Chapter 20 A: Integrative Physiology II: Fluid and Electrolyte Balance

Fluid and Electrolyte Balance

Introduction

Fluid and electrolyte balance is essential for maintaining homeostasis in the human body. The kidneys play a central role in regulating water and solute concentrations, ensuring proper cellular function and overall physiological stability.

Factors Affecting Water Balance

Water Inputs and Outputs

• Water Inputs: Ingestion (2.2 L/day), cellular metabolism (0.3 L/day), total ~2.5 L/day.

• Water Outputs: Excretion in urine (1.5 L/day), feces (0.1 L/day), insensible loss (0.9 L/day via sweat and respiration).

• Distribution: 28 L (2/3) in cells, 11 L in interstitial fluid, 3 L in plasma.

Example: If water intake is less than output, dehydration occurs, affecting blood pressure and cellular function.

Renal Regulation of Body Fluid Volume

Kidneys Conserve Volume

• Glomerular Filtration Rate (GFR): Can be adjusted to regulate fluid volume.

• Volume Loss: Only replaced by intake; kidneys recycle fluid to conserve volume.

• Regulated H2O Reabsorption: Prevents excessive loss in urine.

Example: During dehydration, kidneys reduce urine output to conserve water.

Osmolarity and the Movement of Water

Water Reabsorption in the Nephron

• Proximal Tubules:

  • 70% of filtered water is reabsorbed.

  • Process is not regulated; water follows solute reabsorption (mainly Na+).

• Distal Tubules and Collecting Ducts:

  • Most remaining water is reabsorbed.

  • Regulated by antidiuretic hormone (ADH/vasopressin).

Mechanism of Sodium and Glucose Reabsorption

Proximal Tubule Transport

• Na+ and Glucose: Co-transported across the apical membrane of renal tubule epithelial cells.

• Na+: Actively transported out of the cell into the peritubular fluid via the basolateral membrane.

• K+: Exchanged for Na+ to maintain electrochemical balance.

Example: Glucose reabsorption is coupled to sodium movement, ensuring efficient nutrient recovery.

Establishment of the Medullary Osmotic Gradient

Countercurrent Mechanism in the Loop of Henle

• Osmotic Gradient: Established by countercurrent exchange between the loop of Henle and vasa recta.

• Descending Limb:

  • Permeable to water; water exits into the medulla.

  • No transport of Na+, Cl-, or K+.

• Ascending Limb:

  • Impermeable to water.

  • Active transport of Na+, Cl-, and K+ into the medullary interstitium.

Example: The medullary osmotic gradient allows for the concentration of urine when water conservation is necessary.

Osmolarity Changes Along the Nephron

Fluid Flow and Osmolarity

• Osmolarity increases as fluid descends into the medulla (descending limb), then decreases as it ascends (ascending limb).

• Variable reabsorption of water and solutes occurs in the distal tubule and collecting duct, fine-tuning urine concentration.

Juxtamedullary Nephron and Vasa Recta

Structure and Function

• Juxtamedullary Nephrons: Long loops of Henle extend deep into the medulla, essential for creating the osmotic gradient.

• Vasa Recta: Capillary network that maintains the gradient via countercurrent exchange.

Countercurrent Exchange in the Vasa Recta

Mechanism

• Blood in the vasa recta flows in the opposite direction to filtrate in the loop of Henle.

• Solutes and water are exchanged, preserving the medullary gradient.

Water Reabsorption in the Distal Tubule and Collecting Duct

Regulation by ADH (Vasopressin)

• Dependent on the osmotic gradient established by the countercurrent multiplier.

• Dependent on epithelial permeability to water, which is regulated by water channels (aquaporins).

Role of Vasopressin (ADH) in Water Reabsorption

Mechanism of Action

• Vasopressin binds to receptors on collecting duct cells, triggering insertion of aquaporin-2 water channels into the apical membrane.

• Increased water permeability allows water to be reabsorbed into the medulla, concentrating urine.

Equation:

$\text{Urine Osmolarity} = \frac{\text{Total Solute Excreted}}{\text{Volume of Water Excreted}}$

Effect of Vasopressin Levels

• Low Vasopressin: Collecting duct is impermeable to water; dilute urine is produced.

• High Vasopressin: Collecting duct is highly permeable to water; concentrated urine is produced.

Control of ADH Secretion

Stimuli for ADH Release

• Decreased blood pressure (baroreceptors)

• Decreased atrial stretch (due to low blood volume)

• Increased osmolarity (osmoreceptors in the hypothalamus)

ADH is released from the posterior pituitary, acts on the kidneys to increase water reabsorption, and helps restore blood volume and osmolarity to normal levels.

Summary Table: Major Hormones in Fluid and Electrolyte Balance

Hormone	Source	Primary Target	Main Effect
Vasopressin (ADH)	Posterior pituitary	Kidney collecting ducts	Increases water reabsorption
Aldosterone	Adrenal cortex	Distal tubule, collecting duct	Increases Na+ reabsorption, K+ secretion
Atrial Natriuretic Peptide (ANP)	Atria of heart	Kidney	Increases Na+ and water excretion

Additional info: Other hormones such as angiotensin II and parathyroid hormone also play roles in fluid and electrolyte balance.