Chapter 25: Fluid and Electrolytes

Chapter 25: Fluid and Electrolytes

25.1a Percentage of Body Fluid 1

  • Fluid Compartment Categories:

    • Two main fluid compartments in the body:

    • Intracellular fluid (ICF): fluid contained within cells

    • Extracellular fluid (ECF): fluid outside of cells

  • Fluid Percentage by Weight:

    • The human body is composed of 45% to 75% fluid by weight, which varies based on age and body composition (amount of adipose tissue and muscle tissue).

  • Influencing Variables:

    • Age:

    • Infants have the highest percentage of body fluid.

    • Older adults possess the lowest percentage.

    • Children, young adults, and middle-aged adults lie in the intermediate range.

    • Body fluid content generally decreases with age.

25.1a Percentage of Body Fluid 2

  • Body Composition Influence:

    • The ratio of adipose tissue to skeletal muscle impacts body fluid content:

    • Adipose tissue consists of approximately 20% water.

    • Skeletal muscle is around 75% water.

    • Typically, males possess a greater proportion of skeletal muscle, leading to a slightly higher percentage of body fluid.

    • The percentage of fluid in the body decreases as body fat increases.

25.1b Fluid Compartments 1

  • Compartment Overview:

    • The body's fluids are divided into two main compartments: ICF and ECF.

    • Intracellular fluid (ICF):

    • Comprises two-thirds of the total body fluid.

    • Enclosed by the plasma membrane, controlling substance movement.

25.1b Fluid Compartments 2

  • Extracellular Fluid (ECF):

    • Fluid located outside cells, further divided into:

    • Interstitial Fluid (IF):

      • Surrounds cells and represents two-thirds of ECF.

    • Blood Plasma:

      • Contains fluid within blood vessels, distinguished from IF by the capillary walls.

      • The capillary wall exhibits greater permeability than the plasma membrane.

    • Composition similarities between interstitial fluid and blood plasma.

25.1b Fluid Compartments 3

  • Additional Extracellular Fluids:

    • Notable ECF types include:

    • Cerebrospinal fluid

    • Synovial joint fluid

    • Aqueous and vitreous humor of the eye

    • Fluids of the inner ear

    • Serous fluids within body cavities

    • These fluids typically do not experience significant daily fluctuations in volume.

25.1b Fluid Compartments 4

  • Chemical Composition Distinction:

    • ICF and ECF exhibit distinct chemical compositions:

    • ICF contains more negatively charged proteins.

    • Variations arise from cellular processes and the activity of transport proteins.

    • ECF has two components: IF and blood plasma.

    • The main distinction being the protein content: blood plasma is protein-rich; interstitial fluid holds minimal protein.

    • High concentrations of ions such as:

    • K, Mg, and PO₄³⁻ in ICF.

    • Na, Ca²⁺, Cl⁻, and HCO₃⁻ in ECF; distinctions reflect capillary permeability concerning ions versus proteins.

25.1b Fluid Compartments 5

  • Fluid Movement:

    • Continuous fluid movement between compartments responds to osmolarity changes (concentration shifts).

    • Osmosis dictates water movement:

    • Water shifts from hypotonic to hypertonic solutions to equilibrate osmotic pressures.

  • Fluid Intake & Movement:

    • Drinking water lowers plasma osmolarity, causing water to move into interstitial fluid and subsequently into cells.

    • If dehydration occurs, movement reverses, causing cells to lose water, leading to a concentration of plasma.

25.2a Fluid Intake and Fluid Output 1

  • Fluid Balance:

    • Achieved when fluid intake equals fluid output, maintaining distribution of water and solutes across compartments.

    • Various body systems are involved in the intake of fluid and the regulation of fluid loss.

25.2a Fluid Intake and Fluid Output 2

  • Fluid Intake:

    • Body adds approximately 2500 mL of water daily, categorized as:

    • Ingested Water (Preformed): Approximately 2300 mL, mainly from food and beverages.

    • Metabolic Water: Around 200 mL produced from aerobic respiration and dehydration synthesis.

25.2a Fluid Intake and Fluid Output 3

  • Fluid Output:

    • Daily loss of water also estimated at 2500 mL via several pathways:

    • Urination (approximately 60% of output).

    • Other pathways include breathing, sweating, evaporation from skin (cutaneous transpiration), and feces.

25.2a Fluid Intake and Fluid Output 4

  • Sensible vs. Insensible Loss:

    • Sensible Water Loss: Easily measured, includes urine and fecal loss.

    • Insensible Water Loss: Not easily measured, primarily through expired air and sweat.

25.2a Fluid Intake and Fluid Output 5

  • Obligatory vs. Facultative Loss:

    • Obligatory Water Loss: Always occurs through respiration, skin, feces, and minimal urine required to eliminate waste.

    • Facultative Water Loss: Controlled by body hydration state and regulated hormonally in kidney nephrons. This mechanism allows reduced loss when dehydrated.

25.2b Fluid Imbalance 1

  • Fluid Imbalance:

    • A state where fluid output does not align with fluid intake, categorized into five types:

    • Volume depletion

    • Volume excess

    • Dehydration

    • Hypotonic hydration

    • Fluid sequestration

    • Differentiation criteria:

    • Whether the imbalance alters osmolarity.

    • Whether it results from excess or deficiency of body fluids.

25.2b Fluid Imbalance 2

  • Imbalances with Constant Osmolarity:

    • Occur with isotonic fluid loss or gain:

    • Volume Depletion: Loss exceeds gain, e.g., due to blood loss or severe burns.

    • Volume Excess: Increase in isotonic fluid gain, less urine output, with no osmolarity change.

25.2b Fluid Imbalance 3

  • Imbalances with Changes in Osmolarity:

    • Dehydration: Water loss exceeds solute loss, creates hypertonic plasma due to sweating or insufficient water intake.

    • Fluid shifts from intracellular to interstitial and then plasma, risking cell dehydration.

25.2b Fluid Imbalance 4

  • Hypotonic Hydration:

    • Excess water gain compared to solutes; most common cause is drinking excessive plain water. This leads to hypotonic plasma, risking intracellular swelling (potential cerebral edema).

25.2b Fluid Imbalance 5

  • Fluid Sequestration:

    • Total body fluid remains normal, but distribution is abnormal, exemplified by conditions such as:

    • Edema (interstitial fluid buildup)

    • Ascites (fluid in peritoneal cavity)

    • Pericardial effusion (fluid around the heart)

    • Pleural effusion (fluid in the pleural cavity).

25.2c Regulation of Fluid Balance 1

  • Monitoring Mechanisms:

    • No direct measurement of water volume or solute concentration occurs; regulation is indirect via blood volume, pressure, and osmolality monitoring.

  • Relationships:

    • Fluid Intake > Output: Increases blood volume and pressure, reduces osmolarity.

    • Fluid Intake < Output: Decreases blood volume and pressure, increases blood osmolarity.

25.2c Regulation of Fluid Balance 2

  • Regulating Fluid Intake:

    • Influenced by stimuli activating the thirst center, which include:

    • Decreased blood volume/pressure.

    • Renin release from the kidney increases angiotensin II production, stimulating thirst.

    • Elevated blood osmolarity triggers release of ADH from the hypothalamus.

    • Decreased salivary secretions send sensory information to the thirst center.

25.2c Regulation of Fluid Balance 3

  • Thirst Center Inhibition:

    • Fluid intake surpasses output, leading to increased blood volume/pressure:

    • Inhibition of renin release and reduced angiotensin II action.

    • Decreased blood osmolarity limits thirst center stimulation.

    • Stomach distension from fluid intake generates inhibitory impulses.

25.2c Regulation of Fluid Balance 4

  • Fluid Output Regulation:

    • Governed by kidneys through urine output control, influenced by four major hormones:

    • Angiotensin II, ADH, and aldosterone all act to decrease urine output, enhancing blood volume & pressure.

    • Atrial natriuretic peptide (ANP) stimulates increased urine output, lowering blood volume & pressure.

25.3a Nonelectrolytes and Electrolytes

  • Definition of Nonelectrolytes:

    • Molecules that do not dissociate in solution; mostly covalent compounds.

  • Definition of Electrolytes:

    • Substances that dissociate to form ions in solution and conduct electrical current; these include unique functions and osmotic roles.

    • Concentration expressed as milliequivalents per liter (mEq/L).

25.3b Major Electrolytes: Location, Functions, and Regulation 1

  • Sodium Ion (Na):

    • Primarily distributed as 99% in ECF, 1% in ICF; maintained by sodium pumps.

    • Sodium serves as the principal cation in ECF, exerting significant osmotic pressure (normal range: 135-145 mEq/L).

    • Daily dietary requirement is about 2 g, with losses through urine, feces, and sweat, regulated by aldosterone, ADH, and ANP.

25.3b Major Electrolytes: Location, Functions, and Regulation 2

  • Role in Osmolarity:

    • Sodium primarily regulates blood plasma osmolarity:

    • Increased Na+ concentration leads to hypertonicity; water shifts from ICF to ECF.

    • Decreased Na+ concentration induces hypotonicity; water moves into cells until concentrations equalize.

25.3b Major Electrolytes: Location, Functions, and Regulation 3

  • Sodium Imbalances:

    • Most common electrolyte imbalance:

    • Hypernatremia: elevated sodium levels.

    • Hyponatremia: reduced sodium levels.

    • These changes often result from variations in body water composition.

25.3b Major Electrolytes: Location, Functions, and Regulation 4

  • Potassium Ion (K):

    • 98% located in ICF; critical for intracellular osmotic pressure and neurological activities.

    • Normal concentration: 3.5-5.0 mEq/L; regulated mainly through urinary loss.

25.3b Major Electrolytes: Location, Functions, and Regulation 5

  • Potassium Balance:

    • Dietary requirement is about 40 mEq/L, primarily drawn from fruits and vegetables.

    • Potassium loss occurs through urine, which can increase due to plasma potassium levels, aldosterone secretion, and blood pH changes.

25.3b Major Electrolytes: Location, Functions, and Regulation 6

  • Conditions Influencing Potassium Shifts:

    • Shifts occur based on:

    • Blood plasma concentration changes or hormonal influence.

    • For example, high plasma K+ causes movement into ICF to maintain electrical balance.

25.3b Major Electrolytes: Location, Functions, and Regulation 7

  • Potassium Hormone Impact:

    • Hormonal presence, like insulin, directs potassium shifts from ECF into ICF, thereby improving regulation amid meals and preventing hyperkalemia impacts.

25.3b Major Electrolytes: Location, Functions, and Regulation 8

  • Chloride Ion (Cl⁻):

    • Sum of anions in ECF, following sodium; its output via urine is adjustable with plasma levels, and excessive loss can lead to hypochloremia.

25.3b Major Electrolytes: Location, Functions, and Regulation 9

  • Calcium Ion (Ca²⁺):

    • Predominantly in bones (99% stored), regulating muscle contractions and neurotransmitter functions, with significant losses due to urine, feces, and sweat.

25.3b Major Electrolytes: Location, Functions, and Regulation 10

  • Phosphate Ion (PO₄³⁻):

    • This ion is the most abundant anion in ICF, involved in DNA, RNA, and phospholipid structure, functioning as a buffer in intracellular environments.

25.3b Major Electrolytes: Location, Functions, and Regulation 11

  • Magnesium Ion (Mg²⁺):

    • Found mostly within bones or cells; acts in numerous enzymatic processes and is derived from diet, particularly from leafy greens and legumes.

25.4 Hormonal Regulation

  • Key Hormones:

    • Four principal hormones are integral to regulating fluids and electrolytes:

    • Angiotensin II

    • Antidiuretic hormone (ADH)

    • Aldosterone

    • Atrial natriuretic peptide (ANP)

25.4a Angiotensin II 1

  • Formation and Functions:

    • Generated when renin activates angiotensinogen in response to low blood pressure or nervous system stimulation, leading to:

    • Vasoconstriction of blood vessels to enhance blood pressure.

    • Reduced urine output from kidneys to preserve blood volume.

25.4a Angiotensin II 2

  • Additional Actions:

    • Stimulation of the thirst center in the hypothalamus, enabling increased fluid intake, thus increasing blood volume and pressure.

    • Triggers adrenal cortex to release aldosterone, promoting sodium and water retention.

25.4b Antidiuretic Hormone 1

  • ADH Overview:

    • Synthesized in the hypothalamus and released by the posterior pituitary, primarily as a response to:

    • Low blood pressure, prompting angiotensin II formation.

    • Elevated blood osmolarity is detected by hypothalamic chemoreceptors.

25.4b Antidiuretic Hormone 2

  • Effects of ADH:

    • ADH's action encourages thirst, water reabsorption in the kidneys, leading to reduced fluid loss via urine, enhancing systemic blood pressure by vasoconstriction.

25.4c Aldosterone

  • Aldosterone’s Role:

    • Synthesized by adrenal cortex; activated by low blood sodium levels or angiotensin II, leading to increased sodium and water reabsorption.

    • This hormone reduces urine output and maintains osmolarity.

25.4d Atrial Natriuretic Peptide (ANP)

  • ANP Overview:

    • Secreted by the heart's atria upon blood volume and pressure elevation leads to:

    • Vasodilation of systemic vessels and renal afferent arterioles, which increases glomerular filtration rate (GFR) and lowers urine reabsorption, ultimately decreasing blood volume and pressure.

25.5 Acid-Base Balance

  • Acid-Base Regulation:

    • Critical for body function and requires hydrogen ion concentration regulation.

    • Normal blood pH range is 7.35 to 7.45, with imbalances referred to as acidosis (pH < 7.35) and alkalosis (pH > 7.45).

25.5a Categories of Acid 1

  • Acid Overview:

    • pH inversely correlates with H concentration; two acid categories exist:

    • Fixed Acids: Non-volatile acids produced from metabolic processes, regulated by kidneys.

25.5a Categories of Acid 2

  • Volatile Acids:

    • e.g., carbonic acid formed from CO₂ and water, can be expelled through respiration (the body's primary method of regulating acid-base balance).

25.5b The Kidneys and Regulation of Fixed Acids 1

  • Fixed Acid Regulation:

    • Kidney mechanisms adapt to raise blood H through reabsorption.

    • Input sources include animal-based diets and metabolic waste.

25.5b The Kidneys and Regulation of Fixed Acids 2

  • Decreasing Blood H:

    • Rarely occurs but may happen during extreme conditions; kidney responses ensure excess HCO₃⁻ is excreted.

25.5c Respiration and Regulation of Volatile Acid

  • Respiratory Role:

    • Regulates carbonic acid through ventilation changes; respiratory rate elevates during increased acid production or insufficient oxygen, impacting blood pH.

25.5d Chemical Buffers 1

  • Buffering Systems:

    • Temporary measures to adjust pH fluctuations. Key types include:

    • Protein Buffer System (75% function in body fluids), effective in minimizing pH variations.

25.5d Chemical Buffers 2

  • Phosphate Buffer System:

    • Primarily used in intracellular fluid, assists in buffering metabolic acids.

25.5d Chemical Buffers 3

  • Bicarbonate Buffer System:

    • Dominant in extracellular fluid, regulating pH by interacting with acids and bases.

25.6a Overview of Acid-Base Disturbances 1

  • Definitions:

    • Acid-base disturbances when buffering capacity exceeds are classified based on whether they are respiratory or metabolic with pH changes. Categories include:

    1. Respiratory acidosis

    2. Respiratory alkalosis

    3. Metabolic acidosis

    4. Metabolic alkalosis.

25.6b Respiratory-Induced Acid-Base Disturbances 1

  • Respiratory Acidosis:

    • Resulting from CO₂ retention due to respiratory failure (measured PCO₂ > 45 mm Hg), leads to lowered pH, particularly affecting infants.

25.6b Respiratory-Induced Acid-Base Disturbances 2

  • Respiratory Acidosis Causes:

    • Include hypoventilation due to trauma or airway obstruction, and inadequate lung functionality.

25.6b Respiratory-Induced Acid-Base Disturbances 3

  • Respiratory Alkalosis:

    • Occurs with hypoventilation (< 35 mm Hg PCO₂), often stemming from anxiety, altitude sickness, or salicylate overdose.

25.6c Metabolic-Induced Acid-Base Disturbances 1

  • Metabolic Acidosis:

    • Triggered by fixed acid accumulation or bicarbonate loss, often due to renal issues, diarrhea, or ketoacidosis from uncontrolled diabetes.

25.6c Metabolic-Induced Acid-Base Disturbances 2

  • Metabolic Alkalosis:

    • Arises from bicarbonate retention or significant acid losses (e.g., vomiting). Diagnosis of levels > 26 mEq/L indicates alkalosis.

25.6d Compensation 1

  • Compensatory Mechanisms:

    • Employ buffering systems attempting to normalize pH; outcomes include complete compensation (pH normalization) and incomplete compensation (persistent pH imbalances).

25.6d Compensation 2

  • Renal Compensation:

    • Adjustments occur within the kidneys in response to elevated H, involving cellular processes to maintain pH.

25.6d Compensation 3

  • Additional Renal Responses:

    • When experiencing stress or changes in blood H, actions optimize filtration to balance acid-base levels effectively.

25.6d Compensation 4

  • Respiratory Compensation:

    • Adjustments to respiratory rate occur as H increases or decreases, which can modify CO₂ levels effectively, although less effective than renal methods.

Clinical Views on Acid-Base Disturbances

  • Arterial Blood Gas (ABG):

    • A diagnostic tool for monitoring acid-base disturbances, indicating compensation processes regulating pH in response to systemic changes.