Differences Between Extracellular and Intracellular Fluids. The extracellular fluid contains large amounts of sodium, chloride, and bicarbonate ions plus nutrients for the cells, such as oxygen, glucose, fatty acids, and amino acids. It also contains carbon dioxide that is being transported from the cells to the lungs to be excreted, plus other cellular waste products that are being transported to the kidneys for excretion. The intracellular fluid differs significantly from the extracellular fluid; for example, it contains large amounts of potassium, magnesium, and phosphate ions instead of the sodium and chloride ions found in the extracellular fluid. Special mechanisms for transporting ions through the cell membranes maintain the ion concentration differences between the extracellular and intracellular fluids. These transport processes are discussed in Chapter 4.
Insulin controls glucose metabolism; adrenocortical hormones control sodium ion, potassium ion, and protein metabolism; and parathyroid hormone controls bone calcium and phosphate.
The liver and pancreas regulate the concentration of glucose in the extracellular fluid, and the kidneys regulate concentrations of hydrogen, sodium, potassium, phosphate, and other ions in the extracellular fluid.
That is, when the membrane of a nerve fiber is stimulated, this causes slight leakage of sodium ions through sodium channels in the nerve membrane to the fiber’s interior. The sodium ions entering the fiber then change the membrane potential, which in turn causes more opening of channels, more change of potential, still more opening of channels, and so forth. Thus, a slight leak becomes an explosion of sodium entering the interior of the nerve fiber, which creates the nerve action potential. This action potential in turn causes electrical current to flow along both the outside and the inside of the fiber and initiates additional action potentials. This process continues again and again until the nerve signal goes all the way to the end of the fiber.
In addition to membrane transport of sodium, energy from ATP is required for membrane transport of potassium ions, calcium ions, magnesium ions, phosphate ions chloride ions, urate ions, hydrogen ions, and many other ions and various organic substances.
Potassium channels permit passage of potassium ions across the cell membrane about 1000 times more readily than they permit passage of sodium ions. This high degree of selectivity, however, cannot be explained entirely by molecular diameters of the ions since potassium ions are slightly larger than sodium ions. What is the mechanism for this remarkable ion selectivity? This question was partially answered when the structure of a bacterial potassium channel was determined by x-ray crystallography.
One of the most important of the protein channels, the sodium channel, is only 0.3 by 0.5 nanometer in diameter, but more important, the inner surfaces of this channel are lined with amino acids that are strongly negatively charged, as shown by the negative signs inside the channel proteins in the top panel of Figure 4-5. These strong negative charges can pull small dehydrated sodium ions into these channels, actually pulling the sodium ions away from their hydrating water molecules. Once in the channel, the sodium ions diffuse in either direction according to the usual laws of diffusion. Thus, the sodium channel is specifically selective for passage of sodium ions.
MINERAL METABOLISM
Large number of elements are needed for the functioning of the body. Some elements are needed at high concentrations, required more than 100mg per day. They come under macroelements. Example. Sodium, Potassium, Calcium, Magnesium and Chloride. Sodium and Potassium: They are important in cell, muscle physiology, transmission of messages and other biological processes. Sodium is the principal cation of extra cellular fluid. It is commonly found in all types of foods. Recommended daily allowance (RDA) is 5-10 gms. It is excreted in the urine. The concentrations are maintained by Aldosterone. Potassium is intracellular cation; daily requirement is 1 gm/day. Its excretion is through kidney, linked to sodium excretion. Since both are widely distributed, deficiency of the two elements is rarely found. Functions: • Sodium maintains osmotic pressure of extra cellular fluid and ECF balance. • It has a role , along with others, in the neuro muscular excitability • Sodium is exchanged with Hydrogen in renal tubules to acidify urine. • Sodium pump keeps sodium in far higher concentration outside the cell .This results high polarization, create resting membrane potential. • Sodium and Potassium maintain the degree of hydration of plasma proteins, and there by viscosity of blood. • Potassium is critically important for the functioning of cardiac muscle. Hypernatremia: It occurs nearly always due to water deficiencies rather than Na2+ excess. Increased sodium is found in ECF. It may be due to increased sodium in the body, decreased body water. It is usually seen in patients with dehydration, on steroid therapy or excess sodium intake.
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