Transcript Water, Ions, and Acid/Base Balance

Water, Electrolyte, and Acid-Base
Balance
Function of Water:
Most of cellular activities are performed in
water solutions.
4% TBW
40% TBW
Body Fluid
- makes up
~60% of total
body weight
(TBW)
- distributed
in three fluid
compartments.
16% TBW
4% TBW
40% TBW
Fluid is continually
exchanged between
the three
compartments.
16% TBW
Exchange between
Blood & Tissue Fluid
4% TBW
40% TBW
- determined by
four factors:
capillary blood
pressure
plasma colloid
osmotic
pressure
interstitium
Hydrostatic
Pressure
Interstitium
colloid osmotic
pressure
16% TBW
Exchange between
Blood & Tissue Fluid
4% TBW
40% TBW
- not affected by
electrolyte
concentrations
- Edema = water
accumulation in
tissue fluid
16% TBW
Exchange between
Tissue Fluid &
Intracellular Fluid
4% TBW
40% TBW
- determined by two:
1) intracellular osmotic
pressure
electrolytes
2) interstitial osmotic
pressure
electrolytes
16% TBW
Water Gain
Water is gained from
three sources.
1) food (~700 ml/day)
2) drink – voluntarily
controlled
3) metabolic water (200
ml/day) --- produced as a
byproduct of aerobic
respiration
Routes of water loss
1) Urine – obligatory (unavoidable) and
physiologically regulated, minimum 400
ml/day
2) Feces -- obligatory water loss, ~200 ml/day
3) Breath – obligatory water loss, ~300
ml/day
4) Cutaneous evaporation -- obligatory
water loss, ~400 ml/day
5) Sweat – for releasing heat, varies
significantly
Regulation of Water Intake
- governed by thirst.
blood volume and
osmolarity

peripheral volume sensors
central osmoreceptors

hypothalamus

thirst felt
Regulation of Water Output
- The only physiological control
is through variations in urine
volume.
- urine volume regulated by
hormones
1) ADH
dehydration

blood volume and/or osmolarity

hypothalamic receptors / peripheral
volume sensors

posterior pituitary to release ADH

 H2O reabsorption

Water retention
2) Atrial Natriuretic Factor
 blood volume

atrial volume sensors

atria to release ANF

inhibits Na+ and H2O reabsorption

 water output
Dehydration
-
decrease in body fluid
-
Causes
1) the lack of drinking water
2) excessive loss of body fluid due to:
overheat
diabetes
overuse of diuretics
diarrhea
Edema
- the accumulation of fluid in the
interstitial spaces
caused by:
1) increased capillary filtration,
or
2) reduced capillary reabsorption,
or
3) obstructed lymphatic drainage
ELECTROLYTE BALANCE
Electrolytes = small ions that carry charges
Major cations
Na+
K+
Ca++
H+
Major anions
ClHCO3PO4---
Distribution of Electrolytes
Na+
Ca++
Cl-
Extracellular
space
K+
Cell
PO4---
Na+
Ca++
Sodium
Na+
Cl-
K+ Cell
PO4---
Functions
- involved in generating action membrane
potential of cells
- make a major contribution to extracellular
osmolarity.
Regulation of plasma Na+
1) Aldosterone
plasma Na+

 aldosterone
Na+

renal Na + excretion

 plasma Na +
plasma
2) Renin-angiotensin-II
renin

angiotensin-II

 aldosterone

 renal Na+ excretion

 plasma Na+
Na+
plasma
3) ADH
increases water
reabsorption in
kidneys

water retention

dilute plasma Na+
H2O
Na+
plasma
4) Atrial Natriuretic
Factor
inhibits renal reabsorption
of Na+ and H2O and
the excretion of renin
and ADH

eliminate more sodium
and water

 plasma Na +
Na+
plasma
Na+
Sodium imbalance
hypernatremia
plasma sodium > 145 mEq/L,
hyponatremia
plasma sodium < 130 mEq/L
Potassium
Functions
K+
Na+
Ca++
Cl-
K+ Cell
PO4---
the greatest contributor to intracellular
osmosis and cell volume
- determines the resting membrane potentials
- an essential cofactor for protein synthesis
and some other metabolic processes.
Regulation of Potassium
-
by aldosterone
Aldosterone

stimulates K+
secretion by the kidneys

 Plasma K+
K+
plasma
K+
Potassium Imbalance
hyperkalemia (> 5.5 mEq/L)
hypokalemia (< 3.5 mEq/L)
Na+
Ca++
Chloride
Cl-
K+ Cell
PO4---
- makes a major contribution to extracellular
osmolarity
- required for the formation of stomach acid
(HCl)
Regulation of Cl–
- No direct regulation
- indirectly regulated as an effect of Na+
homeostasis. As sodium is retained or
excreted, Cl– passively follows.
Chloride Imbalance
hyperchloremia (> 105 mEq/L)
hypochloremia (< 95 mEq/L).
Calcium
Na+
Ca++
Cl-
K+ Cell
PO4---
Functions of Ca++
- lends strength to the skeleton
Functions of Ca++
- lends strength to the skeleton
- activates muscle contraction
Excitation
(Action Potentials)
++
[ Ca
]i
Contraction
(shortening)
Functions of Ca++
- lends strength to the skeleton
- activates muscle contraction
- serves as a second messenger for some
hormones and neurotransmitters
Functions of Ca++
- lends strength to the skeleton
- activates muscle contraction
- serves as a second messenger for some
hormones and neurotransmitters
- activates exocytosis
of neurotransmitters and
other cellular secretions
Functions of Ca++
- lends strength to the skeleton
- activates muscle contraction
- serves as a second messenger for some
hormones and neurotransmitters
- activates exocytosis of neurotransmitters
and other cellular secretions
- essential factor
in blood clotting.
Functions of Ca++
- lends strength to the skeleton
- activates muscle contraction
- serves as a second messenger for some
hormones and neurotransmitters
- activates exocytosis of neurotransmitters
and other cellular secretions
- essential factor in blood clotting.
- activates many cellular
enzymes
Dynamics of Calcium
Ca++
Ca++
Ca++
plasma
Ca++
Regulation of calcium
1) parathyroid hormone (PTH):
Regulation of calcium
1) parathyroid hormone (PTH):
- dissolving Ca++ in bones
- reducing renal excretion of Ca++
Ca++
Ca++
plasma
2) calcitonin (secreted by C cells in thyroid
gland):
2) calcitonin (secreted by C cells in thyroid
gland):
depositing Ca++ in bones
Ca++
Ca++
plasma
3) calcitrol (derivative of vitamin D):
- enhancing intestinal absorption of Ca++ from food
Ca++
Ca++
plasma
Ca++
Calcium imbalances
hypocalcemia (< 4.5 mEq/L)
hypercalcemia (> 5.8 mEq/L).
Phosphates
- needed for the synthesis of:
ATP, GTP
DNA, RNA
phospholipids
Regulation of
Phosphate
- by parathyroid hormone
PTH

increases renal excretion
of phosphate

decrease plasma
phosphate
- no real phosphate
imbalances
PO4---
plasma
PO4---
ACID-BASE BALANCE
Acid
An acid is any chemical that releases H+ in
solution.
Base
A base is any chemical that accepts H+.
pH
is the negative logarithm of H+ concentration, and
an indicator of acidity.
pH = - log [H+ ]
Example:
[H+ ] = 0.1 M = 10 –7 M
pH
is the negative logarithm of H+ concentration, and
an indicator of acidity.
pH = - log [10 –7 ] = 7 log 10 = 7
Example:
[H+ ] = 0.1 M = 10 –7 M
pH
is the negative logarithm of H+ concentration, and
an indicator of acidity.
pH = - log [10 –8 ] = 8 log 10 = 8
[H+ ] = 0.01 M = 10 –8 M
Example:
0.01 M [ H+ ]
0.1 M [ H+ ]
= pH 8
= pH 7
 [ H+ ] =  pH
 [ H+ ] =
 pH
Normal functions of proteins (especially
enzymes) heavily depend on an optimal pH.
pH7.35-pH7.45
Regulation of acid-base balance
1) Chemical Buffers
2) Respiratory Control of pH
3) Renal Control of pH
Buffer
is any mechanism that resists changes in
pH.
acid
acid
H2O
pH 7.0
Buffer
pH 7.0
pH 3.0
pH 6.8
base
base
H2O
pH 7.0
Buffer
pH 7.0
pH 11.0
pH 7.2
Chemical Buffers
There are three major buffers in body fluid.
1) The Bicarbonate (HCO3-) Buffer
2) The Phosphate Buffer
3) The Protein Buffer
1) The Bicarbonate (HCO3-) Buffer System
H + HCO3-
H2CO3
H2O
+
CO2
- reversible depending on the equilibrium between
the substrates and products.
- The lungs constantly remove CO2.
2) The Phosphate Buffer System
H + HPO42–
H2PO4– + H
H3PO4
3) The Protein Buffer System
- more concentrated
phosphate buffers
than
either
bicarbonate
or
- accounts for about three-quarters of all chemical
buffering ability of the body fluids.
- The carboxyl groups release H+ when pH rises and
amino groups bind H+ when pH falls.
H+
H+
NH2-CH2-CH2 CH2-CH2-COOH
Properties of Chemical Buffers
- respond to pH changes within a fraction
of a second.
- Bind to H but can not remove H out of
the body
- Limited ability to correct pH changes
1)
H + HCO3-
10
2)
10
H + HCO3-
20
10
3) H + HCO310
0
H2CO3
H2O
10
10
H2CO3
H2O
10
10
H2CO3
H2O
20
10
H2CO3
H2O
10
20
+
CO2
10
+
CO2
10
+
CO2
10
+
CO2
20
Respiratory Control of pH
H + HCO-
H + HCO-
H2CO3
CO2 + H2O
H2CO3
CO2 + H2O
 pH

stimulate peripheral/central chemoreceptors

 pulmonary ventilation

removal of CO2 and  pH
H + HCO3-
H2CO3
H2O + CO2
Limit to respiratory control of pH
The respiratory regulatory mechanism
cannot remove H+ out of the body. Its
efficiency depends on the availability of
HCO3- .
H + HCO3-
H2CO3
H2O
+
CO2
Renal Control of pH
1. The kidneys can neutralize more acid
or base than both the respiratory
system and chemical buffers.
a. Renal tubules secrete hydrogen
ions into the tubular fluid, where most
of it combines with bicarbonate,
ammonia, and phosphate buffers.
b. Bound and free H+ are then
excreted in urine.
2. The kidneys are the only organs
that actually expel H+ from the
body. Other buffering systems only
reduce its concentration by
binding it to another chemical.
3. Tubular secretion of H+ continues
as long as a sufficient
concentration gradient exists
between the tubule cells and the
tubular fluid.
Disorders of Acid-Base Balance
Acidosis: < pH 7.35 , Alkalosis: > pH 7.45
- Mild acidosis
depresses CNS, causing
confusion, disorientation, and coma.
- Mild alkalosis
CNS becomes hyperexcitable.
Nerves fire spontaneously and overstimulate
skeletal muscles.
- Severe acidosis or alkalosis is lethal.
Respiratory vs Metabolic Cause
Respiratory acidosis / alkalosis
-
caused by hypoventilation or hyperventilation
Initial change
H + HCO-
H2CO3
H2O
+
CO2
Emphysema
Respiratory acidosis / alkalosis
-
caused by hypoventilation or hyperventilation
Metabolic acidosis or alkalosis
- result from any causes but respiratory problems
Diabetes

 production of
organic acids

metabolic
acidosis
Chronic vomiting

loss of stomach acid

metabolic alkalosis