Transcript Water , Electrolyte and Acid - Base Balance - A

Water, Electrolyte and AcidBase Balance
Chapter 21
• Balance – a state of equilibrium –
substances are maintained in the right
amounts and in the right place in the body
Water Balance
• Osmosis is the primary method of water
movement into and out of body fluid
compartments.
• Osmosis is the net movement of water
molecules through a selectively permeable
membrane from an area of high water
concentration to an area of lower water
concentration.
• The concentration of solutes determines
the direction of water movement.
• Most solutes in the body are electrolytes –
inorganic compounds which dissociate into
ions in solution.
• “Where sodium goes, water follows.”
• About 40 Liters (10.56 gallons) of body
water
• Babies – 75% water
• Men – 63 %
• Women – 52%
Fluid compartments
• Separated by selectively permeable
membranes
• Intracellular – 2/3 (63%) of total body water
• Extracellular – 1/3 (37%)
– Interstitial fluid – 80 % of extracellular water
– Blood plasma – 20 % of extracellular water
Composition of compartments
• Extracellular fluids:
– High in Na+, Cl-, Ca++, HCO3• Blood plasma has more protein than
interstitial fluid and lymph
• Intracellular fluids:
– High in K+, phosphate, Mg++, and more
protein than plasma
Movement of water
• Hydrostatic pressure – pressure of fluids
• Osmotic pressure – solute concentration
(often Na+)
– In blood referred to as colloid osmotic
pressure (COP)
Water intake = Water loss
• Average adult takes in about 2,500 ml/day
• Sources of water:
– Preformed water: 2,300 ml
• Drinking water: 1,500 ml (60%)
• Moist food :
750 ml (30%)
– Water of metabolism: 250 ml (10%)
• Cellular respiration
• Dehydration synthesis
Regulation of water intake
• Main regulator is thirst.
• Dehydration (output>intake) as little as 1%
decrease in body water causes:
– Decreased production of saliva
– Increased blood osmotic pressure –
stimulates osmoreceptors in the
hypothalamus
– Decreased blood volume – renin is produced
• The thirst center in hypothalamus is
stimulated ( or mistakenly, the hunger
center) and person feels thirsty
• Wetting of the mouth and stretching of
stomach or intestines decrease thirst
before we take in too much water.
• Water is absorbed, and blood osmotic
pressure decreases.
Sources of water loss
• Through kidneys in urine – 1500 ml (60%)
• Through intestines 150 ml (6%)
– Can be significant in vomiting and diarhhea
• From skin (sweat) 150 ml (6%)
• From lungs and skin
700 ml (28%)
• Last is called insensible loss
– (menstruation)
Regulation of Water Output
• Through regulating urine formation
• ADH – production stimulated by ↑ blood
tonicity of decrease in volume.
– Acts on distal convoluted tubules and
collecting ducts of kidney – permits
reabsorption of water
• Aldosterone – production is stimulated by
angiotensin II through renin production
– Causes sodium ( and water) to be reabsorbed
• ANP – causes sodium (and water) loss
when pressure in right atrium is too high
Water imbalances
• Dehydration is the imbalance seen most
often.
– Prolonged diarrhea or vomiting
– Excessive sweating
Water toxicity
• If lose water by sweating, we also lose
sodium.
• Rapidly drinking large quantities of water
decreases plasma sodium concentration
initially, then see decrease in ISF as well.
• Water is drawn into cells
• This increases ISF tonicity, and water is
drawn from blood
• Add salt when replacing fluids like this!
Overhydration
• Can occur if I.V. fluids are given too rapidly
or in too large amounts.
• Extra fluid puts strain on heart
• Water that moves back into capillaries
depends on concentration of plasma
proteins.
• Decrease in blood proteins caused by:
– Dietary deficiency in proteins
– Liver failure
– Blockage of lymphatic system
– Increased capillary permeability
• Burns, infection
• Fluid moves from the blood to the
interstitial fluid.
• Get large amounts of fluid in the
intercellular spaces – Edema
• Of the three main compartments (IVF, ICF
and ISF) the interstitial fluid varies the
most.
Edema
• Can be caused by:
– Decrease in plasma proteins
– Retention of electrolytes, esp. Na+
– Increase in capillary blood pressure
Electrolyte Balance
• Cations – positively charged ions
• Anions – negatively charged ions
• Body fluids also contain charged organic
molecules
• Only a small percentage of molecules in
fluids are non-electrolytes: glucose, urea,
creatinine
Functions of electrolytes
• Certain ions control the osmosis of water
between body compartments
• Ions help maintain the acid-base balance
necessary for cellular activity
• Ions carry electric current, which allows for
action potentials and secretion of
neurotransmitters
• Several ions are cofactors needed for the
optimal activity of enzymes
Electrolyte intake
• Food and water
• Produced by metabolism
• Salt craving
Electrolyte loss
• Sweat
• Feces
• Urine
Osmolarity
• The total concentration of dissolved
particles determines osmolarity.
• Glucose – one dissolved particle
• NaCl – dissolves into two particles
• One mole of NaCl = 2 osmoles
• Osmoles/L = osmolarity of solution
Sodium (Na+)
• 90 % of extracellular cations and half the
osmolarity of extracellular solutions
• Necessary for action potentials in nerve &
muscle cells
• Aldosterone increases reabsorption from
DCT and collecting ducts
– ↓ blood volume, ↓ extracellular Na+ ,↑
extracellular K+
• ANP causes loss of Na+
Potassium (K+)
• Most numerous intracellular cation
• Membrane potential and repolarization
• Controlled by aldosterone – causes loss of
K+ in urine
Calcium (Ca++)
• Part of bone, most abundant mineral in
body. 98% of Ca is in bone
• Extracellular cation
• Needed for blood clotting, nerve and
muscle function
• PTH causes reabsorption of bone and
increases reabsorption from G.I tract and
glomerular filtrate
• Calcitonin inhibits osteoclasts and
stimulates osteoblast, so calcium is
removed from blood
Chloride (Cl-)
• Most common extracellular anions
• Cl- diffuses easily between compartments
– can help balance charges (RBC’s)
• Parietal cells in stomach secrete Cl- & H+
• Aldosterone indirectly adjusts Cl- when it
increases the reabsorption of Na+ - Clfollows the Na+
Bicarbonate (HCO3-)
• Part of the body’s chief buffer and
transports CO2 in blood stream.
• CO2 + H2O ↔H2CO3 ↔ H+ + HCO3• The kidneys are the main regulators of
bicarbonate: they form bicarb when levels
are low and excrete it when levels are
high.
Phosphate (HPO42-)
• Like calcium, most of the phosphate is found in
the bones.
• 15% is ionized
• Found in combination with lipids, proteins,
carbohydrates, nucleic acids and ATP.
• Three different forms
• Part of the phosphate buffer system
• PTH causes phosphate to be released from
bones and to be excreted by the kidneys.
Calcitonin removes phosphate by encouraging
bone formation.
Acid-Base Balance
•
•
•
•
pH – negative log of H+ concentration
Affects functioning of proteins (enzymes)
Can affect concentrations of other ions
Modify hormone actions (proteins)
Acid intake
• Foods
• Produced by cellular metabolism
Strengths of Acids and Bases
• Acids and bases that ionize (break apart)
completely are strong acids and bases.
(HCl; NaOH)
• Acids and bases that do not completely
dissociate in solution are weak acids and
bases. (lactic acid, carbonic acid)
• Remember, blood needs to stay between
7.35 and 7.45 for the body to function
properly.
• Since more acids than bases are formed,
pH balance is mainly a matter of
controlling excess H+.
Control of Acid-Base Balance
1. Buffer systems
2. Exhalation of carbon dioxide
3. Kidney excretion
Buffers
• Are pairs of chemical substances that prevent a
sharp change in the pH of a solution.
• Buffers exchange strong acids for weaker acids
that do not release as much H+ and thus
change the pH less.
Bicarbonate Buffer System
•
NaHCO3
+
sodium bicarbonate
H2CO3
carbonic acid
Addition of a strong acid:
HCl + NaHCO3 → H2CO3 + NaCl
Carbonic acid does not dissociate
completely, and pH is changed much less.
• Addition of a strong base:
• NaOH + H2CO3 → NaHCO3 + H2O
• Water dissociates very little, and pH
remains nearly the same.
• Usually the body is called upon to buffer
weaker organic acids, such as lactic acid.
• Carbonic acid is formed, and amount of
bicarbonate ion decreases.
• Blood needs to maintain a 20:1 ratio of
bicarbonate ion : carbonic acid.
• H+ concentration increases slightly
• pH drops slightly
• Carbonic acid is the most abundant acid in
the body because it is constantly being
formed by buffering fixed acids and by:
H2O + CO2 ↔ H2CO3 ↔ H+ + HCO3-
Phosphate Buffer System
• Is present in extracellular and intracellular
fluids, most important in intracellular fluids
and renal tubules.
• H+ + HPO42- → H2PO4monohydrogen
phosphate
dihydrogen
phosphate
• OH- + H2PO4- → H2O + HPO42-
Protein Buffer System
• The most abundant in body cells and
plasma.
• Carboxyl group -COOH ↔ -COO- + H+
• Amino group –NH2 ↔ -NH3+
Respiratory Mechanisms –
Exhalation of CO2
• Because carbonic acid can be eliminated
by breathing out CO2 it is called a volatile
acid.
• Body pH can be adjusted this way in about
1-3 minutes
• pH also affects breathing rate
• Powerful eliminator of acid, but can only
deal with carbonic acid.
Kidney excretion of
+
H
• Metabolic reactions produce large amounts of
fixed acids.
• Kidneys can eliminate larger amounts of acids
than the lungs
• Can also excrete bases
• Can excrete acids while conserving bicarbonate
ion
• Can produce more bicarbonate ion
• Kidneys are the most effective regulators of pH;
if kidneys fail, pH balance fails
The regulators work at different
rates
• Buffers are the first line of defense because
they work almost instantaneously.
• Secondary defenses take longer to work:
– Respiratory mechanisms take several minutes to
hours
– Renal mechanisms may take several days
pH imbalances
• The normal blood pH range is 7.35 – 7.45
• Any pH below this range is considered to be a
condition of acidosis
• Any pH above this range is considered to be a
condition of alkalosis
• The body response to acid-base imbalance is
called compensation: Compensation may be
complete if the blood pH is brought back to
normal, or partial if it is still outside the norms.
Respiratory problems
• Respiratory acidosis is a carbonic acid
excess (blood CO2 is too high)
• Respiratory alkalosis is a carbonic acid
deficit (blood CO2 is too low)
• Compensation would occur through the
kidneys
Metabolic problems
• Metabolic acidosis is a bicarbonate deficit
• Metabolic alkalosis is a bicarbonate excess
• Compensation would occur through changes
in the depth and rate of respiration.