Fluid

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Fluid Electrolyte Balance
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Fluid Balance

• homeostatic value-must be maintained
• food water are taken in
• what is not needed is excreted
• body is in constant flux
• must be a balance between amount of water gained
  amount lost
• Ideally-should cancel each other out
• digestive system-major source of water gain
• urinary system-primary system for fluid removal

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Electrolyte Balance

• homeostatic value-must be maintained
• electrolytes-Cl, Na, K, etc. are ingested
  everyday
• water sodium regulation are integrated
  defending body against disturbances in volume
  osmolarity
• K imbalance
• trouble with cardiac muscle functioning
• Calcium imbalances
• problems with exocytosis, muscle contraction,
  bone formation clotting
• H HCO3- balance
• determines pH or acid-base balance

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Maintaining Fluid Electrolyte Balance

• homeostasis depends on integration of
  respiratory, cardiovascular, renal behavioral
  systems
• primary route for excretion of water
  ions-kidneys
• essential for regulating volume composition of
  fluids
• lungs remove H HCO3- by excreting CO2
• behavioral mechanisms
• thirst salt appetite aid in fluid electrolyte
  balance

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Osmolarity

• number of solute particles dissolved in 1liter of
  water
• reflected in solutions ability to produce
  osmosis alter osmotic properties of a solvent
• depends only on number of non penetrating solute
  particles in solution
• 10 molecules of Na has same osmotic activity as
  10 glucose or 10 amino acid molecules in same
  amount of fluid

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Osmolarity

• important to maintain water balance since water
  can cross most membranes freely
• water balance determines osmolarity
• as osmolarity of ECF (extra cellular fluid)
  changes?water moves into or out of cells?
  changing intracellular volumes cell function
• excess water intake?osmolarity decreases?water
  moves into cells? swell
• Na intake (osmolarity increases)? water moves out
  of cells?shrink
• changes in cell volume impairs cell function
• swelling
• may cause ion channels to open
• changing membrane permeability

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Water

• major constituent of body
• all operations need water as diffusion medium
• to distribute gas, nutrients wastes
• distributed differently among various body
  compartments
• 63-65-intracellular fluid (ICF)
• 35- 37-extracellular fluid (ECF)
• ECF-composed of three parts
• interstitial or tissue fluid-25
• plasma-8
• transcellular fluid-2
• miscellaneous fluids such as CSF, synovial fluid,
  etc.

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Water Balance

• obtained when daily gains losses are equal
• average intake and loss-2.5L each day
• Gains
• metabolism (200ml/day)
• preformed water-food drink
• Losses
• about 1.5L each day lost via urine
• 200ml elmininated with feces
• 300 ml is lost during breathing
• 100 ml in sweat
• 400ml in cutaneous transpiration
• water that diffuses through epidermis
  evaporates
• output through breath cutaneous transpiration
  is insensible water loss

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Regulation of Intake

• Intake-governed mostly by thirst
• Dehydration
• reduces blood volume blood pressure
• raises blood osmolarity
• Detected by thirst center
• hypothalamus
• salivate less?dry mouth? sense of thirst
• ingest water
• cools moistens mouth
• rehydrates blood
• distends stomach?inhibits thirst

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Regulation of Output

• only way to control water output significantly is
  through urine volume
• kidneys cannot completely prevent water loss or
  replace lost water or electrolytes
• changes in urine volume are usually linked to
  adjustments in sodium reabsorption
• where sodium goes water follows
• ADH is one way to control urine volume without
  sodium
• ADH?collecting ducts? synthesize aquaporins
  (water channels)? water can diffuse out of
  duct?water reabsorbed

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Electrolytes

• participate in metabolism
• determine membrane potentials
• affect osmolarity of body fluids
• major cations
• Na, K, Ca H
• major anions
• Cl, HCO3 P
• intracellular fluid contains more K
• extracellular fluid has more Na Cl-

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Sodium

• crucial role in water electrolyte balance
• involved in excitability of neurons muscle
  cells (resting membrane potentials)
• major solute in extracellular fluid
• determines osmolarity of extracellular fluids

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Sodium Balance

• need about 0.5 grams of sodium each day
• typical American consumes 3-7 g/day
• kidneys regulate Na levels
• hormonal mechanisms control Na concentrations
• Aldosterone
• primary role
• ADH
• ANP

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ADH

• NaCl added to body? increased osmolarity?ADH
  (vaopressin) secretion thirst increased
• thirst?drink?
• osmolarity decreases
• ADH?kidneys?
• conserves water by concentrating urine
• increased water reaborption increases BP
• returned to normal with cardiovascular reflexes

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Aldosterone

• Na regulation also mediated by aldosterone
• steroid hormone produced by adrenal cortex
• stimuli-more closely tied to blood volume
  pressure osmolarity than Na
• Hyponatremia hyperkalemia?adrenal
  cortex?aldosterone
• Hypotension? renin?aldosterone secretion

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Aldosterone

• tells kidneys to reabsorb Na in distal tubule
  collecting ducts
• primary target-last 3rd of distal tubule
• increases activity of Na-K ATPase
• target cell-principal cell
• Apical membranes of P cells have Na K leak
  channels
• Aldosterone enters by simple diffusion ? combines
  with membrane receptors ?Na channels increase
  time they remain open
• as intracellular Na increases?Na-K ATPase speeds
  up transport of Na into ECF?net result-rapid
  increase of Na reaborption that does not require
  synthesis of new channels or ATPase proteins
• slower phase of action?newly made channels
  pumps inserted into epithelial cell membranes

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Renin-Angiotensin-Aldosterone

• primary signal for aldosterone release-angiotensin
  II
• component of renin-angiotensin system
• kidneys sense low blood pressure triggers
  specialized cells-juxtaglomerular cells (JG
  cells) in afferent arterioles to produce renin
• ? angiotensinogen? angiotensin I ? angiotensin II
  by ACE-angiotensin converting enzyme-found in
  lungs on endothelium of blood vessels

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Renin-Angiotensin-Aldosterone Path

• Angiotensin II?adrenal cortex? aldosterone?distal
  tubule? reabsorbs Na
• ADH secretion is also stimulated?water
  reabsorption increases
• because aldosterone is also acting to increase Na
  reabsorption, net effect-retention of fluid that
  is roughly same osmolarity as body fluids
• net effect on urine excretion- decrease in amount
  of urine excreted, with lower osmolarity
• Aldosterone?more NaCl reabsorbed in DCT
  collecting ducts?reduces filtrate osmolarity

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Renin-Angiotensin-Aldosterone

• stimuli that begin renin pathway- related
  directly or indirectly to blood pressure
• JG cells are directly sensitive to pressure
  respond to low pressure by releasing renin
• sympathetic neurons are activated by
  cardiovascular control center when blood pressure
  drops?JG cells?renin release
• paracrine feedback from macula densa cells in
  distal tubule? stimulate renin release
• if fluid flow in distal tubule is high?macula
  densa?NO-nitric oxide?inhibits renin release
• GFR or BP low?fluid flow low? macula densa
  cells?NO lowered?JG cells?renin released

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Sodium Blood Pressure

• Na reaborption does not directly raise blood
  pressure
• retention helps stimulate fluid intake volume
  expansion which increases blood volume blood
  pressure

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Angiotensin Blood Pressure

• Angiotensin II has other effects on blood
  pressure
• increases it directly indirectly through 4
  pathways
• activates angiotensin II receptors in
  brain?increases vasopressin secretion?fluid
  retained in kidneys? constricts blood vessels
• Angiotensin II serves to stimulate thirst?expands
  blood volume increases blood pressure
• Vasoconstriction-also stimulated by angiotensin
  II? increases blood pressure without changing
  blood volume
• angiotensin II activates receptors in
  cardiovascular control center?increases
  sympathetic output to heart blood
  vessels?increases cardio output
  vasoconstriction ? increases blood pressure

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ANP

• Na also regulated by ANP
• atrial natriuretic peptide
• peptide hormone made by heart atrial cells
• released when walls of atria are stretched
• ANP enhances Na excretion urinary water loss
• increases GFR by making more surface area
  available for filtration? decreases Na water
  reabsorption in collecting ducts
• indirectly inhibits renin, aldosterone
  vasopressin release

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K Balance

• most abundant cation of ICF
• must be maintained within narrow range
• changes affect resting membrane potentials
• decreased K?hypokalemia?resting membrane
  potential becomes more negative
• increased K?hyperkalemia?more K inside
  cell?depolarization
• Hypokalemia?muscle weakness
• more difficult for hyperpolarized neurons
  muscles to fire action potentials
• very dangerous
• respiratory heart muscle might fail
• Hyperkalemia
• more dangerous of two situations
• depolarization of excitable tissues make them
  more excited initially?cells unable to repolarize
  fully
• become less excitable?action potentials smaller
  than normal ?may lead to cardiac arrhythmias

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Sodium Water Balance

• Na water reabsorption are separately regulated
  in distal nephron
• water does not automatically follow Na
  reabsorption here
• vasopressin (ADH) must be present
• proximal tubule
• water reabsorption automatically follows Na
  reaborption

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Acid-Base Balance

• water must be strictly monitored to keep it at a
  certain pH
• not too acidic or too alkaline
• metabolism depends on functioning enzymes
• very sensitive to changes in pH
• pH changes also disrupt stability of cell
  membranes
• alter protein structure
• normal pH range 7.35 - 7.45
• neutral side

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pH

• measurement of hydrogen ion concentration
• lower pH indicates higher hydrogen
  concentration-higher acidity
• higher pH indicates lower hydrogen
  concentration-higher alkalinity
• pH-below 7.35-acidosis
• pH-above 7.45-alkalosis
• Strong acids dissociate readily in water giving
  up H which lowers pH
• Weak acids ionized slightly
• keep most of hydrogen bound
• bases accept hydrogen ions
• strong base has strong tendency to bind hydrogen
  ions
• raises pH
• weak base binds less hydrogen ions
• less effect on pH

• HNO2      H    NO2

HNO2         H    NO2
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Disruptions of Acid-Base Balance

• pH imbalances produce problems that can be life
  threatening
• intracellular proteins comprising enzymes,
  membrane channels, etc
• very sensitive to pH
• functions of proteins depend on 3-d shape can
  become altered by pH changes
• must balance gain loss of H ions

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Compensations for Acid-Base Imbalances

• Buffers
• first line of defense
• always present
• attempt to suppress changes in H
• Kidneys
• change in rate of hydrogen ion secretion by
  renal tubules
• greatest effect
• requires days to take effect
• Lungs
• can have rapid effect
• cannot change pH as much as urinary system
• change pulmonary ventilation-expel or retaining
  carbon dioxide

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Chemical Buffers

• any substance that can bind or release H ions
  such that they dampen swings in pH
• three major chemical buffer systems of body
• Bicarbonate System
• Phosphate System
• Protein System

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Carbonic Acid-Bicarbonate Buffer System

• most important extracellular buffer system
• CO2 H2O?H2CO3 H HCO3-__
• add H? equation shifts to left?more HCO3 made?
  increases CO2 H2O

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Phosphate Buffer System

• important in buffering ICF urine
• H2PO4?H HPO4
• H HPO4 ? H2PO4

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Protein Buffer System

• involves amino acids accepting or releasing H
•  ? pH COOH ? COO- H
• ? pH NH2 H? NH3 amino group accepts H

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Respiratory Compensation

• change in respiratory rate directly affects
  carbonic acid-HCO3 buffer system
• any change in PCO2 affects H ion HCO3
  concentrations
• increasing or decreasing rate of respiration
  alters pH by lowering or raising PCO2
• PCO2 increases?pH decreases
• PCO2 decreases?pH increases
• excess CO2 ventilation increases to expel more
• low CO2 ventilation is reduced

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Renal Compensation

• slower than buffers or lung compensation
• changes rate of H HCO3 secretion or
  reabsorption in response to changes in pH
• directly-excretes or reabsorbs H ions
• indirectly-changes reabsorption or excretion of
  HCO3
• during times of acidosis renal tubule secretes H
  into filtrate
• HCO3- K blood pH increases
• pH levels-secretion of H ions decreased
  bicarbonates not reclaimed

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Disorders of Acid-Base Balance

• Acidosis
• low pH?neurons less excitable?CNS
  depression?confusion disorientation? coma?death
• Alkalosis
• high pH?neurons hyperexcitable? numbness
  tingling?muscle twitches? tetanus
• Acid-base imbalances fall into two categories
• Respiratory
• Metabolic

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Respiratory Acidosis

• respiratory system cannot eliminate all CO2 made
  by peripheral tissues
• accumulates in ECF? lowers its pH
• primary symptom of hypercapnia-respiratory
  acidosis
• typical cause
• Hypoventilation-low respiratory rate

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Respiratory Alkalosis

• uncommon
• usually due to hyperventilation (plasma PCO2
  decreases)
• can be modulated by breathing into paper bag
  rebreathing exhaled CO2

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Metabolic Acidosis

• due to drop in blood bicarbonate levels drop
• lost due to renal dysfunction
• lost through severe diarrhea
• due to accumulation of non-volatile acids-organic
  acid
• Lactic acidosis
• Ketoacidosis
• generation of large amount of ketone bodies
• occurs during starvation diabetes
• may also be caused by impaired ability to excrete
  H ions at kidneys or by severe HCO3 loss as
  occurs during diarrhea or overuse of laxatives

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Metabolic Alkalosis

• HCO3 ions become elevated
• Rare
• can be due to non respiratory loss of acid
• excessive intake of alkaline drugs
• excessive vomiting causes a loss of HCl.

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Compensations for Decreased pH
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Compensations for Increased pH
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