Download Fluid, Electrolyte and Acid-Base Balance

Fluid, Electrolyte and AcidBase Balance
Chapter 27
Composition
of the
Human Body
Figure 27–1a
Composition of the Human
Body
Figure 27–1b
Water content varies with age & tissue type
Fat has the lowest water content (~20%).
Bone is close behind (~22 – 25%).
Skeletal muscle is highest at ~65%.
Fluid Compartments
• ECF (extra cellular fluid) and ICF
(intracellular fluid) are called fluid
compartments:
– because they behave as distinct entities
– are separated by cell membranes and active
transport
Water Composition
• Is 60% percent of male body weight
• Is 50% percent of female body weight
• Mostly in intracellular fluid
Water Exchange
• Water exchange between ICF and ECF
occurs across cell membranes by:
– osmosis
– diffusion
– carrier-mediated transport
Major Subdivisions of ECF
• Interstitial fluid of peripheral tissues
• Plasma of circulating blood
Minor Subdivisions of ECF
•
•
•
•
Lymph, perilymph, and endolymph
Cerebrospinal fluid (CSF)
Synovial fluid
Serous fluids (pleural, pericardial, and
peritoneal)
• Aqueous humor
Cations
Body
Fluids
Figure 27–2 (1 of 2)
Anions in
Body
Fluids
Figure 27–2 (2 of 2)
4 Principles of Fluids and Electrolyte
Regulation
1. All homeostatic mechanisms that monitor
and adjust body fluid composition
respond to changes in the ECF, not in
the ICF
2. No receptors directly monitor fluid or
electrolyte balance
4 Principles of Fluids and
Electrolyte Regulation
3. Cells cannot move water molecules by
active transport
4. The body’s water or electrolyte content
will rise if dietary gains exceed
environmental losses, and will fall if
losses exceed gains
Electrolyte concentrations are
calculated in milliequivalents
mEq/L = ion concentration (mg/L) x number of charges on one ion
atomic weight
Na+ concentration in the body is 3300 mg/L
Na+ carries a single positive charge.
Its atomic weight is approximately 23.
Therefore, in a human the normal value for Na+ is:
3300 mg/L = 143 mEq/L
23
Note: One mEq of a univalent is equal to one mOsm whereas one mEq of a bivalent
ion is equal to ½ mOsm. However, the reactivity of 1 mEq is equal to 1 mEq.
Regulation of water balance
• It is not so much water that is regulated, but
solutes.
• osmolality is maintained at between 285 – 300
mOsm.
• An increase above 300 mOsm triggers:
– Thirst
– Antidiuretic Hormone release
3 Primary Regulatory Hormones
•
Affect fluid and electrolyte balance:
1. antidiuretic hormone
2. aldosterone
3. natriuretic peptides
Antidiuretic Hormone (ADH)
• Stimulates water conservation at kidneys:
– reducing urinary water loss
– concentrating urine
• Stimulates thirst center:
– promoting fluid intake
ADH Production
• Osmoreceptors in hypothalamus:
– monitor osmotic concentration of ECF
• Change in osmotic concentration:
– alters osmoreceptor activity
• Osmoreceptor neurons secrete ADH
ADH Release (1 of 2)
• Axons of neurons in anterior
hypothalamus:
– release ADH near fenestrated capillaries
– in posterior lobe of pituitary gland
ADH Release (2 of 2)
• Rate of release varies with osmotic
concentration:
– higher osmotic concentration increases ADH
release
An increase of 2 – 3% in
plasma osmolality triggers
the thirst center of the
hypothalamus.
Secondarily, a 10 – 15%
drop in blood volume also
triggers thirst. This is a
significantly weaker
stimulus.
The Thirst
Mechanism
Aldosterone
• Is secreted by adrenal cortex in response
to:
– rising K+ or falling Na+ levels in blood
– activation of renin–angiotensin system
• Determines rate of Na+ absorption and K+
loss:
– along DCT and collecting system
Dehydration
Chronic dehydration leads to oliguria.
Severe dehydration can result in hypovolemic shock.
Causes include:
•Hemorrhage
•Burns
•Vomiting
•Diarrhea
•Sweating
•Diuresis, which can be caused by diabetes insipidus,
diabetes mellitus and hypertension (pressure diuresis).
Overhydration (Hypotonic hydration)
• Also called water excess
• Occurs when excess water shifts into ICF:
– distorting cells
– changing solute concentrations around
enzymes
– disrupting normal cell functions
Causes of Overhydration
• Ingestion of large volume of fresh water
• Injection into bloodstream of hypotonic
solution
• Endocrine disorders:
– excessive ADH production
Causes of Overhydration
• Inability to eliminate excess water in urine:
– chronic renal failure
– heart failure
– cirrhosis
Signs of Overhydration
• Abnormally low Na concentrations
(hyponatremia)
• Effects on CNS function (water
intoxication)
+
Hyponatremia
• Hyponatremia results in:
– Cerebral edema (brain swelling)
– Sluggish neural activity
– Convulsions, muscle spasms, deranged
behavior.
• Treated with I.V. hypertonic mannitol or
something similar.
Hypotonic
hydration
Blood pressure, sodium, and water
Atrial
Naturetic
Peptide:
The heart’s
own
compensatory
mechanism.
Fluid Gains and Losses
Figure 27–3
Water Balance
Table 27–1
Sources of
intake &
output
Importance of
Electrolyte Balance
• Electrolyte concentration directly affects
water balance
• Concentrations of individual electrolytes
affect cell functions
Sodium
• Is the dominant cation in ECF
• Sodium salts provide 90% of ECF osmotic
concentration:
– sodium chloride (NaCl)
– sodium bicarbonate
Normal Sodium Concentrations
• In ECF:
– about 140 mEq/L
• In ICF:
– is 10 mEq/L or less
Potassium
• Is the dominant cation in ICF
Normal Potassium
Concentrations
• In ICF:
– about 160 mEq/L
• In ECF:
– is 3.8–5.0 mEq/L
2 Rules of Electrolyte Balance
1. Most common problems with electrolyte
balance are caused by imbalance
between gains and losses of sodium ions
2. Problems with potassium balance are
less common, but more dangerous than
sodium imbalance
Sodium Balance in ECF
1. Sodium ion uptake across digestive
epithelium
2. Sodium ion excretion in urine and
perspiration
Sodium Balance in ECF
• Typical Na gain and loss:
+
– is 48–144 mEq (1.1–3.3 g) per day
• If gains exceed losses:
– total ECF content rises
• If losses exceed gains:
– ECF content declines
+
Changes in ECF Na Content
• Do not produce change in concentration
• Corresponding water gain or loss keeps
concentration constant
Homeostatic
Regulation of
+
Na
Concentration
s in Body
Fluids
Figure 27–4
+
Na Balance and ECF Volume
• Na regulatory mechanism changes ECF
volume:
+
– keeps concentration stable
• When Na+ losses exceed gains:
– ECF volume decreases (increased water loss)
– maintaining osmotic concentration
Large Changes in ECF Volume
• Are corrected by homeostatic mechanisms
that regulate blood volume and pressure
• If ECF volume rises:
– blood volume goes up
• If ECF volume drops:
– blood volume goes down
Fluid Volume Regulation
+
and Na Concentrations
Figure 27–5 (1 of 2)
Fluid Volume Regulation
+
and Na Concentrations
Homeostatic Mechanisms (1 of
2)
• A rise in blood volume:
– elevates blood pressure
• A drop in blood volume:
– lowers blood pressure
Homeostatic Mechanisms (2 of
2)
• Monitor ECF volume indirectly by
monitoring blood pressure:
– baroreceptors at carotid sinus, aortic sinus,
and right atrium
+
Abnormal Na
Concentrations in ECF
• Hyponatremia:
– body water content rises (overhydration)
– ECF Na+ concentration < 130 mEq/L
• Hypernatremia:
– body water content declines (dehydration)
– ECF Na+ concentration > 150 mEq/L
ECF Volume
• If ECF volume is inadequate:
– blood volume and blood pressure decline
– renin–angiotensin system is activated
– water and Na+ losses are reduced
– ECF volume increases
Plasma Volume
• If plasma volume is too large:
– venous return increases:
• stimulating natriuretic peptides (ANP and BNP)
• reducing thirst
• blocking secretion of ADH and aldosterone
– salt and water loss at kidneys increases
– ECF volume declines
Potassium Balance
• 98% of potassium in the human body is in
ICF
• Cells expend energy to recover potassium
ions diffused from cytoplasm into ECF
Processes of Potassium
Balance
1. Rate of gain across digestive epithelium
2. Rate of loss into urine
Potassium Loss in Urine
• Is regulated by activities of ion pumps:
– along distal portions of nephron and collecting
system
– Na+ from tubular fluid is exchanged for K+ in
peritubular fluid
Potassium Loss in Urine
• Are limited to amount gained by
absorption across digestive epithelium
(about 50–150 mEq (1.9–5.8 g)/day
3 Factors in Tubular
+
Secretion of K
1. Changes in concentration of ECF:
– higher ECF concentration increases rate of
secretion
3 Factors in Tubular
+
Secretion of K
2. Changes in pH:
– low ECF pH lowers peritubular fluid pH
– H+ rather than K+ is exchanged for Na+ in
tubular fluid
– rate of potassium secretion declines
3 Factors in Tubular
+
Secretion of K
3. Aldosterone levels:
– affect K+ loss in urine
– ion pumps reabsorb Na+ from filtrate in
exchange for K+ from peritubular fluid
•
High K+ plasma concentrations stimulate
aldosterone
Electrolyte Balance
Electrolyte Balance
Table 27–2 (2 of 2)
Calcium
• Is most abundant mineral in the body:
– 1–2 kg (2.2–4.4 lb)
– 99% deposited in skeleton
Functions of Calcium Ion
•
•
•
•
Muscular and neural activities
Blood clotting
Cofactors for enzymatic reactions
Second messengers
+
2
(Ca )
Hormones and
Calcium Homeostasis
• Parathyroid hormone (PTH) and calcitriol:
– raise calcium concentrations in ECF
• Calcitonin:
– opposes PTH and calcitriol
Calcium Absorption
• At digestive tract and reabsorption along
DCT:
– are stimulated by PTH and calcitriol
Calcium Ion Loss
• In bile, urine or feces:
– is very small (0.8–1.2 g/day)
– about 0.03% of calcium reserve in skeleton
Hypercalcemia
• Exists if Ca2+ concentration in ECF is
> 11 mEq/L
• Is usually caused by hyperparathyroidism:
– resulting from oversecretion of PTH
• Other causes:
– malignant cancers (breast, lung, kidney, bone
marrow)
– excessive calcium or vitamin D supplementation
Hypocalcemia
• Exists if Ca2+ concentration in ECF is
< 4 mEq/L
• Is much less common than hypercalcemia
• Is usually caused by chronic renal failure
• May be caused by hypoparathyroidism:
– undersecretion of PTH
– vitamin D deficiency
Magnesium (1 of 3)
• Is an important structural component of
bone
• The adult body contains about 29 g of
magnesium
• About 60% is deposited in the skeleton
Magnesium (2 of 3)
• Is a cofactor for important enzymatic
reactions:
– phosphorylation of glucose
– use of ATP by contracting muscle fibers
Magnesium (3 of 3)
• Is effectively reabsorbed by PCT
• Daily dietary requirement to balance
urinary loss:
– about 24–32 mEq (0.3–0.4 g)
Magnesium Ions
+
2
(Mg )
• In body fluids are primarily in ICF:
– Mg2+ concentration in ICF is about
26 mEq/L
– ECF concentration is much lower
Phosphate Ions (1 of 3)
• Are required for bone mineralization
• About 740 g PO43— is bound in mineral
salts of the skeleton
• Daily urinary and fecal losses:
– about 30–45 mEq (0.8–1.2 g)
Phosphate Ions (2 of 3)
• In ICF,
—
3
PO4
is required for:
– formation of high-energy compounds
– activation of enzymes
– synthesis of nucleic acids
Phosphate Ions (3 of 3)
• In plasma,
—
3
PO4 :
– is reabsorbed from tubular fluid along PCT
– stimulated by calcitriol
• Plasma concentration is 1.8–2.6 mEq/L
—
Chloride Ions (Cl )
• Are the most abundant anions in ECF
• Plasma concentration is 100–108 mEq/L
• ICF concentrations are usually low
—
Chloride Ions (Cl )
• Are absorbed across digestive tract with
Na+
• Are reabsorbed with Na+ by carrier
proteins along renal tubules
• Daily loss is small:
– 48–146 mEq (1.7–5.1 g)
Terms
Relating to
Acid–Base
Balance
Strong or Weak
• Strong acids and strong bases:
– dissociate completely in solution
• Weak acids or weak bases:
– do not dissociate completely in solution
– some molecules remain intact
Acidosis
• Physiological state resulting from
abnormally low plasma pH
• Acidemia:
– plasma pH < 7.35
Alkalosis
• Physiological state resulting from
abnormally high plasma pH
• Alkalemia:
– plasma pH > 7.45
Acidosis and Alkalosis
• Affect all body systems:
– particularly nervous and cardiovascular
systems
• Both are dangerous:
– but acidosis is more common
– because normal cellular activities generate
acids
Relationship between
PCO and Plasma pH
2
Figure 27–6
3 Types of Acids in the Body
1. Volatile acids - Can leave solution and
enter the atmosphere (ex: Carbonic acid)
2. Fixed acids - Are acids that do not leave
solution. Once produced they remain in
body fluids until eliminated by kidneys
• Organic acids -
Sulfuric Acid and Phosphoric
Acid
• Are most important fixed acids in the body
• Are generated during catabolism of:
– amino acids
– phospholipids
– nucleic acids
Organic Acids
• Produced by aerobic metabolism:
– are metabolized rapidly
– do not accumulate
• Produced by anaerobic metabolism (e.g.,
lactic acid)
– build up rapidly
What’ a “buffer”
• Acids are proton donors
• Bases are proton acceptors
– Strong acids & bases dissociate completely
• Acid buffer systems are comprised of
compounds that resist pH changes by
accepting protons from solutions
containing strong acids.
• Base buffer systems accept OH- ions from
solutions. (Not discussed in the text).
Buffers
• Are dissolved compounds that stabilize
pH:
– by providing or removing H+
• Weak acids:
– can donate H+
• Weak bases:
– can absorb H+
A Buffer System
• Consists of a combination of:
– a weak acid
– and the anion released by its dissociation
• The anion functions as a weak base
Buffer
Systems in
Body Fluids
Figure 27–7
3 Major Buffer Systems
1. Protein buffer systems:
– help regulate pH in ECF and ICF
– interact extensively with other buffer systems
2. Carbonic acid–bicarbonate buffer system:
– most important in ECF
3. Phosphate buffer system:
– buffers pH of ICF and urine
Kidney Tubules and pH Regulation
Kidney Tubules and pH Regulation
Generation of new bicarbonate using glutamine
Figure 27–10b
Kidney Tubules and pH Regulation
Excretion of bicarbonate
Figure 27–10c
Respiratory Acid–Base Regulation
Respiratory Acid–Base
Regulation
Figure 27–12b
Metabolic
Alkalosis
Figure 27–14
Diagnostic
Chart for
Acid-Base
Disorders
Figure 27–15 (1 of 2)
Diagnostic Chart
for
Acid–Base
Disorders
Blood Chemistry and
Acid–Base Disorders
Table 27–4
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