Functions of the Urinary System

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Functions of the Urinary System

• Excretion.
• Regulation of blood volume and pressure.
• Regulation of the concentration of solutes in the
  blood.
• Regulation of pH of extracellular fluid.
• Regulation of red blood cell synthesis.
• Vitamin D synthesis.

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Internal Anatomy of the Kidney

• Cortex outer area
• Medulla inner area
• Renal pelvis
• Ureter

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Internal Anatomy of the Kidney

• Cortex outer area
• Medulla inner area
• Renal pelvis
• Ureter
• Urinary Bladder
• Urethra

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Nephron

• Glomerulus
• Bowmans capsule
• Proximal Tubule
• Loop of Henle
• Descending limb, Ascending limb
• Distal Tubule
• Collecting Duct

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Glomerular Filtration Rate

• Volume filtered per unit time
• Averages 180 l/day 125 ml/min
• Total plasma volume 3 l
• Therefore total plasma filtered 60x / day

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Micturition

• Urination
• Kidney ? Ureter ? Bladder ? Urethra
• Detrusor muscle
• Internal Urethral Sphincter
• External Urethral Sphincter

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Micturition Process

• As bladder fills, ? pressure
• ? pressure ? stimulates stretch receptors
• Sensory fibers enter the spinal cord
• Parasympathetic neurons are stimulated
• Sympathetic neurons are inhibited

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Micturition Process

• Stimulation of parasympathetic neurons ?
  contraction of detrusor muscle
• Inhibition of sympathetic neurons relaxes ?
  internal urethral sphincter
• Somatic input to external urethral sphincter is
  also inhibited by a reflexive action

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Micturition Process

• Result

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Micturition Process

• Result urination!

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Micturition Process

• Result urination!
• All of these actions are reflexive
• Central nervous system does have a certain degree
  of control (thankfully!)
• We can also voluntarily initiate or prevent
  urination

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Sodium (Na) Water (H2O) Reabsorption
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Sodium (Na) Water (H2O) Reabsorption

• Both Na and H2O are freely filterable into
  Bowmans space.
• Most Na and H2O is reabsorbed.
• Most reabsorption takes place in the proximal
  tubule (65).
• Na reabsorption is an active process.
• H2O reabsorption is a passive process.

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Sodium (Na) Reabsorption

• 2 phases
• Diffusion down concentration gradient across
  luminal membrane.
• Active transport across basolateral membrane by
  Na/K pump.

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Water (H2O) Reabsorption

• Relies on the movement of Na.
• Na moves from tubule to interstitial fluid.
• Osmolarity of tubular fluid decreases (? water
  concentration).
• Osmolarity of interstitial fluid increases(?
  water concentration).

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Water (H2O) Reabsorption

• As a result, water will move from an area of high
  concentration to an area of low concentration.
• Therefore, there will be a net diffusion of water
  out of the tubule.

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Water (H2O) Reabsorption

• The permeability of the tubular wall varies
  throughout the nephron.
• Due to the presence of protein water channels
  called aquaporins.

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Water (H2O) Reabsorption

• Example Collecting Duct
• Permeability depends on the peptide hormone
  produced by the posterior pituitary known as
  vasopressin (aka anti-diuretic hormone).
• Vasopressin stimulates the insertion of
  aquaporins into the luminal membrane of the
  collecting duct. (? H2O reabsorption).

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Urine Concentration

• Hypoosmotic Total solute concentration less than
  that of normal extracellular fluid.
• Isoosmotic Having the same solute concentration
  as extracellular fluid.
• Hyperosmotic Total solute concentration greater
  than that of normal extracellular fluid.

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Urine Concentration

• When vasopressin is high, urine volume is small.
• The urine is also very concentrated
  (hyperosmotic).
• How does the kidney do this?
• Answer

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Urine Concentration

• When vasopressin is high, urine volume is small.
• The urine is also very concentrated
  (hyperosmotic).
• How does the kidney do this?
• Answer Countercurrent Multiplier System.

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Urine Concentration(Countercurrent Multiplier
System)

• Urine concentration takes place in the medullary
  collecting ducts.
• Interstitial fluid surrounding these ducts is
  very hyperosmotic.
• When vasopressin is present, water diffuses out
  of the ducts into the interstitial fluid and then
  enters the blood.

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Urine Concentration(Countercurrent Multiplier
System)

• How does the interstitial fluid become
  hyperosmotic?
• Answer

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Urine Concentration(Countercurrent Multiplier
System)

• How does the interstitial fluid become
  hyperosmotic?
• Answer The Loop of Henle.

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Urine Concentration(Countercurrent Multiplier
System)

• How does the interstitial fluid become
  hyperosmotic?
• Answer The Loop of Henle.
• The opposing flow in the two limbs of the Loop of
  Henle (countercurrent) creates the hyperosmotic
  interstitial fluid.

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Urine Concentration(Countercurrent Multiplier
System)

• Ascending Limb
• Sodium (Na) and Chloride (Cl-) (i.e. salt) are
  reabsorbed in the ascending limb.
• Ascending limb is also relatively impermeable to
  water, so little water follows the salt.
• Result Interstitial fluid becomes hyperosmotic
  when compared to fluid in ascending limb.

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Urine Concentration(Countercurrent Multiplier
System)

• Descending Limb
• Descending limb is not permeable to sodium (Na)
  and chloride (Cl-) (i.e. salt).
• Descending limb is also highly permeable to
  water.
• Result Net diffusion of water out of the
  descending limb to the more concentrated
  interstitial fluid.

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Urine Concentration(Countercurrent Multiplier
System)

• Descending Limb
• The diffusion will continue until the
  osmolarities inside the descending limb and the
  interstitial fluid are equal.
• This osmolarity is also greater than the
  osmolarity in the ascending limb.
• This is the essence of the system.

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Urine Concentration(Countercurrent Multiplier
System)Multiplication

• Multiplication refers to the fact that the
  osmolarity difference at each horizontal level is
  multiplied to a much higher value at the bend
  in the loop.
• Result Concentrated medullary interstitial fluid.

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Urine ConcentrationDistal Tubule

• The fluid entering the distal tubule is more
  dilute (hypoosmotic) than the plasma.
• The fluid becomes even more dilute while it
  passes through the distal tubule because sodium
  and chloride are pumped out and the tubule is
  relatively impermeable to water.

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Urine ConcentrationCollecting Duct

• The hypoosmotic fluid then enters the cortical
  collecting duct.
• From here on, vasopressin is crucial.
• When vasopressin is present, water is reabsorbed
  until it becomes isoosmotic to the plasma in
  peritubular capillaries (300 mOsmol/L).

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Urine ConcentrationCollecting Duct

• The tubular fluid then enters the medullary
  collecting duct.
• In the presence of vasopressin, water diffuses
  out of the duct into the interstitial fluid due
  to the high osmolarity set up by the
  countercurrent multiplier system.
• This water eventually ends up in the blood.

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Urine ConcentrationCollecting Duct

• The final urine is hyperosmotic.
• The kidneys retain as much water as possible,
  minimizing the rate at which dehydration occurs
  during water deprivation.
• Remember, the kidney relies on vasopressin for
  its function.

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Urine ConcentrationCollecting Duct

• If vasopressin concentration is low, cortical and
  medullary collecting ducts are relatively
  impermeable to water.
• Result

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Urine ConcentrationCollecting Duct

• If vasopressin concentration is low, cortical and
  medullary collecting ducts are relatively
  impermeable to water.
• Result Large volumes of hypoosmotic urine is
  excreted.

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Sodium Regulation

• Sodium is freely filterable into Bowmans Space.
• Total body sodium levels varies by only a few
  percent.
• The body controls the sodium levels reflexively.

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Sodium Regulation

• No specific receptors for sodium.
• Instead, the cardiovascular baroreceptors provide
  feedback for sodium control.
• Baroreceptors respond to pressure changes in the
  cardiovascular system.
• Pressure changes in cardiovascular system are
  linked to sodium levels.
• Low cardiovascular pressures are sensed by
  baroreceptors.

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Sodium Regulation

• Low total-body sodium leads to low cardiovascular
  pressures.
• Low cardiovascular pressures are sensed by
  baroreceptors.
• Result

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Sodium Regulation

• Low total-body sodium leads to low cardiovascular
  pressures.
• Low cardiovascular pressures are sensed by
  baroreceptors.
• Result Lower GFR and increase sodium
  reabsorption.

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Control of Sodium Reabsorption

• More important for long-term regulation of sodium
  levels.
• Major factor in control of sodium levels is the
  hormone aldosterone.

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Aldosterone

• Steroid hormone produced in the adrenal cortex.
• Stimulates reabsorption of sodium by the cortical
  collecting ducts.
• When a person eats a lot of sodium, aldosterone
  secretion is low, and vice versa.

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Aldosterone

• What controls the secretion of aldosterone?
• Answer

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Aldosterone

• What controls the secretion of aldosterone?
• Answer Another hormone called angiotensin II.

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Aldosterone

• What controls the secretion of aldosterone?
• Answer Another hormone called angiotensin II.
• Angiotensin II acts directly at the adrenal
  cortex, stimulating the secretion of aldosterone.

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Renin-Angiotensin System

• Figure 26.19 (page 991)
• Renin is an enzyme secreted by the
  juxtaglomerular cells.
• Renin splits a small peptide (angiotensin I) from
  a larger protein called angiotensinogen (produced
  by the liver).

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Renin-Angiotensin System

• Angiotensin I is converted to angiotensin II by
  another enzyme called angiotensin converting
  enzyme.
• Angiotensin II then stimulates the adrenal cortex
  to secrete aldosterone.
• Therefore, the main determining factor in the
  production of angiotensin II is renin.

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Juxtaglomerular cells
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Juxtaglomerular cells

• Three inputs to the juxtaglomerular cells
• Renal sympathetic nerves
• Intrarenal receptors
• Macula densa

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Renal Sympathetic Nerves

• ? plasma volume
• ?
• ? cardiovascular pressure
• ?
• ? renal sympathetic nerve activity
• ?
• ? renin secretion

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Intrarenal Baroreceptors

• ? blood pressure in kidneys
• ?
• ? stretching of juxtaglomerular cells
• ?
• juxtaglomerular cells secrete less renin

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Macula Densa

• Located near the end of the ascending loops of
  Henle and the distal tubule.
• Senses the sodium concentration in the tubular
  fluid flowing past it.

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Macula Densa

• ? arterial blood pressure
• ?
• ? GFR
• ?
• ? salt concentration (Na Cl-) in tubular fluid
• ?
• ? renin secretion

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Summary
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Renal Water Regulation

• Total-body water levels are regulated mainly by
  reflexes
• The reflexes alter the secretion of vasopressin.

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Vasopressin

• Vasopressin is produced by a group of neurons in
  the hypothalamus that terminate in the posterior
  pituitary.
• Vasopressin is then released into the blood from
  the posterior pituitary.
• The most important inputs for this release are
  from baroreceptors and osmoreceptors.

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Baroreceptor Control of Vasopressin

• ? plasma volume
• ?
• ? blood pressure
• ?
• ? firing rate of cardiovascular baroreceptors
• ?
• ? vasopressin secretion (posterior pituitary)
• ?
• ? plasma vasopressin
• ?
• ? H2O reabsorption (collecting ducts)
• ?
• ? H2O excretion

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Osmoreceptor Control of Vasopressin

• Osmoreceptors are responsive to changes in
  osmolarity.
• Located in the hypothalamus.

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Osmoreceptor Control of Vasopressin

• ? H2O ingested
• ?
• ? body-fluid osmolarity (? H2O concentration)
• ?
• ? firing rate of hypothalamic osmoreceptors
• ?
• ? vasopressin secretion (posterior pituitary)
• ?
• ? plasma vasopressin
• ?
• ? H2O reabsorption (collecting ducts)
• ?
• ? H2O excretion

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Hydrogen Ion Regulation

• Kidneys are ultimately responsible for balancing
  hydrogen ion gains/losses.
• Kidneys excrete excess hydrogen ions or retain
  hydrogen ions to replenish supplies.
• Uses bicarbonate to do this.

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Hydrogen Ion Regulation

• Excretion of bicarbonate in urine results in an
  increase in plasma H.
• This occurs during alkalosis.
• Addition of bicarbonate to plasma decreases
  plasma H.
• This occurs during acidosis.

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Fluid Replacement During Exercise

• During prolonged exercise in the heat, people can
  become dehydrated at a rate of 1-2 L every hour
  (about 2-4 lbs of body weight loss per hour).
• Even a slight amount of dehydration causes
  physiological consequences.

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Fluid Replacement During Exercise

• For example, every liter (2.2 lbs) of water lost
  will cause
• Heart rate to be elevated by about eight beats
  per minute
• Cardiac output to decline by 1 L/min
• Core temperature to rise by 0.3o C when an
  individual participates in prolonged exercise in
  the heat.

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Fluid Replacement During Exercise

• People should attempt to drink fluids at close to
  the same rate that they are losing body water by
  sweating.

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Fluid Replacement During Exercise

• Unfortunately, runners generally drink only
  300-500 mL of fluids per hour and thus allow
  themselves to become dehydrated at rates of
  500-1,000 mL/h.
• Dehydration compromises cardiovascular function
  and places the runner at risk for heat-related
  injury.

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Fluid Replacement During Exercise

• So, the runner must ask him/herself the question
• Will the time I lose by drinking larger volumes
  of fluid be compensated for by the physiological
  benefits the extra fluid produces that may cause
  me to run faster during the last half of the race?

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Fluid Replacement During Exercise

• The prevalent thinking from the turn of the
  century until the 1970's was that participants in
  endurance sports did not need to replace fluids
  lost during exercise.
• However, we know now that drinking fluids reduces
  the increase in body temperature (hyperthermia)
  and the amount of stress on the cardiovascular
  system, especially when exercising in hot
  environments.

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Fluid Replacement During Exercise

• However, many do not appreciate the extent to
  which even a slight degree of dehydration
  adversely affects bodily function during
  exercise.
• Adding carbohydrate and salt to water provides
  added benefit.
• The volume of fluid that most athletes choose to
  drink voluntarily during exercise replaces less
  than one-half of their body fluid losses.

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Fluid Replacement During Prolonged Exercise

• Undoubtedly, the most serious consequence of
  inadequate fluid replacement, i.e., dehydration,
  during exercise is hyperthermia.
• When severe, hyperthermia will cause heat
  exhaustion, heat stroke, and even death.

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Fluid Replacement During Prolonged Exercise

• The risks of too much fluid ingestion are
• Gastrointestinal discomfort.
• Reduced pace during competition associated with
  the physical difficulty of drinking large volumes
  of fluid while exercising.
• The benefits of fluid ingestion are
• Reduced cardiovascular stress.
• Reduced hyperthermia which could improve exercise
  performance.

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Difficulties in Drinking Large Volumes of Fluids
While Running

• Large gastric volumes will no doubt cause
  discomfort in some runners.
• Therefore, in runners, it remains to be
  determined if the performance benefits of high
  rates of fluid replacement outweigh the
  discomfort it may cause.

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Difficulties in Drinking Large Volumes of Fluids
While Running

• Many marathon runners allow themselves to become
  dehydrated to some extent because they feel their
  stomachs cannot tolerate the large volumes of
  fluid that must be drunk to totally offset sweat
  losses.
• In general, most runners drink less than about
  500 mL of fluid per hour.

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Difficulties in Drinking Large Volumes of Fluids
While Running

• Sweat rates often average 1,000-1,500 mL/h.
• Marathon runners commonly become dehydrated at a
  rate of 500-1,000 mL/h, although dehydration
  rates can be much higher when the fastest runners
  compete in hot environments.

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Difficulties in Drinking Large Volumes of Fluids
While Running

• Unfortunately, drinking large volumes of fluid
  cost the runner additional seconds in approaching
  the aid-station table and in attempting to drink
  and breathe while running.
• Furthermore, the added gastrointestinal
  discomfort may cause the competitor to run at a
  slower pace until the discomfort subsides.

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Difficulties in Drinking Large Volumes of Fluids
While Running

• The runner is faced with the same important
  question
• Will the time lost while drinking larger volumes
  of fluid will be compensated for by the
  physiological benefits the extra fluid produces
  that may cause me to run faster during the last
  half of the race?

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Difficulties in Drinking Large Volumes of Fluids
While Running

• However, if the goal is safety, which means
  minimizing hyperthermia, it is clear that the
  closer that the rate of drinking can match the
  rate of dehydration, the better.

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Low Intensity Exercise and Fluid Replacement

• It has been known for over 60 years that fluid
  ingestion during prolonged low-intensity exercise
  such as walking and stair stepping controlled
  deep body (core) temperature and improved
  exercise performance.

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Low Intensity Exercise and Fluid Replacement

• Fluid ingestion equal to the rate of sweating was
  more effective than voluntary or partial fluid
  replacement.
• Furthermore, voluntary fluid ingestion during
  low-intensity exercise is more effective in
  attenuating hyperthermia than when fluid intake
  is totally prohibited or is restricted to small
  volumes.

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Low Intensity Exercise and Fluid Replacement

• Thus, during prolonged, low-intensity,
  intermittent exercise, the optimal rate of fluid
  replacement for reducing hyperthermia appears to
  be the rate that most closely matches the rate of
  sweating.

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Hyponatremia in Athletes

• Hyponetremia is a fluid-electrolyte disorder that
  occurs when the sodium level in blood drops below
  normal.
• The proper blood (plasma) sodium level is
  critical for the body to function normally.

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Hyponatremia in Athletes

• Sodium plays a key role in body fluid balance and
  in the conduction of electrical impulses along
  nerves and across cardiac and skeletal muscle.
• For those reasons, the body is well equipped with
  mechanisms that control blood sodium.

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Hyponatremia in Athletes

• When these mechanisms are overwhelmed, blood
  sodium can drop. If blood sodium falls below an
  acceptable level, the individual is considered to
  be hyponatremic.

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Is Hypernatremia Dangerous?

• Hyponatremia is dangerous and can be deadly.
• The danger of hyponatremia is that it disrupts
  the fluid balance across the blood-brain barrier,
  resulting in a rapid influx of water into the
  brain.

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Is Hypernatremia Dangerous?

• This causes brain swelling and a cascade of
  increasingly severe neurological responses
  (headache, malaise, confusion, seizure, coma)
  that, in some cases, can lead to death.
• The faster and lower the blood sodium falls, the
  greater the risk of fatality.

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Is Hypernatremia Dangerous?

• A decrease in plasma sodium concentration to
  125-135 mEq/L is often benign, with either no
  noticeable symptoms or relatively modest
  gastrointestinal disturbances such as bloating or
  mild nausea.

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Is Hypernatremia Dangerous?

• Below 125 mEq/L, symptoms include throbbing
  headache, vomiting, wheezy breathing, swollen
  hands and feet, restlessness, unusual fatigue,
  confusion and disorientation.

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Is Hypernatremia Dangerous?

• Below 120 mEq/L, seizure, permanent brain damage,
  respiratory arrest, coma and death become more
  likely. However, some athletes have survived
  hyponatremia of lt115 mEq/L, whereas others have
  died at gt120 mEq/L.

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Is Hypernatremia Dangerous?

• Below 120 mEq/L, seizure, permanent brain damage,
  respiratory arrest, coma and death become more
  likely. However, some athletes have survived
  hyponatremia of lt115 mEq/L, whereas others have
  died at gt120 mEq/L.

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What Causes Hypernatremiain Athletes?

• In athletes, hyponatremia is usually caused by
• excessive drinking.
• sodium loss in sweat.
• kidneys limited capacity to excrete water.
• the combination dilutes the sodium content of the
  extracellular fluid (ECF).

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What Causes Hypernatremiain Athletes?

• The ECF contains most of the sodium in the body.
• Large sodium losses in sweat can increase the
  risk for hyponatremia by reducing the sodium
  content of the ECF.

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What Causes Hypernatremiain Athletes?

• However, it is the combination of excessive
  drinking and large sweat sodium losses that poses
  the greatest threat.
• Excessive drinking increases the risk of
  developing hyponatremia in both athletes and
  non-athletes.

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What Causes Hypernatremiain Athletes?

• Some athletes may drink large volumes of fluid in
  a misguided attempt to stay well hydrated.
• For example, Eichner (2002) reports that a woman
  who experienced hyponatremia during a marathon
  drank 10 liters (10.6 quarts) of fluid the
  previous night.

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What Causes Hypernatremiain Athletes?

• Hyponatremia has occurred in people who have
  tried to dilute their urine (to escape being
  detected for drugs) by drinking large amounts of
  fluid.
• The kidneys' limited capacity to excrete water
  can increase the risk of hyponatremia.

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What Causes Hypernatremiain Athletes?

• Most adults can drink 2 quarts of fluid or more
  an hour, but the most we can lose in urine is
  usually less than 1 quart/hour.
• Researchers have shown that plasma sodium levels
  can quickly plummet when resting subjects
  overdrink water.

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What Causes Hypernatremiain Athletes?

• During exercise, it is even easier for an
  overzealous drinker to overwhelm the kidneys'
  ability to excrete excess water because urine
  production normally declines 20 to 60 percent
  from resting values due to a decrease in kidney
  blood flow.

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What Causes Hypernatremiain Athletes?

• This response helps conserve vital water, but
  increases the risk that excessive drinking will
  lead to hyponatremia.

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Symptoms and Treatment of Hypernatremia

• Watch for a combination of these symptoms,
  especially if you or somebody you know is at risk
  for the condition

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Symptoms and Treatment of Hypernatremia
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Symptoms and Treatment of Hypernatremia

• Seek emergency care for hyponatremia victims. In
  most cases, they will be treated with some
  combination of
• An IV of a concentrated sodium solution
• A diuretic medication to speed water loss
• An anticonvulsive medication in the case of
  seizure.

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What Can Be Doneto Prevent Hypernatremia?

• Educate athletes to avoid excessive drinking of
  any beverage and make sure they have enough
  sodium in their diets.

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What Can Be Doneto Prevent Hypernatremia?

• The goal of drinking during exercise is to
• Keep weight loss (dehydration) to a minimum.
  (Losing weight during exercise means athletes are
  not replacing their fluids properly and are at
  risk for dehydration.)
• Make sure athletes don't gain weight during
  exercise, which is a sure sign of drinking too
  much.

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What Can Be Doneto Prevent Hypernatremia?

• The goal of drinking during exercise is to
• An athlete who weighs more after exercise than
  when he or she started has had too much fluid and
  needs to cut back during the next time.
• Assure they're getting enough sodium to replace
  what they're losing in sweat.

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What Can Be Doneto Prevent Hypernatremia?

• The goal of drinking during exercise is to
• Provide athletes with salty foods and snacks.
• During workouts and competitions, athletes should
  favor a sports drink containing at least 100 mg
  of sodium/8-oz serving, (Gatorade), over water to
  assure an additional intake of sodium that will
  help stabilize the sodium content of the ECF.