Fluid, Electrolyte & Fluid, Electrolyte & Acid-Base BalanceAcid-Base Balance

Anatomy & Physiology IIChapter 21

BalanceBalancecellular function requires a fluid

medium with a carefully controlled composition

three types of homeostatic balance◦water balance

average daily water intake and loss are equal

◦electrolyte balance the amount of electrolytes absorbed by the

small intestine balance with the amount lost from the body, usually in urine

◦acid-base balance the body rids itself of acid (hydrogen ion – H+)

at a rate that balances metabolic production

BalanceBalancebalances maintained by the collective

action of the urinary, respiratory, digestive, integumentary, endocrine, nervous, cardiovascular, and lymphatic systems

Body WaterBody Waternewborn baby’s body weight is about

75% water

young men average 55% - 60%

women average slightly less

obese and elderly people as little as 45% by weight

total body water (TBW) of a 70kg (150 lb) young make is about 40 liters

Fluid CompartmentsFluid Compartmentsmajor fluid compartments of the body◦65% intracellular fluid (ICF)

◦35% extracellular fluid (ECF) 25% tissue (interstitial) fluid

8% blood plasma and lymphatic fluid

2% transcellular fluid ‘catch-all’ category cerebrospinal, synovial, peritoneal, pleural, pericardial fluids, vitreous and aqueous humors of the eye, bile, and fluids of the digestive, urinary, and reproductive tracts

Water Movement Between Water Movement Between Fluid CompartmentsFluid Compartmentsfluid continually exchanged between

compartmentswater moves by osmosisbecause water moves so easily through

plasma membranes, osmotic gradients never last for very long

if imbalance arises, osmosis restores balance within seconds so the intracellular and extracellular osmolarity are equal◦ if osmolarity of the tissue fluid rises, water moves

out of the cell

◦ if it falls, water moves in

Water Movement Between Water Movement Between Fluid CompartmentsFluid Compartmentsosmosis from one fluid compartment to

another is determined by the relative concentrations of solutes in each compartment◦ electrolytes – the most abundant solute

particles, by far◦ sodium salts in ECF◦ potassium salts in ICF

electrolytes play the principal role in governing the body’s water distribution and total water content

Water Movement Between Water Movement Between Fluid CompartmentsFluid Compartments

Digestive tract

Bloodstream BloodstreamTissue fluid Lymph

Intracellularfluid

Water GainWater Gainfluid balance - when daily gains and

losses are equal (about 2,500 mL/day)

gains come from two sources:

◦preformed water (2,300 mL/day) ingested in food (700 mL/day) and drink

(1600 mL/day)

◦metabolic water (200 mL/day) by-product of aerobic metabolism and

dehydration synthesis C6H12O6 + 6O2 6CO2 + 6H2O

Water LossWater Losssensible water loss is observable

◦1,500 mL/ day is in urine◦ 200 mL/day is in feces◦ 100 mL/day is sweat in resting adult

insensible water loss is unnoticed◦ 300 mL/day in expired breath◦ 400 mL/day is cutaneous

transpiration diffuses through epidermis and evaporates

does not come from sweat glands

◦loss varies greatly with environment and activity

Fluid BalanceFluid BalanceIntake

2,500 mL/dayOutput

2,500 mL/day

Metabolic water200 mL

Feces200 mL

Expired air300 mL

Cutaneoustranspiration

400 mL

Sweat 100 mL

Urine1,500 mL

Drink1,600 mL

Food700 mL

The Importance of WaterThe Importance of Water

Body fluid maintenance

Thirst mechanism maintains volume

Kidney activity regulates volume and composition

Hormones regulate fluid volume and electrolytes

Buffers, respiration, and kidney function regulate pH

Sense of ThirstSense of Thirst

Control center for thirst

Located in hypothalamus

Regulates total fluid volume

Excessive thirst -- polydipsia

Regulation of Fluid IntakeRegulation of Fluid Intakethirst mainly governs fluid intakedehydration

◦reduces blood volume and blood pressure

◦increases blood osmolarityosmoreceptors in hypothalamus

◦respond to angiotensin II produced when BP drops and to rise in osmolarity of ECF with drop in blood volume

◦hypothalamus produces antidiuretic hormone (ADH)

Regulation of Fluid IntakeRegulation of Fluid Intakecerebral cortex produces conscious

sense of thirst intense sense of thirst with 2-3% increase in

plasma osmolarity or10-15% blood loss

◦salivation is inhibited with thirst

Thirst Satiation Thirst Satiation MechanismsMechanismslong term inhibition of thirst

◦absorption of water from small intestine reduces osmolarity of blood stops the osmoreceptor response, promotes capillary

filtration, and makes the saliva more abundant and watery changes require 30 minutes or longer to take effect

short term inhibition of thirst◦cooling and moistening of mouth quenches

thirst◦distension of stomach and small intestine◦30 to 45 min of satisfaction

must be followed by water being absorbed into the bloodstream or thirst returns

◦short term response designed to prevent overdrinking

Dehydration, Thirst, and Rehydration

Dehydration

Renin

Angiotensin IIDehydration

Thirst

Rehydration

Increasedblood osmolarity

Reducedblood pressure

Stimulateshypothalamic

osmoreceptors

Reducedsalivation

Stimulateshypothalamic

osmoreceptors

Dry mouth?

Sense ofthirst

Cools andmoistens mouth

Ingestionof water

Rehydratesblood

Distendsstomach

and intestines

Short-terminhibitionof thirst

Long-terminhibitionof thirst

Regulation of Water OutputRegulation of Water Outputonly way to control water output

significantly, is through variation in urine volume

◦kidneys can’t replace water or electrolytes

◦only slow rate of water and electrolyte loss until water and electrolytes can be ingested

Regulation of Water OutputRegulation of Water Outputmechanisms:

◦ changes in urine volume linked to adjustments in Na+ reabsorption as Na+ is reabsorbed or excreted, water follows

◦ concentrate the urine through action of ADH ADH secretion stimulated by hypothalamic

osmoreceptors in response to dehydration aquaporins synthesized in response to ADH

membrane proteins in renal collecting ducts whose job is to channel water back into renal medulla, Na+ is still excreted

slows decrease in water volume and increased osmolarity – concentrates urine

◦ ADH release inhibited when blood volume and pressure is too high or blood osmolarity too low effective way to compensate for hypertension

Secretion and Effects of Secretion and Effects of ADHADH

H2O

Elevates blood osmolarity

DehydrationNa+ Na+

Increases water reabsorption

Thirst

Negativefeedback

loop

Negativefeedback

loopWater

ingestion

Reduces urinevolume

Increases ratioof Na+: H2O

in urine

Stimulates distal convolutedtubule and collecting duct

Stimulates posterior pituitaryto release antidiuretic hormone (ADH)

Stimulates hypothalamicosmoreceptors

H2O

Disorders of Water BalanceDisorders of Water Balance the body is in a state of fluid imbalance if there is an abnormality

of total volume, concentration, or distribution of fluid among the compartments

◦ fluid deficiency – fluid output exceeds intake over long period of time

volume depletion (hypovolemia) occurs when proportionate amounts of water and sodium are lost without

replacement total body water declines, but osmolarity remains normal hemorrhage, severe burns, chronic vomiting, or diarrhea

dehydration (negative water balance) body eliminates significantly more water than sodium total body water declines, osmolarity rises lack of drinking water, diabetes, ADH hyposecretion (diabetes insipidus),

profuse sweating, overuse of diuretics infants more vulnerable to dehydration than adults due to high metabolic

rate that demands high urine excretion, immature kidneys cannot concentrate urine effectively, greater ratio of body surface to mass

affects all fluid compartments (ICF, blood, and tissue fluid)◦ most serious effects:

circulatory shock due to loss of blood volume, neurological dysfunction due to dehydration of brain cells, infant mortality from diarrhea

Electrolytes and Their Electrolytes and Their FunctionsFunctions

Conduct electrical current in solution

Positive ions (cations)◦Sodium

◦Potassium

◦Calcium

Negative ions (anions)◦Phosphate

◦Chloride

Electrolyte BalanceElectrolyte BalanceKidneys do majority of balancing

Hormones involved

◦Aldosterone (adrenal cortex)

◦Antidiuretic hormone (ADH) (pituitary)

◦Parathyroid hormone (parathyroid & thyroid glands)

◦Calcitonin hormone (thyroid)

◦Atrial natriuretic peptide (ANP) (from the heart)

Acid–Base BalanceAcid–Base BalancepH scale measures hydrogen ion

(H+) concentration

Body fluids have normal pH of 7.35 to 7.45

Three-tenths of a point shift in either direction is fatal

Regulation of pHRegulation of pHBuffer systems accept or release

ions as needed

◦Bicarbonate

◦Phosphate

◦Proteins

Respiration provides short-term regulation

Kidney function provides long-term regulation

Abnormal pHAbnormal pHAcidosis

◦pH less than 7.35◦Depressed nervous system

Alkalosis◦pH greater than 7.45◦Excited nervous system

Origins◦Respiratory—change in blood CO2

◦Metabolic—change in other acids

Disorders of Body FluidsDisorders of Body FluidsEdema

Water intoxication

Effusion

Ascites

Dehydration

Fluid TherapyFluid TherapyFluids administered intravenouslyNormal (.9%) saline5% dextrose in .45% saline5% dextrose in waterRinger lactate solutionSerum albumin 25%Nutritional solutions

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