acid-base balance. copyright © the mcgraw-hill companies, inc. permission required for reproduction...
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Acid-Base Balance
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Objectives
Explain how the pH of the blood is stabilized by bicarb buffer and define the terms acidosis and alkalosis.
Explain how the acid-base balance of the blood is affected by C02 and HC03
-, and describe the roles of the lungs and kidneys in maintaining acid-base balance.
Explain how C02 affects blood pH, and hypoventilation and hyperventilation affect acid-base balance.
Explain how the interaction between plasma K+ and H+ concentrations affects the tubular secretion of these.
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pH
pH of blood is 7.35 to 7.45 pH = 6.1 + log [HCO3
-] 0.03 x Pco2
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Types of Acids in the Body
Volatile acids: Can leave solution and enter the
atmosphere. H2C03 (carbonic acid). Pco2 is most important factor in pH of
body tissues.
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Types of Acids in the Body
Fixed Acids: Acids that do not leave solution. Sulfuric and phosphoric acid. Catabolism of amino acids, nucleic
acids, and phospholipids.
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Types of Acids in the Body
Organic Acids: Byproducts of aerobic metabolism,
during anaerobic metabolism and during starvation, diabetes.
Lactic acid, ketones.
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Buffer Systems
Provide or remove H+ and stabilize the pH.
Include weak acids that can donate H+ and weak bases that can absorb H+.
Does NOT prevent a pH change.
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Chemical Buffers
Act within fraction of a second. Protein. HCO3
-. Phosphate.
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Proteins
COOH or NH2. Largest pool of buffers in the body. pk. close to plasma. Albumin, globulins such as Hb.
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HCO3-
pk. = 6.1. Present in large quantities. Open system. Respiratory and renal systems act
on this buffer system. Most important ECF buffer.
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HCO3- Limitations
Cannot protect ECF from respiratory problems.
Cannot protect ECF from elevated or decreased CO2.
Limited by availability of HCO3-.
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Phosphates
pk. = 6.8. Low [ ] in ECF, better buffer in ICF,
kidneys, and bone.
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Respiratory System
2nd line of defense. Acts within min. maximal in 12-24
hrs. H2CO3 produced converted to CO2,
and excreted by the lungs. Alveolar ventilation also increases
as pH decreases (rate and depth). Coarse , CANNOT eliminate fixed
acid.
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Urinary Buffers
Nephron cannot produce a urine pH < 4.5.
IN order to excrete more H+, the acid must be buffered.
H+ secreted into the urine tubule and combines with HPO4
-2 or NH3. HPO4
-2 + H+ H2PO4-2
NH3 + H+ NH4+
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Renal Acid-Base Regulation
Kidneys help regulate blood pH by excreting H+ and reabsorbing HC03
-. Most of the H+ secretion occurs across
the walls of the PCT in exchange for Na+. Antiport mechanism.
Moves Na+ and H+ in opposite directions. Normal urine normally is slightly acidic
because the kidneys reabsorb almost all HC03
- and excrete H+. Returns blood pH back to normal range.
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Reabsorption of HCO3-
Apical membranes of tubule cells are impermeable to HCO3
-. Reabsorption is indirect.
When urine is acidic, HCO3- combines
with H+ to form H2C03-, which is catalyzed
by ca located in the apical cell membrane of PCT. As [C02] increases in the filtrate, C02 diffuses
into tubule cell and forms H2C03. H2C03 dissociates to HCO3
- and H+. HCO3
- generated within tubule cell diffuses into peritubular capillary.
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Acidification of Urine
Insert fig. 17.28
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Urinary Buffers
Nephron cannot produce a urine pH < 4.5.
In order to excrete more H+, the acid must be buffered.
H+ secreted into the urine tubule and combines with HPO4
-2 or NH3. HPO4
-2 + H+ H2PO4-
NH3 + H+ NH4+
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Metabolic Acidosis
Gain of fixed acid or loss of HCO3-.
Plasma HCO3- decreases.
PCO2 decreases. pH decreases.
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Metabolic Alkalosis
Loss of fixed acid or gain of HCO3
-. Plasma HCO3
- increases. PCO2 increases.
pH increases.
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Respiratory Acidosis
PCO2 increases. Plasma HCO3
- increases. pH decreases.
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Respiratory Alkalosis
PCO2 decreases. Plasma HCO3
- decreases. pH increases.
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Anion Gap
The difference between [Na+] and the sum of [HC03
-] and [Cl-]. [Na+] – ([HC03
-] + [Cl-]) = 140 - (24 + 105) = 11
Normal = 12 + 2
Clinicians use the anion gap to identify the cause of metabolic acidosis.
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Anion Gap
Law of electroneutrality: Blood plasma contains
an = number of + and – charges.
The major cation is Na+. Minor cations are K+,
Ca2+ , Mg2+. The major anions are
HC03- and Cl-.
(Routinely measured.) Minor anions include
albumin, phosphate, sulfate (called unmeasured anions).
Organic acid anions include lactate and acetoacetate,.
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Anion Gap
In metabolic acidosis, the strong acid releases protons that are buffered primarily by [HC03].
This causes plasma [HC03-]
to decrease, shrinking the [HC03
-] on the ionogram. Anions that remain from
the strong acid, are added to the plasma.
If lactic acid is added, the [lactate] rises.
Increasing the total [unmeasured anions].
If HCL is added, the [Cl-] rises.
Decreasing the [HC03-].
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Salicylates raise the gap to 20. Renal failure raises gap to 25. Diabetic ketoacidosis raises the gap to
35-40. Lactic acidosis raises the gap to > 35
(>50). Largest gaps are caused by ketoacidosis
and lactic acidosis.