Makindo Medical Notes

Sodium is a crucial electrolyte in the human body, playing a key role in maintaining fluid balance, nerve function, and muscle contraction. It is predominantly an extracellular ion, with tightly regulated concentrations to ensure proper physiological function.

Distribution of Sodium

• Sodium is primarily an extracellular ion:
  • The extracellular concentration is approximately 135-145 mEq/L.
  • The intracellular concentration is much lower, around 10-15 mEq/L.
• This distribution is maintained by the sodium-potassium pump (Na⁺/K⁺-ATPase):
  • Actively transports sodium out of cells and potassium into cells.
  • Helps maintain the electrochemical gradient and cellular homeostasis.

Functions of Sodium

• Maintaining Fluid Balance:
  • Sodium helps regulate extracellular fluid volume and osmolarity.
  • Water follows sodium, so changes in sodium levels can affect fluid balance.
• Nerve Impulse Transmission:
  • Sodium ions are crucial for the generation and propagation of action potentials in neurons.
  • The influx of sodium during depolarization initiates the action potential.
• Muscle Contraction:
  • Proper sodium levels are necessary for normal muscle function, including cardiac muscle.
  • Sodium influx triggers the depolarization phase of the muscle action potential.
• Acid-Base Balance:
  • Sodium bicarbonate (NaHCO₃) acts as a buffer, helping to maintain the acid-base balance in the blood.
• Nutrient Absorption and Transport:
  • Sodium plays a role in the absorption of nutrients, such as glucose and amino acids, in the intestines.
  • Sodium-coupled transporters facilitate the uptake of these nutrients into cells.

Regulation of Sodium Levels

• Sodium balance is regulated by several mechanisms:
  • Renal Regulation:
    • The kidneys are the primary organs responsible for maintaining sodium balance.
    • Sodium is filtered by the glomerulus and reabsorbed along the nephron, particularly in the proximal tubule, loop of Henle, distal convoluted tubule, and collecting duct.
    • Hormonal regulation, including aldosterone and atrial natriuretic peptide (ANP), influences sodium reabsorption and excretion.
  • Hormonal Regulation:
    • Aldosterone increases sodium reabsorption in the distal convoluted tubule and collecting duct, promoting water retention and increasing blood pressure.
    • Atrial natriuretic peptide (ANP) decreases sodium reabsorption, promoting sodium and water excretion to lower blood pressure.
  • Thirst Mechanism:
    • Increased plasma osmolarity stimulates the thirst centre in the hypothalamus, leading to increased water intake and dilution of sodium.
  • Antidiuretic Hormone (ADH):
    • ADH, also known as vasopressin, promotes water reabsorption in the kidneys, which can indirectly influence sodium concentration.

Clinical Relevance

• Hyponatremia (low sodium levels):
  • Causes: Excessive water intake, kidney disease, heart failure, syndrome of inappropriate antidiuretic hormone secretion (SIADH), and certain medications.
  • Symptoms: Nausea, headache, confusion, seizures, and coma in severe cases.
  • Treatment: Addressing the underlying cause, fluid restriction, and, in severe cases, hypertonic saline administration.
• Hypernatremia (high sodium levels):
  • Causes: Water loss (e.g., sweating, diarrhoea, diabetes insipidus), inadequate water intake, and excessive sodium intake.
  • Symptoms: Thirst, weakness, confusion, seizures, and coma in severe cases.
  • Treatment: Gradual rehydration with hypotonic fluids and addressing the underlying cause.
• Hypertension:
  • High sodium intake is associated with increased blood pressure and risk of hypertension.
  • Reducing sodium intake is a common dietary recommendation for managing hypertension.
• Heart Failure:
  • In heart failure, the body retains sodium and water, leading to fluid overload and worsening symptoms.
  • Diuretics are often used to manage fluid balance and reduce the workload on the heart.
• Chronic Kidney Disease (CKD):
  • CKD can impair the kidneys' ability to excrete sodium, leading to fluid retention and hypertension.
  • Managing sodium intake and using medications to control blood pressure are important aspects of CKD management.

Summary

Sodium is a vital electrolyte involved in maintaining fluid balance, nerve function, muscle contraction, and overall cellular homeostasis. Its regulation is tightly controlled by renal, hormonal, and thirst mechanisms. Abnormal sodium levels can lead to significant clinical issues, emphasizing the importance of understanding sodium physiology and maintaining its balance.