Dr. Rida Shabbir DPT –IPMR (KMU)

Acid Base Balance Acid: is any chemical that releases H ion in solution. Strong acid: Ionizes freely, gives up most of its hydrogen ions, and can markedly lower the pH of a solution, such as hydrochloric acid (HCl) Weak acid: ionizes only slightly and keeps most hydrogen in a chemically bound form that does not affect pH. Such as carbonic acid (H2CO3)

Acid Base Balance A base is any chemical that accepts H. Strong base: such as the hydroxyl ion (OH) has a strong tendency to bind H and raise the pH. Weak base: such as the bicarbonate ion (HCO3) binds less of the available H and has less effect on pH.

Buffer System Buffer: is any mechanism that resists changes in pH by converting a strong acid or base to a weak one. The normal pH of arterial blood is 7.4 Acidosis: A person is considered to have acidosis when the pH falls below 7.4. Alkalosis: A person is considered to have alkalosis when the pH rises above 7.4. The lower limit of pH at which a person can’t live more than a few hours is about 6.8, and the upper limit is about 8.0. The pH of urine can range from 4.5 to 8.0, depending on the acid-base status of the extracellular fluid.

Buffer System An extreme example of an acidic body fluid is the HCl secreted into the stomach by the oxyntic (parietal) cells of the stomach mucosa.

Types of Buffer System There are three primary systems that regulate the H+ concentration in the body fluids to prevent acidosis or alkalosis: (1) The chemical acid-base buffer systems of the body fluids, which immediately combine with acid or base to prevent excessive changes in H+ concentration. Buffer systems do not eliminate H+ from or add them to the body but only keep them tied up until balance can be reestablished. 1. The bicarbonate buffer system 2. The phosphate buffer system 3. The protein buffer system

Types of Buffer System (2) The respiratory center: It acts within few minutes and regulates the removal of CO2 from the extracellular fluid. It is second line of defense. (3) The kidneys: The kidneys can eliminate the excess acid or base from the body. Although the kidneys are relatively slow to respond compared with the other defenses, over a period of hours to several days, they are by far the most powerful of the acid-base regulatory systems.

The Bicarbonate Buffer System The bicarbonate buffer system is a solution of carbonic acid and bicarbonate ions. Carbonic acid (H2CO3) forms by the hydration of carbon dioxide and then dissociates into bicarbonate (HCO3) and H This is a reversible reaction. When it proceeds to the right, carbonic acid acts as a weak acid by releasing H and lowering pH. When the reaction proceeds to the left, bicarbonate acts as a weak base by binding H, removing the ions from solution, and raising pH.

The Phosphate Buffer System The phosphate buffer system is a solution of HPO4 and H2PO4 It works in much the same way as the bicarbonate system. The following reaction can proceed to the right to liberate H and lower pH, or it can proceed to the left to bind H and raise pH: The phosphate buffer system has a stronger buffering effect than an equal amount of bicarbonate buffer. However, phosphates are much less concentrated in the ECF than bicarbonate, so they are less important in buffering the ECF. They are more important in the renal tubules and ICF, where they are more concentrated.

The Protein Buffer System Proteins are more concentrated than either bicarbonate or phosphate buffers, especially in the ICF. The protein buffer system accounts for about three-quarters of all chemical buffering ability of the body fluids. The buffering ability of proteins is due to certain side groups of their amino acid residues. Some have carboxyl (–COOH) side groups, which release H when pH begins to rise and thus lower pH: Others have amino (–NH2) side groups, which bind H when pH falls too low, thus raising pH toward normal:

Respiratory Control of pH Carbon dioxide is constantly produced by aerobic metabolism and is normally eliminated by the lungs at an equivalent rate. Rising CO2 concentration and falling pH stimulate peripheral and central chemoreceptors, which stimulate an increase in pulmonary ventilation. This expels excess CO2 and thus reduces H concentration. Conversely, a drop in H concentration raises pH and reduces pulmonary ventilation. This allows metabolic CO2 to accumulate in the ECF faster than it is expelled, thus lowering pH to normal.

Acid Base Balance DISORDERS OF ACID BASE BALANCE

Disorder in Acid Base Balance Acid-base imbalances fall into two categories. Respiratory (When Co2 is increased or decreased) Metabolic (When HCO3 is increased or decreased)

NORMAL VALUES paCO2 = (35-45mmHg) PH = ( ) HCO3 = (22-24mEq)

Respiratory Acidosis When pH. becomes lower than 7.35 and CO2 increases more than 45 mmHg it result in respiratory acidosis. It occurs when the rate of alveolar ventilation decreases. Carbon dioxide accumulates in the ECF and lowers its pH.

Respiratory alkalosis When pH. becomes greater than 7.35 and CO2 decreases less than 35 mmHg it result in respiratory alkalosis. Respiratory alkalosis results from hyperventilation, in which CO2 is eliminated faster than it is produced

Metabolic acidosis When pH. becomes less than 7.35 and HCO3 decreases less than 22 mEq it result in Metabolic acidosis. It can result from increased production of organic acids, such as lactic acid in anaerobic fermentation and ketone bodies in alcoholism and diabetes mellitus. It can also result from the ingestion of acidic drugs such as aspirin or from the loss of base due to chronic diarrhea.

Metabolic alkalosis When pH. becomes greater than 7.45 and HCO3 increases more than 24 mEq it result in Metabolic alkalosis. It is rare but can result from overuse of bicarbonates (such as oral antacids and intravenous bicarbonate solutions) Or from the loss of stomach acid by chronic vomiting.