AP2 - Day 13 - Ch 20 pt 1_student.pdf 1

Chapter 20: Water Gain

20.1 Fluid and Electrolyte Homeostasis

• Daily Intake: 2.2 L/day from food and drinks.

20.2 Water Balance

• Water Loss:

  • Skin (Insensible Water Loss): 0.9 L/day

  • Lungs: 0.3 L/day

  • Metabolism: 0.3 L/day

  • Urine: 1.5 L/day

  • Feces: 0.1 L/day

• Total Water Loss: 2.5 L/day

• Balance: Input (2.2 L/day) - Output (2.5 L/day) = 0

20.3 Sodium Balance and ECF Volume

6-4 Fluid and Electrolyte Homeostasis

• Primary Function: Maintain mass balance of water and ions.

• Excretion Mechanisms:

  • Kidneys (primary)

  • Feces and sweat (small amounts)

  • Lungs

Behavioral Responses

• Responses to maintain homeostasis:

  • Need to pee

  • Thirst

  • Salt hunger

Importance of Osmolarity

• Water moves by osmosis in response to solute concentration gradients.

Solutions and Red Blood Cells (RBC)

• Hypotonic Solution: RBC lysed

• Isotonic Solution: RBC normal

• Hypertonic Solution: RBC shriveled

Integrative Physiology

• Involves:

  • Respiratory System

  • Cardiovascular System

  • Renal System

  • Neural System (Behavioral Responses)

Integrated Response to Changes in Blood Volume and Pressure

• Integrating Centers:

  • Cardiovascular

  • Renal

Water Balance Mechanics

• Input = Output.

• Water enters via the digestive system.

• Water Loss: Insensible water loss is undetectable.

• Extreme water loss results in decreased plasma and blood pressure.

Kidneys and Water Conservation

• Kidneys can conserve water but cannot replenish it.

• Must replace water lost to the environment from external sources.

Renal Medulla and Urine Concentration

• Diuresis: Removal of excess water in urine.

• Urine concentration reflects the amount of water excreted.

• Drinking a large volume of water results in more diluted urine (more volume).

Nephron Structure and Function

• Proximal Tubule: Isosmotic fluid leaves here; becomes concentrated in the descending limb.

• Loop of Henle:

  • Descending limb: Only water reabsorbed.

  • Ascending limb: Salts reabsorbed, creating a dilute filtrate.

• Final Urine Osmolarity: Depends on reabsorption in the collecting duct (50-1200 mOsM).

Hormonal Control of Water Permeability

• Vasopressin (ADH): Regulates permeability in distal tubule and collecting duct.

  • Increases water reabsorption through aquaporin channels.

  • Higher vasopressin leads to concentrated urine.

  • Lower vasopressin leads to dilute urine.

Control of Vasopressin Secretion

• Regulated by signals from the hypothalamus; stored and released from the posterior pituitary.

Stimuli for Vasopressin Secretion

• Mechanoreceptors signal when they shrink or swell according to osmolarity (hypertonic vs hypotonic).

The Nephron Loop and Vasa Recta

• Countercurrent Exchange System: Nephron loop and vasa recta pass close to each other, facilitating passive transfer of solutes and water.

Ascending and Descending Limbs

• Descending Limb: Permeable to water, not to solutes.

• Ascending Limb: Permeable to solutes, not to water.

  • Active reabsorption of ions creates a dilute filtrate.

Homeostatic Response to Salt Ingestion

• Ingesting salt affects ECF osmolarity and blood pressure.

• Aldosterone: Regulates sodium reabsorption in distal tubules and collecting ducts.

  • More aldosterone = more Na+ reabsorption.

Sodium Balance

• Aldosterone modulates Na+ reabsorption by modifying and inserting new channels and pumps.

  • K+ is secreted during Na+ reabsorption process.