Physiology of Electrolyte Balance and Regulation

Alterations in Fluid and Electrolytes

• Fluid compartments and sub-compartments of the body, and relative amounts of fluid in each are named
• Mechanisms of fluid movement between compartments are described
• Routes of water intake and output to/from the body are identified
• Thirst mechanism and cessation of thirst are explained
• How shifts in water output occur and how the body compensates for those shifts is indicated
• The activity of antidiuretic hormone (ADH) is discussed
• Imbalances of fluid homeostasis and their consequences are described
• Mechanisms regulating sodium balance in body fluids are identified
• Mechanisms regulating potassium, calcium, and phosphate balance in body fluids are examined
• Mechanisms regulating anions in body fluids are discussed
• Pathophysiology of diabetes insipidus is described
• Pathophysiology of inappropriate ADH syndrome (SIADH) is discussed

Fluids, Ions, Nonelectrolytes, and Electrolytes

• Body fluids, ions, nonelectrolytes, and electrolytes are categorized
• Different compartments of body fluids are specified
• Examples of body fluids, including cerebrospinal fluid, intraocular fluid, digestive juices, serous fluid, and synovial fluid are listed

Functions of Body Fluids

• Body fluids transport gases, nutrients, and wastes
• They help generate electrical activity needed for bodily functions
• Fluids are involved in the transformation of food into energy
• They maintain overall bodily function

Distribution of Body Fluids

• Intracellular compartment (ICF) consists of fluid within cells, making up roughly two-thirds of total body water in healthy adults.
• High concentration of potassium (K+) in the ICF
• Extracellular fluid (ECF) contains the remaining one-third of total body water, including interstitial fluid and plasma located outside of cells.
• High concentration of sodium (Na+) in the ECF

Composition of ECF, Plasma, and Interstitial Fluids

• ECF, plasma, and interstitial fluids contain large amounts of sodium (Na+) and chloride (Cl−)
• These fluids also contain moderate amounts of bicarbonate (HCO3−)
• Small amounts of potassium (K+), magnesium (Mg2+), calcium (Ca2+), and phosphate (PO43−) are present

Composition of ICF

• The intracellular fluid (ICF) contains almost no calcium (Ca2+)
• It contains small amounts of sodium (Na+), chloride (Cl−), bicarbonate (HCO3−), and phosphate (PO43−)
• It has moderate amounts of magnesium (Mg2+)
• A significant amount of potassium (K+) is present in the ICF

Diffusion and Osmosis

• Difference in concentration creates a concentration gradient
• Diffusion moves particles from higher to lower concentrations
• Osmosis moves water across a semipermeable membrane from a lower to higher solute concentration area

Tonicity

• Tonicity refers to the tension or effect of a solution's osmotic pressure on cell size due to water movement across cell membranes
• Isotonic solutions do not cause cells to shrink or swell
• Hypotonic solutions cause cells to swell
• Hypertonic solutions cause cells to shrink

Mechanisms Protecting Extracellular Fluid Volume

• Alterations in hemodynamic variables, like vasoconstriction and increased heart rate, can protect extracellular fluid volume
• Isotonic contraction or expansion of extracellular fluid (ECF) volume relates to concentration changes.
• Changes in extracellular water affect blood volume and circulating blood.

Edema

• Edema is the accumulation of fluid in extracellular spaces
• Pitting edema is characterized by a persistent indentation in the skin after pressure is applied
• Non-pitting edema does not exhibit a persistent indentation in the skin
• Edema forms due to increased capillary filtration pressure, decreased capillary osmotic pressure, increased capillary permeability and decreased lymphatic flow.

Methods for Assessing Edema

• Daily weight, visual assessment, measurement of affected area, and application of finger pressure to detect pitting edema are used for edema assessment.

Physiologic Mechanisms Assisting in Regulating Body Water

• The thirst mechanism regulates water intake, activating when dehydrated.
• The elderly may have reduced thirst mechanisms, leading to easier dehydration.
• ADH (antidiuretic hormone) regulates water output by affecting kidney water reabsorption.

Water and Na+ Balance

• Baroreceptors in blood vessels monitor blood pressure and signal the pituitary gland to release or stop ADH release.
• Atrial natriuretic peptide (ANP) is secreted when extracellular fluid (ECF) volume decreases, increasing renal sodium excretion to counteract decreased volume.
• Renin-angiotensin-aldosterone system (RAAS) conserves sodium and water by stimulating aldosterone secretion.

Regulators of Sodium

• The kidneys play a key role in regulating sodium levels, responding to blood pressure changes by retaining or eliminating sodium.
• This regulation is coordinated by the sympathetic nervous system and the renin-angiotensin-aldosterone system (RAAS).

Assessment of Body Fluid Loss

• Conditions affecting fluid loss, such as heart rate, blood pressure, postural hypotension, venous volume, capillary refill rate, and urine output are assessed.

Causes of Fluid Volume Deficit

• Abnormal losses through the gastrointestinal tract or kidneys (such as from perspiration, vomiting, diarrhea, or bleeding).
• Movement of fluid into third spaces (e.g., abdominal cavity).
• Insufficient fluid intake.

Extracellular Fluid Volume Excess

• Extracellular fluid (ECF) volume excess occurs when ECF becomes expanded due to increases in both interstitial and vascular fluid volumes.
• It is often coupled with increased body sodium content.

Causes of Fluid Volume Excess

• Inadequate sodium and water elimination, excessive sodium intake, and excessive fluid intake are potential causes of ECF volume excess.

ADH: Antidiuretic Hormone

• ADH, also called arginine vasopressin (AVP), is produced in the hypothalamus and stored in the posterior pituitary gland.
• It's released into the bloodstream when needed to help the body retain water and reduce urination.

SIADH versus Diabetes Insipidus

• These conditions have opposite effects and treatments.

Diabetes Insipidus

• Diabetes insipidus is a disease where the body loses the ability to correctly retain water and leads to increased urination and intense thirst.
• It results from problems with ADH causing the kidneys to excrete large amounts of dilute urine.
• Causes include conditions like head injuries and tumors disrupting ADH function.

Symptoms

• Symptoms of diabetes insipidus include high urine production, insatiable thirst, and constant desire to drink fluids.

Diagnosis

• Diagnosis for diabetes insipidus involves testing urine specific gravity, serum sodium, and urine osmolality to detect a lack of ADH.

Treatment

• Treatment involves administering ADH to maintain proper fluid and electrolyte balances.

SIADH

• SIADH occurs when the body retains too much water.
• Common causes are related to the excess production or inappropriate release of ADH, often including conditions after surgery or due to certain medications.

Symptoms of SIADH

• Symptoms include weight gain, edema, nausea, muscle cramps, and confusion.

Treatment of SIADH

• Treatment of SIADH aims to correct the underlying cause and limit water intake. Restricting water and administering certain medications are common treatment approaches.

Complications Of Treatment

• Possible complications include central pontine myelinolysis that can cause neurological issues such as spastic paralysis and quadriparesis. Other potential complications include airway problems and various neuromuscular issues.

Treatment Complications

• Individuals at greater risk of complications from treatment include alcoholics, malnourished individuals, elderly people taking thiazide medications, and those with low potassium levels.

Potassium Distribution and Regulation

• Potassium levels are regulated to maintain balance in two compartments: intracellular and extracellular, where extracellular potassium is at 3.5–5.0 mEq/L.
• Body potassium stores are related to body size and muscle mass.
• Renal mechanisms and transcellular shifts regulate potassium levels.

Abnormal Potassium

• Hypokalemia is the decrease in plasma potassium levels below 3.5 mEq/L, and hyperkalemia is the increase above 5.0 mEq/L.
• Causes include inadequate intake, excessive losses, and intracellular-extracellular shifts.

Vitamin D, Calcitonin, and Parathyroid Hormone

• Calcium, phosphate, and magnesium are major cations in the body, and vitamin D helps maintain their normal levels by promoting their absorption from food.
• Calcitonin regulates calcium concentrations in the blood by reducing calcium mobilization from bone.
• The parathyroid hormone (PTH) and vitamin D work together to regulate calcium absorption and excretion, keeping levels within a specific range.

Mechanisms Regulating Calcium, Phosphate, and Magnesium Balance

• Calcium, phosphate, and magnesium are ingested in the diet, absorbed from the intestine, filtered in the glomerulus, and reabsorbed by the kidney tubules.
• These minerals are essential for bodily functions and play a critical role in maintaining proper levels.

Calcium Gain and Losses

• Dietary calcium intake is a key source.
• Parathyroid hormone (PTH) and vitamin D influence calcium reabsorption within the kidneys to maintain blood calcium levels.
• Calcium losses can occur when too much calcium is excreted through the intestines.

Causes and Symptoms of Hypocalcemia

• Hypocalcemia, or low calcium levels, occurs due to impaired calcium mobilization from bone, abnormal kidney calcium loss, or increased protein binding.
• Symptoms include neuromuscular excitability issues (Chvostek's and Trousseau's signs).
• Cardiovascular and nerve effects can occur as well.

Hypocalcemia: Chvostek's and Trousseau's Signs

• This section discusses diagnostic signs (Chvostek's Sign, Trousseau’s Sign) used to assess hypocalcemia.

Causes and Symptoms of Hypercalcemia

• Hypercalcemia, or high calcium levels, can arise from increased intestinal calcium absorption, bone resorption, or decreased calcium elimination.
• Such high levels lead to effects on neural and smooth muscle, as well as kidney health.

Role of Phosphate in the Body

• Important in bone formation, metabolism processes, cell structures, and as an acid-base buffer.
• Facilitates oxygen delivery by red blood cells and other blood cell function.

Common Causes of Hypo/Hyperphosphatemia

• Causes related to phosphate imbalances, such as impaired intestinal absorption, transcompartmental shifts in phosphate, and increased renal losses of phosphate, are discussed.
• Excess phosphate results from kidney failure and or increased intake of phosphate.

Magnesium Balance

• Magnesium is essential in various bodily functions and is regulated in the kidney.
• Sources of magnesium are ingested in the diet and absorbed from the intestine.
• Excretion occurs through the kidneys.

Hypomagnesemia

• Hypomagnesemia, a low magnesium level, can produce significant neuromuscular and vascular issues.

Manifestations of Hypomagnesemia

• Symptoms of hypomagnesemia include various neuromuscular, mental, cardiovascular, and neurological issues.

Athetoid and Choreiform Movements

• Athetoid and choreiform movements are involuntary movements of the body.

Nystagmus

• Description of involuntary eye movements, nystagmus.

Causes of Hypermagnesemia

• Risk factors for hypermagnesemia, such as excessive intake, decreased excretion, and kidney disease, are explained.

Signs & Symptoms of Hypermagnesemia

• Symptoms of hypermagnesemia, including decreased respiration, depressed mental status, bradycardia, and neurological effects, are discussed.

Practice Questions

• Practice questions on the topics learned to aid in testing comprehension of the subject matter.