1. 26
Fluid, Electrolyte, and Acid-Base Balance

2. Infants: 73% or more water (low body fat, low bone mass)
Body Water Content
Infants: 73% or more water (low body fat, low bone mass)
Adult males: ~60% water
Adult females: ~50% water (higher fat content, less skeletal muscle mass)
Adipose tissue least hydrated of all
Water content declines to ~45% in old age
© 2013 Pearson Education, Inc.

3. Two main fluid compartments
Total body water = 40 L
Two main fluid compartments
Intracellular fluid (ICF) compartment: 2/3 in cells
Extracellular fluid (ECF) compartment: 1/3 outside cells
Plasma: 3 L
Interstitial fluid (IF): 12 L in spaces between cells
Usually considered part of IF: lymph, CSF, humors of the eye, synovial fluid, serous fluid, and gastrointestinal secretions
© 2013 Pearson Education, Inc.

4. Intracellular fluid (ICF) Volume = 25 L 40% of body weight Plasma
Figure The major fluid compartments of the body.
Total body water
Volume = 40 L
60% of body weight
Interstitial
fluid (IF)
Volume = 12 L
80% of ECF
Intracellular fluid (ICF)
Volume = 25 L
40% of body weight
Plasma
Volume = 3 L, 20% of ECF
Extracellular
fluid (ECF)
Volume = 15 L
20% of body weight
© 2013 Pearson Education, Inc.

5. Composition of Body Fluids
Water: universal solvent
Solutes: what is dissolved in water
Classified as nonelectrolytes and electrolytes
Nonelectrolytes: most organic
Do not dissociate in water: e.g., glucose, lipids, creatinine, and urea
No charged particles created
© 2013 Pearson Education, Inc.

6. Composition of Body Fluids
Electrolytes
Dissociate into ions in water; e.g., inorganic salts, all acids and bases, some proteins
Ions conduct electrical current
Greater osmotic power than nonelectrolytes
Greatest ability to cause fluid shifts
© 2013 Pearson Education, Inc.

7. Electrolyte Concentration
© 2013 Pearson Education, Inc.

8. Electrolyte Concentration
For single charged ions (e.g. Na+), 1 mEq = 1 mOsm
For bivalent ions (e.g. Ca2+), 1 mEq = 1/2 mOsm
1mEq of either provides same amount of charge
© 2013 Pearson Education, Inc.

9. Extracellular and Intracellular Fluids
Each fluid compartment has distinctive pattern of electrolytes
ECF
All similar
Major cation: Na+
Major anion: Cl–
Except: higher protein, lower Cl– content of plasma
© 2013 Pearson Education, Inc.

10. Extracellular and Intracellular Fluids
ICF:
Low Na+ and Cl–
Major cation: K+
Major anion HPO42–
More soluble proteins than in plasma
© 2013 Pearson Education, Inc.

11. Extracellular and Intracellular Fluids
Electrolytes most abundant solutes in body fluids; determine most chemical and physical reactions
Bulk of dissolved solutes are proteins, phospholipids, cholesterol, and triglycerides
90% in plasma
60% in IF
97% in ICF
© 2013 Pearson Education, Inc.

12. Total solute concentration (mEq/L)
Figure Electrolyte composition of blood plasma, interstitial fluid, and intracellular fluid.
160
140
120
Blood plasma
Interstitial fluid
Intracellular fluid
100
Na+
Sodium
Total solute concentration (mEq/L) 80 K+
Potassium
Ca2+
Calcium
Mg2+
Magnesium 60
HCO3–
Bicarbonate
Cl–
Chloride
HPO42–
Hydrogen
phosphate 40
SO42–
Sulfate 20
Na+ K+
Ca2+
Mg2+
HCO3–
Cl–
HPO42–
SO42–
Protein
anions
© 2013 Pearson Education, Inc.

13. Fluid Movement Among Compartments
Osmotic and hydrostatic pressures regulate continuous exchange and mixing of fluids
Water moves freely along osmotic gradients
All body fluid osmolality almost always equal
Change in solute concentration of any compartment leads to net water flow
 ECF osmolality  water leaves cell
 ECF osmolality  water enters cell
© 2013 Pearson Education, Inc.

14. Fluid Movement Among Compartments
Between plasma and IF across capillary walls
Fluid leaks from arteriole end of capillary, reabsorbed at venule end; lymphatics pick up remaining and return to blood
Between IF and ICF across cell membrane
Two-way osmotic flow of water
Ions move selectively; nutrients, wastes, gases unidirectional
© 2013 Pearson Education, Inc.

15. Lungs Gastrointestinal Kidneys tract Blood O2 CO2 Nutrients H2O, H2O,
Figure Exchange of gases, nutrients, water, and wastes between the three fluid compartments of the body.
Lungs
Gastrointestinal
tract
Kidneys
Blood
plasma O2
CO2
Nutrients
H2O,
Ions
H2O,
Ions
Nitrogenous
wastes O2
CO2
Nutrients
H2O
Ions
Nitrogenous
wastes
Interstitial
fluid
Intracellular
fluid in tissue cells
© 2013 Pearson Education, Inc.

16. Water Balance and ECF Osmolality
Water intake must = water output = ~ 2500 ml/day
Water intake: beverages, food, and metabolic water
Water output: urine (60%), insensible water loss (lost through skin and lungs), perspiration, and feces
© 2013 Pearson Education, Inc.

17. 100 ml Feces 4% Metabolism 10% 250 ml Sweat 8% 200 ml Insensible loss
Figure Major sources of water intake and output.
100 ml
Feces 4%
Metabolism 10%
250 ml
Sweat 8%
200 ml
Insensible loss
via skin and
lungs 28%
Foods 30%
750 ml
700 ml
2500 ml
Urine 60%
Beverages 60%
1500 ml
1500 ml
Average intake
per day
Average output
per day
© 2013 Pearson Education, Inc.

18. Maintenance of Body fluid Osmolality
Osmolality maintained at ~ 280 – 300 mOsm
Rise in osmolality 
Stimulates thirst
ADH release
Decrease in osmolality 
Thirst inhibition
ADH inhibition
© 2013 Pearson Education, Inc.

19. Regulation of Water Intake
Thirst mechanism driving force for water intake
Governed by hypothalamic thirst center
Hypothalamic osmoreceptors detect ECF osmolality; activated by
 Plasma osmolality of 1 – 2%
Dry mouth
Decreased blood volume or pressure
Angiotensin II or baroreceptor input
© 2013 Pearson Education, Inc.

20. Regulation of Water Intake
Drinking water creates inhibition of thirst center
Inhibitory feedback signals include
Relief of dry mouth
Activation of stomach and intestinal stretch receptors
© 2013 Pearson Education, Inc.

21. Figure 26.5 The thirst mechanism for regulating water intake.
ECF osmolality
Plasma volume
(5 –10%)
Blood pressure
Osmoreceptors
in hypothalamus
Saliva
Granular cells
in kidney
Renin-angiotensin-
aldosterone
mechanism
Dry mouth
Angiotensin II
Hypothalamic
thirst center
Sensation of thirst;
person takes a
drink
Water moistens
mouth, throat;
stretches stomach,
intestine
Water absorbed
from GI tract
Initial stimulus
ECF osmolality
Plasma volume
Physiological response
Result
Increases, stimulates
© 2013 Pearson Education, Inc.
Reduces, inhibits

22. Regulation of Water Output
Obligatory water losses
Insensible water loss from lungs and skin
Feces
Minimum daily sensible water loss of 500 ml in urine to excrete wastes
Solute concentration and volume of urine a function of fluid intake, diet, and water loss via other avenues
© 2013 Pearson Education, Inc.

23. Regulation of Water Output: Influence of ADH
Water reabsorption in collecting ducts proportional to ADH release
 ADH  dilute urine and  volume of body fluids
 ADH  concentrated urine; reabsorption of water   volume of body fluids
Hypothalamic osmoreceptors sense ECF solute concentration and regulate ADH accordingly
© 2013 Pearson Education, Inc.

24. Regulation of Water Output: Influence of ADH
Other factors may trigger ADH release
Large changes in blood volume or pressure
E.g.,  BP   ADH release due to blood vessel baroreceptors and renin-angiotensin-aldosterone mechanism
Factors lowering blood volume: intense sweating, vomiting, or diarrhea; severe blood loss; traumatic burns; and prolonged fever
© 2013 Pearson Education, Inc.

25. Figure 26.6 Mechanisms and consequences of ADH release.
ECF osmolality
Na+ concentration
in plasma
Stimulates
Plasma volume
(5–10%), BP
Osmoreceptors
in hypothalamus
Inhibits
Negative
feedback
inhibits
Baroreceptors
in atria and
large vessels
Stimulates
Stimulates
Posterior pituitary
Releases
ADH
Antidiuretic
hormone (ADH)
Targets
Collecting ducts
of kidneys
Effects
Water reabsorption
Results in
ECF osmolality
Plasma volume
Scant urine
© 2013 Pearson Education, Inc.

26. Disorders of Water Balance
Principal abnormalities of water balance
Dehydration
Hypotonic hydration
Edema
© 2013 Pearson Education, Inc.

27. Disorders of Water Balance: Dehydration
Negative fluid balance
ECF water loss due to: hemorrhage, severe burns, prolonged vomiting or diarrhea, profuse sweating, water deprivation, diuretic abuse, endocrine disturbances
Signs and symptoms: "cottony" oral mucosa, thirst, dry flushed skin, oliguria
May lead to weight loss, fever, mental confusion, hypovolemic shock, and loss of electrolytes
© 2013 Pearson Education, Inc.

28. Figure 26.7a Disturbances in water balance.
Slide 1
Excessive
loss of H2O
from ECF 1
ECF osmotic
pressure rises 2
Cells lose
H2O to ECF
by osmosis;
cells shrink 3
Consequences of dehydration. If more water than solutes is lost, cells shrink.
© 2013 Pearson Education, Inc.

29. Figure 26.7a Disturbances in water balance.
Slide 2
Excessive
loss of H2O
from ECF
Excessive
loss of H2O
from ECF 1
Consequences of dehydration. If more water than
solutes is lost, cells shrink.
Consequences of dehydration.
© 2013 Pearson Education, Inc.

30. Figure 26.7a Disturbances in water balance.
Slide 3
Excessive
loss of H2O
from ECF 1
ECF osmotic
pressure rises 2
Consequences of dehydration. If more water than
solutes is lost, cells shrink.
© 2013 Pearson Education, Inc.

31. Figure 26.7a Disturbances in water balance.
Slide 4
Excessive
loss of H2O
from ECF 1
ECF osmotic
pressure rises 2
Cells lose
H2O to ECF
by osmosis;
cells shrink 3
Consequences of dehydration. If more water than
solutes is lost, cells shrink.
© 2013 Pearson Education, Inc.

32. Disorders of Water Balance: Hypotonic Hydration
Cellular overhydration, or water intoxication
Occurs with renal insufficiency or rapid excess water ingestion
ECF osmolality   hyponatremia  net osmosis into tissue cells  swelling of cells  severe metabolic disturbances (nausea, vomiting, muscular cramping, cerebral edema)  possible death
Treated with hypertonic saline
© 2013 Pearson Education, Inc.

33. Figure 26.7b Disturbances in water balance.
Slide 1
Excessive
H2O enters
the ECF 1
ECF osmotic
pressure falls 2
H2O moves
into cells by osmosis; cells swell 3
Consequences of hypotonic hydration (water gain).
If more water than solutes is gained, cells swell.
© 2013 Pearson Education, Inc.

34. Figure 26.7b Disturbances in water balance.
Slide 2
Excessive
H2O enters
the ECF 1
Consequences of hypotonic hydration (water gain).
If more water than solutes is gained, cells swell.
© 2013 Pearson Education, Inc.

35. Figure 26.7b Disturbances in water balance.
Slide 3
Excessive
H2O enters
the ECF 1
ECF osmotic
pressure falls 2
Consequences of hypotonic hydration (water gain).
If more water than solutes is gained, cells swell.
© 2013 Pearson Education, Inc.

36. Figure 26.7b Disturbances in water balance.
Slide 4
Excessive
H2O enters
the ECF 1
ECF osmotic
pressure falls 2
H2O moves
into cells by osmosis; cells swell 3
Consequences of hypotonic hydration (water gain).
If more water than solutes is gained, cells swell.
© 2013 Pearson Education, Inc.

37. Disorders of Water Balance: Edema
Atypical accumulation of IF  tissue swelling (not cell swelling)
Result of  fluid out of blood or  fluid into blood
 fluid out of blood caused by
Increased capillary hydrostatic pressure and permeability
Capillary hydrostatic pressure increased by incompetent venous valves, localized blood vessel blockage, congestive heart failure,  blood volume
Capillary permeability increased by ongoing inflammatory response
© 2013 Pearson Education, Inc.

38.  fluid returning to blood result of
Edema
 fluid returning to blood result of
Imbalance in colloid osmotic pressures, e.g., hypoproteinemia ( plasma protein levels  low colloid osmotic pressure)
Fluids fail to return at venous ends of capillary beds
Results from protein malnutrition, liver disease, or glomerulonephritis
© 2013 Pearson Education, Inc.

39. Also caused by blocked (or surgically removed) lymphatic vessels
Edema
Also caused by blocked (or surgically removed) lymphatic vessels
Cause leaked proteins to accumulate in IF
Colloid osmotic pressure of IF draws fluid from blood
Increases diffusion distance for nutrients and oxygen
Results in low blood pressure and severely impaired circulation
© 2013 Pearson Education, Inc.

40. Electrolytes are salts, acids, bases, some proteins
Electrolyte Balance
Electrolytes are salts, acids, bases, some proteins
Electrolyte balance usually refers only to salt balance
Salts control fluid movements; provide minerals for excitability, secretory activity, membrane permeability
Salts enter body by ingestion and metabolism; lost via perspiration, feces, urine, vomit
© 2013 Pearson Education, Inc.

41. Most abundant cation in ECF
Central Role of Sodium
Most abundant cation in ECF
Sodium salts in ECF contribute 280 mOsm of total 300 mOsm ECF solute concentration
Only cation exerting significant osmotic pressure
Controls ECF volume and water distribution
Changes in Na+ levels affects plasma volume, blood pressure, and ECF and IF volumes
© 2013 Pearson Education, Inc.

42. Na+ leaks into cells; pumped out against its electrochemical gradient
Central Role of Sodium
Na+ leaks into cells; pumped out against its electrochemical gradient
Na+ moves back and forth between ECF and body secretions (e.g., digestive secretions)
Renal acid-base control mechanisms are coupled to sodium ion transport
© 2013 Pearson Education, Inc.

43. Sodium Concentration Versus Sodium Content
Concentration of Na+
Determines osmolality of ECF; influences excitability of neurons and muscles
Remains stable due to water shifts out of or into ICF
Content of Na+
Total body content determines ECF volume and therefore blood pressure
© 2013 Pearson Education, Inc.

44. Table 26.2 Sodium Concentration and Sodium Content
© 2013 Pearson Education, Inc.

45. Regulation of Sodium Balance
No known receptors that monitor Na+ levels in body fluids
Na+-water balance is linked to blood pressure and blood volume control mechanisms
Changes in blood pressure or volume trigger neural and hormonal controls to regulate Na+ content
© 2013 Pearson Education, Inc.

46. Regulation of Sodium Balance: Aldosterone
Regardless of aldosterone presence
65% Na+ reabsorbed in proximal tubules; 25% reclaimed in loops of Henle
Na + never secreted into filtrate
Water in filtrate follows Na+ if ADH is present
 Na+ in urine   water loss
© 2013 Pearson Education, Inc.

47. Aldosterone  decreased urinary output; increased blood volume
By active reabsorption of remaining Na+ in distal convoluted tubule and collecting duct
Also causes increased K+ secretion
© 2013 Pearson Education, Inc.

48. Regulation of Sodium Balance: Aldosterone
Renin-angiotensin-aldosterone mechanism main trigger for aldosterone release
Granular cells of JGA secrete renin in response to
Sympathetic nervous system stimulation
 filtrate osmolality
 stretch (due to  blood pressure) of granular cells
© 2013 Pearson Education, Inc.

49. Regulation of Sodium Balance: Aldosterone
Renin catalyzes production of angiotensin II
Prompts aldosterone release from adrenal cortex
 Na+ reabsorption by kidney tubules
Aldosterone release also triggered by elevated K+ levels in ECF
Aldosterone brings about its effects slowly (hours to days)
© 2013 Pearson Education, Inc.

50. triggers renin release, increasing angiotensin II
Figure Mechanisms and consequences of aldosterone release.
K+ concentration
in the ECF
Body Na+ content
triggers renin release,
increasing angiotensin II
Stimulates
Adrenal cortex
Releases
Aldosterone
Targets
Kidney tubules
Effects
Na+ reabsorption
K+ secretion
Restores
Homeostatic plasma
levels of Na+ and K+
© 2013 Pearson Education, Inc.

51. Regulation of Sodium Balance: ANP
Released by atrial cells in response to stretch ( blood pressure)
Effects
Decreases blood pressure and blood volume
 ADH, renin and aldosterone production
 excretion of Na+ and water
Promotes vasodilation directly and also by decreasing production of angiotensin II
© 2013 Pearson Education, Inc.

52. Atrial natriuretic peptide Na+ and H2O reabsorption
Figure Mechanisms and consequences of ANP release.
Stretch of atria
of heart due to BP
Releases
Negative
feedback
Atrial natriuretic peptide
(ANP)
Targets
JG complex
of the kidney
Hypothalamus and
posterior pituitary
Adrenal cortex
Effects
Effects
Renin release*
ADH release
Aldosterone
release
Angiotensin II
Inhibits
Inhibits
Collecting ducts
of kidneys
Vasodilation
Effects
Na+ and H2O reabsorption
Results in
Blood volume
Results in
Blood pressure
© 2013 Pearson Education, Inc.

53. Influence of other Hormones
Female sex hormones
Estrogens:  NaCl reabsorption (like aldosterone)
 H2O retention during menstrual cycles and pregnancy
Progesterone:  Na+ reabsorption (blocks aldosterone)
Promotes Na+ and H2O loss
Glucocorticoids:  Na+ reabsorption and promote edema
© 2013 Pearson Education, Inc.

54. Cardiovascular Baroreceptors
Baroreceptors alert brain of increases in blood volume and pressure
Sympathetic nervous system impulses to kidneys decline 
Afferent arterioles dilate 
GFR increases 
Na+ and water output increase 
Reduced blood volume and pressure
© 2013 Pearson Education, Inc.

55. Figure Mechanisms regulating sodium and water balance help maintain blood pressure homeostasis.
Systemic
blood pressure/volume
Stretch in afferent
arterioles
Filtrate NaCl
concentration in ascending
limb of nephron loop
Inhibits baroreceptors
in blood vessels
(+)
(+)
(+)
Granular cells
of kidneys
(+)
Sympathetic
nervous system
Release
(+)
Renin
Systemic arterioles
Catalyzes conversion
Causes
Angiotensinogen
(from liver)
Angiotensin I
Vasoconstriction
Results in
Converting enzyme (in lungs)
Peripheral resistance
(+)
Angiotensin II
Posterior pituitary
(+)
(+)
(+)
Releases
Systemic arterioles
Adrenal cortex
ADH (antidiuretic
hormone)
Causes
Secretes
(+)
Vasoconstriction
Aldosterone
Collecting ducts
of kidneys
Results in
Targets
Causes
Peripheral resistance
Distal kidney tubules
Causes
H2O reabsorption
Na+ (and H2O)
reabsorption
Results in
Blood volume
(+) stimulates
Blood pressure
Renin-angiotensin-aldosterone
Mechanism
Neural regulation (sympathetic
nervous system effects)
ADH release and effects
© 2013 Pearson Education, Inc.

56. Regulation of Potassium Balance
Importance of potassium
Affects RMP in neurons and muscle cells (especially cardiac muscle)
 ECF [K+]  RMP  depolarization  reduced excitability
 ECF [K+]  hyperpolarization and nonresponsiveness
© 2013 Pearson Education, Inc.

57. Regulation of Potassium Balance
Hyperkalemia - too much K+
Hypokalemia - too little K+
Both disrupt electrical conduction in heart 
Sudden death
© 2013 Pearson Education, Inc.

58. Regulation of Potassium Balance
K+ part of body's buffer system
H+ shifts in and out of cells in opposite direction of K+ to maintain cation balance, so
ECF K+ levels rise with acidosis
ECF K+ levels fall with alkalosis
Interferes with activity of excitable cells
© 2013 Pearson Education, Inc.

59. Regulation of Potassium Balance
K+ balance controlled in cortical collecting ducts by regulating amount secreted into filtrate
High K+ content of ECF favors principal cell secretion of K+
Kidneys have low ability to retain K+ so when levels low
Principal cells reduce secretion and excretion of K+; type A intercalated cells reabsorb some K+ left in filtrate
© 2013 Pearson Education, Inc.

60. Influence of Plasma Potassium Concentration
Most important factor affecting K+ secretion is its concentration in ECF
High K+ diet   K+ content of ECF  K+ entry into principal cells  K+ secretion
Low K+ diet or accelerated K+ loss reduces its secretion
© 2013 Pearson Education, Inc.

61. Regulation of Potassium Balance
Influence of aldosterone
Stimulates K+ secretion (and Na+ reabsorption) by principal cells
Adrenal cortical cells directly sensitive to K+ content of ECF
Increased K+ in adrenal cortex causes
Release of aldosterone  K+ secretion
Abnormal aldosterone levels severely influence K+ levels
© 2013 Pearson Education, Inc.

62. 99% of body's calcium in bones Ca2+ in ECF important for
Regulation of Calcium
99% of body's calcium in bones
Calcium phosphate salts
Ca2+ in ECF important for
Blood clotting
Cell membrane permeability
Secretory activities
Neuromuscular excitability - most important
© 2013 Pearson Education, Inc.

63. Hypocalcemia   excitability and muscle tetany
Regulation of Calcium
Hypocalcemia   excitability and muscle tetany
Hypercalcemia  inhibits neurons and muscle cells, may cause heart arrhythmias
Calcium balance controlled by parathyroid hormone (PTH) from parathyroid gland
Rarely deviates from normal limits
© 2013 Pearson Education, Inc.

64. PTH promotes increase in calcium levels by targeting
Influence of PTH
PTH promotes increase in calcium levels by targeting
Bones – osteoclasts break down matrix, releasing calcium and phosphate to blood
Kidneys – increases calcium reabsorption; decreases phosphate ion reabsorption
Small intestine – increases calcium absorption (indirectly through stimulation of kidney to activate vitamin D precursor)
© 2013 Pearson Education, Inc.

65. 98% filtered calcium reabsorbed due to PTH
Influence of PTH
98% filtered calcium reabsorbed due to PTH
If ECF calcium levels normal PTH secretion inhibited
75% of filtered phosphates reabsorbed in PCT
PTH inhibits this by decreasing the Tm
Phosphate reabsorption also affected by insulin (increases it) and glucagon (decreases it)
© 2013 Pearson Education, Inc.

66. Figure 16.13 Effects of parathyroid hormone on bone, the kidneys, and the intestine.
Hypocalcemia
(low blood Ca2+)
PTH release from
parathyroid gland
Osteoclast activity
in bone causes Ca2+
and PO43- release
into blood
Ca2+ reabsorption
in kidney tubule
Activation of
vitamin D by kidney
Ca2+ absorption
from food in small
intestine
Ca2+ in blood
Initial stimulus
Physiological response
Result
© 2013 Pearson Education, Inc.

67. Cl– is major anion in ECF
Regulation of Anions
Cl– is major anion in ECF
Helps maintain osmotic pressure of blood
99% of Cl– is reabsorbed under normal pH conditions
When acidosis occurs, fewer chloride ions are reabsorbed
Other anions have transport maximums and excesses are excreted in urine
© 2013 Pearson Education, Inc.

68. Normal pH of body fluids
Acid-base Balance
pH affects all functional proteins and biochemical reactions, so closely regulated
Normal pH of body fluids
Arterial blood: pH 7.4
Venous blood and IF fluid: pH 7.35
ICF: pH 7.0
Alkalosis or alkalemia: arterial pH >7.45
Acidosis or acidemia: arterial pH <7.35
© 2013 Pearson Education, Inc.

69. Most H+ produced by metabolism
Acid-base Balance
Most H+ produced by metabolism
Phosphorus-containing protein breakdown releases phosphoric acid into ECF
Lactic acid from anaerobic respiration of glucose
Fatty acids and ketone bodies from fat metabolism
H+ liberated when CO2 converted to HCO3– in blood
© 2013 Pearson Education, Inc.

70. Concentration of hydrogen ions regulated sequentially by
Acid-base Balance
Concentration of hydrogen ions regulated sequentially by
Chemical buffer systems: rapid; first line of defense
Brain stem respiratory centers: act within 1–3 min
Renal mechanisms: most potent, but require hours to days to effect pH changes
© 2013 Pearson Education, Inc.

71. Acid-base Balance: Chemical Buffer Systems
Strong acids dissociate completely in water; can dramatically affect pH
Weak acids dissociate partially in water; are efficient at preventing pH changes
Strong bases dissociate easily in water; quickly tie up H+
Weak bases accept H+ more slowly
© 2013 Pearson Education, Inc.

72. completely into its ions. A weak acid such as H2CO3 does not
Figure Dissociation of strong and weak acids in water.
A strong acid such as
HCI dissociates
completely into its ions.
A weak acid such as
H2CO3 does not
dissociate completely.
© 2013 Pearson Education, Inc.

73. Chemical Buffer Systems
Chemical buffer: system of one or more compounds that act to resist pH changes when strong acid or base is added
Bind H+ if pH drops; release H+ if pH rises
Bicarbonate buffer system
Phosphate buffer system
Protein buffer system
© 2013 Pearson Education, Inc.

74. Bicarbonate Buffer System
Mixture of H2CO3 (weak acid) and salts of HCO3– (e.g., NaHCO3, a weak base)
Buffers ICF and ECF
Only important ECF buffer
© 2013 Pearson Education, Inc.

75. Bicarbonate Buffer System
If strong acid added:
HCO3– ties up H+ and forms H2CO3
HCl + NaHCO3  H2CO3 + NaCl
pH decreases only slightly, unless all available HCO3– (alkaline reserve) used up
HCO3– concentration closely regulated by kidneys
© 2013 Pearson Education, Inc.

76. Bicarbonate Buffer System
If strong base added
It causes H2CO3 to dissociate and donate H+
H+ ties up the base (e.g. OH–)
NaOH + H2CO3  NaHCO3 + H2O
pH rises only slightly
H2CO3 supply is almost limitless (from CO2 released by respiration) and subject to respiratory controls
© 2013 Pearson Education, Inc.

77. Phosphate Buffer System
Action nearly identical to bicarbonate buffer
Components are sodium salts of:
Dihydrogen phosphate (H2PO4–), a weak acid
Monohydrogen phosphate (HPO42–), a weak base
Unimportant in buffering plasma
Effective buffer in urine and ICF, where phosphate concentrations are high
© 2013 Pearson Education, Inc.

78. Protein Buffer System
Intracellular proteins are most plentiful and powerful buffers; plasma proteins also important
Protein molecules are amphoteric (can function as both weak acid and weak base)
When pH rises, organic acid or carboxyl (COOH) groups release H+
When pH falls, NH2 groups bind H+
Hemoglobin functions as intracellular buffer
© 2013 Pearson Education, Inc.

79. Physiological Buffering Systems
Respiratory and renal systems
Regulate amount of acid or base in body
Act more slowly than chemical buffer systems
Have more capacity than chemical buffer systems
© 2013 Pearson Education, Inc.

80. Chemical buffers cannot eliminate excess acids or bases from body
Acid-Base Balance
Chemical buffers cannot eliminate excess acids or bases from body
Lungs eliminate volatile carbonic acid by eliminating CO2
Kidneys eliminate nonvolatile (fixed) acids produced by cellular metabolism (phosphoric, uric, and lactic acids and ketones) to prevent metabolic acidosis
Kidneys also regulate blood levels of alkaline substances; renew chemical buffers
© 2013 Pearson Education, Inc.

81. Respiratory Regulation of H+
Respiratory system eliminates CO2 (an acid)
A reversible equilibrium exists in blood
CO2 + H2O  H2CO3  H+ + HCO3–
During CO2 unloading reaction shifts to left (and H+ incorporated into H2O)
During CO2 loading reaction shifts to right (and H+ buffered by proteins)
© 2013 Pearson Education, Inc.

82. Respiratory Regulation of H+
Hypercapnia activates medullary chemoreceptors
 Increased respiratory rate and depth
Rising plasma H+ activates peripheral chemoreceptors
More CO2 is removed from the blood
H+ concentration is reduced
© 2013 Pearson Education, Inc.

83. Respiratory Regulation of H+
Alkalosis depresses respiratory center
Respiratory rate and depth decrease
H+ concentration increases
Respiratory system impairment causes acid-base imbalances
Hypoventilation  respiratory acidosis
Hyperventilation  respiratory alkalosis
© 2013 Pearson Education, Inc.

84. Renal Mechanisms of Acid-Base Balance
Most important renal mechanisms
Conserving (reabsorbing) or generating new HCO3–
Excreting HCO3–
Generating or reabsorbing one HCO3– same as losing one H+
Excreting one HCO3– same as gaining one H+
© 2013 Pearson Education, Inc.

85. Renal Mechanisms of Acid-Base Balance
To reabsorb bicarbonate kidney must secrete H+
To excrete excess bicarbonate kidney must retain (not secrete) H+
© 2013 Pearson Education, Inc.

86. Renal Mechanisms of Acid-base Balance
Renal regulation of acid-base balance depends on kidney's ability to secrete H+
H+ secretion occurs in PCT and collecting duct type A intercalated cells:
The H+ comes from H2CO3 produced in reactions catalyzed by carbonic anhydrase inside cells
As H+ secreted, Na+ reabsorbed
See Steps 1 and 2 of following figure
© 2013 Pearson Education, Inc.

87. Primary active transport Secondary active transport
Figure Reabsorption of filtered HCO3– is coupled to H+ secretion.
Slide 1
CO2 combines with water
within the tubule cell, forming
H2CO3. 1
H2CO3 is quickly split, forming H+ and bicarbonate ion (HCO3−). 2
H+ is secreted
into the filtrate. 3a
Nucleus
Filtrate in
tubule
lumen
Peri-
tubular
capillary
PCT cell 3b
For each H+ secreted,
a HCO3− enters the peritubular capillary
blood either via symport with Na+ or via antiport with CI−.
ATPase 3a 3b 4
Secreted H+ combines with HCO3− in the filtrate, forming carbonic acid (H2CO3). HCO3− disappears from the filtrate at the same rate that HCO3− (formed
within the tubule cell) enters the peritubular capillary blood. 2 4
ATPase 5
CA * 1 CA 6
Tight
junction
Primary active transport
Transport protein 6
CO2 diffuses into the tubule
cell, where it triggers further H+
secretion. 5
Secondary active transport
The H2CO3 formed in the filtrate dissociates to release CO2 and H2O. CA
Carbonic anhydrase
Simple diffusion
© 2013 Pearson Education, Inc.

88. Primary active transport Secondary active transport
Figure Reabsorption of filtered HCO3– is coupled to H+ secretion.
Slide 2
CO2 combines with water
within the tubule cell, forming
H2CO3. 1
Nucleus
Filtrate in
tubule
lumen
Peri-
tubular
capillary
PCT cell
ATPase
ATPase
CA * 1 CA
Tight
junction
Primary active transport
Transport protein
Secondary active transport CA
Carbonic anhydrase
Simple diffusion
© 2013 Pearson Education, Inc.

89. Primary active transport Secondary active transport
Figure Reabsorption of filtered HCO3– is coupled to H+ secretion.
Slide 3
CO2 combines with water
within the tubule cell, forming
H2CO3. 1
H2CO3 is quickly split, forming H+ and bicarbonate ion (HCO3−). 2
Nucleus
Filtrate in
tubule
lumen
Peri-
tubular
capillary
PCT cell
ATPase 2
ATPase
CA * 1 CA
Tight
junction
Primary active transport
Transport protein
Secondary active transport CA
Carbonic anhydrase
Simple diffusion
© 2013 Pearson Education, Inc.

90. Primary active transport Secondary active transport
Figure Reabsorption of filtered HCO3– is coupled to H+ secretion.
Slide 4
CO2 combines with water
within the tubule cell, forming
H2CO3. 1
H2CO3 is quickly split, forming H+ and bicarbonate ion (HCO3−). 2
H+ is secreted
into the filtrate. 3a
Nucleus
Filtrate in
tubule
lumen
Peri-
tubular
capillary
PCT cell
ATPase 3a 2
ATPase
CA * 1 CA
Tight
junction
Primary active transport
Transport protein
Secondary active transport CA
Carbonic anhydrase
Simple diffusion
© 2013 Pearson Education, Inc.

91. Primary active transport Secondary active transport
Figure Reabsorption of filtered HCO3– is coupled to H+ secretion.
Slide 5
CO2 combines with water
within the tubule cell, forming
H2CO3. 1
H2CO3 is quickly split, forming H+ and bicarbonate ion (HCO3−). 2 3a
H+ is secreted
into the filtrate.
Nucleus
Filtrate in
tubule
lumen
Peri-
tubular
capillary
PCT cell
For each H+ secreted,
a HCO3− enters the peritubular capillary
blood either via symport with Na+ or via antiport with CI−. 3b
ATPase 3a 3b 2
ATPase
CA * 1 CA
Tight
junction
Primary active transport
Transport protein
Secondary active transport CA
Carbonic anhydrase
Simple diffusion
© 2013 Pearson Education, Inc.

92. Primary active transport Secondary active transport
Figure Reabsorption of filtered HCO3– is coupled to H+ secretion.
Slide 6
CO2 combines with water
within the tubule cell, forming
H2CO3. 1
H2CO3 is quickly split, forming H+ and bicarbonate ion (HCO3−). 2
H+ is secreted
into the filtrate. 3a
Nucleus
Filtrate in
tubule
lumen
Peri-
tubular
capillary
PCT cell 3b
For each H+ secreted,
a HCO3− enters the peritubular capillary
blood either via symport with Na+ or via antiport with CI−.
ATPase 3a 3b 4
Secreted H+ combines with HCO3− in the filtrate, forming carbonic acid (H2CO3). HCO3− disappears from the filtrate at the same rate that HCO3− (formed
within the tubule cell) enters the peritubular capillary blood. 2 4
ATPase
CA * 1 CA
Tight
junction
Primary active transport
Transport protein
Secondary active transport CA
Carbonic anhydrase
Simple diffusion
© 2013 Pearson Education, Inc.

93. Primary active transport Secondary active transport
Figure Reabsorption of filtered HCO3– is coupled to H+ secretion.
Slide 7
CO2 combines with water
within the tubule cell, forming
H2CO3. 1
H2CO3 is quickly split, forming H+ and bicarbonate ion (HCO3−). 2 3a
H+ is secreted
into the filtrate.
Nucleus
Filtrate in
tubule
lumen
Peri-
tubular
capillary
PCT cell
For each H+ secreted,
a HCO3− enters the peritubular capillary
blood either via symport with Na+ or via antiport with CI−. 3b
ATPase 3a 3b 4
Secreted H+ combines with HCO3− in the filtrate, forming carbonic acid (H2CO3). HCO3− disappears from the filtrate at the same rate that HCO3− (formed
within the tubule cell) enters the peritubular capillary blood. 2 4
ATPase 5
CA * 1 CA
Tight
junction
Primary active transport
Transport protein 5
Secondary active transport
The H2CO3 formed in the filtrate dissociates to release CO2 and H2O. CA
Carbonic anhydrase
Simple diffusion
© 2013 Pearson Education, Inc.

94. Primary active transport Secondary active transport
Figure Reabsorption of filtered HCO3– is coupled to H+ secretion.
Slide 8
CO2 combines with water
within the tubule cell, forming
H2CO3. 1
H2CO3 is quickly split, forming H+ and bicarbonate ion (HCO3−). 2 3a
H+ is secreted
into the filtrate.
Nucleus
Filtrate in
tubule
lumen
Peri-
tubular
capillary
PCT cell 3b
For each H+ secreted,
a HCO3− enters the peritubular capillary
blood either via symport with Na+ or via antiport with CI−.
ATPase 3a 3b 4
Secreted H+ combines with HCO3− in the filtrate, forming carbonic acid (H2CO3). HCO3− disappears from the filtrate at the same rate that HCO3− (formed
within the tubule cell) enters the peritubular capillary blood. 2 4
ATPase 5
CA * 1 CA 6
Tight
junction
Primary active transport
Transport protein 6
Secondary active transport
CO2 diffuses into the tubule
cell, where it triggers further H+
secretion. 5
The H2CO3 formed in the filtrate dissociates to release CO2 and H2O. CA
Carbonic anhydrase
Simple diffusion
© 2013 Pearson Education, Inc.

95. Renal Mechanisms of Acid-base Balance
Rate of H+ secretion changes with ECF CO2 levels
 CO2 in peritubular capillary blood   rate of H+ secretion
System responds to both rising and falling H+ concentrations
© 2013 Pearson Education, Inc.

96. Bicarbonate Reabsorption
To maintain alkaline reserve kidneys must replenish bicarbonate
Yet tubule cells cannot reabsorb bicarbonate
Must conserve filtered bicarbonate in roundabout way
© 2013 Pearson Education, Inc.

97. Reabsorption of Bicarbonate
CO2 combines with water in PCT cells, forming H2CO3
H2CO3 dissociates
H+ is secreted, and HCO3– shunted into capillary blood
Secreted H+ unites with HCO3– to form H2CO3 in filtrate, which generates CO2 and H2O
HCO3– disappears from filtrate at same rate it enters peritubular capillary blood
© 2013 Pearson Education, Inc.

98. Primary active transport Secondary active transport
Figure Reabsorption of filtered HCO3– is coupled to H+ secretion.
Slide 1
CO2 combines with water
within the tubule cell, forming
H2CO3. 1
H2CO3 is quickly split, forming H+ and bicarbonate ion (HCO3−). 2
H+ is secreted
into the filtrate. 3a
Nucleus
Filtrate in
tubule
lumen
Peri-
tubular
capillary
PCT cell 3b
For each H+ secreted,
a HCO3− enters the peritubular capillary
blood either via symport with Na+ or via antiport with CI−.
ATPase 3a 3b 4
Secreted H+ combines with HCO3− in the filtrate, forming carbonic acid (H2CO3). HCO3− disappears from the filtrate at the same rate that HCO3− (formed
within the tubule cell) enters the peritubular capillary blood. 2 4
ATPase 5
CA * 1 CA 6
Tight
junction
Primary active transport
Transport protein 6
CO2 diffuses into the tubule
cell, where it triggers further H+
secretion. 5
Secondary active transport
The H2CO3 formed in the filtrate dissociates to release CO2 and H2O. CA
Carbonic anhydrase
Simple diffusion
© 2013 Pearson Education, Inc.

99. Primary active transport Secondary active transport
Figure Reabsorption of filtered HCO3– is coupled to H+ secretion.
Slide 2
CO2 combines with water
within the tubule cell, forming
H2CO3. 1
Nucleus
Filtrate in
tubule
lumen
Peri-
tubular
capillary
PCT cell
ATPase
ATPase
CA * 1 CA
Tight
junction
Primary active transport
Transport protein
Secondary active transport CA
Carbonic anhydrase
Simple diffusion
© 2013 Pearson Education, Inc.

100. Primary active transport Secondary active transport
Figure Reabsorption of filtered HCO3– is coupled to H+ secretion.
Slide 3
CO2 combines with water
within the tubule cell, forming
H2CO3. 1
H2CO3 is quickly split, forming H+ and bicarbonate ion (HCO3−). 2
Nucleus
Filtrate in
tubule
lumen
Peri-
tubular
capillary
PCT cell
ATPase 2
ATPase
CA * 1 CA
Tight
junction
Primary active transport
Transport protein
Secondary active transport CA
Carbonic anhydrase
Simple diffusion
© 2013 Pearson Education, Inc.

101. Primary active transport Secondary active transport
Figure Reabsorption of filtered HCO3– is coupled to H+ secretion.
Slide 4
CO2 combines with water
within the tubule cell, forming
H2CO3. 1
H2CO3 is quickly split, forming H+ and bicarbonate ion (HCO3−). 2
H+ is secreted
into the filtrate. 3a
Nucleus
Filtrate in
tubule
lumen
Peri-
tubular
capillary
PCT cell
ATPase 3a 2
ATPase
CA * 1 CA
Tight
junction
Primary active transport
Transport protein
Secondary active transport CA
Carbonic anhydrase
Simple diffusion
© 2013 Pearson Education, Inc.

102. Primary active transport Secondary active transport
Figure Reabsorption of filtered HCO3– is coupled to H+ secretion.
Slide 5
CO2 combines with water
within the tubule cell, forming
H2CO3. 1
H2CO3 is quickly split, forming H+ and bicarbonate ion (HCO3−). 2 3a
H+ is secreted
into the filtrate.
Nucleus
Filtrate in
tubule
lumen
Peri-
tubular
capillary
PCT cell
For each H+ secreted,
a HCO3− enters the peritubular capillary
blood either via symport with Na+ or via antiport with CI−. 3b
ATPase 3a 3b 2
ATPase
CA * 1 CA
Tight
junction
Primary active transport
Transport protein
Secondary active transport CA
Carbonic anhydrase
Simple diffusion
© 2013 Pearson Education, Inc.

103. Primary active transport Secondary active transport
Figure Reabsorption of filtered HCO3– is coupled to H+ secretion.
Slide 6
CO2 combines with water
within the tubule cell, forming
H2CO3. 1
H2CO3 is quickly split, forming H+ and bicarbonate ion (HCO3−). 2
H+ is secreted
into the filtrate. 3a
Nucleus
Filtrate in
tubule
lumen
Peri-
tubular
capillary
PCT cell 3b
For each H+ secreted,
a HCO3− enters the peritubular capillary
blood either via symport with Na+ or via antiport with CI−.
ATPase 3a 3b 4
Secreted H+ combines with HCO3− in the filtrate, forming carbonic acid (H2CO3). HCO3− disappears from the filtrate at the same rate that HCO3− (formed
within the tubule cell) enters the peritubular capillary blood. 2 4
ATPase
CA * 1 CA
Tight
junction
Primary active transport
Transport protein
Secondary active transport CA
Carbonic anhydrase
Simple diffusion
© 2013 Pearson Education, Inc.

104. Primary active transport Secondary active transport
Figure Reabsorption of filtered HCO3– is coupled to H+ secretion.
Slide 7
CO2 combines with water
within the tubule cell, forming
H2CO3. 1
H2CO3 is quickly split, forming H+ and bicarbonate ion (HCO3−). 2 3a
H+ is secreted
into the filtrate.
Nucleus
Filtrate in
tubule
lumen
Peri-
tubular
capillary
PCT cell
For each H+ secreted,
a HCO3− enters the peritubular capillary
blood either via symport with Na+ or via antiport with CI−. 3b
ATPase 3a 3b 4
Secreted H+ combines with HCO3− in the filtrate, forming carbonic acid (H2CO3). HCO3− disappears from the filtrate at the same rate that HCO3− (formed
within the tubule cell) enters the peritubular capillary blood. 2 4
ATPase 5
CA * 1 CA
Tight
junction
Primary active transport
Transport protein 5
Secondary active transport
The H2CO3 formed in the filtrate dissociates to release CO2 and H2O. CA
Carbonic anhydrase
Simple diffusion
© 2013 Pearson Education, Inc.

105. Primary active transport Secondary active transport
Figure Reabsorption of filtered HCO3– is coupled to H+ secretion.
Slide 8
CO2 combines with water
within the tubule cell, forming
H2CO3. 1
H2CO3 is quickly split, forming H+ and bicarbonate ion (HCO3−). 2 3a
H+ is secreted
into the filtrate.
Nucleus
Filtrate in
tubule
lumen
Peri-
tubular
capillary
PCT cell 3b
For each H+ secreted,
a HCO3− enters the peritubular capillary
blood either via symport with Na+ or via antiport with CI−.
ATPase 3a 3b 4
Secreted H+ combines with HCO3− in the filtrate, forming carbonic acid (H2CO3). HCO3− disappears from the filtrate at the same rate that HCO3− (formed
within the tubule cell) enters the peritubular capillary blood. 2 4
ATPase 5
CA * 1 CA 6
Tight
junction
Primary active transport
Transport protein 6
Secondary active transport
CO2 diffuses into the tubule
cell, where it triggers further H+
secretion. 5
The H2CO3 formed in the filtrate dissociates to release CO2 and H2O. CA
Carbonic anhydrase
Simple diffusion
© 2013 Pearson Education, Inc.

106. Generating New Bicarbonate Ions
Two mechanisms in PCT and type A intercalated cells
Generate new HCO3– to be added to alkaline reserve
Both involve renal excretion of acid (via secretion and excretion of H+ or NH4+)
© 2013 Pearson Education, Inc.

107. Excretion of Buffered H+
Dietary H+ must be balanced by generating new HCO3–
H+ not excreted when filtered bicarbonate reclaimed
Most filtered HCO3– is "used up" before filtrate reaches the collecting duct
Generating new bicarbonate counteracts acidosis from dietary H+
© 2013 Pearson Education, Inc.

108. Excretion of Buffered H+
Most important urine buffer is phosphate buffer system
Intercalated cells actively secrete H+ into urine, which is buffered by phosphates and excreted
Generated "new" HCO3– moves into interstitial space via cotransport system, then moves passively into peritubular capillary blood
© 2013 Pearson Education, Inc.

109. H2CO3 is quickly split, forming H+ and bicarbonate ion (HCO3−).
Figure New HCO3– is generated via buffering of secreted H+ by HPO42– (monohydrogen phosphate).
Slide 1
CO2 combines with water within the type A intercalated cell, forming H2CO3.
H2CO3 is quickly split, forming H+ and bicarbonate ion (HCO3−). 1 2
H+ is secreted into the filtrate by a H+ ATPase pump. 3a
Nucleus
Filtrate in
tubule
lumen
Tight junction
Peri-
tubular
capillary
For
each H+ secreted, a HCO3− enters
the peritubular capillary blood via an antiport carrier in a HCO3− -CI− exchange process. 3b 1 2 3a 3b
(new)
ATPase 4
Secreted
H+ combines with HPO42−
in the tubular filtrate,
Forming
H2PO4−. 4
Type A
intercalated
cell of collecting duct 5
out in urine
Primary active transport
Transport protein
Secondary active transport
The H2PO4−
is excreted
in the urine. 5
Ion channel
Simple diffusion
Carbonic anhydrase
Facilitated diffusion
© 2013 Pearson Education, Inc.

110. cell of collecting duct
Figure New HCO3– is generated via buffering of secreted H+ by HPO42– (monohydrogen phosphate).
Slide 2
CO2 combines with water within the type A intercalated cell, forming H2CO3. 1
Nucleus
Filtrate in
tubule
lumen
Tight junction
Peri-
tubular
capillary 1
(new)
ATPase
Type A
intercalated
cell of collecting duct
out in urine
Primary active transport
Transport protein
Secondary active transport
Ion channel
Simple diffusion
Carbonic anhydrase
Facilitated diffusion
© 2013 Pearson Education, Inc.

111. H2CO3 is quickly split, forming H+ and bicarbonate ion (HCO3−).
Figure New HCO3– is generated via buffering of secreted H+ by HPO42– (monohydrogen phosphate).
Slide 3
CO2 combines with water within the type A intercalated cell, forming H2CO3.
H2CO3 is quickly split, forming H+ and bicarbonate ion (HCO3−). 1 2
Nucleus
Filtrate in
tubule
lumen
Tight junction
Peri-
tubular
capillary 1 2
(new)
ATPase
Type A
intercalated
cell of collecting duct
out in urine
Primary active transport
Transport protein
Secondary active transport
Ion channel
Simple diffusion
Carbonic anhydrase
Facilitated diffusion
© 2013 Pearson Education, Inc.

112. H2CO3 is quickly split, forming H+ and bicarbonate ion (HCO3−).
Figure New HCO3– is generated via buffering of secreted H+ by HPO42– (monohydrogen phosphate).
Slide 4
CO2 combines with water within the type A intercalated cell, forming H2CO3.
H2CO3 is quickly split, forming H+ and bicarbonate ion (HCO3−). 1 2
H+ is secreted into the filtrate by a H+ ATPase pump. 3a
Nucleus
Filtrate in
tubule
lumen
Tight junction
Peri-
tubular
capillary 1 2 3a
(new)
ATPase
Type A
intercalated
cell of collecting duct
out in urine
Primary active transport
Transport protein
Secondary active transport
Ion channel
Simple diffusion
Carbonic anhydrase
Facilitated diffusion
© 2013 Pearson Education, Inc.

113. H2CO3 is quickly split, forming H+ and bicarbonate ion (HCO3−).
Figure New HCO3– is generated via buffering of secreted H+ by HPO42– (monohydrogen phosphate).
Slide 5
CO2 combines with water within the type A intercalated cell, forming H2CO3.
H2CO3 is quickly split, forming H+ and bicarbonate ion (HCO3−). 1 2
H+ is secreted into the filtrate by a H+ ATPase pump. 3a
Nucleus
Filtrate in
tubule
lumen
Tight junction
Peri-
tubular
capillary
For
each H+ secreted, a HCO3− enters
the peritubular capillary blood via an antiport carrier in a HCO3− -CI− exchange process. 3b 1 2 3a 3b
(new)
ATPase
Type A
intercalated
cell of collecting duct
out in urine
Primary active transport
Transport protein
Secondary active transport
Ion channel
Simple diffusion
Carbonic anhydrase
Facilitated diffusion
© 2013 Pearson Education, Inc.

114. H2CO3 is quickly split, forming H+ and bicarbonate ion (HCO3−).
Figure New HCO3– is generated via buffering of secreted H+ by HPO42– (monohydrogen phosphate).
Slide 6
CO2 combines with water within the type A intercalated cell, forming H2CO3.
H2CO3 is quickly split, forming H+ and bicarbonate ion (HCO3−). 1 2
H+ is secreted into the filtrate by a H+ ATPase pump. 3a
Nucleus
Filtrate in
tubule
lumen
Tight junction
Peri-
tubular
capillary
For
each H+ secreted, a HCO3− enters
the peritubular capillary blood via an antiport carrier in a HCO3− -CI− exchange process. 3b 1 2 3a 3b
(new)
ATPase 4
Secreted
H+ combines with HPO42−
in the tubular filtrate,
Forming
H2PO4−. 4
Type A
intercalated
cell of collecting duct
out in urine
Primary active transport
Transport protein
Secondary active transport
Ion channel
Simple diffusion
Carbonic anhydrase
Facilitated diffusion
© 2013 Pearson Education, Inc.

115. H2CO3 is quickly split, forming H+ and bicarbonate ion (HCO3−).
Figure New HCO3– is generated via buffering of secreted H+ by HPO42– (monohydrogen phosphate).
Slide 7
CO2 combines with water within the type A intercalated cell, forming H2CO3.
H2CO3 is quickly split, forming H+ and bicarbonate ion (HCO3−). 1 2
H+ is secreted into the filtrate by a H+ ATPase pump. 3a
Nucleus
Filtrate in
tubule
lumen
Tight junction
Peri-
tubular
capillary
For
each H+ secreted, a HCO3− enters
the peritubular capillary blood via an antiport carrier in a HCO3− -CI− exchange process. 3b 1 2 3a 3b
(new)
ATPase 4
Secreted
H+ combines with HPO42−
in the tubular filtrate,
Forming
H2PO4−. 4
Type A
intercalated
cell of collecting duct 5
out in urine
Primary active transport
Transport protein
Secondary active transport
The H2PO4−
is excreted
in the urine. 5
Ion channel
Simple diffusion
Carbonic anhydrase
Facilitated diffusion
© 2013 Pearson Education, Inc.

116. Ammonium Ion Excretion
More important mechanism for excreting acid
Involves metabolism of glutamine in PCT cells
Each glutamine produces 2 NH4+ and 2 "new" HCO3–
HCO3– moves to blood and NH4+ is excreted in urine
Replenishes alkaline reserve of blood
© 2013 Pearson Education, Inc.

117. the filtrate, taking the place of H+ on a Na+ -H+ antiport carrier. 2a
Figure New HCO3– is generated via glutamine metabolism and NH4+ secretion.
Slide 1
PCT cells metabolize
glutamine to
NH4+and HCO3−. 1
This weak acid NH4+ (ammonium) is secreted into
the filtrate, taking the place of
H+ on a Na+ -H+ antiport carrier. 2a
For each NH4+ secreted,
a bicarbonate ion (HCO3−) enters the peritubular capillary blood via a symport carrier. 2b
Nucleus
Filtrate in
tubule lumen
Peri-
tubular
capillary
PCT tubule cells
Glutamine
Glutamine
Glutamine
Deamination,
oxidation, and
acidification
(+H+) 1 2a 2b
(new) 3
out in urine
ATPase
Tight junction
Primary active transport
Simple diffusion
The NH4+ is excreted in the urine. 3
Secondary active transport
Transport protein
© 2013 Pearson Education, Inc.

118. Primary active transport Simple diffusion Secondary active transport
Figure New HCO3– is generated via glutamine metabolism and NH4+ secretion.
Slide 2
PCT cells metabolize
glutamine to
NH4+and HCO3−. 1
Nucleus
Filtrate in
tubule lumen
Peri-
tubular
capillary
PCT tubule cells
Glutamine
Glutamine
Glutamine
Deamination,
oxidation, and
acidification
(+H+) 1
(new)
out in urine
ATPase
Tight junction
Primary active transport
Simple diffusion
Secondary active transport
Transport protein
© 2013 Pearson Education, Inc.

119. the filtrate, taking the place of H+ on a Na+ -H+ antiport carrier. 2a
Figure New HCO3– is generated via glutamine metabolism and NH4+ secretion.
Slide 3
PCT cells metabolize
glutamine to
NH4+and HCO3−. 1
This weak acid NH4+ (ammonium) is secreted into
the filtrate, taking the place of
H+ on a Na+ -H+ antiport carrier. 2a
For each NH4+ secreted,
a bicarbonate ion (HCO3−) enters the peritubular capillary blood via a symport carrier. 2b
Nucleus
Filtrate in
tubule lumen
Peri-
tubular
capillary
PCT tubule cells
Glutamine
Glutamine
Glutamine
Deamination,
oxidation, and
acidification
(+H+) 1 2a 2b
(new)
out in urine
ATPase
Tight junction
Primary active transport
Simple diffusion
Secondary active transport
Transport protein
© 2013 Pearson Education, Inc.

120. the filtrate, taking the place of H+ on a Na+ -H+ antiport carrier. 2a
Figure New HCO3– is generated via glutamine metabolism and NH4+ secretion.
Slide 4
PCT cells metabolize
glutamine to
NH4+and HCO3−. 1
This weak acid NH4+ (ammonium) is secreted into
the filtrate, taking the place of
H+ on a Na+ -H+ antiport carrier. 2a
For each NH4+ secreted,
a bicarbonate ion (HCO3−) enters the peritubular capillary blood via a symport carrier. 2b
Nucleus
Filtrate in
tubule lumen
Peri-
tubular
capillary
PCT tubule cells
Glutamine
Glutamine
Glutamine
Deamination,
oxidation, and
acidification
(+H+) 1 2a 2b
(new) 3
out in urine
ATPase
Tight junction
Primary active transport
Simple diffusion
The NH4+ is excreted in the urine. 3
Secondary active transport
Transport protein
© 2013 Pearson Education, Inc.

121. Bicarbonate Ion Secretion
When body in alkalosis, type B intercalated cells
Secrete HCO3–
Reclaim H+ to acidify blood
© 2013 Pearson Education, Inc.

122. Bicarbonate Ion Secretion
Mechanism is opposite of bicarbonate ion reabsorption process by type A intercalated cells
Even during alkalosis, nephrons and collecting ducts conserve more HCO3– than they excrete
© 2013 Pearson Education, Inc.

123. Abnormalities of Acid-Base Balance
All classed as respiratory or metabolic
Respiratory acidosis and alkalosis
Caused by failure of respiratory system to perform pH-balancing role
Single most important indicator is blood PCO2
Metabolic acidosis and alkalosis
All abnormalities other than those caused by PCO2 levels in blood; indicated by abnormal HCO3– levels
© 2013 Pearson Education, Inc.

124. Respiratory Acidosis and Alkalosis
Most important indicator of adequacy of respiratory function is PCO2 level (normally 35–45 mm Hg)
PCO2 above 45 mm Hg  respiratory acidosis
Common cause of acid-base imbalances
Due to decrease in ventilation or gas exchange
CO2 accumulates in blood
Characterized by falling blood pH and rising PCO2
© 2013 Pearson Education, Inc.

125. Respiratory Acidosis and Alkalosis
PCO2 below 35 mm Hg  respiratory alkalosis
Common result of hyperventilation often due to stress or pain
CO2 eliminated faster than produced
© 2013 Pearson Education, Inc.

126. Metabolic Acidosis and Alkalosis
Metabolic acidosis – low blood pH and HCO3–
Causes
Ingestion of too much alcohol ( acetic acid)
Excessive loss of HCO3– (e.g., persistent diarrhea)
Accumulation of lactic acid (exercise or shock), ketosis in diabetic crisis, starvation, and kidney failure
© 2013 Pearson Education, Inc.

127. Metabolic Acidosis and Alkalosis
Metabolic alkalosis much less common than metabolic acidosis
Indicated by rising blood pH and HCO3–
Causes include vomiting of acid contents of stomach or by intake of excess base (e.g., antacids)
© 2013 Pearson Education, Inc.

128. Effects of Acidosis and Alkalosis
Blood pH below 6.8  depression of CNS  coma  death
Blood pH above 7.8  excitation of nervous system  muscle tetany, extreme nervousness, convulsions, death often from respiratory arrest
© 2013 Pearson Education, Inc.

129. Respiratory and Renal Compensations
If acid-base imbalance due to malfunction of physiological buffer system, other one tries to compensate
Respiratory system attempts to correct metabolic acid-base imbalances
Kidneys attempt to correct respiratory acid-base imbalances
© 2013 Pearson Education, Inc.

130. Respiratory Compensation
Changes in respiratory rate and depth
In metabolic acidosis
High H+ levels stimulate respiratory centers
Rate and depth of breathing elevated
Blood pH is below 7.35 and HCO3– level is low
As CO2 eliminated by respiratory system, PCO2 falls below normal
© 2013 Pearson Education, Inc.

131. Respiratory Compensation
Respiratory compensation for metabolic alkalosis revealed by:
Slow, shallow breathing, allowing CO2 accumulation in blood
High pH (over 7.45), elevated HCO3– levels, PCO2 above 45 mm Hg
© 2013 Pearson Education, Inc.

132. Renal Compensation for Respiratory Acid-Base Imbalance
Hypoventilation causes elevated PCO2
Respiratory acidosis
Renal compensation indicated by high PCO2 (causes acidosis) and HCO3– levels (indicates kidneys compensating)
Respiratory alkalosis exhibits low PCO2 and high pH
Renal compensation is indicated by decreasing HCO3– levels
© 2013 Pearson Education, Inc.

133. Acid-base Imbalances
Respiratory system cannot compensate for respiratory acidosis or alkalosis
Renal system cannot compensate for acid-base imbalances caused by renal problems
© 2013 Pearson Education, Inc.

134. Developmental Aspects
Infants have proportionately more ECF than adults until about 2 years of age
Problems with fluid, electrolyte, and acid-base balance are most common in infancy, reflecting
Low residual lung volume
High rate of fluid intake and output
High metabolic rate, yielding more metabolic wastes
High rate of insensible water loss
Inefficiency of kidneys, especially during first month
1st month newborns risk dehydration and acidosis
© 2013 Pearson Education, Inc.

135. Developmental Aspects
At puberty, sexual differences in body water content arise as males develop greater muscle mass
In old age, total body water often decreases
Homeostatic mechanisms slow down with age
Elderly may be unresponsive to thirst cues  at risk for dehydration; risk of congestive heart failure and diabetes mellitus that cause fluid, electrolyte or acid-base problems
© 2013 Pearson Education, Inc.