Acids, Bases, and the pH Scale

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Chapter: Anatomy and Physiology for Health Professionals: Levels of Organization : Chemical Basics of Life

1. Define electrolytes, acids, and bases. 2. What does pH measure? 3. Define the pH of a solution.

Acids, Bases, and the pH Scale

Acids, Bases, and the pH Scale

Acids are electrolytes that release hydrogen ions (H+) in water. An example of an acid is hydrochloric acid, made up of hydrogen and chloride ions. Bases are electrolytes that release hydroxide (OH–) ions that bond with hydrogen ions. An example of a base is sodium hydroxide (NaOH), made up of sodium, oxy-gen, and hydrogen ions. In body fluids, the concentra-tions of hydrogen and hydroxide ions affect chemical reactions by a considerable amount. These reactions control certain physiological functions such as blood pressure and breathing rates.

The bicarbonate ion (HCO3–) is important in the human body, and found in large amounts in the blood. Ammonia (NH3) is a base that is also a waste product of the body’s breakdown of protein.

Ammonia has one pair of unshared electrons that have a strong ability to attract protons. When a pro-ton is accepted, ammonia becomes an ammonium ion (NH4+) in the following process:

NH3+ H+ NH4+

Hydrogen ion concentrations in body fluids are vital, and can be measured by the value pH. It is expressed in a type of mathematical shorthand based on concentrations calculated in moles per liter (with a mole representing an amount of solute in a solution). The pH of a solution is defined as the level of acidity or basicity. The pH scale ranges from 0 to 14, with 7 being the midpoint (meaning it has equal numbers of hydrogen and hydroxide ions). Pure water has a pH of 7, and this midpoint is considered to be neutral (nei-ther acidic nor basic). Therefore, a solution contains an equal number of hydrogen ions and hydroxide ions are neutral. Measurements of less than 7 pH are con-sidered acidic, meaning there are more hydrogen ions than hydroxide ions. Measurements of more than 7 pH are considered basic, also known as alkaline, meaning there are more hydroxide ions than hydrogen ions.

The pH of blood usually ranges from 7.35 to 7.45. Abnormal fluctuations in pH can damage cells and tis-sues, change the shapes of proteins, and alter cellular functions. Acidosis is an abnormal physiological state caused by blood pH that is lower than 7.35. If pH falls below 7, coma may occur. The two different types of acidosis are metabolic and respiratory. Metabolic aci-dosis is a condition in which the kidneys are not able to remove ketone bodies, which are metabolites of fats. In patients with type 1 diabetes, the body may be producing too many ketone bodies. Respiratory aci-dosis occurs in patients suffering from chronic lung diseases such as emphysema or chronic bronchitis. Higher carbon dioxide concentrations in the blood results in blood pH decreasing.


Alkalosis results from blood pH that is higher than7.45. If pH rises above 7.8, it generally causes uncon-trollable and sustained skeletal muscle contractions. Alkalosis also may be metabolic or respiratory. Meta-bolic alkalosis involves elevations of tissue pH, either as a result of decreased hydrogen ion concentration or a direct result of increased bicarbonate concen-trations. Respiratory alkalosis is caused by increased respiration, which elevates the blood pH. It may be caused by pneumonia, stroke, meningitis, fever, and pregnancy. 

Chemicals that resist pH changes are called buf-fers. They combine with hydrogen ions when theseions are excessive and contribute hydrogen ions when these ions are reduced. FIGURE 2-6 shows the pH val-ues of acids and bases. An example of a buffer that is important in body fluids is sodium bicarbonate.


1. Define electrolytes, acids, and bases.

2. What does pH measure?

3. Define the pH of a solution.

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