Acid-Base Balance

Acid-Base Balance

Electrolytes that dissociate in water to release hydro-gen ions are called acids. Electrolytes that release ions that combine with hydrogen ions are called bases. Homeostasis requires control of acid and base concentrations in body fluids. Most hydrogen ions in body fluids begin as byproducts of metabolic processes. The major sources of hydrogen ions are as follows:

■■ Aerobic respiration of glucose: produces carbon dioxide and water, forms carbonic acid, and releases hydrogen and bicarbonate ions

■■ Anaerobic respiration of glucose: produces lactic acid, adding hydrogen ions to body fluids

■■ Incomplete oxidation of fatty acids: produces acidic ketone bodies to increase hydrogen ion concentration

■■ Oxidation of sulfur-containing amino acids: yields sulfuric acid, releasing hydrogen ions

■■ Hydrolysis of phosphoproteins and nucleic acids: produces phosphoric acid, releasing hydrogen ions

Strengths of Acids and Bases

Strong acids dissociate to release hydrogen ions more completely, whereas weak acids release them less com-pletely. An example of a strong acid is hydrochloric acid and of a weak acid is carbonic acid. Bases release ions, such as hydroxide ions, that combine with hydrogen ions, lowering their own concentration. Examples of bases include sodium hydroxide and sodium bicarbonate. Strong bases dissociate to release more hydroxide ions than weak bases. Negative ions are often referred to as bases. They may combine with strong acids; for example, bicarbonate ions may com-bine with hydrogen ions from hydrochloric acid to form carbonic acid.

All functioning proteins, including enzymes, cytochromes, and hemoglobin, are influenced by hydrogen concentrations. This is true because of their abundant hydrogen bonds. Therefore, nearly all bio-chemical reactions are influenced by fluid environ-ment pH, and there is close regulation of acid-base balance. Variations in optimal pH are not excessive. Although the normal pH of intracellular fluid is on average 7.0, in the arterial blood it is 7.4 and in the venous blood and interstitial fluid, 7.35. The lower pH in venous blood and the cells is because of their larger amounts of carbon dioxide and acidic metab-olites. Carbon dioxide combines with water to form carbonic acid. The partial pressure of carbon dioxide (Pco₂) is the most important factor affecting the pH of body tissues.

Acid-Base Buffer Systems

Acid-base buffer systems consist of chemicals that combine with excess acids or bases. Buffer sys-tem chemicals can combine with strong acids, which release more hydrogen ions, to convert them into weak acids, which release fewer hydrogen ions. The three most important acid-base buffer systems in the body’s fluids are as follows:

■■ Bicarbonate buffer system: This system is present in both intracellular and extracellular fluids, using the bicarbonate ion as a weak base and carbonic acid as a weak acid. It is sometimes called the cabonic acid bicarbonate buffer system. Carbonic acid is formed when hydrogen ions are excessive and dissociates when conditions are basic or alkaline. The body maintains a readily available bicarbonate reserve.

■■ Phosphate buffer system: This system also operates in both intracellular and extracellular fluids and is very important in controlling hydrogen ion concentrations in the fluid of the nephrons and in urine. It consists of monohydrogen phosphate and dihydrogen phosphate.

■■ Protein buffer system: Consists of plasma proteins and certain cell proteins. When the solution pH falls, amino groups accept hydrogen ions; when it rises, carboxyl groups release hydrogen ions. For red blood cells, which are densely packed with hemoglobin molecules that buffer hydrogen ions, a chloride shift occurs. This involves dissociation of carbonic acid and bicarbonate ions diffusing into the plasma in exchange for chloride ions. In the lungs this occurs in reverse and is known as the hemglobin buffer system. TABLE 23-3 summarizes the three major buffer systems.

Carbonic acid production increases when cells increase carbon dioxide production. As carbonic acid dissociates, hydrogen ions increase and the inter-nal environment pH drops. These actions stimulate chemoreceptors in the medulla oblongata, increas-ing breathing so the lungs can excrete more carbon ­dioxide. If cells are less active, production of these components is low and breathing may be closer to resting levels. Nephrons excrete hydrogen ions in urine to help regulate hydrogen ion concentration. Epithelial cells in the renal tubules secrete hydrogen ions into the tubular fluid.

1. Explain the difference between an acid and a base.

2. List the body’s major chemical buffer systems.

3. Describe two additional ways the body balances acids and bases.