Chapter 20 B:Fluid, Electrolyte, and Acid-Base Balance: Sodium Regulation and Hormonal Control

Fluid and Electrolyte Balance

Overview of Fluid and Electrolyte Homeostasis

Fluid and electrolyte balance is essential for maintaining cellular function, blood pressure, and overall homeostasis. The kidneys play a central role in regulating the volume and osmolarity of body fluids by controlling the reabsorption and excretion of water and solutes.

• Osmolarity: The concentration of solutes in body fluids, measured in osmoles per liter.

• Volume: The total amount of fluid present in the body compartments.

• Changes in fluid volume and osmolarity can result from various physiological and pathological conditions.

Volume and Osmolarity Changes

The following table summarizes how different scenarios affect fluid volume and osmolarity:

Volume	Osmolarity Decrease	Osmolarity No Change	Osmolarity Increase
Increase	Drinking large amount of water	Ingestion of isotonic saline	Ingestion of hypertonic saline
No change	Replacement of sweat loss with plain water	Normal volume and osmolarity	Eating salt without drinking water
Decrease	Incomplete compensation for dehydration	Hemorrhage	Dehydration (e.g., sweat loss or diarrhea)

Sodium Balance

Regulation of Sodium in the Kidneys

Sodium (Na+) is the major extracellular cation and is crucial for fluid balance, nerve conduction, and muscle function. The kidneys regulate sodium levels through filtration and reabsorption processes.

• Hypernatremia: Elevated plasma sodium concentration.

• Hyponatremia: Reduced plasma sodium concentration.

• Approximately 70% of filtered sodium is reabsorbed in the proximal tubules (unregulated).

• Further reabsorption is regulated in the distal tubules and collecting ducts.

• The Na+/K+ pump on the basolateral membrane drives sodium reabsorption.

Mechanisms of Sodium Reabsorption

Sodium reabsorption in the proximal tubule involves several transporters and channels:

• Na+ enters proximal tubule cells via symporters and antiporters on the apical membrane.

• Na+ is actively transported out of the cell into the peritubular fluid by the Na+/K+ ATPase pump on the basolateral membrane.

• Other solutes, such as glucose and amino acids, are co-transported with Na+.

Normal Rates of Filtration and Reabsorption

Substance	Filtration rate	Reabsorption rate	Percentage of filtered load reabsorbed
Water	180 liters/day	178.5 liters/day	99.2%
Glucose	800 millimoles/day	800 millimoles/day	100%
Urea	54 millimoles/day	24 millimoles/day	44%
Na+	25.50 moles/day	25.05 moles/day	98.2%
Cl-	18.00 moles/day	17.70 moles/day	98.3%
K+	0.70 moles/day	0.60 moles/day	86.1%
Ca2+	0.54 moles/day	0.50 moles/day	98.1%
HCO3-	4.32 moles/day	4.31 moles/day	99.8%

Hormonal Regulation of Sodium and Water Balance

The Effects of Aldosterone

Aldosterone is a steroid hormone produced by the adrenal cortex that increases sodium reabsorption and potassium secretion in the distal nephron.

• Stimulates synthesis of Na+ channels and Na+/K+ pumps in principal cells of the distal tubule and collecting duct.

• Increases sodium reabsorption and potassium secretion simultaneously.

• Helps regulate blood pressure and extracellular fluid volume.

Control of Aldosterone Secretion

Aldosterone secretion is primarily controlled by the renin-angiotensin-aldosterone system (RAAS) and plasma potassium levels.

• High plasma K+ directly stimulates aldosterone release.

• Low blood pressure or low blood volume activates the RAAS pathway.

• Renin is released from juxtaglomerular cells in the kidney in response to decreased perfusion, sympathetic stimulation, or decreased NaCl delivery to the distal tubule.

• Renin converts angiotensinogen (from the liver) to angiotensin I, which is then converted to angiotensin II by angiotensin-converting enzyme (ACE) in the lungs.

• Angiotensin II stimulates aldosterone secretion from the adrenal cortex.

Renin-Angiotensin-Aldosterone System (RAAS)

• Regulates blood pressure, sodium, and water balance.

• Increases sodium reabsorption, water retention, and potassium excretion.

• Key steps: Renin release → Angiotensinogen conversion → Angiotensin I → Angiotensin II → Aldosterone secretion.

Atrial Natriuretic Peptide (ANP)

ANP is a hormone secreted by atrial cells in response to increased atrial stretch (high plasma volume). It promotes salt and water excretion by the kidneys.

• Increases glomerular filtration rate (GFR).

• Inhibits sodium reabsorption in the distal nephron.

• Suppresses renin and aldosterone secretion.

• Results in increased sodium and water excretion, lowering blood volume and pressure.

Summary of Key Concepts

• Fluid and electrolyte balance is tightly regulated by renal mechanisms and hormones.

• Sodium reabsorption is a major determinant of extracellular fluid volume and blood pressure.

• Aldosterone and the RAAS system increase sodium reabsorption and potassium secretion.

• ANP counteracts the effects of RAAS by promoting sodium and water excretion.

Example: Dehydration Response

• Dehydration (loss of water and/or sodium) triggers compensatory mechanisms to restore blood volume and osmolarity.

• RAAS activation increases sodium and water reabsorption.

• ADH (antidiuretic hormone) increases water reabsorption in the collecting ducts.

• ANP secretion is suppressed during dehydration.

Additional info: These notes integrate textbook figures and lecture slides to provide a comprehensive overview of sodium and water regulation, including hormonal control and clinical relevance.