CLINICAL BIOCHEMISTRY Lecture No 1

Water balance, Osmolality, Electrolytes,pH and Blood gases Internal environment In medical usage the term electrolytes is applied to any of the four ions in plasma Na+ , K+ , Cl- and Hco3- that exert the greatest influence upon water balance and acid-base relationship. This refer to the concentration of ions and other constituents in the body fluids it also include pH.

Homeostasis Osmolality The maintenance of the steady state in the body and a relatively constant concentration of ions ,pH and osmotic pressure in the various body fluid. Is a measurement of the of dissolved particles ( ions and undissolved molecules such as glucose or protein) per unit of water.

Osmotic pressure Active Transport When two compartments of different osmolality are separated by semipermeable membrane that allows any small molecules to pass through it, the osmotic pressure represent the hydrostatic pressure that would have to be applied to the compartment of higher osmolality to prevent water passage into it from the lower one. Active transport is the transport of ions or molecules a cross a cell membrane against a concentration gradient , it involves energy-linked, enzyme reaction, because energy in form of ATP is usually required.

Body water Water is found both inside and outside cells and can be divided into main two spaces or compartments : intracellular (water within cells) and extracellular (water outer of cells). The extracellular water can be interstitial fluid between cells and plasma. The body water is about 60% of total body weight.

In many illnesses the water and electrolyte balance is severely disturbed because of heavy fluid and electrolyte losses caused by : - Vomiting - Diarrhea - excessive urination - fistulas (ناسور )

Electrolytes distribution Na+ is the principal cation of extracellular fluid 92 % Cl- is the principal anion of extracellular fluid 67%

Sodium Na+ normal range (136-145meq/L) or 136-145mmol/L Hypernatremia Hyponatremia Sever dehydration Hyperadrenalism (Cushing's syndrome) Comatose diabetics Hypothalamic injury interfering with thirst mechanism Nasogastric feeding Diabetes insipidus (deficiency of antidiuretic hormone) Large loss of gastrointestinal secretion (diarrhea, intestinal fistulas) Acidosis of diabetes mellitus Addison's disease(depressed secretion of aldosterone and corticosteroids) Renal disease malfunction (salt-losing nephritis)

Potassium K+ normal range (3.8-5.4meq/L) or 3.8-5.4mmol/L Hyperkalemia Hypokalemia Anoxia and acidosis Decrease output of urine Shock or circulatory failure Decrease production of aldosterone Chronic renal insufficiency Increase loss of potassium (vomiting, diarrhea, gastrointestinal fistulas) Long-term therapy with diuretics Certain carcinomas that secrete ACTH (adrenocorticotropic hormone) cause a lowering of serum potassium

Chloride Cl- normal range (98-108meq/L) or 98-108mmol/L Increased concentration Decreased concentration Dehydration Certain type of renal tubular acidosis Over breathing (respiratory alkalosis) following stimulation of the respiratory center by some drugs, hysteria, anxiety or fever Metabolic acidosis Uncontrolled diabetes In renal disease (impaired glomerular filtration Prolonged vomiting Salt-losing nephritis Metabolic alkalosis (Hco3- increases and Cl- decreases)

Cerebrospinal fluid(CSF) Chloride Cl- Cerebrospinal fluid(CSF) Normal range 120-132 meq/L The concentration of chloride decreases in cases of bacterial meningitis Urine chloride The amount of urinary chloride varies greatly with the intake. An adult eating an average diet may excrete from 110-250 meq/L.

Blood buffer systems When carbohydrates, fats and proteins are catabolized, the hydrogen atoms of the carbon chains are converted to H+ . The H+ is transported with an electron chain along a chain of coenzymes in mitochondria (the respiratory chain)

Bicarbonate/Carbonic Acid buffer System The most important buffer system in plasma because of the high concentration of HCO3- and the readiness with which H2CO3 may be increase through lung activity and decrease by blowing off CO2 .Carbonic anhydrase (carbonic dehydratase) The increased HCO3- in blood cells is followed by diffusion of HCO3- into plasma, accompanied by passage of Cl- into the erythrocytes to maintain ionic balance. Supplies about 95% of the buffering capacity.

Hemoglobin buffer system Hemoglobin (Hb), the protein present in high concentration in red blood cells, bind to oxygen in the lungs and releases it in the tissues. The oxygenated form of Hb is much stronger acid than the deoxygenated form. The reaction take place This buffer buffer system accounts for about 30% of the buffering capacity of the whole blood.

Phosphate buffer system The phosphate buffer system is a minor component of the total blood buffer system, but it does play an important role in the elimination of H+ in urine. In the plasma at pH 7.4, 80% of the phosphate in form of HPO4-- , but in an acid urine the bulk of it exist as H2PO4- .

Compensation of acid-base disturbance Plasma protein can also accept H+ to a minor extent and therefore serve as a buffer. Disturbance s in blood pH are usually compensated to greater or lesser extent by appropriate responses of respiratory and renal systems.

The direction of the changes in values of the acid-base parameter in various state of imbalance are summarized above.

Blood gases : pH, pCO2 and pO2 the normal values of acid-base and blood gas shown above (pO2 the partial pressure of oxygen)