194 THE ARMY FOR HOME DEFENCE 



tubules, and finally opens into the pelvis of the kidney. As the 

 structure of the kidney is intimately related to its function we 

 must briefly follow the course of a tubule from capsule to ureter 

 and note the type of epithelium with which it is lined. 



Capsule, flat thin endothelial cells ; > neck of tubule, cylindrical epithe- 

 lial cells ; > first convoluted tubule, columnar epithelium, indefinite out- 

 lines, rows of granules arranged vertically to base of cells, striated free border ; 



> U-shaped loop of Henle, descending limb has flattened epithelium while 



ascending limb is similar to convoluted tubules, but with less striation 



second convoluted tubule, as first ; > junctional tubule, cubical or 



columnar cells with no granules ; > straight collecting tubule, same as 



junctional. 



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

 the cortex into a large number of arterioles, 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. In the mammal, the capillaries surround- 

 ing the tubules may receive some blood which has not passed 

 through the glomeruli. 



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

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

 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. (See Chap. XXII., O.P. of 

 Plasma.) 



Function of Capsule. 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, arguments against the supposition may 

 be found. For instance, the thin layer of epithelial cells is not 

 strengthened in any way to stand a large filtration pressure. Again 

 it is doubtful whether any such pressure exists in the glomeruli. 

 Measurements are given of general arterial pressure, say in the 

 carotid artery. The capillary pressure niay be under one-fifth of 

 this. 



