Acid Base Balance Physiology slideshowAcid Base Balance Physiology slideshow

Learning objectives

  • By the end of the session should be able to comprehend:
  • Regulation of Hydrogen ion
  • Production
  • Elimination
  • Concept of acid and a base
  • Defense mechanisms
  • Buffer and its types

Acid Base Balance

• Precise regulation of hydrogen ions
• Balance between intake, production and excretion of H+ ions is
• Optimum hydrogen ion balance is maintained for homeostasis


• pH:

Measure of hydrogen ions,
Normal pH= 7.35-7.45

• pCO2 :
– Partial Pressure of CO2

: is the concentration of CO2

in the blood

– High pCO2

indicates acidosis and vice versa

• HCO3 or Bicarbonate:
– Measures the concentration of HCO3

ions only
– High values indicate alkalosis and vice versa


• Acids: Liberate free H+


– Strong Acid: Greater tendency to dissociate into free H+ & Anions e.g. HCl
– Weak Acid: Only less no of molecules dissociate in solution e.g.: H2 CO3
• Base: Accepts free H+ ions
– Strong Base: is able to bind H+

ions more readily than weak base

• K : Dissociation constant
– Every acid has constant degree of dissociation, when in solution
Dissociated ions [H+

] [(HCO-3

] = K

[H2 CO3

Undissociated Acid


• Normal pH = 7.4
– pH of Arterial Blood = 7.45
– pH of Venous Blood = 7.35
• Normal Range = 6.8 – 8.0 Beyond which death occurs in seconds.
– Acidosis = Blood pH < 7.35 – Alkalosis = Blood pH > 7.45
• pH of ICF is lower than plasma i.e. 6.0 – 7.4
• pH of urine ranges between 4.5 – 8.0
• Extreme Acidic pH in body is of gastric Acid = 0.8

• As a result of metabolic processes ‘Acids’ are produced
• Daily production of H+ ions by metabolism or ingestion with food [e.g.
Citric Acid in oranges] = 80 mEq/L
• Types
i. Respiratory or Volatile acids; CO2
ii. Metabolic or fixed acids
• To maintain balance ‘acids’ are to be; “Excreted” or “Metabolized”
• Non-volatile acids (fixed acids), not excreted by lungs, but excreted by

• Lungs regulate amount of CO2

in the blood

• Kidneys regulate the bicarbonate ions

• Both acidosis and alkalosis can be of two different types:

– Respiratory
– Metabolic

• Respiratory disturbances caused by various malfunctions of the

• Metabolic disturbances are caused by metabolic disorders
which result in an excessive build up or loss of acids or bases

Defense Mechanisms

• Hydrogen ions in the body are regulated by Three Defense
st defense: Chemical buffering
nd defense: Respiratory (alteration in arterial CO2)
rd defense: Renal (alteration in HCO-3 excretion)


• A substance that can reversibly bind H+ ions

Buffer + H+ H Buffer
– If excess reaction shifts to the right
– If deficient reaction shifts to the left
• Normal H+ ions concentration in body = 0.00004mEq/L
– Can alter from 10nEq/L to 160nEq/L
• Daily intake or by metabolism = 80mEq/L
• Buffering system is needed to maintain the concentration


  1. Chemical Buffer system:
    – Responds within seconds
    – Does not eliminate or add H+

from body
– Operates by binding or to tied up H+

till balance is reestablished.


a. In ECF:
• Mainly HCO-3/CO2 Buffer
• Plasma Proteins
• HPO–4/H2PO-4 Buffer system

b. In ICF:
• Proteins Mainly e.g.: Hb in
• HPO–4/H2PO-4 Buffer

• Routes of excretion of acids
– Lungs
– Kidneys

  1. Respiratory Mechanisms:
    – Responds within minutes
    – Takes 6-12 hours to be fully effective
    – Operates by excreting CO2 or (adding H2CO3/HCO-3)
  1. Renal Mechanisms:
    • Responds slowly (effectively in 3-5 days)
    • Eliminates excess Acids or Bases from body
    • The most powerful mechanism
    e.g. i. HCO3/CO2 Buffer system

ii. NH3/NH+

4 Buffer system

iii. HPO–4/H2PO4 Buffer system

Bicarbonate buffer system

• Consist of
– A weak acid
– A bicarbonate salt

CO2+ H2 O H2 CO3

• Slow reaction without carbonic anhydrase
• Present in walls of alveoli and renal tubules

H2 CO3 H++ HCO-3

• Second component

NaHCO3 Na + HCO-3

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