162 



THE ARMY FOR HOME DEFENCE 



3. Blood Supply. The artery supplying the kidney breaks up in 

 the cortex into a large number of capillaries, each of which forms 

 a nodule or glomerulus invaginated in Bowman's capsule. The 

 capillaries again coalesce to form the efferent vessel, and this 

 again breaks into a number of capillaries entwining round the 

 tubules. After this the blood leaves the kidney by way of the 

 renal vein. That is, the blood is first supplied to the glomeruli 

 and then to the tubules. 



4. Blood Pressure and Secretion. If the blood pressure is 

 lowered to between 40 and 30 mm. Hg. secretion stops. Starling 



Afferent 

 vessel 



Efferent 

 vessel 



Glomerular 

 capsule 



Capillary 

 tuft 



Capsule 



Tubule 



FIG. 28. Diagram of Malpighian corpuscle. (From Cushny's Secretion of Urine.) 



measured the osmotic pressure of plasma and found it to be about 

 30 mm. Hg. It is generally inferred from this that unless the 

 blood pressure be greater than the osmotic pressure of the plasma 

 colloids no secretion will take place. Starling confirmed this by 

 obstructing the ureter so that the Hydrostatic pressure therein 

 was equal to 92 mm. Hg. when secretion stopped. The blood 

 pressure was 133. That gives a filtration pressure of 133 minus 

 the osmotic pull back of the colloids (30), i.e. 103, approximately 

 equal to the pressure in the ureter. 



It is only fair to point out that, though modern theorists are 

 at one regarding the forced filtration of colloid free blood through 

 the capsule by means of glomerular pressure, i.e. heart work, 



