1. Chapter 29 Fluid and Electrolyte Balance

2. Interrelationship of Fluid and Electrolyte Balance
Fluid and electrolyte balance—implies homeostasis
Electrolytes have chemical bonds that allow dissociation into ions, which carry an electrical charge; of critical importance in fluid balance
Fluid balance and electrolyte balance are interdependent

3. Total Body Water
Fluid content of the human body ranges from 40% to 60% of its total weight
Fluid content varies according to age, gender, weight, and fat content of the body

4. Body Fluid Compartments
Two major fluid compartments (Figure 29-1)
Extracellular fluid (ECF) constitutes the internal environment of the body
Consists mainly of plasma and interstitial fluid
Lymph, cerebrospinal fluid, and specialized joint fluids are considered extracellular
Functions of ECF provide a relatively constant environment for cells and transport substances to and from the cells

5. Body Fluid Compartments
Intracellular fluid (ICF)—water inside the cells
Functions to facilitate intracellular chemical reactions that maintain life
By volume, ICF is the largest body fluid compartment

6. Chemical Content, Distribution, and Measurement of Electrolytes in Body Fluid
Extracellular vs. intracellular fluids
Plasma and interstitial fluid (ECFs) are almost identical in chemical make-up, with intracellular fluid showing striking differences (Figure 29-3)
Extracellular fluids
Difference between blood and interstitial fluid—blood contains a slightly larger total of ions than interstitial fluid
Functionally important difference between blood and interstitial fluid is the number of protein anions; blood has an appreciable amount, whereas interstitial fluid has hardly any; because the capillary membrane is practically impermeable to proteins, almost all protein anions remain in the blood
Intracellular fluids—ICF and ECF are more dissimilar than similar
Chemical structure of plasma, interstitial fluid, and intracellular fluid helps control water and electrolyte movement between them

7. Chemical Content, Distribution, and Measurement of Electrolytes in Body Fluid
Measuring electrolyte concentration—concentration or weight of an electrolyte can be expressed as the number of milligrams per 100 ml of solution (mg%); conversion of mg% to milliequivalents per liter (mEq/L) provides information on actual physiological activity
Measuring electrolyte reactivity—milliequivalent— measures the number of ionic charges or electrocovalent bonds in a solution; accurately measures the physiological combining power of an electrolyte solution

8. Avenues by Which Water Enters and Leaves the Body
Water enters the body via the digestive tract; water is also added to the total fluid volume from each cell as it catabolizes food, and the resulting water enters the bloodstream (Figure 29-5)
Water leaves the body via four exits (Figure 29-5):
As urine through the kidney
As water in expired air through the lungs
As sweat through the skin
As feces from the intestine

9. Some General Principles about Fluid Balance
Cardinal principle of fluid balance: fluid balance can be maintained only if intake equals output
Mechanisms are available to adjust output and intake to maintain fluid balance; e.g., aldosterone mechanism (Figure 29-6), renin-angiotensin mechanism (Figure 29-8)
Most rapid fluid balance devices are mechanisms for controlling water movement between fluid compartments of the body; will maintain normal blood volume at the expense of interstitial fluid volume

10. Mechanisms That Maintain Homeostasis of Total Fluid Volume
Under normal conditions, homeostasis of total volume of water is maintained or restored primarily by adjusting urine volume and secondarily by fluid intake (Figure 29-7)
Regulation of fluid intake—when dehydration begins to develop, salivary secretion decreases, producing the sensation of thirst; increased fluid intake to offset increased output tends to restore fluid balance

11. Mechanisms That Maintain Homeostasis of Total Fluid Volume
Regulation of urine volume—two factors determine urine volume:
Glomerular filtration rate, except under abnormal conditions, remains fairly constant
Rate of tubular reabsorption of water fluctuates considerably; normally adjusts urine volume to fluid intake; influenced by amount of antidiuretic hormone and of aldosterone
Decrease in fluid intake causes osmoreceptors in “thirst center”—wall of third ventricle (subfornical organ) and in supraoptic and paraventricular nuclei of hypothalamus—to increase secretion of ADH (Figure 29-16).
Factors that alter fluid loss under abnormal conditions—rate of respiration and volume of sweat secreted may alter fluid output under certain abnormal conditions; vomiting, diarrhea, or intestinal drainage can produce fluid and electrolyte imbalances

12. Regulation of Water and Electrolyte Levels in Plasma and Interstitial Fluid (ECF)
Law of capillaries—the control mechanism for water exchange between plasma and interstitial fluid consists of four pressures: blood hydrostatic and colloid osmotic pressures on one side of the capillary membrane and interstitial fluid hydrostatic and colloid osmotic pressures on the other side; two of the pressures promote a vector in one direction and the other two in the opposite direction
Blood hydrostatic pressure (BHP) forces fluid out of capillaries into interstitial fluid (IF)
Blood colloid osmotic pressure (BCOP) draws fluid from IF into capillaries
Interstitial fluid hydrostatic pressure (IFHP) forces fluid out of IF into capillaries
Interstitial fluid colloid osmotic pressure (IFCOP) draws fluid from capillaries to IF

13. Regulation of Water and Electrolyte Levels in Plasma and Interstitial Fluid (ECF)
The rate and direction of fluid exchange between capillaries and interstitial fluid are determined by the hydrostatic and colloid osmotic pressures of the two fluids (Figure 29-10)
Some principles about transfer of water between blood and interstitial fluid
No net transfer of water occurs as long as (BHP + IFCOP) = (IFHP + BCOP)
A net transfer of fluid occurs when (BHP + IFCOP) ≠ (IFHP + BCOP)
Fluid shifts out of blood into interstitial fluid whenever (BHP + IFCOP) > (IFHP + BCOP)
Fluid shifts out of interstitial fluid into blood whenever (BHP + IFCOP) < (IFHP + BCOP)

14. Edema
Edema—presence of abnormally large amounts of fluid in the intercellular tissue spaces of the body (Figure 29-11)
Classic example of fluid imbalance; may be due to any of the following:
Retention of electrolytes in the extracellular fluid
Increase in capillary blood pressure
Decrease in the concentration of plasma proteins normally retained in the blood (Figure 29-12)

15. Regulation of Water and Electrolyte Levels in Intracellular Fluid (ICF)
Plasma membrane plays critical role in regulating ICF composition
IF and ICF hydrostatic and colloid pressures regulate water transfer between ECF and ICF; colloid osmotic pressures are chief regulators of water transfer across cell membranes, and these are directly related to the electrolyte concentration gradients maintained across cell membranes (Figure 29-14)

16. Regulation of Sodium and Potassium Levels in Body Fluids
Normal sodium concentration in IF and potassium concentration in ICF depends on various factors, but especially on amount of ADH and aldosterone secreted
ADH regulates ECF electrolyte concentration and colloid osmotic pressure by regulating amount of water reabsorbed into blood by renal tubules
Aldosterone regulates ECF volume by regulating the amount of sodium reabsorbed into blood by renal tubules
When conservation of body sodium is required, the kidneys excrete an essentially sodium-free urine; kidneys are considered the chief regulator of sodium levels

17. Regulation of Sodium and Potassium Levels in Body Fluids
Chloride—most important extracellular anion, is almost always linked to sodium; chloride ions are generally excreted in urine associated with potassium; thus hypochloremia is often associated with cases of potassium loss
Hypokalemia occurs where there is cell breakdown; as cells disintegrate, potassium enters ECF and is rapidly excreted because it is not reabsorbed efficiently by the kidney