26 fluid, electrolyte and acid base balance

7/29/2019 26 Fluid, Electrolyte and Acid Base Balance

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Copyright 2009 All Rights Reserved Presented by: Dr. Patrick Garrett DC, B Sci, DABFM

Human Anatomy & Physiology

Chapter 26

Fluid, Electrolyte, and Acid-Base Balance

Dr. Patrick Garrett D.C., B. Sci, D.A.B.F.M


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Body Water Content

Infants have low body fat, low bone mass, and are

73% or more water

Total water content declines throughout life

Healthy males are about 60% water; healthy

females are around 50%


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Body Water Content

This difference reflects females:

Higher body fat

Smaller amount of skeletal muscle

In old age, only about 45% of body weight is water


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Fluid Compartments

Water occupies two main fluid compartments

Intracellular fluid (ICF) about two thirds by

volume, contained in cells

Extracellular fluid (ECF) consists of two major

subdivisions Plasma the fluid portion of the blood

Interstitial fluid (IF) fluid in spaces between cells

Other ECF lymph, cerebrospinal fluid, eye

humors, synovial fluid, serous fluid, and

gastrointestinal secretions


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Fluid Compartments

PLAY InterActive Physiology :

Introduction to Body Fluids, page 10Figure 26.1


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Composition of Body Fluids

Water is the universal solvent

Solutes are broadly classified into:

Electrolytes inorganic salts, all acids and bases,and some proteins

Nonelectrolytes examples include glucose, lipids,creatinine, and urea

Electrolytes have greater osmotic power than

nonelectrolytes Water moves according to osmotic gradients


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Electrolyte Concentration

Expressed in milliequivalents per liter (mEq/L), a

measure of the number of electrical charges in one

liter of solution

mEq/L = (concentration of ion in [mg/L]/the

atomic weight of ion) number of electrical

charges on one ion

For single charged ions, 1 mEq = 1 mOsm

For bivalent ions, 1 mEq = 1/2 mOsm


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Extracellular and Intracellular Fluids

Each fluid compartment of the body has adistinctive pattern of electrolytes

Extracellular fluids are similar (except for the highprotein content of plasma)

Sodium is the chief cation

Chloride is the major anion

Intracellular fluids have low sodium and chloride

Potassium is the chief cation

Phosphate is the chief anion


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Sodium and potassium concentrations in extra- and

intracellular fluids are nearly opposites

This reflects the activity of cellular ATP-dependent sodium-potassium pumps

Electrolytes determine the chemical and physicalreactions of fluids

PLAY InterActive Physiology :Introduction to Body Fluids, page 12


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Proteins, phospholipids, cholesterol, and neutral

fats account for:

90% of the mass of solutes in plasma

60% of the mass of solutes in interstitial fluid

97% of the mass of solutes in the intracellularcompartment


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Electrolyte Composition of Body Fluids

Figure 26.2


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Fluid Movement Among Compartments

Compartmental exchange is regulated by osmotic

and hydrostatic pressures

Net leakage of fluid from the blood is picked up bylymphatic vessels and returned to the bloodstream

Exchanges between interstitial and intracellularfluids are complex due to the selective

permeability of the cellular membranes

Two-way water flow is substantial


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Ion fluxes are restricted and move selectively byactive transport

Nutrients, respiratory gases, and wastes moveunidirectionally

Plasma is the only fluid that circulates throughout

the body and links external and internalenvironments

Osmolalities of all body fluids are equal; changesin solute concentrations are quickly followed byosmotic changes

PLAY InterActive Physiology :

Introduction to Body Fluids, pages 1922


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Continuous Mixing of Body Fluids

Figure 26.3


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Water Balance and ECF Osmolality

To remain properly hydrated, water intake must

equal water output

Water intake sources

Ingested fluid (60%) and solid food (30%)

Metabolic water or water of oxidation (10%)


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Water Balance and ECF Osmolality

Water output

Urine (60%) and feces (4%)

Insensible losses (28%), sweat (8%)

Increases in plasma osmolality trigger thirst and

release of antidiuretic hormone (ADH)


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Water Intake and Output

Figure 26.4


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Regulation of Water Intake

The hypothalamic thirst center is stimulated:

By a decline in plasma volume of 10%15%

By increases in plasma osmolality of 12%

Via baroreceptor input, angiotensin II, and other

stimuli

PLAY InterActive Physiology :Water Homeostasis, page 18


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Regulation of Water Intake

Thirst is quenched as soon as we begin to drink

water

Feedback signals that inhibit the thirst centersinclude:

Moistening of the mucosa of the mouth and throat Activation of stomach and intestinal stretch

receptors

PLAY InterActive Physiology :Water Homeostasis, page 19


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Regulation of Water Intake: Thirst Mechanism

Figure 26.5


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Regulation of Water Output

Obligatory water losses include:

Insensible water losses from lungs and skin

Water that accompanies undigested food residuesin feces

Obligatory water loss reflects the fact that:

Kidneys excrete 900-1200 mOsm of solutes tomaintain blood homeostasis

Urine solutes must be flushed out of the body inwater

PLAY InterActive Physiology :Water Homeostasis, pages 310


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Influence and Regulation of ADH

Water reabsorption in collecting ducts is proportional to

ADH release

Low ADH levels produce dilute urine and reduced volumeof body fluids

High ADH levels produce concentrated urine

Hypothalamic osmoreceptors trigger or inhibit ADH

release

Factors that specifically trigger ADH release include

prolonged fever; excessive sweating, vomiting, or diarrhea;

severe blood loss; and traumatic burns



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Mechanisms and Consequences of ADH

Release

Figure 26.6


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Disorders of Water Balance: Dehydration

Water loss exceeds water intake and the body is innegative fluid balance

Causes include: hemorrhage, severe burns,prolonged vomiting or diarrhea, profuse sweating,water deprivation, and diuretic abuse

Signs and symptoms: cottonmouth, thirst, dryflushed skin, and oliguria

Prolonged dehydration may lead to weight loss,fever, and mental confusion

Other consequences include hypovolemic shock

and loss of electrolytes


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Disorders of Water Balance: Dehydration

Figure 26.7a


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Disorders of Water Balance:

Hypotonic Hydration Renal insufficiency or an extraordinary amount of

water ingested quickly can lead to cellular

overhydration, or water intoxication

ECF is diluted sodium content is normal butexcess water is present

The resulting hyponatremia promotes net osmosisinto tissue cells, causing swelling

These events must be quickly reversed to preventsevere metabolic disturbances, particularly inneurons


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Disorders of Water Balance:

Hypotonic Hydration

Figure 26.7b


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Disorders of Water Balance: Edema

Atypical accumulation of fluid in the interstitialspace, leading to tissue swelling

Caused by anything that increases flow of fluidsout of the bloodstream or hinders their return

Factors that accelerate fluid loss include:

Increased blood pressure, capillary permeability

Incompetent venous valves, localized blood vessel

blockage Congestive heart failure, hypertension, high blood

volume


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Edema

Hindered fluid return usually reflects an imbalance

in colloid osmotic pressures

Hypoproteinemia low levels of plasma proteins

Forces fluids out of capillary beds at the arterial

ends

Fluids fail to return at the venous ends

Results from protein malnutrition, liver disease, or

glomerulonephritis


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Edema

Blocked (or surgically removed) lymph vessels:

Cause leaked proteins to accumulate in interstitial

fluid

Exert increasing colloid osmotic pressure, which

draws fluid from the blood

Interstitial fluid accumulation results in low blood

pressure and severely impaired circulation

PLAY InterActive Physiology :Electrolyte Homeostasis, pages 1216


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Sodium in Fluid and Electrolyte Balance

Sodium holds a central position in fluid andelectrolyte balance

Sodium salts: Account for 90-95% of all solutes in the ECF

Contribute 280 mOsm of the total 300 mOsm ECFsolute concentration

Sodium is the single most abundant cation in the

ECF Sodium is the only cation exerting significant

osmotic pressure


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Sodium in Fluid and Electrolyte Balance

The role of sodium in controlling ECF volume and

water distribution in the body is a result of:

Sodium being the only cation to exert significantosmotic pressure

Sodium ions leaking into cells and being pumped

out against their electrochemical gradient

Sodium concentration in the ECF normally

remains stable


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Regulation of Sodium Balance: Aldosterone

Sodium reabsorption

65% of sodium in filtrate is reabsorbed in the

proximal tubules

25% is reclaimed in the loops of Henle

When aldosterone levels are high, all remainingNa+ is actively reabsorbed

Water follows sodium if tubule permeability has

been increased with ADH


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The renin-angiotensin mechanism triggers therelease of aldosterone

This is mediated by the juxtaglomerular apparatus,which releases renin in response to:

Sympathetic nervous system stimulation

Decreased filtrate osmolality

Decreased stretch (due to decreased bloodpressure)

Renin catalyzes the production of angiotensin II,which prompts aldosterone release


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Adrenal cortical cells are directly stimulated to

release aldosterone by elevated K+ levels in the

ECF

Aldosterone brings about its effects (diminished

urine output and increased blood volume) slowly



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Figure 26.8


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Cardiovascular System Baroreceptors

Baroreceptors alert the brain of increases in blood

volume (hence increased blood pressure)

Sympathetic nervous system impulses to thekidneys decline

Afferent arterioles dilate

Glomerular filtration rate rises

Sodium and water output increase


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Cardiovascular System Baroreceptors

This phenomenon, called pressure diuresis,

decreases blood pressure

Drops in systemic blood pressure lead to oppositeactions and systemic blood pressure increases

Since sodium ion concentration determines fluid

volume, baroreceptors can be viewed as sodium

receptors


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Maintenance of Blood Pressure Homeostasis

Figure 26.9


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Atrial Natriuretic Peptide (ANP)

Reduces blood pressure and blood volume byinhibiting:

Events that promote vasoconstriction Na+ and water retention

Is released in the heart atria as a response to stretch

(elevated blood pressure)

Has potent diuretic and natriuretic effects

Promotes excretion of sodium and water

Inhibits angiotensin II production


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Mechanisms and Consequences of ANP

Release

Figure 26.10


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Influence of Other Hormones on Sodium

Balance Estrogens:

Enhance NaCl reabsorption by renal tubules

May cause water retention during menstrual cycles

Are responsible for edema during pregnancy


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Influence of Other Hormones on Sodium

Balance Progesterone:

Decreases sodium reabsorption

Acts as a diuretic, promoting sodium and water

loss

Glucocorticoids enhance reabsorption of sodium

and promote edema

R l i f P i B l


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Regulation of Potassium Balance

Relative ICF-ECF potassium ion concentration

affects a cells resting membrane potential

Excessive ECF potassium decreases membranepotential

Too little K+ causes hyperpolarization and

nonresponsiveness

R l ti f P t i B l


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Regulation of Potassium Balance

Hyperkalemia and hypokalemia can:

Disrupt electrical conduction in the heart

Lead to sudden death

Hydrogen ions shift in and out of cells

Leads to corresponding shifts in potassium in the

opposite direction

Interferes with activity of excitable cells

R l t Sit C ti l C ll ti D t


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Regulatory Site: Cortical Collecting Ducts

Less than 15% of filtered K+ is lost to urineregardless of need

K+

balance is controlled in the cortical collectingducts by changing the amount of potassium secretedinto filtrate

Excessive K+ is excreted over basal levels by corticalcollecting ducts

When K+ levels are low, the amount of secretion and

excretion is kept to a minimum

Type A intercalated cells can reabsorb some K+ leftin the filtrate

I fl f Pl P t i C t ti


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Influence of Plasma Potassium Concentration

High K+ content of ECF favors principal cells to

secrete K+

Low K+ or accelerated K+ loss depresses itssecretion by the collecting ducts

PLAY InterActive Physiology :Electrolyte Homeostasis, pages 2832


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Regulation of Calcium


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Ionic calcium in ECF is important for:

Blood clotting

Cell membrane permeability

Secretory behavior

Hypocalcemia:

Increases excitability

Causes muscle tetany



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Hypercalcemia:

Inhibits neurons and muscle cells

May cause heart arrhythmias

Calcium balance is controlled by parathyroid

hormone (PTH) and calcitonin


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Acid-Base Balance


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Acid Base Balance

Normal pH of body fluids

Arterial blood is 7.4

Venous blood and interstitial fluid is 7.35

Intracellular fluid is 7.0

Alkalosis or alkalemia arterial blood pH risesabove 7.45

Acidosis or acidemia arterial pH drops below7.35 (physiological acidosis)

Sources of Hydrogen Ions


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Sources of Hydrogen Ions

Most hydrogen ions originate from cellularmetabolism

Breakdown of phosphorus-containing proteinsreleases phosphoric acid into the ECF

Anaerobic respiration of glucose produces lacticacid

Fat metabolism yields organic acids and ketonebodies

Transporting carbon dioxide as bicarbonatereleases hydrogen ions

Hydrogen Ion Regulation


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Hydrogen Ion Regulation

Concentration of hydrogen ions is regulated

sequentially by:

Chemical buffer systems act within seconds

The respiratory center in the brain stem acts

within 1-3 minutes

Renal mechanisms require hours to days to effect

pH changes


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Strong and Weak Acids


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g

Figure 26.11

Chemical Buffer Systems


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y

One or two molecules that act to resist pH changes

when strong acid or base is added

Three major chemical buffer systems

Bicarbonate buffer system

Phosphate buffer system Protein buffer system

Any drifts in pH are resisted by the entire chemicalbuffering system


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Bicarbonate Buffer System


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If strong base is added:

It reacts with the carbonic acid to form sodium

bicarbonate (a weak base) The pH of the solution rises only slightly

This system is the only important ECF buffer

PLAY InterActive Physiology :Acid/Base Homeostasis, pages 1617

Phosphate Buffer System


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Nearly identical to the bicarbonate system

Its components are:

Sodium salts of dihydrogen phosphate (H2PO4 ), a

weak acid

Monohydrogen phosphate (HPO42

), a weak base This system is an effective buffer in urine and

intracellular fluid

PLAY InterActive Physiology :Ac id/Base Homeostas is, page 18


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Physiological Buffer Systems


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During carbon dioxide unloading, hydrogen ions areincorporated into water

When hypercapnia or rising plasma H+ occurs:

Deeper and more rapid breathing expels more carbon dioxide

Hydrogen ion concentration is reduced

Alkalosis causes slower, more shallow breathing, causing H+

to increase

Respiratory system impairment causes acid-base imbalance

(respiratory acidosis or respiratory alkalosis)

PLAY InterActive Physiology :Ac id/Base Homeostasis, page 2728


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Renal Mechanisms of Acid-Base Balance


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The most important renal mechanisms for

regulating acid-base balance are:

Conserving (reabsorbing) or generating newbicarbonate ions

Excreting bicarbonate ions

Losing a bicarbonate ion is the same as gaining a

hydrogen ion; reabsorbing a bicarbonate ion is the

same as losing a hydrogen ion

Renal Mechanisms of Acid-Base Balance


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Hydrogen ion secretion occurs in the PCT and in

type A intercalated cells

Hydrogen ions come from the dissociation ofcarbonic acid


Reabsorption of Bicarbonate


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Carbon dioxide combines with water in tubulecells, forming carbonic acid

Carbonic acid splits into hydrogen ions andbicarbonate ions

For each hydrogen ion secreted, a sodium ion and

a bicarbonate ion are reabsorbed by the PCT cells Secreted hydrogen ions form carbonic acid; thus,bicarbonate disappears from filtrate at the same

rate that it enters the peritubular capillary blood

Reabsorption of Bicarbonate


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Carbonic acid formedin filtrate dissociatesto release carbon

dioxide and water

Carbon dioxide thendiffuses into tubule

cells, where it acts totrigger furtherhydrogen ion

secretion

Figure 26.12

Generating New Bicarbonate Ions


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Two mechanisms carried out by type Aintercalated cells generate new bicarbonate ions

Both involve renal excretion of acid via secretionand excretion of hydrogen ions or ammonium ions

(NH4+)


Hydrogen Ion Excretion


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Dietary hydrogen ions must be counteracted bygenerating new bicarbonate

The excreted hydrogen ions must bind to buffers inthe urine (phosphate buffer system)

Intercalated cells actively secrete hydrogen ions

into urine, which is buffered and excreted Bicarbonate generated is:

Moved into the interstitial space via a cotransportsystem

Passively moved into the peritubular capillaryblood

Hydrogen Ion Excretion


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In response toacidosis:

Kidneys generatebicarbonate ions

and add them to the

blood

An equal amount of

hydrogen ions are

added to the urine

Figure 26.13


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Ammonium Ion Excretion


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When the body is in alkalosis, type B intercalatedcells:

Exhibit bicarbonate ion secretion

Reclaim hydrogen ions and acidify the blood

The mechanism is the opposite of type A

intercalated cells and the bicarbonate ionreabsorption process

Even during alkalosis, the nephrons and collecting

ducts excrete fewer bicarbonate ions than theyconserve



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Metabolic Acidosis


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All pH imbalances except those caused byabnormal blood carbon dioxide levels

Metabolic acid-base imbalance bicarbonate ionlevels above or below normal (22-26 mEq/L)

Metabolic acidosis is the second most common

cause of acid-base imbalance Typical causes are ingestion of too much alcohol

and excessive loss of bicarbonate ions

Other causes include accumulation of lactic acid,shock, ketosis in diabetic crisis, starvation, andkidney failure

Metabolic Alkalosis


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Rising blood pH and bicarbonate levels indicatemetabolic alkalosis

Typical causes are: Vomiting of the acid contents of the stomach

Intake of excess base (e.g., from antacids)

Constipation, in which excessive bicarbonate is

reabsorbed


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Respiratory Compensation


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In metabolic acidosis:

The rate and depth of breathing are elevated

Blood pH is below 7.35 and bicarbonate level islow

As carbon dioxide is eliminated by the respiratory

system, PCO2 falls below normal In respiratory acidosis, the respiratory rate is often

depressed and is the immediate cause of the

acidosis


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Renal Compensation

Alk l i h L P d hi h H


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Alkalosis has Low PCO2 and high pH

The kidneys eliminate bicarbonate from the body

by failing to reclaim it or by actively secreting it

Developmental Aspects

W t t t f th b d i t t t bi th (70


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Water content of the body is greatest at birth (70-80%) and declines until adulthood, when it is about58%

At puberty, sexual differences in body watercontent arise as males develop greater muscle mass

Homeostatic mechanisms slow down with age Elders may be unresponsive to thirst clues and are

at risk of dehydration

The very young and the very old are the mostfrequent victims of fluid, acid-base, and electrolyteimbalances

Problems with Fluid, Electrolyte, and Acid-

B B l


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Base Balance Occur in the young, reflecting:

Low residual lung volume

High rate of fluid intake and output

High metabolic rate yielding more metabolic

wastes

High rate of insensible water loss

Inefficiency of kidneys in infants

26 fluid, electrolyte and acid base balance

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