1. Fluid, Electrolyte, and Acid-Base Balance
Chapter 27, part 1
Fluid, Electrolyte, and Acid-Base Balance

2. Learning Objectives
Explain what is meant by “fluid balance,” “electrolyte balance,” and “acid-base balance”
Compare the compositions of intracellular and extracellular fluids
Identify the hormones that play important roles in regulating fluid and electrolyte balance
Describe the movement of fluid that takes place within the ECF, between the ECF and the ICF, and between the ECF and the environment

3. Learning Objectives
Discuss how sodium, potassium, calcium and chloride ions are regulated to maintain electrolyte balance
Explain the buffering systems that balance the pH of the intracellular and extracellular fluids
Describe the compensatory mechanisms involved in acid-base balance

4. SECTION 27-1 Fluid, Electrolyte and Acid-base Balance: An Overview

5. Maintenance of normal fluid volume and composition is vital
Extracellular fluid (ECF)
Interstitial fluid, plasma, and other body fluids
Intracellular fluid (ICF)
The cytosol

6. Fluid and electrolyte balance
Fluid balance
The amount of water gained each day equals the amount lost
Electrolyte balance
The ion gain each day equals the ion loss
Acid-base balance
H+ gain is offset by their loss

7. SECTION 27-2 An Introduction to Fluid and Electrolyte Balance

8. The ECF and the ICF are two distinct fluid compartment
The cytosol of cells
Makes up about two-thirds of the total body water
ECF
Major components include the interstitial fluid and plasma
Minor components include all other extracellular fluids

9. Figure 27.1 The Composition of the Human Body
Figure 27.1a

10. Regulation of fluids and electrolytes
Homeostatic mechanisms respond to changes in ECF
No receptors directly monitor fluid or electrolyte balance
Respond to changes in plasma volume or osmotic concentrations
All water moves passively in response to osmotic gradients
Body content of water or electrolytes rises if intake exceeds outflow

11. Figure 27.2 Cations and Anions in Body Fluids

12. Primary regulatory hormones
Antidiuretic hormone (ADH)
Stimulates water conservation and the thirst center
Aldosterone
Controls Na+ absorption and K+ loss along the DCT
Natriuretic peptides (ANP and BNP)
Reduce thirst and block the release of ADH and aldosterone

13. Interplay between fluid balance and electrolyte balance
Different mechanisms regulate fluid and electrolyte balance
This distinction is vital in the clinical setting

14. SECTION 27-3 Fluid Balance

15. Fluid movement within the ECF
Fluid moves freely within ECF compartment
Water losses are normally balanced by gains
Eating
Drinking
Metabolic generation

16. Figure 27.3 Fluid Exchanges

17. Fluid exchange with the environment
The major routes of fluid exchange with the environment include:
Water loss
Temperature rise from fever
Water gains

18. Water excess and depletion
Hyponatremia
Na+ concentration in the ECF is reduced (overhydration)
Hypernatremia
Na+ in the ECF is abnormally high
Dehydration
Develops when water loss outpaces water gains

19. Fluid shifts Water movement between ECF and ICF
If ECF becomes hypertonic relative to ICF, water moves from ICF to ECF
If ECF becomes hypotonic relative to ICF, mater moves from ECF into cells
PLAY
Animation: Introduction to Body Fluids

20. SECTION 27-4 Electrolyte Balance

21. Problems with Electrolyte Balance
Usually result from sodium ion imbalances
Potassium imbalances are less common, but more dangerous

22. Sodium balance
Rate of sodium uptake across digestive tract directly proportional to dietary intake
Sodium losses occur through urine and perspiration
Shifts in sodium balance result in expansion or contraction of ECF
Large variations corrected by homeostatic mechanisms
Too low, ADH / aldosterone secreted
Too high, ANP secreted

23. Figure 27.4 The Homeostatic Regulation of Normal Sodium Ion Concentrations in Body Fluids

24. Figure 27.5 The Integration of Fluid Volume Regulation and Sodium Ion Concentrations in Body Fluids

25. Potassium balance Potassium ion concentrations in ECF are low
Not as closely regulated as sodium
Potassium ion excretion increases as
ECF concentrations rise
Aldosterone secreted
pH rises
Potassium retention occurs when pH falls

26. ECF Concentrations of other electrolytes
Calcium balance
Bone reserves, absorption in the digestive tract, and loss at kidneys
Magnesium balance
Absorbed by the PCT to keep pace with urinary losses

27. ECF Concentrations of other electrolytes
Phosphate balance
Absorbed by the PCT in response to calcitriol
Chloride balance
Absorbed at digestive tract to balance losses in urine and sweat
PLAY
Animation: Electrolyte homeostasis