howMed Know Yourself
Acids: Liberate free H+ ions
• 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.: H2CO3
Bases: Accept 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
[H+] [(HCO-3]/ [H2 CO3] = K
ACID BASE REGULATION
Buffer: A substance that can reversibly bind H+
The ‘pair of substances’ are involved in reversible reaction:-
– One substance can yield free H+ ions when [H+] decreases.
– Other substance can bind free H+ ions when [H+] starts rising.
Buffer + H+ ↔ H.Buffer
Buffering:
Process by which a strong acid (or base) is replaced by a weaker acid/ base with a consequent reduction in a number of ‘free H+ ions’
i.e. ‘Shock’ of H+ ions is taken up by buffer.
Therefore, change in pH after addition of acid is less than it would be in absence of buffer.
“Buffer solution resists a change in pH”
H+ Cl- + Na+ HCO-3 → H2 CO3 + Na+ Cl-
Strong Acid Buffer weak Acid Neutral salt.
pH
• It is a measure of H+ ion activity, expressed as
–ve log [H+] or = log 1/[H+]
• pH = log to the base 10 to the reciprocal of [H+]
• Thus High [H+] = Low pH
Low [H+] = High pH
• Every unit change in pH actually represents a ten fold change in [H+], because of log relationship.
• Normal [H+] in body = 40 x 10-9 Eq/l
or 40 ± 3 = nEq/l
or 0.00004 mEq/l
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
Addition of H+ into the Body Fluids
• Body’s metabolic processes always produce ‘Acids’
• 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 kidneys
Fixed acids:
• Acids other than H2CO3 are ‘fixed acids’.
• Commonly referred as their ‘anions’ e.g. lactate, phosphate, sulphate, acetoacetate or β-hydroxybutarate. [Amount of anions reflect the no. of H+].
• Net production of fixed acids = 1-1.5 mmoles /kg/day
= 70-100 mmoles of H + /day in adults.
• Fixed acids: produced due to incomplete metabolism of carbohydrates (lactate), fats (ketones) & proteins (sulphate, phosphate)
• Lactate: not commonly included in ‘net fixed acids’ because it is metabolized & not excreted.
Three sources of Continuous Addition of H+ ions:
1. Carbonic Acid Formation : i.e. Hydration of CO2
– Cellular oxidation of nutrients yield = Energy + CO2 + H20
– CO2 is a volatile Acid (Respiratory Acid; H2CO3); 13,000 – 20,000 mM/day
– CO2 itself is ‘neither an acid’ & ‘nor the proton donor’, instead has the
potential to create equivalent amount of H2CO3
2. Inorganic Acid formation:
During break down of proteins, phospholipids
Meat proteins contain sulpher (Sulphuric Acid) & phosphorus (Phosphoric
Acid)
Fixed Acids = Inorganic + Organic Acids = 40 – 60 mM/day
‘Vegetables & Fruits yield bases to neutralize acids’.
3. Organic Acid:
Resulting from intermediary metabolism e.g.
Lactic Acid from Carbohydrates
Fatty Acids from Fats
β-hydroxy butyric & Acetoacetic Acid
Effects of Fluctuations of H+
Only narrow range of pH (6.8 – 8.0) is compatible with life.
1. Changes in excitability of nerve & muscle cells:
↑ [H+] i.e. Acidosis → Depression of CNS
Disorientation & Death.
↓ [H+] i.e. Alkalosis → Over excitability of CNS (convulsions)
First in peripheral Nerves than in CNS
Sensory effects: Tingling (‘pins & needle’ sensation)
Motor effects : Muscle twitches/spasm
Extreme Alkalosis → Spasm of Resp. muscles & Death
2. Changes in Enzyme Activity:
By altering shape & activity of protein molecules.
Some reactions are accelerated, others are depressed.
3. Change in K+ levels in Body Fluids:
Normally in nephron;
* Na+ reabsorption exchanges with secretion of H+ or K+
* K+ ions are secreted more than H+
In Acidosis: H+ ion are eliminated more than K+
i.e. Hyperkalemia → Cardiac dysfunction
Body’s Response to Change in Acid-Base Status
Precise regulation or maintenance of ‘free H+ ions’ in body fluids.
Balance is Achieved by Three Defense Mechanisms:-
• First defense: Chemical buffering
• 2nd defense: Respiratory (alteration in arterial CO2)
• 3rd defense: Renal (alteration in HCO-3 excretion)
