Basic Fluid, Electrolyte Balance

Ardi Pramono, SpAn, MKes

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Body Fluids

Body Water ContentTotal body water is a function of age,

body mass, and body fat • Adipose tissue is ~20% water• Skeletal muscle tissue is ~75% water• Due to their low body fat and bone mass,

infants are about 73% water• The body water content of men is about

60%; women about 50%• Body water declines throughout life,

ultimately comprising about 45% of total body mass in old age

Body Fluids

Fluid Compartments There are 2 main fluid compartments of the

body:• The intracellular compartment (ICF)

• Contains slightly less than two-thirds by volume

• The extracellular compartment (ECF)• The remaining third of the body’s water

There are two sub-compartments of the ECF:

• Blood plasma• Fluid portion of the blood

• Interstitial fluid• Found between cells

TBW 60%

ICF 35%

ECF 25%

Plasma 5% ISF 20%

Body Fluids – Composition Electrolytes

Dissociate in water to ions Include inorganic salts, acids and bases,

and some proteins Have stronger osmotic power because they

dissociate in water; contribute at least two particles to solution

• Great ability to cause shifts in fluids Non-electrolytes

Do not dissociate in water; carry no net electrical charge

Include most organic molecules, such as glucose & lipids

Berkeringat menyebabkan hilangnya elektrolit tubuh

Body fluid composition

ECF composition Sodium (Na+; a cation) and chloride (Cl-; an anion) Interstitial fluid contains more Cl- ions than plasma

does ICF composition

Potassium (K+; a cation) and phosphate (P-; an anion)

Electrolytes are the most abundant solutes in body fluids

Proteins and some non-electrolytes (cholesterol, triglycerides) account for 60–97% of dissolved solutes

Komposisi utama elektrolit tubuh

Body Fluids – Movement

Water moves freely between compartments Anything that changes solute concentration

in any compartment leads to net water movement

Establishment of a gradient (!) Plasma vs. interstitial fluid

The protein-free portion of plasma is forced out of the blood by hydrostatic pressure

Reabsorption into plasma due to colloid osmotic pressure of plasma proteins

Interstitial fluid vs. ICF Ion movement depends on membrane

permeability, selectivity, and active transport Two-way osmotic flow of water is equal in

both directions Movement of nutrients, respiratory gases,

and wastes usually occur in one direction Solute concentration changes of ECF and/or

ICF will move water

Figure 25.3

Water Balance

For the body to remain properly hydrated, water intake must equal water output Water input

• Most water enters the body through ingested liquids and food

• Some is also produced by cellular metabolism Water output

• Due to evaporative loss from lungs and skin (insensible water loss), sweating, defecation, and urination (~60% of total output)

Osmolality (solute concentration) increases cause:

• Thirst mechanism• Release of ADH to conserve water through

kidneys

Water Balance Regulation of Water Intake

Thirst mechanism• Triggered by an increase in plasma osmolality

• Leads to a dry mouth (water remains in bloodstream and less saliva is produced)

• Also triggered by decline in blood volume• Excites the hypothalamic thirst center

Thirst is quenched before water can even be absorbed to bloodstream

• Mucosa of the mouth is moistened• Distention of the stomach and intestines• Inhibits of the hypothalamic thirst center

Regulation of Water Output Drinking is necessary since due to obligatory water

losses• Include the insensible water losses (through lungs and

skin), feces, urine Beyond obligatory water losses, solute concentration

and volume of urine depend on fluid intake

Water Balance Influence of ADH (antidiuretic hormone)

The amount of water reabsorbed in the renal collecting ducts is proportional to ADH release

• Low ADH levels:• Most water in the collecting ducts is not

reabsorbed, resulting in large quantities of dilute urine

• High ADH levels:• Filtered water is reabsorbed, resulting in a lower

volume of concentrated urine

Under hypothalamic control• Monitors solute concentration of ECF, large

changes in blood volume or pressure, or vascular baroreceptors

• Promotes or inhibits ADH secretion

Water Balance – Disorders Dehydration

Occurs when water output exceeds water intake• ICF move to the extracellular space• Accompanied by electrolyte loss

May be due to hemorrhage, burns, vomiting, diarrhea, sweating, water deprivation

Can lead to weight loss, fever, mental confusion, or hypovolemic shock

Hypotonic hydration A result of renal insufficiency, or intake of an excessive

amount of water very quickly Diluted ECF or hyponatremia leads to water movement into

the cells Leads to metabolic disturbances, especially to neurons

Edema Accumulation of fluid in the interstitial space, which may

impair tissue function Often due to changes in capillary pressure and permeability Cardiovascular system may be seriously hindered due to a

decline in blood volume and pressure

Electrolyte Balance

Sodium The most important cation to regulation of

fluid and electrolyte balance • **Water follows salt**

Most abundant cation in the ECF Contributes significant osmotic pressure Body fluids are in chemical equilibrium

• Any change in Na+ levels causes a compensatory shift in water

• This affects plasma volume, blood pressure, and intracellular and interstitial fluid volumes

Secretions containing Na+ enter the digestive tract; Na+ is almost entirely reabsorbed

Electrolyte Balance – Regulation of Sodium Aldosterone

High levels lead to nearly all the filtered sodium to be reabsorbed in the kidneys

Will also lead to water retention of ADH is present The most important trigger for the release of aldosterone

is the renin-angiotensin mechanism• Initiated in response to sympathetic stimulation, decrease

in filtrate osmolality, or decreased blood pressure Cardiovascular baroreceptors

Monitor blood volume so that blood pressure remains stable

Atrial natriuretic peptide (ANP) Reduces blood pressure and blood volume by inhibiting

release of ADH, renin, and aldosterone, and directly causing vasodilation

Estrogens Chemically similar to aldosterone; enhance reabsorption

of salt by the kidneys

Electrolyte Balance – Regulation of Potassium Critical to the maintenance of the membrane potential of

neurons and muscle cells Acts as a buffer that compensates for shifts of hydrogen

ions in or out of the cell Renal mechanisms regulate

K+ may be secreted into the filtrate to be eliminated in urine

Na+ is never secreted; only reabsorbed as needed Blood plasma levels of K+ are the most important factor

regulating K+ secretion High ECF K+ levels promote secretion; deficiencies

inhibit secretion Aldosterone influences K+ secretion, since K+ secretion is

simultaneously enhanced when Na+ reabsorption increases

Electrolyte Balance

Regulation of Calcium Ca2+ ion levels are closely regulated by

parathyroid hormone and calcitonin; about 98% is reabsorbed

• PTH is released when blood Ca2+ levels decline, and targets the bones, small intestine, and kidneys

• Calcitonin is an antagonist to parathyroid hormone, and is released when blood Ca2+ rises, targeting bone

Regulation of Anions Chloride is the major anion reabsorbed with

Na+, and helps maintain the osmotic pressure of the blood

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