Chapter 27

Fluid, Electrolyte and Acid-Base Homeostasis

 

Lecture Outline

Chapter 27
Fluid, Electrolyte and Acid-Base Homeostasis

•       Body fluid

–    all the water and dissolved solutes in the body’s fluid compartments

•       Mechanisms regulate

–    total volume

–    distribution

–    concentration of solutes and pH

•       Regulatory mechanisms insure homeostasis of body fluids since their malfunction may seriously endanger nervous system and organ functioning.

FLUID COMPARTMENTS AND FLUID BALANCE

Balance Between Fluid Compartments

•       Only 2 places for exchange between compartments:

–    cell membranes separate intracellular from interstitial fluid.

–    only in capillaries are walls thin enough for exchange between plasma and interstitial fluids

Introduction

•      In lean adults body fluids comprise about 55-60% (Figure 27.1) of total body weight.

–   Water is the main component of all body fluids.

–   About two-thirds of the body’s fluid is located in cells and is called intracellular fluid (ICF).

–   The other third is called extracellular fluid (ECF).

–   About 80% of the ECF is interstitial fluid and 20% is blood plasma.

•       Some of the interstitial fluid is localized in specific places, such as lymph; cerebrospinal fluid; gastrointestinal tract fluids; synovial fluid; fluids of the eyes (aqueous humor and vitreous body) and ears (endolymph and perilymph); pleural, pericardial, and peritoneal fluids between serous membranes; and glomerular filtrate in the kidneys.

Membranes

•      Selectively permeable membranes separate body fluids into distinct compartments.

–   Plasma membranes of  individual cells separate intracellular fluid from interstitial fluid.

–   Blood vessel walls divide interstitial fluid from blood plasma.

•      Although fluids are in constant motion from one compartment to another, the volume of fluid in each compartment remains fairly stable – another example of homeostasis.

Fluid and Solute Balance

•      Fluid balance means that the various body compartments contain the required amount of water, proportioned according to their needs.

–   Fluid balance, then, means water balance, but also implies electrolyte balance; the two are inseparable.

•      Osmosis is the primary way in which water moves in and out of body compartments. The concentrations of solutes in the fluids is therefore a major determinant of fluid balance.

•      Most solutes in body fluids are electrolytes, compounds that dissociate into ions.

Body Water Gain and Loss (Figure 27.2)

•      45-75% body weight

–   declines with age since fat contains almost no water

•      Gain from ingestion and  metabolic water formed during aerobic respiration & dehydration synthesis reactions (2500 mL/day)

•      Normally loss = gain

–   urine, feces, sweat, breathe

Dehydration Stimulates Thirst

•      Regulation of fluid gain is by regulation of thirst.

Regulation of Water Gain

•      Metabolic water volume depends mostly on the level of aerobic cellular respiration, which reflects the demand for ATP in body cells.

•      The main way to regulate body water balance is by adjusting the volume of water intake.

•      When water loss is greater than water gain, dehydration occurs (Figure 27.3).

•      The stimulus for fluid intake (gain) is dehydration resulting in thirst sensations; one mechanism for stimulating the thirst center in the hypothalamus is the renin-angiotensin II pathway, which responds to decreased blood volume (therefore, decreased blood pressure) (Figure 27.3).

•      Drinking occurs è body water levels return to normal

Regulation of Water and Solute Loss

•      Although increased amounts of water and solutes are lost through sweating and exhalation during exercise, loss of body water or excess solutes depends mainly on regulating how much is lost in the urine (Figure 27.4).

 

•      Under normal conditions, fluid output (loss) is adjusted by

–   antidiuretic hormone (ADH)

–   atrial natriuretic peptide (ANP)

–   aldosterone

     all of which regulate urine production.

 

•      Table 27.1 summarizes the factors that maintain body water balance.

Regulation of Water and Solute Loss

•      Elimination of excess water or solutes occurs through urination

•      Consumption of very salty meal demonstrates function of three hormones

•      Demonstrates how

–   “water follows salt”

–   excrete Na+ and water will follow and decrease blood volume

 

Movement of Water Between Body Fluid Compartments

•      A fluid imbalance between the intracellular and interstitial fluids can be caused by a change in their osmolarity.

•      Most often a change in osmolarity is due to a change in the concentration of Na⁺.

–   When water is consumed faster than the kidneys can excrete it, water intoxication may result (Figure 27.5).

–   Repeated use of enemas can increase the risk of fluid and electrolyte imbalances. (Clinical Application)

Hormone Effects on Solutes

•      Angiotensin II and aldosterone promote reabsorption of Na+ and Cl- and an increase in fluid volume

–   stretches atrial volume and promotes release of ANP

–   slows release of renin & formation of angiotensin II

•   increases filtration rate & reduces water & Na+ reabsorption

•   decreases secretion of aldosterone slowing reabsorption of Na+ and Cl- in collecting ducts

•      ANP promotes natriuresis or the increased excretion of Na+ and Cl- which decreases blood volume

Hormone Regulation of Water Balance

•      Antidiuretic hormone (ADH) from the posterior pituitary

–    stimulates thirst

–   increases permeability of principal cells of collecting ducts to assist in water reabsorption

–   very concentrated urine is formed

•      ADH secretion shuts off after the intake of water

•      ADH secretion is increased

–   large decrease in blood volume

–   severe dehydration and drop in blood pressure

–   vomiting, diarrhea, heavy sweating or burns

Movement of Water

•       Intracellular and interstitial fluids
normally have the same osmolarity,
so cells neither swell nor shrink

•       Swollen cells of water intoxication
because Na+ concentration of plasma
falls below normal

–    drink plain water faster than kidneys can
excrete it

–    replace water lost from diarrhea or vomiting
with plain water

–    may cause convulsions, coma & death unless oral rehydration includes small amount salt in water intake

ELECTROLYTES IN BODY FLUIDS

•      Electrolytes serve four general functions in the body.

–   Because they are more numerous than nonelectrolytes, electrolytes control the osmosis of water between body compartments.

–   maintain the acid-base balance required for normal cellular activities.

–   carry electrical current, which allows production of action potentials and graded potentials and controls secretion of some hormones and neurotransmitters. Electrical currents are also important during development.

–   cofactors needed for optimal activity of enzymes.

 

•      Concentration expressed in mEq/liter or milliequivalents per liter for plasma, interstitial fluid and intracellular fluid

Concentrations of Electrolytes in Body Fluids

•      To compare the charge carried by ions in different solutions, the concentration is typically expressed in milliequivalents/liter (mEg/Liter), which gives the concentration of cations or anions in a solution.

•      The chief difference between plasma and interstitial fluid

–   plasma contains quite a few protein anions

–   interstitial fluid has hardly any since plasma proteins generally cannot move out of impermeable blood vessel walls

–   plasma also contains slightly more sodium ions but fewer chloride ions than the interstitial fluid. In other respects, the two fluids are similar.

Concentrations of Electrolytes in Body Fluids

•      Intracellular fluid (ICF) differs considerably from extracellular fluid (ECF), however.

•      Figure 27.6 compares the concentrations of the main electrolytes and protein anions in plasma, interstitial fluid, and intracellular fluid.

Comparison Between Fluid Components

•       Plasma contains many proteins, but interstitial fluid does not

–    producing blood colloid osmotic pressure

•       Extracellular fluid contains Na+ and Cl-

•       Intracellular fluid contains K+ and phosphates (HPO₄ ^-2)

Sodium (Na⁺) is the most abundant extracellular ion.

•      Most abundant extracellular ion

–   accounts for 1/2 of osmolarity of ECF

•      Average daily intake exceeds normal requirements

•      Hormonal controls

–   aldosterone causes increased reabsorption Na+

–   ADH release ceases if Na+ levels too low--dilute urine lost until Na+ levels rise

–   ANP increases Na+ and water excretion if Na+ levels too high

 

•       Excess Na⁺ in the body can result in edema. Excess loss of Na⁺ causes excessive loss of water, which results in hypovolemia, an abnormally low blood volume. (Clinical Application)

Edema, Hypovolemia and Na+ Imbalance

•      Sodium retention causes water retention

–   edema is abnormal accumulation of interstitial fluid

•      Causes of sodium retention

–   renal failure

–   hyperaldosterone

•      Excessive loss of sodium causes excessive loss of water (low blood volume)

–   due to inadequate secretion of aldosterone

–   too many diuretics

 

Chloride (Cl^-) is the major extracellular anion.

•      Regulation of Cl^-  balance in body fluids is indirectly controlled by aldosterone. Aldosterone regulate sodium reabsorption; the negatively charged chloride follows the positively charged sodium passively by electrical attraction.

Chloride (Cl^-) is the major extracellular anion.

•      Most prevalent extracellular anion

•      Moves easily between compartments due to Cl- leakage channels

•      Helps balance anions in different compartments

•      Regulation

–   passively follows Na+ so it is regulated indirectly by aldosterone levels

–   ADH helps regulate Cl- in body fluids because it controls water loss in urine

•      Chloride shift across red blood cells with buffer movement

•      It plays a role in forming HCl in the stomach.

Potassium (K⁺) is the most abundant cation in intracellular fluid.

•      It is involved in maintaining fluid volume, impulse conduction, muscle contraction.

•      Exchanged for H+ to help regulate pH in intracellular fluid

•      The plasma level of K⁺ is under the control of mineralocorticoids, mainly aldosterone.

•      Helps establish resting membrane potential & repolarize nerve & muscle tissue

•      Control is mainly by aldosterone which stimulates principal cells to increase K+ secretion into the urine

•      abnormal plasma K+ levels adversely affect cardiac and neuromuscular function

 

Bicarbonate (HCO₃^-) is a prominent ion in the plasma.

•      It is a significant plasma anion in electrolyte balance.

•      It is a major component of the plasma acid-base buffer system.

–   Concentration increases as blood flows through systemic capillaries due to CO₂ released from metabolically active cells

–   Concentration decreases as blood flows through pulmonary capillaries and CO₂ is exhaled

•      Kidneys are main regulator of plasma levels

–   intercalated cells form more if levels are too low

–   excrete excess in the urine

Calcium (Ca⁺²), the most abundant ion in the body, is principally an extracellular ion.

•      It is a structural component of bones and teeth.

•      Important role in blood clotting, neurotransmitter release, muscle tone & nerve and muscle function

•      Regulated by parathyroid hormone

–   stimulates osteoclasts to release calcium from bone

–   increases production of calcitriol (Ca+2 absorption from GI tract and reabsorption from glomerular filtrate)

 

Magnesium (Mg⁺²) is primarily an intracellular cation.

•      It activates several enzyme systems involved in the metabolism of carbohydrates and proteins and is needed for operation of the sodium pump.

•      It is also important in neuromuscular activity, neural transmission within the central nervous system, and myocardial functioning.

•      Several factors regulate magnesium ion concentration in plasma. They include hypo- or hypercalcemia, hypo- or hypermagnesemia, an increase or decrease in extracellular fluid volume, an increase or decrease in parathyroid hormone, and acidosis or alkalosis.

Phosphate

•      Present as calcium phosphate in bones and teeth, and in phospholipids, ATP, DNA and RNA

•      HPO₄ ^-2 is important intracellular anion and acts as buffer of H+ in body fluids and in urine

–   mono and dihydrogen phosphate act as buffers in the blood

•      Plasma levels are regulated by parathyroid hormone & calcitriol

–   resorption of bone releases phosphate

–   in the kidney, PTH increase phosphate excretion

–   calcitriol increases GI absorption of phosphate

Review

•      Table 27.2 describes the imbalances that result from the deficiency or excess of several electrolytes.

Clinical Application

•      Individuals at risk for fluid and electrolyte imbalances include those dependent on others for fluid and food needs; those undergoing medical treatment involving intravenous infusions, drainage or suction, and urinary catheters, those receiving diuretics, and post-operative individuals, burned individuals, individuals with chronic disease, and those with altered states of consciousness.

Acid-Base Balance

•      The overall acid-base balance of the body is maintained by controlling the H⁺ concentration of body fluids, especially extracellular fluid.

•      Homeostasis of H+ concentration is vital

–   proteins 3-D structure sensitive to pH changes

–   normal plasma pH must be maintained between  7.35 - 7.45

–   diet high in proteins tends to acidify the blood

•      3 major mechanisms to regulate pH

–   buffer system

–   exhalation of CO₂ (respiratory system)

–   kidney excretion of H+  (urinary system)

Actions of Buffer Systems

•      Prevent rapid, drastic changes in pH

•      Change either strong acid or base into weaker one

•      Work in fractions of a second

•      Found in fluids of the body

•      3 principal buffer systems

–   protein buffer system

–   carbonic acid-bicarbonate buffer system

–   phosphate buffer system

 

Protein Buffer System

•      Abundant in intracellular fluids & in plasma

–   hemoglobin very good at buffering H+ in RBCs

–   albumin is main plasma protein buffer

•      Amino acids contains at least one carboxyl group     (-COOH) and at least one amino group (-NH₂)

–   carboxyl group acts like an acid & releases H+

–   amino group acts like a base & combines with H+

–   some side chains can buffer H+

•      Hemoglobin acts as a buffer in blood by picking up CO₂ or H+

Carbonic Acid-Bicarbonate Buffer System

•      Acts as extracellular & intracellular buffer system

–   bicarbonate ion (HCO₃-) can act as a weak base

•   holds excess H+

–   carbonic acid (H₂CO₃) can act as weak acid

•   dissociates into H+ ions

•      At a pH of 7.4, bicarbonate ion concentration is about 20 times that of carbonic acid

•      Can not protect against pH changes due to respiratory problems

Phosphate Buffer System

•      Most important intracellularly, but also acts to buffer acids in the urine

•      Dihydrogen phosphate ion acts as a weak acid that can buffer a strong base

•      Monohydrogen phosphate acts a weak base by buffering the H+ released by a strong acid

 

Exhalation of Carbon Dioxide

•      The pH of body fluids may be adjusted by a change in the rate and depth of respirations, which usually takes from 1 to 3 minutes.

•      An increase in the rate and depth of breathing causes more carbon dioxide to be exhaled, thereby increasing pH.

•      A decrease in respiration rate and depth means that less carbon dioxide is exhaled, causing the blood pH to fall.

•      The pH of body fluids, in turn, affects the rate of breathing (Figure 27.7).

•      The kidneys excrete H⁺ and reabsorb HCO₃^- to aid in maintaining pH.

Exhalation of Carbon Dioxide

•      pH modified by changing rate & depth of breathing

–   faster breathing rate, blood pH rises

–   slow breathing rate, blood pH drops

•      H+ detected by chemoreceptors in medulla oblongata, carotid & aortic bodies

•      Respiratory centers inhibited or stimulated by changes is pH

Kidney Excretion of H+

•       Metabolic reactions produce 1mEq/liter of nonvolatile acid for every kilogram of body weight

•       Excretion of H+ in the urine is only way to eliminate huge excess

•       Kidneys synthesize new bicarbonate and  save filtered bicarbonate

•       Renal failure can cause death rapidly due to its role in pH balance

Regulation of Acid-Base Balance

•       Cells in the PCT and collecting ducts secrete hydrogen ions into the tubular fluid.

•       In the PCT Na⁺/H⁺ antiporters secrete H⁺ and reabsorb Na⁺ (Figure 26.13).

•       The apical surfaces of some intercalated cells include proton pumps (H⁺  ATPases) that secrete H⁺  into the tubular fluid and HCO₃^– antiporters in their basolateral membranes to reabsorb HCO₃^–  (Figure 27.8).

•       Other intercalated cells have proton pumps in their basolateral membranes and Cl^–/HCO₃^– antiporters in their apical membranes.

•       These two types of cells help maintain body fluid pH by excreting excess H⁺  when pH is too low or by excreting excess HCO₃^– when the pH is too high.

•       Table 27.3 summarizes the mechanism that maintains pH of body fluids.

Acid-Base Imbalances

•      The normal pH range of systemic arterial blood is between 7.35-7.45.

•      Acidosis is a blood pH below 7.35. Its principal effect is depression of the central nervous system through depression of synaptic transmission.

•      Alkalosis is a blood pH above 7.45. Its principal effect is overexcitability of the central nervous system through facilitation of synaptic transmission.

Acid-Base Imbalances

•      Compensation is an attempt to correct the problem

–   respiratory compensation

–   renal compensation

•      Acidosis causes depression of CNS---coma

•      Alkalosis causes excitability of nervous tissue---spasms, convulsions & death

Acid-Base Imbalances

•      Compensation refers to the physiological response to an acid-base imbalance.

•      Respiratory acidosis and respiratory alkalosis are primary disorders of blood P_CO2.

•      metabolic acidosis and metabolic alkalosis are primary disorders of bicarbonate concentration.

 

•      A summary of acidosis and alkalosis is presented in Table 27.4.

Diagnosis

•      Diagnosis of acid-base imbalances employs a general four-step process.

–   Note whether the pH is high or low relative to the normal range.

–   Decide which value of P_CO2 or HCO₃^- could cause the abnormality.

–   Specify the problem source as respiratory or metabolic.

–   Look at the noncausative value and determine if it is compensating for the problem.

Summary of Causes

•      Respiratory acidosis & alkalosis are disorders involving changes in partial pressure of CO₂ in blood

•      Metabolic acidosis & alkalosis are disorders due to changes in bicarbonate ion concentration in blood

Respiratory Acidosis

•      Cause is elevation of pCO₂ of blood

•      Due to lack of removal of CO₂ from blood

–   emphysema, pulmonary edema, injury to the brainstem & respiratory centers

•      Treatment

–   IV administration of bicarbonate (HCO₃-)

_–     ventilation therapy to increase exhalation of CO₂

Respiratory Alkalosis

•      Arterial blood pCO₂ is too low

•      Hyperventilation caused by high altitude, pulmonary disease, stroke, anxiety, aspirin overdose

•      Renal compensation involves decrease in excretion of H+ and increase reabsorption of bicarbonate

•      Treatment

–   breathe into a paper bag

Metabolic Acidosis

•      Blood bicarbonate ion concentration too low

–   loss of ion through diarrhea or kidney dysfunction

–   accumulation of acid (ketosis with dieting/diabetes)

–   kidney failing to remove H+ from protein metabolism

•      Respiratory compensation by hyperventilation

•      Treatment

–   IV administration of sodium bicarbonate

–   correct the cause

 

Metabolic Alkalosis

•      Blood bicarbonate levels are too high

•      Cause is nonrespiratory loss of acid

–   vomiting, gastric suctioning, use of diuretics, dehydration, excessive intake of alkaline drugs

•      Respiratory compensation is hypoventilation

•      Treatment

–   fluid and electrolyte therapy

–   correct the cause

 

Diagnosis of Acid-Base Imbalances

•      Evaluate

–   systemic arterial blood pH

–   concentration of bicarbonate (too low or too high)

–   P_CO2 (too low or too high)

•      Solutions

–   if problem is respiratory, the pCO₂ will not be normal

–   <