Fluid, Electrolyte & Acid-Base Balance

What is the body water content of infants?

Infants are 73% or more water (low body fat, low bone mass).

What is the body water content of adult males?

Adult males are ~60% water.

What is the body water content of adult females?

Adult females are ~50% water (higher fat content, less skeletal muscle mass).

Which tissue is least hydrated of all?

Adipose tissue is least hydrated of all.

What does total body water in adults average?

Total body water in adults averages approximately 40 L.

What happens to water content in old age?

Water content declines to ~45% in old age.

What are the two main fluid compartments?

Intracellular fluid (ICF) compartment and extracellular fluid (ECF) compartment.

What is the intracellular fluid (ICF) compartment?

Fluid inside cells; accounts for 2/3 of total body fluid.

What is the extracellular fluid (ECF) compartment?

Fluid outside cells; accounts for one-third of total body fluid.

How much of the ECF is plasma?

 Plasma: 3 L.

How much of the ECF is interstitial fluid?

Interstitial fluid (IF): 12 L in spaces between cells.

What fluids are also considered part of IF?

Lymph, CSF, humors of the eye, synovial fluid, serous fluid, and gastrointestinal secretions.

What are the functions of bodily fluids?

Carry nutrients and waste; medium for chemical reactions.

What is the universal solvent?

Water

What are solutes?

substances dissolved in water.

How are solutes classified?

Solutes are classified as nonelectrolytes and electrolytes.

What are nonelectrolytes?

Most are organic molecules that do not dissociate in water.

What are examples of nonelectrolytes?

Glucose, lipids, creatinine, and urea.

What happens to charged particles with nonelectrolytes?

No charged particles are created.

What are electrolytes?

Electrolytes dissociate into ions in water.

What are examples of electrolytes?

 Inorganic salts, all acids and bases, some proteins.

Why do electrolytes have a greater ability to cause fluid shifts?

Due to ability to dissociate into two or more ions.

How does NaCl dissociate?

NaCl → Na⁺ + Cl⁻ (electrolyte; 2 particles).

How does MgCl₂ dissociate?

MgCl₂ → Mg²⁺ + 2Cl⁻ (electrolyte; 3 particles).

How does glucose behave in water?

Glucose → glucose (nonelectrolyte; 1 particle).

Does each fluid compartment have its own electrolyte pattern?

Yes, each fluid compartment has a distinctive pattern of electrolytes.

How are ECF electrolyte contents similar?

ECF electrolyte contents are all similar except for higher protein, lower Cl⁻ content of plasma.

What is the major cation in ECF?

Na⁺

What is the major anion in ECF?

Cl⁻

How does ICF compare to plasma?

ICF contains more soluble proteins than plasma.

What is ICF low in?

Low in Na⁺ and Cl⁻

What is the major cation in ICF?

K⁺

What is the major anion in ICF?

HPO₄²⁻

What generates the potential between ICF and ECF?

Differences in ionic composition of intracellular fluid (ICF) and extracellular fluid (ECF), differences in plasma membrane permeability, and unequal ionic distribution between ECF and ICF.

What is the unequal ionic distribution the result of?

Result of Na⁺/K⁺ pump and membrane permeability.

Where is Na⁺ highly concentrated?

High concentration of Na⁺ outside.

Where is K⁺ highly concentrated?

High concentration of K⁺ inside.

What regulates the continuous exchange and mixing of fluids?

Osmotic and hydrostatic pressures regulate the continuous exchange and mixing of fluids.

How does water move between compartments?

Water moves freely along osmotic gradients.

What is true of all body fluid osmolality?

All body fluid osmolality is almost always equal.

What happens if solute concentration changes in any compartment?

Change in solute concentration of any compartment leads to net water flow.

What happens when ECF osmolality increases?

↑ ECF osmolality → water leaves cell.

What happens when ECF osmolality decreases?

↓ ECF osmolality → water enters cell.

Where do exchanges between plasma and interstitial fluid occur?

Across capillary walls.

What happens at the arteriolar end of a capillary?

Fluid leaks from the arteriolar end of a capillary.

What happens at the venule end of a capillary?

Fluid is reabsorbed at the venule end.

What do lymphatics do?

Lymphatics pick up remaining fluid and return it to the blood.

Where do exchanges between IF and ICF occur?

Across cell membranes.

How does water move between IF and ICF

Two-way osmotic flow of water (H₂O).

How do ions move between IF and ICF?

Ions move selectively into or out of cell.

How do nutrients, wastes, and gases move?

Nutrients, wastes and gases have unidirectional flow.

What exception is there for gases?

Gases may move either in or out the cell.

How are water intake and output regulated?

Both intake and output of water are regulated.

What must water intake equal?

Water intake must equal water output: ~2500 mL/day.

How is most water taken in?

Most water is taken in via ingested foods and beverages.

What is a small additional source of water intake?

A small amount comes from metabolism.

What is metabolic water (water of oxidation)?

Water produced by cellular metabolism.

What are the routes of water output?

Urine (60%), insensible water loss (lost through skin and lungs), perspiration, and feces.

Around what value is osmolality maintained?

Around 280–300 mOsm.

What does a rise in osmolality do?

Stimulates thirst and causes ADH release.

What does a decrease in osmolality do?

Water inhibits the thirst center and causes ADH inhibition.

What is maintenance of plasma osmolarity?

Refers to the ratio of solutes to water in blood plasma.

How is water intake regulated?

Stimulate thirst to encourage adequate water intake

What contributes to water loss?

Urine, perspiration, breathing, and fecal matter contribute to water loss.

What is the primary regulator of water balance?

Urine serves as the primary regulator of water balance.

How does ADH regulate water balance?

ADH concentrates urine by retaining water.

How does aldosterone help regulate water balance?

Aldosterone also contributes to urine concentration.

What does ADH release dictate?

ADH release dictates water reabsorption in the collecting ducts (kidneys).

What happens when ADH decreases?

↓ ADH → dilute urine, decreased body fluid volume.

What happens when ADH increases?

↑ ADH → concentrated urine, increased body fluid volume.

What regulates ADH based on ECF solute concentration?

Hypothalamic osmoreceptors regulate ADH based on ECF solute concentration.

What other triggers cause ADH release?

Large changes in blood volume or pressure.

How does decreased BP/volume stimulate ADH release?

Decreased BP/volume prompts ADH release via baroreceptors and renin-angiotensin-aldosterone mechanism.

What can cause decreased BP/volume?

Intense sweating, vomiting, diarrhea, blood loss, burns, and fever.

What are obligatory water losses?

Obligatory water losses explain why we cannot live without water very long.

What do obligatory water losses include?

Insensible water loss from lungs or skin (sweat); sensible water loss from urine to excrete wastes (60%), obvious sweat (8%), and feces (4%).

What does the volume of urine excreted and solute concentration depend on?

Fluid intake, diet, and water loss via other avenues.

What are unusual water losses?

These can be due to excessive urination and diarrhea, for example.

What happens with high osmolarity?

Thirst → increased water intake; ADH release → water reabsorbed from urine.

What happens with low osmolarity?

Lack of thirst → decreased water intake; decreased ADH release → water lost in urine.

What is dehydration?

Dehydration is negative fluid balance.

What can cause dehydration

ECF water loss due to hemorrhage, severe burns, prolonged vomiting or diarrhea, profuse sweating, water deprivation, diuretic abuse, endocrine disturbances.

What are signs and symptoms of dehydration?

“Cottony” oral mucosa, thirst, dry flushed skin, oliguria.

What can dehydration lead to?

Weight loss, fever, mental confusion, hypovolemic shock, and loss of electrolytes.

What is edema?

Positive fluid balance; atypical accumulation of IF, resulting in tissue swelling (not cell swelling).

Which fluid compartment increases in edema?

Only the volume of the IF is increased, not of other compartments.

How can edema impair tissue function?

By increasing distance for diffusion of oxygen and nutrients from blood into cells.

What can cause edema?

Increased fluid flow out of blood or decreased return of fluid to blood.

What is hypotonic hydration?

Cellular overhydration, or water intoxication.

When does hypotonic hydration occur?

Occurs with renal insufficiency or rapid excess water ingestion.