Chapter 26

Body Fluid Composition

Body Fluids

  • Composed of water and various solutes.

  • Distributed among:

    • Intracellular Fluid (ICF): Water within cells.

    • Extracellular Fluid (ECF): Water outside cells, including plasma and interstitial fluid (IF).

Body Water Content

  • Varies by age, body mass, and fat content:

    • Infants: High percentage of body water (73 or more %).

    • Adults: 60% water with males typically higher due to muscle mass and females 50% water generally higher fat content and less skeletal

      • skeletal muscle is 75% water

      • adipose tissue less than 20% water

    • Older Adults: Decreased total body water due to increased fat, 45% in old age

  • Muscle tissue has higher water content than adipose tissue.

Fluid Compartments

Total Body Fluid

  • Average body fluid in adults: approximately 40 liters.

Major Fluid Compartments

  • Plasma: About 3 liters in adults.

  • Interstitial Fluid (IF): About 12 liters, occupies spaces between cells.

  • Other Compartments: Lymph, cerebrospinal fluid (CSF), synovial fluid, etc.

Composition of Body Fluids

  • Solvent: Water is considered a universal solvent.

  • Solutes:

    • Electrolytes: Dissociate into ions in water (e.g., sodium chloride, potassium).

    • Nonelectrolytes: Do not dissociate (e.g., glucose, urea).

  • Electrolyte concentrations are vital for maintaining osmotic balance.

Fluid Movement

Principles of Fluid Exchange

  • Fluid movement between compartments is governed by osmotic and hydrostatic pressures.

    • Water moves freely between compartments, while solutes may not.

  • Exchange mechanisms:

    • Between plasma and IF across capillary walls.

    • Between IF and ICF across plasma cell membranes.

Water Regulation

Intake and Output

  • Water intake must match output (2500 ml/day)

  • Water gains :

    • 90% or more taken in via ingested foods or beverages

    • small amount produced via cellular metabolism (250 ml/day) → called metabolic water or water of oxidation

      • Intake Sources: Foods and beverages; metabolic water.

      • Water Loss: Insensible losses (skin/airways), urine, sweat, feces.

        • unavoidable obligatory water loss explains why we cannot live without water very long

        • sensible water loss from urine (500 ml/day) and sweat and feces

  • Osmolality: Regulated within a narrow range; influences thirst and ADH release.

  • Volume and solute concentration of urine depend on fluid intake, diet, and variable water loss via sweat (substantial on hot days) and feces (substantial with diarrhea)

    • kidneys begin to eliminate excess water - 30 minutes after ingestion

Regulation of Water Intake

  • Controlled by hypothalamic thirst center activated by:

    • Osmoreceptors sensing plasma osmolality.

    • Dry mouth and decreased blood pressure.

  • Drinking water reduces thirst drive through feedback mechanisms.

Influence of Antidiuretic Hormone (ADH)

  • Water reabsorption in collecting ducts is proportional to ADH release

    • Fall in ADH increases urine output and decreases volume of body fluids

      • less ADH = fewer aquaporins

    • Rise in ADH decreases urine output and increases volume of body fluids

      • more aquaporins

    • ADH release stimulated by:

      • relatively small rise in ECF osmolality (via hypothalamic osmoreceptors)

      • relatively large drop in blood volume or BP (via baroreceptors and renin-antiotensin-aldoterone system)

Disorders of Water Balance

Common Disorders

  1. Dehydration (ECF fluid loss) -

    • due to hemorrhage, sever burns, prolonged vomiting or diarrhea, profuse sweating, water deprivation, diuretic abuse, endocrine disturbances

    • early signs/symptoms : thirst, “cotton” mouth, dry flushed skin, reduced urine output (oliguria)

    • may lead to : weight loss, mental confusion, and even hypovolemic shock

  2. Hypotonic Hydration -

    • Overhydration that occurs with renal insufficiency or rapid excess water ingestion

    • ECF osmolality decreases, causing hyponatremia

      • results in net osmosis of water into tissue cells (cells swell)

      • symptoms : severe metabolic disturbances, nausea, vomiting, muscular cramping, cerebral edema (which can lead to death)

  3. Edema: Accumulation of IF leading to tissue swelling, not cells (no change to ICF compartment)

    • only volume of IF is increased

    • increases distance for diffusion of oxygen and nutrients from blood into cells (can impair tissue function)

    • caused by anything that increases fluid flow out of blood (or decreases return of fluid to it)

Regulation of Electrolytes

Key Electrolytes

  • Regulatory importance of Sodium, Potassium, Calcium, and Phosphate in body functions:

    • Sodium maintains fluid balance and BP, influenced by hormones like aldosterone and ANP.

    • Potassium vital for muscle and nerve function; regulated primarily by secretion in the kidneys.

    • Calcium's balance controlled by parathyroid hormone (PTH) affecting bones and kidneys

  • Electrolyte balance usually refers only to salt balance even though electrolytes also include acids and bases, and some proteins

  • Salts control fluid movements, and provide minerals (ions) for excitability, secretory activity, and permeability of cell membranes

  • Salts enter body by ingestion (some liberated during metabolism)

    • Lost via perspiration, feces, urine, vomit

  • Severe electrolyte deficiencies may prompt craving for salty foods

    • common with addison’s disease - a disorder in which too little aldosterone is produced by adrenal cortex (too much Na+ lost in urine)

  • Pica - abnormal cravings, eating substances like chalk or clay

    • cause by deficiency in minerals like iron

Acid-Base Balance

pH Regulation

  • Normal pH: 7.35 - 7.45 (arterial blood).

  • Acidosis: pH < 7.35; Alkalosis: pH > 7.45.

  • Mechanisms include chemical buffers (e.g., bicarbonate), respiratory adjustments, and renal control.

Metabolic and Respiratory Disturbances

  • Respiratory issues can lead to acid-base imbalances, necessitating renal compensation.

  • Metabolic acidosis can stem from heavy exercise or kidney failure, while alkalosis often results from vomiting.

Clinical Insights

Electrolyte Imbalances

  • Could arise from dietary issues, dehydration, or renal dysfunction leading to clinical symptoms like weakness, confusion, and arrhythmias.

Hormonal Influence

  • Hormones (e.g., aldosterone, ANP) play crucial roles in regulating sodium and fluid balance, influencing blood pressure.

Developmental Aspects

Infants

  • Higher risk of dehydration and acid-base problems due to high metabolic rate and immature kidney function.

Older Adults

  • Total body water decreases, making them more vulnerable to imbalances.