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Chapter 26
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Varies with weight, age, and sex: Early embryo (97%) Newborn (77%) Adult male (60%) Adult female (54%) Elderly (45%)
Adipose tissue versus skeletal muscle largely determines adult differences.
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Electrolytes• Cations: sodium, potassium, hydrogen,
magnesium, and calcium• Anions: chloride, bicarbonate, phosphate, and
sulfate
Non-electrolytes• Glucose• Urea• Protein• Lipids• Creatinine
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Exchange between blood and interstitial spaces
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Exchange between extracellular fluids and cells
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Obligatory Water Losses• Skin and lungs• Urine and feces
Fluid intake
Diet
Levels of Anti-diuretic hormone (ADH)
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Dehydration
Hypotonic hydration
Edema
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Regulation of Sodium Balance
Regulation of Potassium Balance
Regulation of Calcium and Phosphate Balance
Regulation of Magnesium Balance
Regulation of Anions
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Aldosterone ANP Baroreceptors Other hormones:
• Estrogen=enhances Na+ reabsorption• Progesterone=decreases Na+ reabsorption• Glucocorticoids=enhances Na+
reabsorption
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75-80% of sodium (NaCl) in renal filtrate is reabsorbed in proximal tubules of kidneys.
Aldosterone aids in actively reabsorbing remaining Na+Cl- in distal convoluted tubule/collecting tubule by increasing tubule permeability; therefore aldosterone promotes both sodium and water retention
Mechanism: • increase in K or decease in Na in blood plasma
renin-angiotensin Mechanism • stimulates adrenal cortex to release aldosterone • aldosterone targeted towards the kidney tubules • increase in Na reabsorption increase in K secretion • restores homeostatic plasma levels of Na and K
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Influences on aldosterone synthesis and release:• Elevated potassium levels in ECF directly stimulates
adrenal cells to secrete aldosterone• Juxtaglomerular apparatus of renal tubes release renin in
response to: decreased stretch (due to decrease in blood pressure) decreased filtrate osmolarity sympathetic nervous system stimulation
Cardiovascular system
As blood volume (and pressure) rises, the baroreceptors in the heart and in the large vessels of the neck and thorax (carotid arteries and aorta) communicate to the hypothalamus
Sympathetic nervous system impulses to kidneys decrease, allowing afferent arterioles to dilate; as the glomerular filtration rate rises, sodium and water output increases (causing pressure diuresis)
Reduced blood volume and pressure results
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Influence of ADH
Amount of water reabsorbed in the distal segments of the kidney tubules is proportional to ADH release (increase in ADH secretion = increase in water resorption)
Osmoreceptors of the hypothalamus sense the ECF solute concentrations and trigger or inhibit ADH release from the pituitary
Mechanism:• decrease in sodium concentration in plasma (decreased
osmolarity)• stimulates osmoreceptors in hypothalamus• stimulates posterior pituitary to release ADH• ADH targeted toward distal and collecting tubules of
kidney• the effect is increased water resorption• plasma volume increases, osmolarity decreases• scant urine produced
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Influence of atrial natriuretic factor (ANF)
Reduces blood pressure and blood volume by inhibiting nearly all events that promote vasoconstriction and sodium and water retention
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The regulatory site of potassium is in the renal tubules
Influence of aldosterone
Influence of plasma potassium concentrations
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Regulation of Potassium Balance Potassium is the chief intracellular cation Relative intracellular-extracellular potassium
concentrations directly affects a cell's resting membrane potential, therefore a slight change on either side of the membrane has profound effects (ie. on neurons and muscle fibers)
Potassium is part of the body's buffer system, which resists changes in pH of body fluids; ECF potassium levels rise with acidosis (decrease pH) as potassium leave cells and fall with alkalosis (increase pH) as potassium moves into cells
Potassium balance is maintained primarily by renal mechanisms
Potassium reabsorption from the filtrate is constant - 10-15% is lost in urine regardless of need; because potassium content of ECF is low (compared to sodium concentration), potassium balance is accomplished by changing amount of potassium secreted into the filtrate; therefore regulated by collecting tubules
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Influence of Parathyroid Hormone
Influence of Calcitonin
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Regulation of Calcium Balance 99% of calcium found in bones as an apatite Calcium needed for blood clotting, nerve
transmission, enzyme activation, etc... Calcium ion concentration is regulated by
interaction of two hormones: parathyroid hormone and calcitonin
Calcium ion homeostasis: effects of PTH and calcitonin
• PTH - released by the parathyroid cells, promotes increase in calcium levels by targeting... bones - PTH activates osteoclasts, which breakdown the
matrix small intestines - PTH enhances intestinal absorption of
calcium ions indirectly by stimulating the kidneys to transform vitamin D to its active form which is a necessary cofactor for calcium absorption
Kidneys - PTH increases calcium reabsorption by renal tubes while simultaneously decreasing phosphate ion reabsorption
• Calcitonin - targets bone to encourage deposition of calcium salts and inhibits bone reabsorption (therefore an antagonist of PTH
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Influence of Parathyroid Hormone• Decreases plasma phosphate
concentrations while increasing calcium concentrations
Influence of Calcitonin• Increases plasma phosphate concentration
while decreasing calcium concentrations
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Magnesium• PTH increases plasma magnesium
concentrations by causing a decrease in the amount of magnesium excreted by the kidneys
Anions• Chloride is indirectly increased by
Aldosterone because it passively follows sodium
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Respiratory acidosis Increased CO2=increased H+=decreased pH Hypoventilation To compensate: increase excretion of H+ or by
increased reabsorption of HCO3-
Respiratory alkalosis Decreased CO2=decreased H+=increased pH Hyperventilation To compensate: decreased H+ excretion or by
decreased reabsorption of HCO3-
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Metabolic acidosis Decreased HCO3
-=increased H+=decreased pH Diarrhea, ketosis, renal dysfunction Hyperventilation
Metabolic alkalosis Increased HCO3
-=decreased H+=increased pH Vomiting, diuretics, alkaline drug use Hypoventilation
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Three Types:Bicarbonate Buffers
Phosphate BuffersProtein Buffers
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Major extracellular buffering system
HCO3- functions as a weak base while
H2CO3 functions as a weak acid.
Example:HCl + NaHCO3- H2CO3 + NaCl
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Important in urine and intracellular buffering systems
However NaH2PO4 acts as the weak acid and Na2HPO4 serves as the weak base.
Example:HCl + Na2HPO4 NaH2PO4 + NaCl
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Most abundant buffering system in the body including intracellular and extracellular compartments.
Carboxyl groups (COOH) and amine groups (NH3) act as either an acid or a base respectively.
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Two Types:Respiratory Buffering System
Renal Buffering System
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Respiratory System• Rising plasma H+ causes deeper, rapid breathing which
decreases CO2 blood thereby decreasing H+ ions.
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Renal System• To counteract acidosis,
H+ is secreted into the renal tubules and excreted in urine or NH4
+ is excreted rather than reabsorbed.
• To counteract alkalosis, bicarbonate ions are secreted into the filtrate and H+ is reabsorbed.
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