Biological function of inorganic elements Dr. Sherin Bakhashab

Body Fluids Components Intracellular fluid (ICF) compartment: fluid found in the cells (cytoplasm, nucleoplasm) comprises 60% of all body fluids. Extracellular fluid (ECF) compartment: all fluids found outside the cells, comprises 40% of all body fluids. It is distributed between plasma and interstitial fluid.

Composition of Body Fluids Water is the main component of all body fluids making up 45-75% of the total body weight. Solutes are broadly classified into: Electrolytes are inorganic salts, all acids and bases, and some proteins Nonelectrolytes – examples include glucose, lipids, creatinine, and urea Electrolytes have greater osmotic power than nonelectrolytes Water moves according to osmotic gradients

Electrolyte Composition of Body Fluids Extracellular Fluids ECFs are similar except for the high protein content of plasma Sodium (Na+) is the major cation                    Chloride (Cl-)is the major anion Intracellular Fluids Have low sodium and chloride Potassium (K+) is the chief cation Phosphate (PO4-) is the chief anion

Extracellular and Intracellular Fluids Sodium and potassium concentrations in extra- and intracellular fluids are nearly opposites

Sodium, Potassium and Chloride Sodium is the principle cation in the ECF. Potassium is the principle cation in ICF. Chloride is the main anion in the ECF. Sources and requirements: Na and Cl are obtained from NaCl of food (cheese, bread, whole grain). K is found in large quantities in beef, chicken, some fruits and potatoes. The daily intake of NaCl is about 10-15g and 98% is eliminated by the faeces. The amount of K in diet is 2-4 g.

Sodium, Potassium and Chloride Distribution: The total amount of Na in the body is about 4000 mmoles. 50% are present in ECF (2000 mmoles) 3% are present in ICF (140 mmoles) 47% are present in bone (1900 mmoles) The total amount of K in the body is about 4300 mmoles. 98% are present in ICF (4200 mmoles) 1% is present in ECF (50 mmoles) 1% is present in bones (50 mmoles)

Plasma level (mmole/L) Red cell level (mmole/L) Blood levels Most Na and Cl are present in the blood plasma, while most of the K is present in the red blood cells. Plasma level (mmole/L) Red cell level (mmole/L) Sodium 140 ± 6 37 Chloride 100 ± 6 53 Potassium 4.4 ± 0.9 110

Functions Production of gastric HCl by the parietal cells: The process of acid secretion begins with the hydrolysis of water to form one H+ and OH- in the cytoplasm of parietal cells. Secretion of H+ into the lumen is an active process driven by H+ /K+ ATPase. The pump exchanges H+ for K+ . Then Cl- ions diffuse through open chloride channel. * CA = Carbonic anhydrase

Functions b) Maintenance of normal acid-base balance (Chloride shift) During normal metabolic activity, acids are continually being formed, which should be neutralized and excreted through lungs and kidneys to maintain the acid-base equilibrium. Plasma

Functions  

Functions The red cell membrane is permeable to HCO3- but impermeable to K+ . HCO3- diffuse outside the RBCs in exchange for chloride ions (Cl-) which shifts into the cell in order to maintain electrical ions neutrality across the erythrocyte membrane. Cl- ions are neutrilized by K+ while sodium bicarbonate is formed in the plasma. This process occurs when CO2 tensions is increased and this explains the higher chloride content in venous RBCs than arterial RBCs. In arteries, where CO2 tension is reduced, the reverse occurs, i.e., the Cl- leaves the cells and enters the plasma.

Active transport system for Na+ and K+ Pumping Na+ ions out and K+ in against strong concentration gradients called Na+ - K+ pump. It requires ATP as source of energy Active transport in cells controls the concentration gradient, muscle contraction, nerve impulse, and drives the active transport of sugars and amino acids. ATP drives the transfer of Na+ and K+ across the membrane by the following mechanism:

Important features of the pump For each ATP hydrolysed, 3 Na+ are removed from the cell and 2K+ enters the cell. Na+ triggers phosphorylation, whereas K+ triggers dephosphorylation.

Transport of sugars and amino acid by Na+ flow Transport of sugar (glucose) into the cell is coupled by simultaneous entry of Na+ .

Transport of sugars and amino acid by Na+ flow Na+ and glucose bind to a specific transport protein and enter together by symport carriers. Na+ enters the cell are pumped out by Na+ - K+ pump. The rate of glucose transport depends on the Na+ concentration gradient across the membrane. Symports driven by Na+ are widely used by the animal cells to transport amino acids. This symport system is present in the plasma membrane of intestinal and kidney cells. Na+ is a driving force in antiport that releases Ca++ from variety of cells. Most symports and antiports are driven by Na+ gradients generated by Na+ - K+ pump.

Na, K and Cl absorption

References http://www.austincc.edu/apreview/EmphasisItems/Electrolytef luidbalance.html#bodyfluids