Porth's Essentials of Pathophysiology, 4e

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Integrative Body Functions

U N I T 2

Laboratory Tests Disorders of Acid–Base Balance Primary Versus Compensatory Changes in pH Single Versus Mixed Acid–Base Disorders

Acid–Base Balance

Acid–Base Chemistry

Acid and Base Production Calculation of pH

Metabolic Acidosis Metabolic Alkalosis Respiratory Acidosis Respiratory Alkalosis

Regulation of pH

Chemical Buffer Systems Respiratory Control Mechanisms Renal Control Mechanisms

Movement of Body Fluids and Electrolytes Between Compartments The lipid bilayer and transport proteins serve as the primary barriers to the movement of substances across the cell membrane that separates the ECF and ICF compartments (see Chapter 1, Fig. 1-11). Lipid- soluble substances (e.g., oxygen [O 2 ] and carbon diox- ide [CO 2 ]), which dissolve in the lipid layer of the cell membrane, pass directly through the membrane. Many ions (e.g., sodium [Na + ] and potassium [K + ]) rely on transport proteins located in the cell mem- brane for movement across the membrane, accounting for the compartmental difference in their concentra- tions. Water crosses the cell membrane by osmosis using special transmembrane protein channels called aquaporins. Dissociation of Electrolytes Electrolytes are substances that dissociate in solution to form charged particles, or ions . For example, the sodium chloride (NaCl) molecule dissociates to form a positively charged Na + and a negatively charged Cl – ion. Particles that do not dissociate into ions such as glucose and urea are called nonelectrolytes . Positively charged ions are called cations because they are attracted to the cathode of a wet electric cell, and nega- tively charged ions are called anions because they are attracted to the anode. The ions found in body fluids carry one charge (i.e., monovalent ion) or two charges (i.e., divalent ion). Because of their attraction forces, positively charged cations are always accompanied by negatively charged anions. Thus, both the ICF and ECF contain equal amounts of anions and cations. Cations and anions may be exchanged for one another, provid- ing they carry the same charge. For example, a posi- tively charged hydrogen ion (H + ) may be exchanged for a positively charged K + , and a negatively charged bicar- bonate ion (HCO 3 – ) may be exchanged for a negatively charged chloride ion (Cl – ). The concentration of electrolytes in the ICF and ECF can be expressed in several ways; for example, milli- grams per deciliter (mg/dL), milliequivalents per liter (mEq/L), or millimoles per liter (mmol/L) (Box 8-1).

fluid contained within all of the trillions of cells in the body, contains about two thirds of the body water in healthy adults. The remaining one third is in the extra- cellular fluid (ECF) compartment , which contains all the fluids outside the cells, including those in the inter- stitial or tissue spaces and the plasma in the blood ves- sels (Fig. 8-1). The composition of the ECF and ICF are strikingly different. The ECF contains large amounts of sodium and chloride, moderate amounts of bicarbonate, but only small quantities of potassium, magnesium, cal- cium, and phosphate. In contrast, the ICF contains almost no calcium; small amounts of sodium, chloride, bicarbonate, and phosphate; moderate amounts of mag- nesium; and large amounts of potassium (Table 8-1). Although blood levels usually are representative of the total body levels of an electrolyte, this is not always the case, particularly with potassium, which is approxi- mately 28 times more concentrated inside the cell than outside.

Intracellular water

Extracellular (plasma) water

Extracellular (interstitial) water

FIGURE 8-1. Distribution of body water.The extracellular space includes the vascular compartment and the interstitial spaces.