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Acid-Base Balance

Ming-Jyh Sheu, PhD

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Purpose of acid-base balance & significance

To maintain the pH of the body fluids within 7.35~7.45

The kidney maintains the pH of the body fluids within the normal range– [H+] in body fluids 小於其它離子的百萬分之一倍 , its small size results in a high reactivity with binding sites on proteins

– [H+]plasma = 40 nEq/L or nM, pH = 7.4

– [Na+]plasma = 140 mEq/L

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Regulation of [H+]

Chemical buffering by buffers in both ECF and ICF （快）

Regulation of CO2 concentration in the blood by alveolar ventilation in the lung （中）

Control of [HCO3-]ECF by the kidney （慢）

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CO2-HCO3- Buffering system

CO2 + H2O ↔ H2CO3 ↔ HCO3- + H+

– K’ = [H+] [HCO3-] / [CO2] [H2O]

• K’: depends on – Temperature （ at 37 OC, K’ = 10-6.1, pK’ = 6.

1 ）

– [CO2] [H2O] 所代表的意思 • 代表 total amount of CO2 dissolved in solution

• Most of this CO2 is in gaseous form （ only 0.3% contained in H2CO3）

• The amount of CO2 is solution depends on its partial pressure （ pCO2） and its solubility （ α ）

CA

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Buffering CO2-HCO3- system: 在 ECF 中是

一個重要的緩衝系統

K’ = [H+] [HCO3-] / α pCO2

– (37 OC 時： α= 0.03)

[H+] = k’ α pCO2 / [HCO3-]

– - Log[H+] = - log[K’] + - log α pCO2 / - log[HCO3

-]

– pH = pK’ + log [HCO3-] / α pCO2

– pH = 6.1 + log [HCO3-] / 0.03pCO2（ He

nderson-Hasselbalch equation）

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[ H+] = K （ pCO2 / [HCO3-] p ）

This equation is useful in rapid interpretation of clinicalacid-base balance disturbances

[H+] is directly related to pCO2

[H+] is inversely related to [HCO3-]p

1. Metabolic acid-base disorder 代謝性酸減不平衡 : ECF [HCO3

-] 的改變2. Respiratory acid-base disorder 呼吸性酸減不平衡 : a change in PCO2

如果 pCO2 or [HCO3-] ↑, then the other must als

o ↑for [H+] to remain constant如果 pCO2 or [HCO3

-] ↓, then the other must also ↓for [H+] to remain constant

Henderson-Hasselbalch equation

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Buffering CO2-HCO3- system

CO2 + H2O ↔ H2CO3 ↔ HCO3- + H+

– CO2 enters blood → ↑[H+]– CO2 leaves blood, and is exhaled from the lungs →

[H+]

Hypoventilation occurs, [H+] p increases → respiratory acidosis

Hyperventilation occurs, [H+] p decreases → respiratory alkalosis

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Metabolic production of acid and alkali

Nonvolatile acid – Cysteine, methionine → H2SO4

– Lysine, arginine, and histidine → HCl– Catabolism of dietary lipids → phosphori

c acid– Anaerobic metabolism → lactic acid– Aspartate and glutamate → HCO3

-

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Renal regulation of H+ balance

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Renal acid excretion

To maintain acid-base balance– Kidney must excrete an amount of acid e

qual to the production of nonvolatile acid

Replenish ( 補充 ) of HCO3-

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Net acid excretion (NAE)

[(UNH4+ V) + (UTA V)] - (UHCO3

- V)

– UNH4+ V: rate of excretion of NH4

+

– UTA V: titrable acid

– UHCO3- V: amount of HCO3

- lost in the urine

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Reabsorption of HCO3- along the nephron

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HCO3- 於集尿

管中再吸收的機制

此處反應之發生與代謝性鹼中毒有關 ;　

Intercalated cells

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Factors regulating HCO3- reabsorption:

針對增加 H+ secretion 部分探討

HCO3- (increased in filter load)

Na+ balance– Volume contraction → Na+ reabsorption, HCO3

- reabsorption ( 增加 H+ secretion)

[HCO3-]p ↓ （ pH ↓ ） ---metabolic acidosis

pCO2 in plasma ↑---------respiratory acidosis

↑Aldosterone secretion

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Aldosterone

Aldosterone 1.間接作用

a. Na+ reabsorption 所以 magnitude of

lumen-negative voltage 增加

• 直接作用a.增加 H+ 之 tran

sporter

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Addition of new HCO3- to plasma

2 major buffers of H+ in urine

– Ammonia（ NH3）– Dibasic phosphate（ HPO4

-2）

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在此地方幾乎已經無 HCO3

-

之存在

此處 buffer 之產生應體內酸鹼平衡之需要而有 buffer之產生

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Renal tubule acidosis（ RTA） Urine acidification is impaired 無法分泌 H+以平衡過多的代謝酸（ nonvolatile

acid ），所以產生代謝性酸中毒 Defect in PT H+-secretion （ proximal RTA ）

– Cystinosis 胱胺酸症– Fanconi’s syndrome– Carbonic anhydrase inhibitors

Defect in distal tubule H+-secretion （ distal RTA ）– Medullary sponge kidney– Amphotericin B– Secondary to urinary obstruction

Treatment: ingest alkali (HCO3-)

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Response to acid-base disorders Respiratory defense

– Metabolic acidosis: ↑H+↑ventilation rate• Type-I-DM patient (keto acid 製造增加 ): Kussmaul respiration: 呼吸肌會疲勞 , 所以呼吸代償會受損 , 酸中毒更嚴重

– Metabolic alkalosis:↓H+ ↓ventilation rate

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Response to acid-base disorders

Renal defense

– Secretion of H+

– HCO3- reabsorbed

– Production and excretion of NH4+

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DM patient: ketone bodiesDiarrhea:

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