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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.