck. 



Since it is by the osmotic pressure of the colloids in the blood 

 that the water is prevented from escaping into the tissues, and 

 causing what is known as "oedema," we see the object of adding a 

 colloid, such as gum arabic, to a liquid used for intravenous 

 injection to replace blood lost, or increase the volume in actual 

 circulation. Solutions containing crystalloids only have been 

 found useless, since they rapidly escape from the circulation. 

 Whereas, if 6 or 7 per cent, of gum be added, they remain in the 

 circulation, maintaining the volume and pressure of the blood at 



