Porth's Essentials of Pathophysiology, 4e

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Integrative Body Functions

U N I T 2

Proportionate changes in sodium and water

Disproportionate changes in sodium and water

Loss of sodium and water

Gain of sodium and water

Loss of sodium or gain of water

Gain of sodium or loss of water

Isotonic fluid deficit in ECF compartment

Isotonic fluid excess in ECF compartment

Hyponatremia

Hypernatremia

Contraction of fluids in interstitial and vascular compartments of the ECF

Expansion of fluids in the interstitial and vascular compartments of the ECF

Water movement from extracellular to intracellular compartment

Water movement from intracellular to extracellular compartment

280 mOsm

300 mOsm

280 mOsm

280 mOsm

280 mOsm

280 mOsm

260 mOsm

280 mOsm

Water

Water

Intracellular fluid (ICF)

Extracellular fluid (ECF)

FIGURE 8-8. Effect of isotonic fluid excess and deficit and of hyponatremia and hypernatremia on movement of water between the extracellular fluid (ECF) and intracellular fluid (ICF) compartment.

gastrointestinal fluids such as occurs with severe vom- iting, diarrhea, or gastrointestinal suction; excessive urinary losses, such as occurs with osmotic diuresis or injudicious use of diuretic therapy; excessive sweat- ing due to fever and exercise; or endocrine disorders, such as adrenal insufficiency, in which reduced levels of aldosterone cause excessive sodium loss in the urine (see Chapter 32). Third-space losses cause sequestering of ECF in the serous cavities or extracellular spaces of injured tissue. Isotonic fluid volume deficit is manifested by a decrease in ECF volume, as evidenced by a decrease in body weight. A mild ECF deficit exists when weight loss equals 2% of body weight. A moderate deficit equates to a 5% loss in weight and a severe deficit to an 8% or greater loss in weight (Table 8-3). 3 Because the ECF is trapped in the transcellular compartment of persons with third-space losses, their body weight may not decrease. Thirst is a common symptom of fluid deficit, although it is not always present in early stages of isotonic fluid deficit. Urine output decreases and urine osmolality and specific gravity increase as ADH levels rise because of a decrease in vascular volume. Although there is an iso- tonic loss of fluid from the vascular compartment, blood components such as red blood cells and BUN become more concentrated. The fluid content of body tissues decreases as fluid is removed from the interstitial spaces. The eyes assume a sunken appearance and feel softer than nor- mal as the fluid content in the anterior chamber of the eye decreases. Fluids add resiliency to the skin and

underlying tissues that is referred to as tissue turgor . Tissue turgor is assessed by pinching a fold of skin between the thumb and forefinger (Fig. 8-9). The skin should immediately return to its original configuration when the fingers are released. A loss of 3% to 5% of body water in children causes the resiliency of the skin to be lost, and the tissue remains elevated for several seconds. 3 Decreased tissue turgor is less predictive of fluid deficit in older persons (>65 years) because of the loss of tissue elasticity. In infants, fluid deficit may be evidenced by depression of the anterior fontanel due to a decrease in cerebrospinal fluid. Arterial and venous volumes decline during periods of fluid deficit, as does filling of the capillary circula- tion, which can be assessed by applying pressure to a fingernail for 5 seconds and then releasing the pres- sure and observing the time (normally 1 to 2 seconds) that it takes for the color to return to normal (capil- lary refill time). 3 As the volume in the arterial system declines, the blood pressure decreases, the heart rate increases, and the pulse becomes weak and thready. Postural hypotension (a drop in blood pressure on standing) is an early sign of fluid deficit. On the venous side of the circulation, the veins become less promi- nent. When volume depletion becomes severe, signs of hypovolemic shock and vascular collapse appear (see Chapter 20). Treatment of fluid volume deficit consists of fluid replacement and measures to correct the underlying cause. Usually, isotonic electrolyte solutions are used for fluid replacement. Acute hypovolemia and hypovo- lemic shock can cause renal damage; therefore, prompt