BIOLOGICAL TRANSPORT 



cellular organism living in the sea may elaborate and maintain an 

 extracellular fluid— an internal medium bathing its cells— having an 

 entirely different composition from the salt or fresh water of the 

 outside environment. In this case, barriers of cells lie between the 

 internal and the external environment, principally at the gills, in the 

 alimentary canal, and in the kidneys, and determine what solutes 

 pass in each direction. Similarly, the land animal maintains the com- 

 position of its extracellular fluid by controlling exchanges across 

 mucosal barriers. 



Within the organism, secretions of still-different compositions 

 are produced from the extracellular fluid. Typically, in this process 

 a pool of the normal environment is first separated by a preliminary 

 filtration or secretion; then a barrier of cells lying between this pool 

 and the main portion of the extracellular fluid proceeds to enrich the 

 secretion with certain solutes and to impoverish it with regard to 

 others. In the specific case of the kidney, the blood passes through 

 two successive capillary networks: In the first, ultrafiltration at the 

 glomerulus (Figure 1) produces the primary flow; in the second, 

 the blood is again brought into intimate contact with this flow, 

 separated this time by a barrier of secretory cells; the cells transfer 

 solutes from one aqueous phase to the other. Such a circulatory 

 arrangement, passing through two successive capillary networks, is 

 called a portal circulation. 



Because the composition of secretions usually differs strikingly 

 from that of the main bulk of the extracellular fluid, secretion was 

 probably the first instance of transport to be recognized— over a cen- 

 tury ago; the process still serves to convince the most critical ob- 

 server that living cells may have highly developed potentialities 

 for producing concentration gradients. Much more serious diffi- 

 culties are encountered in proving that a given solute can be trans- 

 ported against concentration gradients into the cell itself. 



We should note further that the enrichment of a secretion with 

 a solute is often achieved indirectly, merely by withdrawing solvent 

 from the stream of filtrate while preventing the solute in question 

 from following the solvent. In the case of renal secretion, the ex- 

 tracting of water is also indirect, being produced by the transport 

 of an important osmolite, especially the sodium ion, out of the ultra- 

 filtrate; the water follows osmotically. This sequence is the one 

 that causes many solutes, e.g., creatinine, to be far more concentrated 

 in the urine than in the blood plasma. 



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