Pi1.0-1.4(total)mmol/L 0.5-0.7(ionized)0.5-0.7(ionized) *Ca++ andPi(H2PO4−/HPO4−2)areboundtoproteinsandotherorganicmolecules.Inaddition,largeamountsofCa++ canbesequesteredwithincells.LargeamountsofPiarepresentincellsaspartoforganicmolecules,suchasadenosinetriphosphate(ATP).
composition and cell volume is accomplished through the activity of specific transporters in the plasma membrane of the cells. This section is a review of the mechanisms by which cells maintain their intracellular ionic environment and their membrane potential and by which they control their volume.
Ionic Composition of Cells
The intracellular ionic composition of cells varies from tissue to tissue. For example, the intracellular composition of neurons is different from that of muscle cells, both of which differ from that of blood cells. Nevertheless, there are similar patterns, and these are presented in . In comparison with the ECF, the ICF is characterized by a low [Na+] and a high [K+]. This is the result of the activity of the Na+,K+-ATPase, which transports 3 Na+ ions out of the cell and 2 K+ ions into the cell for each ATP molecule hydrolyzed. As discussed later in this chapter, the activity of the Na+,K+-ATPase not only is important for establishing the cellular Na+ and K+ gradients but also is involved in determining, indirectly, the cellular gradients for many other ions and molecules. Of importance is that the cellular K+ gradient generated by the activity of the Na+,K+-ATPase is a major determinant of the membrane voltage because of the leak of K+ out of the cell through K+selective channels (see the section “
Membrane Potential”).
Thus the Na+,K+-ATPase converts the energy in ATP into ion gradients (i.e., Na+ and K+), and a voltage gradient (i.e., membrane voltage).
The Na+,K+-ATPase–generated ion and electrical gradients are used to drive the transport of other ions and molecules into or out of the cell (
Fig. 2.3 ). For example, as described in , a number of solute carriers couple the transport of Na+ to that of other ions or molecules. The Na+-glucose and Na+–amino acid symporters use the energy in the Na+ electrochemical gradient, directed to bring Na+
CHAPTER 2 Homeostasis: Volume and Composition of Body Fluid Compartments •Fig. 2.3 Cell Model Depicting How Cellular Gradients and the Membrane Potential (Vm) Are Established. (1)TheNa+,K+-ATPasedecreasestheintracellular[Na+]andincreasestheintracellular[K+].SomeK+ exitsthecellviaK+-selectivechannelsandgeneratestheVm(cell’sinterioriselectricallynegative).(2)TheenergyintheNa+ electrochemicalgradientdrivesthetransportofotherionsandmoleculesthroughtheuseofvarioussolutecarriers.(3)TheVmdrivesCl− outofthecellviaCl−-selectivechannels.(4)TheCa++-ATPaseandthe3Na+-Ca++ antiportersmaintainthelowintracellular[Ca++].
into the cell, to drive the secondary active cellular uptake of glucose and amino acids. Similarly, the inwardly directed Na+ gradient drives the secondary active extrusion of H+ from the cell and thus contributes to the maintenance of intracellular pH. The 3Na+-Ca++ antiporter, along with the plasma membrane Ca++-ATPase, extrudes Ca++ from the cell and thus contributes to the maintenance of a low intracellular [Ca++].
In addition, the membrane voltage drives Cl− out of the cell through Cl−-selective channels, thus lowering the intracellular concentration below that of the ECF.