Extracellular fluid sodium ions

Sodium and Potassium. Whereas sodium ion is the most abundant cation in the extracellular fluid, potassium ion is the most abundant in the intracellular fluid. Small amounts of K" are requited in the extracellular fluid to maintain normal muscle activity. Some sodium ion is also present in intracellular fluid (see Fig. 5). Common food sources rich in potassium may be found in Table 7. Those rich in sodium are Hsted in Table 8. 

Sodium is the most abundant extracellular cation in the body and is the major osmotically active ion in the extracellular fluid. Sodium concentration determines the distribution of... 

Interest in the use of ion-selective electrodes in the biomedical field is a natural consequence of the electrolyte composition of bulk body and cell fluids (Table 2.1), a proportion of which is in the ionised form. In extracellular fluids, sodium is the principal cation with chloride as the major anion. In intracellular fluids, potassium is the major cation and phosphate the principal anion — except in erythrocytes where chloride predominates. Of special interest is ionised calcium, because of its importance in various physiological and biochemical processes such as bone formation, nerve conduction, cerebral function, cardiac conduction and contraction, membrane phenomena, muscle contraction and relaxation, blood coagulation, and enzyme activation [2—4]. 

Active Transport. Maintenance of the appropriate concentrations of K" and Na" in the intra- and extracellular fluids involves active transport, ie, a process requiring energy (53). Sodium ion in the extracellular fluid (0.136—0.145 AfNa" ) diffuses passively and continuously into the intracellular fluid (<0.01 M Na" ) and must be removed. This sodium ion is pumped from the intracellular to the extracellular fluid, while K" is pumped from the extracellular (ca 0.004 M K" ) to the intracellular fluid (ca 0.14 M K" ) (53—55). The energy for these processes is provided by hydrolysis of adenosine triphosphate (ATP) and requires the enzyme Na" -K" ATPase, a membrane-bound enzyme which is widely distributed in the body. In some cells, eg, brain and kidney, 60—70 wt % of the ATP is used to maintain the required Na" -K" distribution. 

Diuretics are a group of therapeutic agents designed to reduce the volume of body fluids. Their mechanism of action is at the level of the kidney and involves an increase in the excretion of Na+ and Cl ions and, consequently, an increase in urine production. As discussed in Chapter 2, sodium is the predominant extracellular cation and, due to its osmotic effects, a primary determinant of extracellular fluid volume. Therefore, if more sodium is excreted in the urine, then more water is also lost, thus reducing the volume of extracellular fluids including the plasma. 

The basic answer to this question is that ions move across the plasma membrane of the neuron. Recall that ions are charged particles, frequently derived from single atoms by the gain or loss of electrons. The ions that are most important to us in understanding nervous system function are sodium ion, Na+, potassium ion, K+, calcium ion, Ca +, and chloride ion, Cl . If we compare the concentrations of these ions on the inside of the neuron and in the extracellular fluid that bathes the neuron, we find the neuron interior has a higher concentration of potassium ion than does the exterior fluid. In contrast, the exterior fluid has higher concentrations of sodium, calcium, and chloride ions than does the neuron interior. These concentration differences are referred to as concentration gradients. 

Pharmacology Normal osmolarity of the extracellular fluid ranges between 280 to 300 mOsm/L it is primarily a function of sodium and its accompanying ions, chloride, and bicarbonate. Sodium chloride is the principal salt involved in maintenance of plasma tonicity. One gram of sodium chloride provides 17.1 mEg sodium and 17.1 mEq chloride. 

Pharmacokinetics Sodium bicarbonate in water dissociates to provide sodium and bicarbonate ions. Sodium is the principal cation of extracellular fluid. Bicarbonate is a normal constituent of body fluids and normal plasma level ranges from 24 to 31 mEq/L. Plasma concentration is regulated by the kidney. Bicarbonate anion is considered labile because, at a proper concentration of hydrogen ion, it may be converted to carbonic acid, then to its volatile form, carbon dioxide, excreted by lungs. Normally, a ratio of 1 20 (carbonic acid bicarbonate) is present in extracellular fluid. In a healthy adult with normal kidney function, almost all the glomerular filtered bicarbonate ion is reabsorbed less than 1% is excreted in urine. 

Potassium, along with nitrogen and phosphorus, is an essential element needed for plant growth. In plants, it occurs mostly as K+ ion in cell juice. It is found in fruit or seed. Deficiency can cause curling leaves, yellow or brown coloration of leaves, weak stalk and diminished root growth. Potassium deficiency has been associated with several common animal ailments. Potassium is in extracellular fluid in animals at lower concentrations than sodium. 

The concentrations and distribution of electrolytes are not fixed, because cell membranes are permeant to ions and to water. Movement of ions and water in and out of cells is determined by the balance of thermodynamic forces, which are normally close to equilibrium. Selective changes of ion concentrations cause movement of water in or out of cells to compensate for these alterations. The kidneys are a major site where changes in salt or water are sensed. The loss of fluids due to illness or disease may alter intracellular and extracellular electrolyte concentrations, with attendant changes in fluid movement in or out of cells. Changes of extracellular or intracellular ion concentrations, particularly for potassium, sodium, and calcium, can have profound effects on neuronal excitability and contractility of the heart and other muscles. 

Mecftanism of Action Sodium Is a major cat ion of extracellular fluid that controls water distribution, fluid and electrolyte balance, and osmotic pressure of body fluids it also maintains acid-base balance. 

Figure 12.4 Mechanism of action of Na+/K+symport inhibitors (thiazides) on the distal convoluted tubule. As in the other parts of the nephron, Na+movement is powered by the energy-requiring sodium pump (P) in the basolateral membrane which exchanges intracellular Na+for K-i-in the extracellular fluid (ECF). The transport of Na-rand Cl- into the cell from the filtrate against the prevailing electrochemical gradient is facilitated by the symporter (S). The Na-Hons are then transported by the pump mechanism described above and the Cl- ions diffuse passively Into the ECF through ion channels in the basolateral membrane. Thiazide diuretics inhibit the symporter by disabling the Cl- binding site with the loss of Na-rand Cl- in the urine.

The AChR is one of the best characterized of all cell-surface receptors for hormones or neurotransmitters (Figure 2-9). One form of this receptor is a pentamer made up of four different polypeptide subunits (eg, two chains plus one B, one 7, and one 5 chain, all with molecular weights ranging from 43,000 to 50,000). These polypeptides, each of which crosses the lipid bilayer four times, form a cylindrical structure that is 8 nm in diameter. When acetylcholine binds to sites on the subunits, a conformational change occurs that results in the transient opening of a central aqueous channel through which sodium ions penetrate from the extracellular fluid into the cell. 

It will be apparent that if normal extracellular fluids were subjected to an isotonic resorption of sodium and chloride ions by the process, the net effect would be to concentrate other ions and precipitate minerals. This suggestion was made613 to explain one of the methods of forming deposits in the calciferious glands of earthworms. It was proposed that the posterior glands received blood directly from the intestine. Fluid was formed in these glands by a process of filtration and saline was then resorbed by the epithelial cells. This resulted in the formation of calcareous deposits (Fig. 5). 

For example, sodium ion is the principal cation of the extracellular fluid of the mammalian body, comprising, as the chloride and bicarbonate, more than 90% of the total solute in that fluid. Ingestion of sodium chloride solutions is used to replace salt lost by excessive perspiration. More sophisticated preparations have been proposed for this purpose one such preparation5 comprises mainly sodium chloride, supplemented with smaller amounts of potassium and phosphate ions to approximate the average composition of sweat in a sweetened glucose solution. 

Maintenance of unequal concentrations of ions across membranes is a fundamental property of living cells. In most cells, the concentration of K+ inside the cells is about 30 times that in the extracellular fluids, while sodium ions are present in much higher concentration outside the cells than inside. These concentration gradients are maintained by the Na+-K+-ATPase by means of the expenditure of cellular energy. Since the plasma membrane is more permeable to K+ than to other ions, a K+ diffusion potential maintains membrane potentials which are usually in the range of -30 to -90 mV. H+ ions do not behave in a manner different from that of other ions. If passively distributed across the plasma membrane, then the equilibrium intracellular H+ concentration can be calculated from the Nernst equation via... 

Two different types of membrane-based osmosensors have been proposed for animal cells extracellular solute sensors and membrane stretch-activated sensors. The former sensors are conjectured to function by detecting changes in the concentration of specific ions, for instance, sodium ion, in the external fluids. There is some indirect evidence for sodium-specific sensors in animal cells, and sodium-gated cation channels have been proposed as candidates for this role. However, no direct evidence for their involvement as upstream osmoregulatory elements has yet been presented. 

The nerve cell membrane separates the external from the internal cell fluid, as does any cell membrane. As is true of virtually all cells, the intra- and extracellular fluids are electrolytic solutions of almost equal conductivity, but their chemical composition is very different. The ions present in largest quantities are sodium and potassium. The species in the external fluid are made up of more than 90 per cent sodium and chloride ions in the cell interior there are principally potassium and organic ions that cannot pass through the membrane, only 10 per cent of the ions being sodium and chloride. 

All cells, including muscle and nerve cells, have inside them an intracellular fluid (ICF) which contains high levels of potassium, K+, phosphate ions, PC>43+, and protein and small amounts of Na+ ions and chlorine ions. Outside the cells in the extracellular fluid (ECF) consists mostly of sodium ions, Na+, chloride, Cl, and bicarbonate ions, HC03, but no protein, plus low concentrations of potassium ions. The inner layer of the cell membrane is negatively charged relative to the outside. When activity occurs then an ionic pumping action takes place to try to maintain the balance within the cells between the intra and extra flow of sodium and potassium... 

Except for respiratory and dermal insensible water-vapor losses, all remaining water lost by the body contains electrolytes, mainly sodium and chloride. The normal cation and anion constituent composition of the fluid spaces is given in Table IV. In the extracellular fluid space, sodium is the major cation and chloride the major anion. Those two ions constitute 95 of the extracellular fluid osmolality. Changes in plasma sodium concentration reflect changes in extracellular fluid volume. Potassium is the major cellular cation and phosphates and proteins comprise the major anions. The total cellular osmolality (175 + 135 = 310 mosraol/kg H2O) is equal to the total extracellular osmolality (155 + 155 = 310 mosmol/kg HaO) therefore, equal total osmotic concentrations are maintained between two fluid compartments of widely different ionic contents (Table IV). 

Neurons (as all cells) maintain a resting potential across their plasma membranes, typically —60 to —75 mV relative to the extracellular fluid. This resting potential is generated by the active and passive diffusion of ions across the plasma membrane. In the resting state, the sodium ion (Na" ") concentration is higher in the extracellular fluid than in the intracellular fluid. In contrast, the K+ concentration is higher... 

The most common molecules in the body are water and inorganic molecules such as sodium, potassium and chloride ions. A feature that is common among all living cells is that the concentrations of these ions are different in the extracellular and intracellular compartments. The extracellular fluid is high in sodium (Na+) and chloride (CT) ions, but low in potassium (K" ) ions (Figure 10.1). In contrast, the intracellular solution is... 

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