Fluid balance

Week 2 lecture pt 1

tissue water levels as we age

decrease

child / adult / elderly water vs solid level

child 70-80 water 20-30 solid

adult 50-60 water 40-50 solid

elderly 45-55 water 45-55 solid

water and body composition

muscle holds more water than fat

a fat person will have a lower body water than a muscular person at the same weight

sources of water intake

oral intake as water and as food

water of oxidation (metabolism)

sources of water loss

sensible - fecal, urine

insensible - lungs, skin

what is the best indicator of fluid volume changes

weight

what is the best weight

dry weight

• normal weight without any extra fluid in the body

fluid balance depends on

age

body mass / surface area

sex

hydration status

hormones (ADH, aldosterone, cortisol, ANP)

cardiac function

renal function

comorbidities

intake

sweat glands

what are the fluid compartments containing solution in the body

intracellular

extracellular

• interstitial

• intravascular

• transcellular

intercellular fluid

inside of the cell

accounts for 2/3 of water in body

extracellular fluid

outside the cell

accounts for 1/3 body water

interstitial fluid

majority of extracellular fluid

between tissues

intravascular fluid

plasma

inside blood vessel

transcellular fluid

third space

CSF, joint fluid, ocular fluid, peritoneal cavity, pericardial sac, pleural cavity, lymph

if total body water is 60% of body weight what percent is intracellular vs extracellular

40% intracellular

20% extracellular

• 14% interstitial

• 5% plasma

• 1% transcellular

mechanisms of fluid movement in the body

diffusion - go with the flow

active transport - swimming upstream

osmosis - lets fluids through

diffusion

movement of MOLECULES from an area of high to low concentration with aim to make both sides equal in concentration

active transport

movement of MOLECULES against a pressure gradient using energy

osmosis

movement of WATER between 2 compartments separated by a semipermeable membrane

what is the principle extracellular solute, what does this mean for water

Sodium

water will always follow sodium

capillary hydrostatic pressure (CHP)

force inside an artery/capillary that forces fluids and solutes outward into the interstitial space

PUSHING OUT

CHP directly correlates with what body measures

Blood pressure

fluid volume

CHP is set by the

heart

capillary oncotic pressure (COP) / plasma colloid osmotic

inward pulling pressure

• from interstitial into artery/capillary

PULLING IN

COP is set by

plasma proteins

what is the contributing factor to COP

plasma proteins (do not pass through the capillary)

CHP is ___ COP is ____

CHP is variable COP is constant

interstitial hydrostatic pressure IHP

pressure of water in the tissue to move water into the capillary

• PUSHING out of interstitial

• negligible

Interstitial oncotic pressure (IOP) / colloid osmotic

pressure of albumin in the tissues that attracts water from capillary and keeps it in tissues

• PULLING into interstitial

• negligible

as blood flows through the arteries into capillary and into vein

hydrostatic forces ____ while oncotic forces _____ because

hydrostatic forces decrease

while

oncotic forces increase

because

there is less water available the further you travel, so the plasma protein concentration is higher

with normal pressures what is happening in the arteriole / venule

in the arteriole end CHP > COP so fluid exits

in the venule COP > CHP so fluid enters

with fluid deficit what is happening in the arteriole / venule

in arteriole CHP < COP so fluid enters

in venule CHP < COP so fluid enters

with fluid overload what is happening in the arteriole / venule

in arteriole CHP way > COP so fluid exits

in venule CHP slightly < COP so fluid enters

Edema in interstitial space

lymphatic systems role in fluid control

the lymphatics take extra fluid and flush back to heart

help to prevent edema

causes of fluid volume deficit

increased urine output

vomiting

diarrhea

decreased PO intake

increased insensible loss

surgery, drains

wounds

burns

hemorrhage

hyperthermia

endocrine disorders (diabetes insipidus)

symptoms of fluid volume deficit

tachycardia

decreased blood pressure

decreased urine output

decreased skin turgor

decreased cardiac output

decreased perfusion to vital organs

weakness

confusion headaches

changes in lab values

thirst

causes of fluid volume excess

heart failure (increased sodium)

renal failure

high salt intake

increased water intake

medication side effects

endocrine disorders

liver disease

symptoms of fluid volume excess

increased blood pressure

increased cardiac output within compensatory limits

weight gain

edema

muffled heart sounds

confusion

Edema results from

decreased capillary oncotic pressure

increased capillary permeability

increased tissue oncotic pressure

lymph obstruction

increased capillary hydrostatic pressure

in edema fluid is moving from _______ into ______

in edema fluid is moving from capillaries or lymph channels into tissues (interstitial space / cells)

lymphedema

lymphatic system is blocked

protein and fluid accumulate in interstitial space

third spacing

too much fluid moves from intravascular space into the transcellular or interstitial space

what are third spaces

non-functional area between the cells

unavailable for metabolic processes to occur

third spacing can cause

edema hypotension and shock

third spacing is commonly seen with

burn patients, liver patients (ascites)

what mechanisms does the body use to maintain fluid/solute balance

KIDNEYS

antidiuretic hormone

renin and angiotensin

aldosterone

atrial natriuretic peptide

thirst

osmolarity is

concentration of a solution

# of solute particles per liter

Osm/L

normal serum osmolarity

280-294 mOsm

higher osmolarity means

more solutes less water

high osmolarity causes

thirst - increased water intake

ADH release - water reabsorbed from urine

low osmolarity causes

lack of thirst - decreased water intake

decreased ADH release - water lost in urine

role of the kidneys

the nephron is the workhorse

capillary blood pressure forces fluid into the proximal end of the kidney tubule

when the body needs more water the tubule retains fluid, when the body needs less water tubule excretes more water

antidiuretic hormone ADH role

water retainer or vasopressin

made by the hypothalamus, stored and released by the posterior pituitary gland based on serum osmolarity

controls water reabsorption and BP

too little ADH can occur with

Diabetes insipidus

• neurogenic / central - brain not released enough ADH

• nephrogenic - communication of ADH from brain to kidneys is not occuring, receptor problem

too much ADH can occur with

syndrome of inappropriate ADH

Diabetes Insipidus (DI)

• extreme thirst

• enormous urinary losses

SIADH

INAPPropriate ADH

Increased NA (sodium) PP (urine output)

• ADH released when unneeded - water retention and sodium excretion (in urine) causes cerebral edema

• major cause of low sodium levels

caused by

• cancer

• cns disorders

• pulmonary disorders

• medications

treatment

• correct underlying cause

renin and angiotensin

works in similar ways to ADH but role is to maintain BP

renin (kidneys) + angiotensin II (blood/lungs)

• vasoconstriction (angio II)

• stimulates production of aldosterone (adrenal gland)

• ELEVATES BP

amount of renin secretion depends on

• blood flow to kidneys

• level sodium in blood stream

Aldosterone

produced as a result of renin-angiotensin system

secreted by adrenal cortex

regulates sodium and water within nephron

your body has low BP what will aldosterone do

moves Na from tubules into bloodstream by active transport

water is reabsorbed and blood volume expands

increases BP

atrial natriuretic peptide (ANP)

ANP is a cardiac hormone stored in cells of the atria

• released with atrial pressures increase

• opposes the RAS system by decreasing BP and intravascular blood volume

mechanism of action

• suppresses Renin levels

• decreases Aldosterone release

• increases glomerular filtration

• decreases ADH release

• Vasodilation

how is thirst stimulated

dry mouth causes stimulation of thirst center in hypothalamus (osmoreceptor)

what happens to thirst in elderly

less effective in elderly - more prone to dehydration

neural control of fluid volume

• osmoreceptors - thirst center in anterior hypothalamus, affected by serum osmolarity

• baroreceptors in carotid sinus, aortic arch, atria, thoracic vein, medulla

endocrine control of fluid volume

• ADH from hypothalamus posterior pituitary : WATER RETENTION

• Aldosterone : REGULATION Na BALANCE

• Atrial natriuretic peptide : hormone released by atria in response to increasing volume : INCREASES Na AND H2O EXCRETION TO DECREASE BP

3 types of IV solutions and their osmolality ranges

Isotonic 280-294 mOsm

hypotonic < 280 mOsm

hypertonic > 294 mOsm

what are isotonic IV solutions used for

no water shifts

used to maintain blood volume

what is hypertonic IV solution used for

water shifts from cells to blood (cells shrink)

used to reduce tissue swelling and increase blood volume

what is hypotonic IV solution used for

water shift from blood to cells (swell)

used to hydrate cells

tonicity

osmolality of a solution

normal = 285 mOsm

volume contraction

decrease in total body water

fluid deficit

volume expansion

increase in total body water

fluid excess

isotonic contraction

volume contraction in which sodium and water are lost in isotonic proportions

decrease in total volume but no changes in osmolality

causes of isotonic contraction

vomiting, diarrhea, kidney disease, misuse diuretics

treatments of isotonic contraction

fluid administration - isotonic to plasma - 0.9% normal saline

replenish slowly to prevent pulmonary edema

hypertonic contraction

loss of water exceeds loss of sodium

reduced extracellular fluid volume and increased osmolality

causes of hypertonic contraction

excessive sweating, osmotic diuresis (urination) concentrated food given to infants (more formula than necessary)

extensive burns or CNS disorders that interfere with thirst

treatment of hypertonic contraction

initial therapy - drink water

hypotonic fluids (0.45% NaCl) or fluids without solutes (D5W)

hypotonic contraction

loss of sodium exceeds loss of water

both volume and osmolality of ECF decrease

causes of hypotonic contraction

excessive loss of Na through the kidney

• diuretic therapy, chronic renal insufficiency, lack of aldosterone

treatment of hypotonic contraction

mild: infusing isotonic sodium chloride solution for injection

severe: hypertonic solution 3% NaCl

watch for signs of fluid overload

volume expansion causes

overdose with therapeutic fluids

disease states

• congestive heart failure CHF

• nephrotic syndrome

• cirrhosis with ascites

• decreased urine formation

symptoms of volume expansion

cerebral edema - confusion and convulsions

weakness

muscle twitching

nausea

headache

weight gain

treatment of volume expansion

diuretics

agents used for heart failure

decreasing urine intake