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

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

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

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

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

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

SECTION 27-2 An Introduction to Fluid and Electrolyte Balance

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

Figure 27.1 The Composition of the Human Body Figure 27.1a

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

Figure 27.2 Cations and Anions in Body Fluids

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

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

SECTION 27-3 Fluid Balance

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

Figure 27.3 Fluid Exchanges

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

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

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

SECTION 27-4 Electrolyte Balance

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

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

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

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

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

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

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