WEEK 1: FLUIDS

Age, gender, and body fat.

Factors influencing body fluid amount

Lean body

has higher water content;

skeleton

has low water content;

muscle, skin, and blood

contain the highest amounts of water.

Intracellular Fluid (ICF)

Fluid inside the cells; about 40% of body weight and contains about two-thirds of body fluid.

Extracellular Fluid (ECF)

Fluid outside the cells; about 20% of body weight and contains about one-third of body fluid.

Interstitial space

About 15% of body fluid; surrounds the cells and totals about 11–12 L in an adult.

Lymph

is an interstitial fluid.

Intravascular space

About 5% of body fluid; fluid within blood vessels and contains plasma or effective circulating volume.

About 6 L: approximately 3 L plasma and 3 L blood cells.

Average blood volume

Transcellular space

The smallest division of ECF; contains about 1 L.

About 2,500 mL/day.

Average adult oral fluid intake

1,500–3,000 mL/day.

Normal adult daily fluid intake range

100 mL/kg.

Daily fluid requirement: first 10 kg

50 mL/kg.

Daily fluid requirement: next 10 kg

15 mL/kg.

Daily fluid requirement: remaining kilograms

About 2,500 mL/day

Average daily fluid loss

About 1,500 mL/day or 30–50 mL/hr.

Urinary fluid loss

200ml

Bowel elimination -

Sensible fluid loss

Fluid loss that can be seen, measured, or noticed.

Insensible fluid loss

Fluid loss that is not easily seen or measured.

1,400–1,500 mL/day.

Average adult urine output

350–400 mL/day.

Average adult insensible loss through lungs & skin

About 100 mL/day.

Average adult sweat loss

100–200 mL/day.

Average adult fecal fluid loss

2,300–2,600 mL/day.

Average total adult fluid output

pota, pho, mag

Major ICF electrolytes

Potassium (K⁺).

Major ICF cation

Phosphate (PO₄³⁻).

Major ICF anion

Magnesium (Mg²⁺)

Important for enzymes and muscle function.

Sod, chlo, bi

Major ECF

Sodium (Na⁺).

Major ECF cation

Chloride (Cl⁻).

Major ECF anion

Bicarbonate (HCO₃⁻)

Helps maintain acid-base balance.

275–300 mOsm/L.

Normal tonicity/osmolarity of body fluids

● Transport of nutrients and oxygen

● Body temperature regulation

● Lubricant of musculoskeletal joints

● Medium or milieu for metabolic processes

● Insulator and shock absorber

Functions of body fluids

Translocation

The movement back and forth of fluid and exchange of chemicals from one location to another. It is a continuous process occurring in and among all areas where water is located.

1. Electrolytes

2. Acids

3. Bases

chemicals involve in translocation

Electrolytes

Substances that, when dissolved in fluid, carry an electrical charge.

Acids

Substances that release hydrogen ions (H⁺) into fluid.

Bases

Substances that bind with hydrogen ions (H⁺).

Osmoreceptors

Specialized neurons on the surface of the hypothalamus that sense sodium concentration and serum osmolality and trigger thirst = increase fluid intake.

Also sensitive to changes in Blood Volume (BV) & Blood Pressure (BP)

○ A decrease in BV by 10%

○ Systolic BP falls below 90 mmHg

○ Right Atrium is underfilled

Events that cause ADH release

○ Neurons become dehydrated

○ Osmoreceptors send impulses to the posterior pituitary

○ ↑ ADH release

Effect of increased osmotic pressure

ADH

Travels through the blood to the kidneys, alters kidney permeability to water increases water reabsorption, and decreases urine output.

Dilutes the ECF and returns ECF concentration toward normal.

Effect of retained water

When normal osmotic pressure is restored, negative feedback to osmoreceptors further ADH release is inhibited.

Normal osmotic pressure restored

RAAS Renin-Angiotensin-Aldosterone System;

helps maintain fluid balance and restore blood volume and blood pressure.

Low kidney perfusion or decreased blood pressure, blood volume.

Trigger for renin release

Renin

Released by the kidneys from the juxtaglomerular apparatus and converts angiotensinogen to angiotensin I.

Angiotensin-converting enzyme (ACE)

Converts angiotensin I into angiotensin II.

Angiotensin II

Causes potent peripheral arterial vasoconstriction, increasing arterial blood pressure and stimulating aldosterone release.

Aldosterone

Increases sodium and water reabsorption, increases blood volume and BP, and increases potassium secretion/excretion by the kidneys.

Natriuretic peptides

Help regulate fluid volume and cardiovascular function; oppose RAAS.

Inhibit renin, aldosterone, and ADH; increase water and sodium excretion; cause vasodilation; decrease blood volume, BP, and thirst.

Effects of natriuretic peptides

Atrial Natriuretic Peptide (ANP)

Produced, stored, and released by atrial muscle cells.

BNP and NT-proBNP

Released by ventricular muscle cells and commonly measured for diagnosis and management of heart failure.

NT-proBNP

Has a longer half-life than BNP and remains elevated in the blood longer.

Kidney failure, coronary heart disease, valvular heart disease, constrictive pericarditis, pulmonary hypertension, and sepsis.

Conditions that may increase natriuretic peptides

Hydrostatic pressure

Pressure exerted by fluid within a closed system that pushes fluid out of the capillary into the interstitial space.

Colloid osmotic/oncotic pressure

Osmotic pressure exerted mainly by albumin and large plasma proteins that pulls fluid into the capillary.

Albumin

Major plasma protein responsible for colloid oncotic pressure; helps keep water inside the bloodstream.

Colloid

A fluid containing nonsoluble substances evenly distributed within a solvent.

Blood as a colloid solution

Contains blood cells and plasma, including water, proteins, enzymes, and other solutes.

Osmosis

Movement of water/solvent across a semipermeable membrane from lower solute concentration to higher solute concentration until concentrations become balanced.

Osmolality

Number of milliosmoles of solute per kilogram of solvent; expressed as mOsm/kg. More commonly used to evaluate blood and urine

Osmolarity

Number of milliosmoles of solute per liter of solution; expressed as mOsm/L.

Filtration

Movement of both solute and solvent across a semipermeable membrane from higher pressure to lower pressure, driven by hydrostatic pressure. When HP > OP → filtration occurs

Active transport

Movement of solute from lower to higher concentration using energy (ATP).

Sodium–potassium pump

■ 3 Na⁺ pumped out of the cell

■ 2 K⁺ pumped into the cell

■ Uses Na⁺/K⁺-ATPase

Diffusion

Movement of particles, solutes, or molecules from higher concentration to lower concentration without ATP.

Factors affecting the rate of diffusion: molecule size

Larger molecules move more slowly than smaller molecules.

Factors affecting the rate of diffusion: concentration gradient

A greater difference in concentration results in a faster rate of diffusion.

Factors affecting the rate of diffusion: temperature

An increase in temperature increases the rate of diffusion.

Simple/passive diffusion

Movement directly from high to low concentration without a carrier or energy; example is O₂ and CO₂ exchange between alveoli and pulmonary capillaries.

Facilitated diffusion

Movement from high to low concentration without ATP but requires a carrier or channel protein. Ex: Glucose entering cells through GLUT transporters.

Insulin

helps activate/translocate GLUT4 in muscle and fat cells.

Crystalloid solutions

Aqueous solutions of mineral salts or other water-soluble molecules commonly used for fluid replacement/hypovolemia.

0.9% NaCl, 0.45% NaCl, Lactated Ringer’s, and Plasma-Lyte.

Examples of crystalloid solutions

Colloid solutions

Gelatinous solutions containing large molecules/proteins that maintain high osmotic/oncotic pressure in the blood. Help keep or pull fluid into the bloodstream; Used for temporary volume replacement

Human albumin, hyperoncotic starch, and dextran.

Examples of colloid solutions

Tonicity

The ability of a solution to cause water movement between ICF and ECF; determines cell hydration and cell size.

Isotonic solutions

Have approximately the same concentration as blood and cause no major water shift between ICF and ECF. Primarily expand plasma volume

0.9% NaCl, Lactated Ringer’s, Ringer’s Solution, and 5% Dextrose in water.

Examples of isotonic solutions

Hypotonic solutions

Have less solute and more water than blood; move water from ECF to ICF, causing cells to swell and helping hydrate cells.

0.45% NaCl, 0.33% NaCl, 0.22% NaCl, and 2.5% dextrose water.

Examples of hypotonic solutions

Hypertonic solutions

Have more solute and less water than blood; pull water from ICF to ECF, causing cells to lose water and shrink. May be used for severe edema, especially cerebral edema

Examples of hypertonic solutions

3% NaCl, 5% NaCl, 3% or 5% NaCl + D/W, and >5% D/W.

Hypovolemia (Fluid Volume Deficit)

Low volume of fluid in the body; occurs when ECF fluid loss exceeds fluid intake and may result in dehydration.

Blood clots, urinary stones, and compromised kidney function in excreting nitrogenous wastes.

Complications of hemoconcentration

Third-space fluid shifts

Burns with edema and ascites with liver dysfunction.

Diabetes insipidus

- ADH deficiency or kidney resistance to ADH

Thirst.

Earliest symptom of hypovolemia

Lactated Ringer’s or 0.9% NaCl.

Isotonic fluids for hypovolemic hypotension

0.45% NaCl may be used.

Fluid used once the patient is normotensive

Rise slowly, avoid alcohol and caffeine, and include a moderate amount of table salt or sodium-containing foods each day.

Health teaching for hypovolemia

Hypervolemia (Fluid Volume Excess)

Expansion of ECF from abnormal retention of water and sodium, usually caused by increased total body sodium. Usually results from increased total body sodium, causing increased body water. Considered an isotonic fluid accumulation, so serum sodium may remain normal

Circulatory overload may compromise cardiopulmonary function; the heart compensates by increasing BP and force of contraction.

Pathophysiology of hypervolemia

Approximately 3 L.

Amount of excess intravascular fluid associated with pitting edema

Weight gain, elevated BP, and dependent edema of the feet, ankles, sacrum, and buttocks.

Early signs of hypervolemia

Rings, shoes, or stockings leaving marks; prominent/distended jugular veins; moist breath sounds or crackles.

Other assessment findings of hypervolemia