130 ELECTROLYTES IN BIOLOGICAL SYSTEMS 



lipid and protein components. Hillier and Hoffman (41) have presented elec- 

 tron microscopic data which they interpret to indicate that the surface of the 

 human red cell ghost is composed of many glycolipoprotein plaques, each about 

 250 A in diameter and 30 A thick, which are bound to a deeper, tangentially, 

 oriented layer 20 A thick which may be composed of fibrous proteins. In view 

 of the high permeability of the red cell membrane to water (46), it is probable 

 that continuous aqueous channels or pores connect the exterior and interior 

 of the cell. The spaces between the lipoid plaques, areas roughly 10-20 A in 

 diameter, may possibly correspond to these pores. 



Human red cells contain approximately 140 mM K/kg H2O and 15 mM 

 Na/kg H2O. Since they are normally suspended in plasma containing 4 mM/1. 

 K and 140 mn/l. Na, the concentration gradients across the cell membrane 

 for both ions are large. It is highly probable that large differences between the 

 electro-chemical activities of Na and K inside and outside the cell also exist. 



There are two main arguments in favor of this deduction. First, it is probable 

 that there are no large differences in the activity coefficients of K and Na 

 between the inside and outside of the cell. This follows from the absence of an 

 appreciable osmotic pressure across the cell membrane (e.g. as measured by 

 freezing point depression (4)), and the approximate equality of total cation 

 concentration inside and outside the cell (117). Secondly, it is probable that 

 the electrical potential difference across the red cell membrane is far too small 

 to account for the observed asymmetry in K distribution. As mentioned 

 above, chloride ions penetrate the human red cell with extreme rapidity 

 (16, 54) and are probably always at thermodynamic equilibrium across the 

 cell membrane (39, 116). Since the ratio of cell to plasma chloride concentra- 

 tion (per kg H2O) is about .8 at pH 7.4 (36) the membrane potential cannot 

 be more than 8-10 mv, inside negative (100). This is far too small to account 

 for the K concentration ratio of about 30. 



From experiments with radioactive tracers (87, 94, 100), we now know that 

 K ions are constantly transported into and out of human red cells at a rate 

 of about 1.6 mM/(l. RBC) X (hr.) at 37°. Streeten and Solomon (103) have 

 recently presented evidence that 2.1 is a more correct figure for K influx when 

 the K concentration in the medium is 4.5 mn/l. The Na transport rate under 

 identical conditions is 3.0 mM/(l. RBC) X (hr.) (95, 100). In view of the 

 probable presence of large electro-chemical activity gradients for both ions, 

 these transport processes must require the performance of thermodynamic 

 work. 



We may state the Na and K transport properties of the human red cell 

 membrane in another way. Inward and outward rate constants for transport 

 may be defined as before: 'kn = 'MK/[K]m and °kK = "Mk/[K]c where k is the 

 rate constant in (hours)~\ M is the flux in mM/(l. RBC) X (hr.), and [K] 

 is the concentration of potassium in mn/kg HoO. Superscripts are i, inward. 



