bio 169 fluid, electrolyte, and acid-base balance chapter 27

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BIO 169

FLUID, ELECTROLYTE, AND ACID-BASE

BALANCE

Chapter 27

created by Dr. C. Morgan

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TOPICS

Introduction

Fluid and Electrolyte Balance

Acid-Base Balance

Disturbances in Acid-Base Balance

Age Related Considerations

Resource: IPCD Fluid, Electrolyte, and

Acid/Base Balance

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Introduction Objectives

Review the importance of water to cellular life.

Distinguish between extracellular and intracellular fluids.

Discuss the meaning of fluid balance.

Discuss the meaning of electrolyte balance.

Discuss the meaning of acid-base balance.

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Introduction

Cellular life consists of an unending chain of chemical reactions we collectively call metabolism.

These chemical reactions occur in an aqueous medium.

If cells are to survive, the volume and composition of the fluids within and outside cells must be maintained within tolerable limits.

Extracellular fluids (ECF) are those present outside cells (blood, lymph, interstitial fluid, kidney filtrate, etc).

Intracellular fluids (cytosol, mitochondrial matrix, nucleoplasm, etc) are those present within cells.

Exchanges of water and solutes regularly occur between the two fluid compartments.

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Introduction (cont)

Homeostasis of all systems depends upon the dailymaintenance of fluid, electrolyte, and acid-base balance.

Fluid balance is achieved when water gain = water loss along with the proper allocation to ECF and ICF.

Electrolyte balance is achieved when ion gain = ion loss which is mostly influenced by activities of the digestive and renal systems.

Acid-base balance refers to the maintenance of H+ concentration in body fluids within normal limits.

Proteins will denature if there is an extensive acid-base imbalance.

Without functional proteins (enzymes), our cells will not survive.

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TOPICS

Introduction


Acid-Base Balance



7

Fluid and Electrolyte Balance Objectives

Discuss the composition of the human body with respect to water and other components.

Describe the distribution of fluids in the ECF and ICF.

Show the main cations and anions in body fluids.

Present four principles that apply to fluid and electrolyte balance.

Discuss the main regulatory hormones.

Discuss fluid balance.

Discuss electrolyte regulation of Na+, K+, Ca2+, and Cl–.

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Our body consists of liquids and solids.

Water comprises about 50% (females) to 60% (males) of the total body weight with the gender differences

attributed to variations in adipose tissue versus muscle mass.

Proteins comprise most of the solids with lipids second.

About ⅔ of the body H2O is present in the ICF.

Most of the ECF is interstitial fluid with plasma second.

Fluid exchanges between the ICF and ECF occur by both passive and active transport processes.

Osmotic concentrations of the ICF and ECF are equal.

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Fluid and Electrolyte Balance (cont)

solids = 31.5 kg

Fig. 1 a

Solid components of the body.

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Fig. 1 b

water = 38.5 l

Fluid components of the body (H2O + ions)

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ICF and ECF contain similar ions

but they differ in concentrations.

Among the cations, Na+ is high in ECF

and K+ is high in the ICF compartment.

11Fig. 2 a

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Fig. 2 b 12

Among the anions, Cl– is the most important in the

ECF while phosphate ions

predominate in the ICF.

Proteins are abundant in the ICF

and many are present in ECF.

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Important principles:

*Homeostatic mechanisms respond to conditions in the ECF (not the ICF).

*Receptors monitor plasma volume and osmotic concentration which reflect fluid and electrolyte balance in the entire body.

*Water moves by osmosis between the ECF and ICF.

*Water content and / or electrolytes will vary with intake and the effects of circulating hormones – especially

antidiuretic hormone (ADH), aldosterone, and atrial natriuretic peptide (ANP).

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Osmoreceptors in the hypothalamus are sensitive to even slight changes in the osmotic concentration of the ECF.

The rate of ADH release from hypothalamic neurons varies directly with ECF osmotic concentration.

ADH results in increased reabsorption of water by the kidneys and stimulates thirst to increase H2O volume.

Aldosterone is secreted when K+ rises, Na+ falls, or by the RAA response.

Renin is secreted by the JGA when BP drops, filtrate osmotic concentration in the DCT, or the K+ / Na+ concentration changes as noted above.

Aldosterone results in enhanced Na+ reabsorption with Cl– following along with water by osmosis.

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Fluid Balance

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water constantly moves between

cells, cavities and compartments

Fig. 3

pericardial

thoracic

peritoneal

synovial

blood / CSF

blood / interstitium

80% in interstitium+ compartments

20% in plasma

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Net filtration pressure determines fluid exchanges between the blood and interstitium.

Inability to return adequate amounts of fluid from the interstitium may result in edema (tissue swelling).

Fluids also exchange with the outside environment.

Ideally, water losses will be balanced by water gains.

You lose about 2500 ml / day of water in urine, feces, insensible perspiration (lungs and skin), and sensible perspiration (sweat gland activity).

Water is gained by intake (2200 ml / day) and during metabolism (300 ml / day).

TABLE 1 summarizes fluid gains and losses.

Fluid Balance (cont)

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Body water varies with the amount of Na+ present in the ECF which is ideally around 136 – 142 mEq / l.

When the Na+ level rises above 150 mEq / l, the body is in a state of hypernatremia.

When the Na+ level falls below 130 mEq / l, the body is in a state of hyponatremia.

The Na+ rise and fall in concentration is due the loss or gain of water respectively.

Dehydration will cause Na+ concentration to rise while overhydration will dilute the Na+ concentration

The interdependence of solute concentration and fluid volume is evident.


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Fluid shifts between the ICF and ECF are due to changes in the osmotic concentration of the ECF.

The ICF represents a water reserve.

When the ECF becomes hypertonic to the interior of cells, water will move from the ICF to the ECF by osmosis.

Conversely, when the ECF becomes hypotonic to the interior of cells, water will move into cells from the ECF.

Within volume limits, homeostasis with regard to the osmotic concentration of the ICF and ECF is constantlymaintained by fluid shifts.


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Electrolyte balance is important since electrolytes affect water balance and cell functions depend on a balance of electrolytes, especially Na+ and K+.

At 136 – 142 mEq / l, Na+ salts (NaCl and NaHCO3) account for more than 90% of the ECF solutes.

ICF Na+ is only about 10 mEq / l.

At 160 mEq / l, K+ predominates in the ICF and in the ECF it is present from 3.8 – 5 mEq / l.

*Most electrolyte imbalances involve Na+

*K+ problems do occur and are more dangerous than Na+ balance problems.

Electrolyte Balance

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Total ECF Na+ is determined by absorption across the intestinal villi epithelium (48 – 144 mEq / day) and kidney function in reabsorption and excretion plus other losses such as in sweat.

Osmosis and hormonal regulation of fluid shifts keep the ECF concentration of Na+ within normal limits.

Kidney function in the presence of the regulatory hormones ADH and aldosterone is the most important

homeostatic regulatory mechanism for sodium balance and ECF fluid volume.

ANP inhibits mechanisms that elevate blood pressure.

The ECF Na+ concentration and fluid volume are closely integrated.

Electrolyte Balance (cont) – Na+ issues

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21Fig. 4

Response mechanisms to rising Na+ in the ECF.

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22Fig. 4

Response mechanisms to falling Na+ in the ECF.

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Fig. 5

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Other hormonal effects:

Estrogens have aldosterone-like effects.

Progesterones block aldosterone.

Glucocorticoids may mimic aldosterone.

Electrolyte Balance (cont) – Na+ issues

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98% of the potassium in the body is contained in the ICF.

The ECF K+ level is due to absorption across the digestive epithelium (50 – 150 mEq / day) and urinary losses.

There are no other routes of appreciable K+ losses.

Renal tubular secretion varies

*directly with K+ ECF concentration,

*preferential H+ secretion for pH maintenance rather than K+ secretion (meaning K+ is retained), and

*aldosterone levels which influence Na+ reabsorption and K+ secretion.

Electrolyte Balance (cont) – K+ issues

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K+ imbalances cause numerous physiological problems.

Hypokalemia, a plasma K+ level < 2 mEq / l, may result from inadequate dietary intake, diuretic administration, excessive aldosterone secretion, or the renal response to pH of the ECF.

Muscular weakness and paralysis may result.

Hyperkalemia, a plasma K+ level > 8 mEq / l, may result from renal failure, diuretics that block Na+ reabsorption, and a in pH.

Cardiac arrhythmias accompany hyperkalemia.

Electrolyte Balance (cont) – K+ issues

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Calcium, the most abundant body mineral, is absorbed across the intestinal mucosa, deposited and released from bone, reabsorbed from the filtrate as it is

processed in the kidneys, and slightly lost in the bile.

The normal serum range is 4.5 – 5.3 mEq / l (plasma range is 8.5 – 11 mEq/l) (why the difference?)

Parathyroid hormone and calcitriol stimulate absorption across the intestinal mucosa and reabsorption from the DCT.

Hypercalcemia exists when the ECF concentration exceeds 11 mEq / l with hyperparathyroidism the most common cause.

Severe cardiac arrhythmias may result.

Electrolyte Balance (cont) – other regulated ions

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Hypocalcemia exists when the ECF serum concentration is less than 4 mEq / l with hypoparathyroidism, vitamin D deficiency, or renal failure the main causes.

The condition results in cardiac arrhythmias and decreased contractility , osteoporosis, muscle spasm, and possibly convulsions.

Magnesium, an enzymatic cofactor, is present in the ICF at about 26 mEq / l and at 1.5 – 2.5 mEq / l in the ECF.

The skeleton contains 60% of the Mg2+ in the body.


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Phosphate ions are used in bone mineralization, as cofactors, in making ATP and other high energy compounds, and for the synthesis of nucleic acids.

The serum concentration of PO4 3¯ is 1.8 – 2.6 mEq / l.

Calcitriol stimulates reabsorption along the PCT but some is lost in urine and feces.

Chloride ion serum concentration is from 100 – 108 mEq / l and only 3 mEq / l in the ICF.

Cl– moves with Na+ and provides electrical balance.

You only need 48 – 146 mEq / day (1.7 – 5.1 g).

TABLE 2 lists normal ECF range of the ions discussed.


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TOPICS

Introduction


Acid-Base Balance



31

Acid – Base Balance Objectives

Review some terms related to acid – base balance.

Discuss the importance of pH control in body fluids.

Describe the types of acids in the body.

Discuss the mechanisms of pH control by buffer systems.

Discuss the maintenance of acid – base balance.

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Acid – Base Balance

By definition, an acid releases H+ ions into solution and a base removes H+ from solution usually by supplying

OH– which combines with the H+ to form H2O.

The strongest acids and bases dissociate completely; weaker acids and bases do not have all molecules dissociated.

Salts dissociate to release ions that are not H+ or OH– .

Buffers are substances that oppose pH changes by supplying or removing H+ from body fluids (TABLE 3).

The normal pH of ECF is 7.35 – 7.45.

Deviations from the normal pH range threaten life and mechanisms constantly operate to maintain pH homeostasis.

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Acid – Base Balance (cont)

Acidemia is a plasma pH < 7.35 resulting in acidosis.

Alkalemia is a plasma pH > 7.45 resulting in alkalosis.

The CNS and cardiovascular system are very sensitive to pH fluxuations.

Acidosis is a more prevalent deviation from normal.

There are three types of acids in the body.

*A volatile acid will leave the body as a gas.

Carbonic acid is our only volatile acid.

CO2 + H2O H2CO3 H+ + HCO3–

The CO2 you exhale is the key component of the volatile acid that may leave the body and enter the atmosphere.

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The CO2 – pH balancing act

34Fig. 6

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*Fixed acids do not leave solution and are eliminated by the kidneys.

Sulfuric and phosphoric acid are the main fixed acids which are generated during metabolism of amino acids and compounds containing phosphate groups.

*Organic acids are those that participate in the TCA cycle or evolve as byproducts of metabolism.

Lactic acid and ketone bodies are the main forms.

Lactic acid builds up as an end product of anaerobic metabolism and excess ketone bodies are generated during prolonged lipid and amino acid catabolism (as in starvation).

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Buffer systems exist to maintain acid–base balance.

Buffers are able to donate H+ to solution or remove H+ from solution.

H+ ions are absorbed across the intestinal villi and are generated during metabolism.

H+ ions are eliminated by the kidneys and indirectly by the lungs as CO2 is exhaled and water is formed.

There are three major buffer systems:

*the protein buffer system

*the carbonic acid–bicarbonate buffer system

*the phosphate buffer system

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Fig. 7

The three buffer systems

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Protein Buffers

Amino acids are able to donate an H+ from their carboxyl end or bind an H+ on their amino end.

Peptides have buffering ability only at the ends of their amino acid chain or side groups of histidine and cysteine.

In the ECF, plasma proteins and hemoglobin are buffers.

In the ICF, intracellular proteins are efficient buffers.

Fig. 8

If pH falls

If pH rises

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Protein Buffers (cont)

Proteins present in the plasma and interstitial fluids act as buffers.

Intracellular functional and structural proteins serve to buffer acids produced during metabolism.

Since ion exchanges occur between the ECF and ICF, intracellular functional and structural proteins are able to assist in buffering the ECF.

H+ ions may be pumped from an acidic ECF into cells where intracellular proteins act as buffers.

ECF K+ ions may also exchange for intracellular H+ ions when the ECF pH rises above normal.

These buffering processes are relatively slow.

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Protein Buffers (cont)

Hemoglobin molecules have the ability to bind H+ on their polypeptide chains and supply bicarbonate which moves into the plasma where it may buffer H+.

The H+ evolves from the familiar equation:

CO2 + H20 H2CO3 H+ + HCO3–

Recall that in the lungs, the equation shifts to the left.

The hemoglobin buffering system helps to maintain plasma pH homeostasis when there are fluxuations in

PCO2.

This is the only intracellular buffering system that is able to immediately change the pH of the ECF.

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Carbonic Acid – Bicarbonate Buffer System

The main role of this system is to buffer organic and fixed acids that move into the ECF.

The supply of bicarbonate ions may be quickly consumed so must be replaced from the bicarbonate reserve

(NaHCO3) present in body fluids.

Fig. 9 a

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As the bicarbonate is consumed in buffering the H+ from acids, more carbonic acid is formed driving the equation to the left when the blood reaches the lungs.

The amount of available HCO3– ions is the limiting factor.

The respiratory system is able to compensate for the metabolic generation of H+ ions.

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lungs

replacement required

Fig. 9 b

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Kidney Function

Transport processes along the DCT result in the generation of bicarbonate while

secreting H+ ions into the tubular fluid.

This new bicarbonate becomes part of

the plasma bicarbonate reserve.

Fig. 26-14 c43

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Phosphate Buffer System

Dihydrogen phosphate (H2PO4– ) is a weak acid that will

dissociate to supply an anion that is able to buffer H+.

H2PO4– H+ + HPO4

2–

As the monohydrogen phosphate is consumed during buffering, more is released from a phosphate reserve.

Na2HPO4 2 Na+ + HPO4 2–

This system has greatest importance in the ICF.

This system is also employed as a buffer in the urine.

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Buffering is only a short term solution to pH imbalances.

Long term maintenance of acid–base balance depends on eliminating excess H+ ions from the ECF so buffers are free to be used again.

Likewise, if H+ ions need to be supplied to correct an alkalotic condition, the supply of H+ ions obtained from

buffers may be rapidly exhausted unless the H+ ions are replaced.

The solution is to integrate activities of the respiratory and urinary systems to eliminate or supply H+ ions to the ECF while buffers solve immediate pH imbalances.

pH homeostasis is critically important in maintaining the integrity of functional and structural proteins.

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Respiratory compensatory mechanisms involve a change in respiratory rate and depth of ventilation which results in an alteration of the amount of CO2 leaving the body.

This mechanism manipulates the carbonic acid – bicarbonate buffer system.

CO2 + H20 H2CO3 H+ + HCO3–

Recall that central and peripheral chemoreceptors send information to the respiratory center in the medulla

oblongata which results in appropriate adjustments to ventilation for pH maintenance of the ECF.

pH ventilation

pH ventilation


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Renal compensatory mechanisms involve changing the rates of secretion or reabsorption of H+ and newly

generated HCO3– ions.

Kidney tubular cells are able to secrete H+ until the pH falls to about 4.0 to 4.5 after which buffering is required.

The kidneys eliminate H+ in the form of ammonium ions (NH4

+), dihydrogen phosphate (H2PO4– ), and utilizing the

carbonic acid – bicarbonate system.

When alkalosis develops, the kidneys decrease H+

secretion, reclaim fewer bicarbonates, and excrete HCO3

– while reabsorbing H+ and Cl–.

48


Renal compensation using buffer systems

1. bicarbonate 2. monohydrogen phosphate 3. ammonia

Fig. 10 a

Acidosis response

Collecting duct

PCT

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The amino acid glutamine supplies ammonia and bicarbonate for buffering.

The toxic NH3 is detoxified to ammonium ion (NH4

+) during buffering.

renal tubule cell of the PCT

Fig. 10 b

Acidosis response

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Renal response to alkalosis

Reclaim H+

Secrete HCO3–

Fig. 10 c

51

TOPICS

Introduction


Acid-Base Balance



52

Acid – Base Disturbances Objectives

Describe the most common causes of acid–base disturbances.

Discuss respiratory acidosis.

Discuss respiratory alkalosis.

Discuss metabolic acidosis.

Discuss combined respiratory and metabolic acidosis.

Discuss metabolic alkalosis.

Describe methods to detect acidosis and alkalosis.

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Acid – Base Disturbances

Balancing pH means controlling the gains and losses of H+.

Buffers and the respiratory ventilation and renal mechanisms all have a role in maintaining pH balance.

The acid–base disorders include acidosis and alkalosis from respiratory or metabolic causes.

These disorders arise when the pH cannot be controlled by the various available mechanisms.

Factors that affect any one of the regulatory mechanisms may lead to an acid–base disturbance.

In respiratory acid–base imbalances the CO2 level in the ECF is abnormal.

In metabolic imbalances, the ECF HCO3– level is abnormal.

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NORMAL BLOOD VALUES

pH = 7.35 – 7.45

PCO2 = 35 – 45 mmHg

HCO3– = 24 – 28 mEq / l (text)

HCO3– = 22 – 26 mEq / l (other sources)

acidosis = pH < 7.35

alkalosis = pH > 7.45

Acid – Base Disturbances

TABLE 4

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Acid – Base Disturbances (cont)

Interactions in response to acidosis.

55Fig. 11 a

56


Interactions in response to alkalosis.

Fig. 11 b 56

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Respiratory acidosis, the most common acid–base disturbance, develops when there is more CO2 produced by the metabolism of cells than can be eliminated from the body.

The plasma pH drops below 7.35 because of hypercapnea which drives the carbonic acid equation to the right.

Hypoventilation is the most common cause.

Acute respiratory acidosis is severe and develops quickly.

Chronic respiratory acidosis persists with COPD, paralysis, or any disorder that decreases ventilation .

The kidneys attempt to compensate for the H+ buildup.

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Respiratory acidosis: characteristics and responses

Fig. 12 aFig. 12 a 58

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Respiratory alkalosis, an infrequent disturbance, results in a rise in pH above 7.45 due to hypocapnia caused by hyperventilation.

Hyperventilation is usually associated with stress and pain or infrequently with adaptation to high altitude, brain stem injuries, or mechanical ventilation.

Hyperventilation does not usually result in acid–base disturbances that require medical intervention.

Chemoreceptor responses will result in changes in ventilation that correct for respiratory acidosis and alkalosis in healthy individuals.

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Respiratory alkalosis: characteristics and responses

Fig. 12 b 60

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Metabolic acidosis, the second most common acid–base disturbance, may be due to

* a buildup of fixed and organic acids in the ECF

lactic acidosis due to strenuous exercise; hypoxia

ketoacidosis in uncontrolled diabetes mellitus or starvation

* the loss of HCO3– (usually from chronic diarrhea)

* failure of the kidneys to secrete adequate H+

diuretic therapy

kidney disease

Compensation is by the respiratory and renal systems.

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Fig. 13

Responses to metabolic acidosis

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Respiratory and metabolic acidosis often occur simultaneously since hypoventilation leads to tissue hypoxia so cells must resort to anaerobic metabolism.

During anaerobic metabolism (glycolysis only), pyruvic acid is converted into lactic acid.

Metabolic alkalosis is due to increased HCO3– which

remove H+.

The most common cause is vomiting resulting in losses of HCl which is replaced while generating a prolonged alkaline tide into the ECF.

Ingestion of alkaline drugs and gastric suctioning are also causes.

Compensation is by the respiratory and renal systems.

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Responses to metabolic alkalosis

Fig. 14

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A study of the blood chemistry will reveal information about acid–base disturbances (see TABLE 4).

The usual test is to analyze a sample of arterial blood (ABG = arterial blood gas) which reveals pH, blood gas partial pressures, and the bicarbonate level.

Analysis of ABGs may be done in a series of steps.

• check pH for acidosis or alkalosis

• check to see of PCO2 or HCO3– is abnormal

• if the PCO2 is abnormal, the cause is respiratory

• if the HCO3– is abnormal, the cause is metabolic

• examine the value that does not correspond with the pH change to determine if compensation is occurring.

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ABG correlation guide

pH PCO2 HCO3–

Normal plasma values 7.35 – 7.45 35 – 45 mm Hg 22 – 26 mEq/l

Respiratory acidosis normalrenal compensation

Respiratory alkalosis normalrenal compensation

Metabolic acidosis normal respiratory compensation

Metabolic alkalosis normal respiratory compensation

Hb saturation 95% or >

PO2 = 80 – 100 mm Hg

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Fig. 15anion gap= (Na+) – (HCO3

¯ ) + (Cl¯ ) normal = 10-12 mEq/l

68

TOPICS

Introduction


Acid-Base Balance



69

Age Related Considerations Objectives

Describe considerations with fluid balance.

Describe considerations with renal function.

Discuss concerns with mineral content of the body.

Discuss concerns with respiratory function.

Describe some concerns with infants.

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More fluid, electrolyte, and acid–base balance problems accompany aging than occur in infants and children.

*The total amount of body water decreases after age 60.

This decrease results in less ability to dilute toxins, waste products of metabolism, and drugs.

*The total number of functional nephrons decreases which reduces the GFR.

This decrease reduces renal compensatory mechanisms.

*There is a reduced sensitivity to ADH and aldosterone.

This reduces reabsorption of fluid and loss of body water.

*Total mineral content of the body declines.

This affects buffering ability.

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*Reduced vital capacity limits the ability of the respiratory system to compensate for metabolic acidosis.

Infancy considerations:

The volume of the ECF is about equal to the ICF volume.

Changes in the ECF status will quickly alter the intracellular conditions and intracellular buffering ability may be quickly exhausted.

Infants have less minerals in reserve.

Metabolic rate of infants is twice that seen in adults.

Kidneys cannot produce a urine that exceeds 450 mOsm/l.

Because the functional residual capacity is small in infants, they may quickly develop respiratory disturbances that lead to acidosis and alkalosis.

72

TOPICS

Introduction


Acid-Base Balance



bio 169 fluid, electrolyte, and acid-base balance chapter 27

Documents

meaning of fluid balance

meaning of electrolyte

electrolyte balance

electrolyte balance

electrolyte balance

interstitial fluid

fluid exchanges

ipcd fluid