Porth's Essentials of Pathophysiology, 4e

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Disorders of Fluid, Electrolyte, and Acid–Base Balance

C h a p t e r 8

at a concentration approximately 3.5 million times that of the H + . Because it is cumbersome to work with such a small number, the H + concentration is commonly expressed in terms of the pH . Specifically, pH repre- sents the negative logarithm (log 10 ) of the H + concen- tration expressed in mEq/L. Since the pH is inversely related to the H + concentration, a low pH indicates a high concentration of H + and a high pH a low concen- tration of H + . Acid and Base Production Acids are continuously generated as by-products of met- abolic processes (Fig. 8-16). Physiologically, these acids fall into two groups: volatile H 2 CO 3 and all other non- volatile or fixed acids (e.g., sulfuric, hydrochloric, and

Acid–Base Balance Metabolic activities of the body require precise regula- tion of acid–base balance as reflected in the pH of the ECF, which is normally maintained within a very narrow range of 7.35 to 7.45. 1–3 Membrane excitability, enzyme systems, and chemical reactions all depend on the pH being regulated within a narrow physiologic range to function in an optimal way. Acid–Base Chemistry Acids and bases have their own chemical properties and definitions. An acid is defined as a compound that can dissociate and release a hydrogen (H + ) ion and a base as a compound that can accept or combine with H + . 1–3 For example, hydrochloric acid (HCl) dissociates in water to form H + and Cl – ions. The bicarbonate ion (HCO 3 – ) is a base because it can combine with H + to form carbonic acid (H 2 CO 3 ). Most of the body’s acids and bases are weak; the most important are H 2 CO 3 , which is a weak acid derived from carbon dioxide (CO 2 ), and HCO 3 – , which is a weak base. The concentration of H + in body fluids is low com- pared with other ions. For example, the Na + is present ■■ Magnesium, which is the second most abundant ICF cation, acts as a cofactor in many intracellular enzyme reactions and is required for cellular energy metabolism, functioning of the Na + /K + - ATPase membrane pump, nerve conduction, ion transport, and potassium and calcium channel activity. Hypomagnesemia produces a decrease in serum calcium due to suppression of PTH release and a decrease in serum potassium due to renal wasting, both of which contribute to an increase in neuromuscular exitability. Hypermagnesemia causes neuromuscular dysfunction with hyporeflexia, muscle weakness, and confusion. ■■ Parathyroid hormone disorders impact both calcium and phosphate homeostasis. Acute hypoparathyroidism causes hypocalcemia, manifested by signs of increased neuromuscular excitability such as muscle cramps and tetany. Chronic hypoparathyroidism is manifested by lethargy and fatigue. Hyperparathyroidism can occur as a primary disorder causing elevated serum calcium levels and increased urinary excretion of both calcium and phosphorus, which provides the potential for development of kidney stones. Secondary hyperparathyroidism, which associated with chronic kidney disease, exerts its effects on bone, causing renal osteodystrophies.

Food intake

Digestion

Absorption

Chemical buffering

Respiratory response

Renal response

Cell metabolism of food

H +

H +

Bound by body buffer bases

CO 2

Sulfate Phosphate Chloride

CO 2

Extracellular fluid [HCO 3 - ]

New

-

HCO 3

Extracellular fluid [HCO 3 - ]

CO 2

H +

Excreted (combined with urinary buffer bases)

Excreted

Sulfate Phosphate Chloride

Sulfate Phosphate Chloride

FIGURE 8-16. The maintenance of normal blood pH by chemical buffers, the respiratory system, and the kidneys. On a mixed diet, pH is threatened by the production of strong acids (sulfuric, hydrochloric, and phosphoric) mainly as the result of protein metabolism.These strong acids are buffered in the body by chemical buffer bases such as extracellular fluid (ECF) bicarbonate (HCO 3 – ).The kidney eliminates hydrogen ions (H + ) combined with urinary buffers and anions in the urine. At the same time, they add new HCO 3 – to the ECF to replace the HCO 3 – consumed in buffering strong acids.The respiratory system disposes of carbon dioxide (CO 2 ). (From Rhoades RA, Bell DR. Medical Physiology: Principles of Clinical Medicine. 4th ed. Philadelphia, PA: Wolters Kluwer Health | Lippincott Williams & Wilkins; 2013:454.) ( text continues on page 193 )