Acid-base homeostasis

Acid-base Homeostasis
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Overview
Mechanism
Blood Gas Analysis
Relationship between pH and H+
Compensation
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1] Associate Editor(s)-in-Chief: Sadaf Sharfaei M.D.[2]; Priyamvada Singh, M.D. [3]

Overview

Acid-base homeostasis is the part of human homeostasis concerning the proper balance between acids and bases, in other words the pH. The body is very sensitive to its pH level. Outside the range of pH that is compatible with life, proteins are denatured and digested, enzymes lose their ability to function, and the body is unable to sustain itself.

Mechanism

The kidneys maintain acid-base homeostasis by regulating the pH of the blood plasma. Gains and losses of acid and base must be balanced. The study of the acid-base reactions in the body is acid base physiology.

Buffering agents

Any substance that can reversibly bind hydrogen ions is called a buffering agent. They function to impede any change in pH. Hydrogen ions are buffered by extracellular (e.g., bicarbonate, ammonia) and intracellular buffering agents (including proteins and phosphate).

Blood Gas Analysis

Blood gas analysis	Vessel	Range	Interpretation
Oxygen Partial Pressure (pO2)	Arterial	80 to 100 mmHg	Normal
		<80  mmHg	Hypoxia
	Venous	35 to 40 mmHg	Normal
Oxygen Saturation (SO2)	Arterial	>95%	Normal
		<95%	Hypoxia
	Venous	70 to 75%	Normal
pH	Arterial	<7.35	Acidemia
		7.35 to 7.45	Normal
		>7.45	Alkalemia
	Venous	7.26 to 7.46	Normal
Carbon Dioxide Partial Pressure (pCO2)	Arterial	<35 mmHg	Low
		35 to 45 mmHg	Normal
		>45 mmHg	High
	Venous	40 to 45 mmHg	Normal
Bicarbonate (HCO3−)	Arterial	<22 mmol/L	Low
		22 to 26 mmol/L	Normal
		>26 mmol/L	High
	Venous	19 to 28 mmol/L	Normal
Base Excess (BE)	Arterial	<−3.4	Acidemia
		−3.4 to +2.3 mmol/L	Normal
		>2.3	Alkalemia
	Venous	−2 to −5 mmol/L	Normal
Osmolar gap = Osmolality – Osmolarity		>10	Abnormal
Anion gap = Na+ - [Cl−+ HCO3−] Corrected AG = (measured serum AG) + (2.5 x [4.5 − Alb])		<8	Low
		8 to 16	Normal
		>16	High

Relationship between pH and H⁺

An inverse relationship between the H+ concentration (nmol/L) and the pH is given as follows:

pH	[H+]
7.80	16
7.70	20
7.60	26
7.50	32
7.40	40
7.30	50
7.20	63
7.10	80
7.00	100
6.90	125
6.80	160

Compensation Mechanism

• There are compensation mechanisms in the body in order to normalizing the pH inside the blood.
• The amount of compensation depends on proper functioning of renal and respiratory systems. However, it is uncommon to compensate completely. Compensatory mechanisms might correct only 50–75% of pH to normal.
• Acute respiratory compensation usually occurs within first day. However, chronic respiratory compensation takes 1 to 4 days to occur.
• Renal compensation might occur slower than respiratory compensation.

Primary disorder	pH	PaCO2	[HCO3−]	Compensation	Compensation formula
Metabolic acidosis	↓	↓	↓	Respiratory	Expected paCO2 = 1.5 x serum HCO3− + 8 ± 2 (Winters' formula) Expected paCO2 = Serum HCO3− + 15
Metabolic alkalosis	↑	↑	↑	Respiratory	Expected paCO2 = 0.5 − 1 increase/ every 1 unit increase in serum HCO3− from 24
Respiratory acidosis	↓	↑	↑	Renal	Acute: HCO3− increases by 1mEq/L for every 10 mmHg increase in paCO2 above 40 Chronic: HCO3− increases by 3.5mEq/L for every 10 mmHg increase in paCO2 above 40
Respiratory alkalosis	↑	↓	↓	Renal	Acute: HCO3− decreases by 2mEq/L for every 10 mmHg derease in paCO2 above 40 Chronic: HCO3− decreases by 5mEq/L for every 10 mmHg decrease in paCO2 above 40

Related Chapters

• Renal physiology
• Acid–base imbalance
• Arterial blood gas
• Metabolic acidosis
• Metabolic alkalosis
• Respiratory acidosis
• Respiratory alkalosis
• Anion gap