The document discusses acid-base balance and homeostasis in the human body. It covers three main mechanisms for maintaining pH levels: 1) Buffer systems like carbonic acid-bicarbonate and proteins which temporarily bind excess hydrogen ions. 2) Exhalation of carbon dioxide through breathing, which reduces carbonic acid levels and raises blood pH within minutes. 3) Kidney excretion of hydrogen ions in urine over longer periods, which is needed to eliminate non-volatile acids produced by metabolism. Together these mechanisms tightly regulate pH through feedback loops and keep levels appropriate for cellular functions.

2.  various ions play different roles that help
maintain homeostasis.
 A major homeostatic challenge is keeping the
H concentration (pH) of body fluids at an
appropriate level.
 This task, the maintenance of acid–base
balance, is of critical importance to normal
cellular function.

3.  Most enzymes work only in specific pH (change
in pH → enzymes become inactive)
 Change in pH cause disturbance in electrolytes
-Can affect some hormones
 Acidosis can cause depression of synaptic
ending and lead to coma such as a patient with
diabetes keto acidosis and Hypercalcaemia
 Alkalosis can cause convulsion , muscle
twitching, tetanyand hypocalcemia.

4. A number of processes can alter [H+]
concentration in the body, such as;
 Metabolism of ingested food.
 GI secretions.
 Generation of acids & bases from metabolism
of stored fat & glycogen.
 Changes in CO2 production.

5. •An Acid = a molecule that can release H+ in a solution.
•H2CO3 (carbonic acid)
•HCl (hydrochloric acid)
•A base = a molecule that accepts H+ in a solution.
•Bicarbonate ions (HCO3-).
•Hydrogen phosphate (HPO4-2)
•Strong acid = HCL (complete dissociation)
•Weak acid = Lactic acid,CO2,H2CO3 “Carbonic acid” (Partial
dissociation)
•Strong base = NaOH (complete dissociation)
•Weak base = NaHCO3,HCO3(Partial dissociation)

6.  Food that contain proteins and lipids are rich in acids
 The end cellular metabolism in mitochondria produced
CO2 which source of H+ from the following reaction:
CO2 + H20 ↔ H2CO3 ↔ H+ + HCO3
 Acids in the body are of two kinds:
1.Volatile
Produced by oxidative metabolism of CHO,Fat,Protein
Average 15000-20000 mmol of CO₂ per day
Excreted through LUNGS as CO₂ gas
2.Non-volatile “fixed”
Acids that do not leave solution ,once produced they
remain in body fluids Until eliminated by KIDNEYS
Eg: Sulfuric acid ,phosphoric acid , Organic acids

7. •H+ ion concentrations are expressed as pH.
•pH = - Log [H+]
• If the [H+] increase → pH will decrease
(more acidic)
• If the [H+] decrease → pH will increase
(more alkaline)
What is the normal range of pH?
-in general: 0-14
-in the blood: 7.35-7.45
-Extracellular fluid (ECF): 7.4

9. 1.Buffer systems.
 Buffers act quickly to temporarily bind H+ ,removing
the highly reactive, excess H+ from solution.
 Buffers thus raise pH of body fluids but do not remove
H from the body.
2. Exhalation of carbon dioxide.( Lung )
 By increasing the rate and depth of breathing, more
carbon dioxide can be exhaled.
 Within minutes this reduces the level of carbonic acid in
blood, which raises the blood pH (reduces blood H level).
3. Kidney excretion of H+
 The slowest mechanism, but the only way to eliminate
acids other than carbonic acid, is through their
excretion in urine.

11.  A buffer is a mixture of a weak acid and a weak
base that are in equilibrium.
•A weak acid and its conjugated base (H2CO3 &
NaHCO3).
•A weak base and its conjugated acid (NH3 &
NH4+).
 Buffers prevent rapid, drastic changes in the pH of
body fluids by converting strong acids and bases
into weak acids and weak bases within fractions of
a second.
 Strong acids lower pH more than weak acids
because strong acids release H more readily and
thus contribute more free hydrogen ions.
 Similarly, strong bases raise pH more than weak
ones

12.  There are 3 chemical buffers in the body;
1. The protein buffer system.
2. The carbonic acid – bicarbonate buffer
system.
3. Phosphate buffer system
•They are the 1st line of defense against
changes in pH i.e. [H+], act within seconds.

13.  Is the most abundant buffer in intracellular fluid
and blood plasma
 For example, the protein hemoglobin is an
especially good buffer within red blood cells, and
albumin is the main protein buffer in blood plasma.
 Helps prevent major changes in pH when plasma
PCO2 is rising or falling
 Proteins are composed of amino acids, organic
molecules that contain at least one carboxyl group
(–COOH) and at least one amino group (–NH2);

14.  The free carboxyl group at one end of a
protein acts like an acid by releasing H when
pH rises;
 The free amino group at the other end of a
protein can act as a base by combining with H
when pH falls
 So proteins can buffer both acids and bases.
 As we have already noted, the protein
hemoglobin is an important buffer of H in red
blood cells.

15.  As blood flows through the systemic capillaries,
carbon dioxide (CO2) passes from tissue cells
into red blood cells, where it combines with
water (H2O) to form carbonic acid (H2CO3).
 Once formed, H2CO3 dissociates into H and
HCO3 . At the same time that CO2 is entering
red blood cells, oxyhemoglobin (Hb–O2) is
giving up its oxygen to tissue cells.
 Reduced hemoglobin (deoxyhemoglobin) picks
up most of the H. For this reason, reduced
hemoglobin usually is written as Hb–H.

16. H2O + CO2 → H2CO3
Water Carbon dioxide Carbonic acid
 (entering RBCs)
H2CO3→ H + HCO3
Carbonic acid Hydrogen ion Bicarbonate ion
Hb–O2 + H → Hb–H + O
Oxyhemoglobin Hydrogen ion Reduced Oxygen
(in RBCs) (from carbonic hemoglobin (released to
acid) tissue cells)

19.  carbonic acid–bicarbonate buffer system is
based on the bicarbonate ion (HCO3), which
can act as a weak base, and carbonic acid
(H2CO3), which can act as a weak acid
 Prevents changes in pH caused by organic acids
and fixed acids in ECF
 The reason it is the most important
extracellular buffer system is because it
regulated by kidney and lungs.

20.  If there is an excess of H, the HCO3 can
function as a weak base and remove the excess
H as follows:
H2O + CO2 → H2CO3
 Then, H2CO3 dissociates into water and
carbon dioxide, and the CO2 is exhaled from
the lungs.
 Conversely, if there is a shortage of H, the
H2CO3 can function as a weak acid and
provide H
H2CO3→ H + HCO3
Each element of the buffer system is regulated

22.  Cannot protect ECF from changes in pH that
result from elevated or depressed levels of
CO2
Because CO2 and H2O combine to form
H2CO3, this buffer system cannot protect
against pH changes due to respiratory
problems in which there is an excess or
shortage of CO2.
 Functions only when respiratory system and
respiratory control centers are working
normally
 Ability to buffer acids is limited by availability
of bicarbonate ions

23.  The phosphate buffer system acts via a
mechanism similar to the one for the carbonic
acid–bicarbonate buffer system.
 The components of the phosphate buffer
system are the ions dihydrogen phosphate
(H2PO4) and monohydrogen phosphate (HPO4).
 Plays a major role in buffering intracellular &
renal tubular fluid.
 The dihydrogen phosphate ion acts as a weak
acid and is capable of buffering strong bases
such as OH

24. OH + H2PO4 → H2O + HPO4
Hydroxide ion Dihydrogen Water Monohydrogen
(strong base) phosphate phosphate
(weak acid) (weak base)
 The monohydrogen phosphate ion is capable of
buffering the H released by a strong acid such as
hydrochloric acid (HCl) by acting as a weak base
H + HPO4 → H2PO4
(strong acid) (weak base) (weak acid)
 Because the concentration of phosphates is highest
in intracellular fluid, the phosphate buffer system
is an important regulator of pH in the cytosol

25.  It also acts to a smaller degree in
extracellular fluids and buffers acids in urine.
 H2PO4 is formed when excess H in the kidney
tubule fluid combines with HPO4
 The H that becomes part of the H2PO4 passes
into the urine. This reaction is one way the
kidneys help maintain blood pH by excreting
H in the urine.

27.  Provide only temporary solution to acid base
imbalance
 Buffers do not correct changes in [H+] or
[HCO3-], they only limit the effect of change
on body pH until their concentration is
properly adjusted by either the lungs or the
kidney.
 Supply of buffer molecules is limited

28.  When chemical buffers alone cannot prevent
changes in blood pH, the respiratory system is
the second line of defense against changes.
 The only component regulated here is CO2
which is volatile acids. It cannot deal with fixed
acids such lactic acids that accumulate in
skeletal muscles, which is regulated by
kidneys.
 The simple act of breathing plays an important
role in maintaining the pH of body fluids.

29.  An increase in the CO2 concentration in body
fluids increases H concentration and thus
lowers the pH (makes body fluids more acidic).
 Conversely, a decrease in the CO2
concentration of body fluids raises the pH
(makes body fluids more alkaline).
 Changes in the rate and depth of breathing can
alter the pH of body fluids within a couple of
minutes. With increased ventilation, more CO2
is exhaled.

30.  The pH of body fluids and the rate and depth
of breathing interact via a negative feedback
loop
 When the blood acidity increases, the decrease
in pH (increase in concentration of H) is
detected by central chemoreceptors in the
medulla oblongata and peripheral
chemoreceptors in the aortic and carotid
bodies, both of which stimulate the dorsal
respiratory group in the medulla oblongata.
 As a result, the diaphragm and other
respiratory muscles contract more forcefully
and frequently, so more CO2 is exhaled. As
less H2CO3 forms and fewer H are present,
blood pH increases.

31.  The same negative feedback loop operates if the
blood level of CO2 increases.
 Ventilation increases, which removes more CO2,
reducing the H concentration and increasing the
blood’s pH.
 By contrast, if the pH of the blood increases, the
respiratory center is inhibited and the rate and
depth of breathing decrease. A decrease in the
CO2 concentration of the blood has the same
effect.
 When breathing decreases, CO2 accumulates in
the blood so its H concentration increases.
 •↑↑ [H+] → ↑↑ ventilation (RR) → ↓↓ PCO2
 •↓↓ [H+] → ↓↓ ventilation (RR) → accumulation of
CO2→↑↑ PCO2.

33.  The kidneys are the third line of defense
against wide changes in body fluid PH.
 Metabolic reactions produce nonvolatile acids
such as sulfuric acid at a rate of about 1 mEq
of H per day for every kilogram of body mass.
 The only way to eliminate this huge acid load is
to excrete H in the urine.
Three ways
1.Secreting H+
2.Reabsorbing HCO3-
3.Generating “new” bicarbonate ions.

34.  Hydrogen ion secretion and HCO3− reabsorption
occur in virtually all parts of the tubules except
the descending and ascending thin limbs of the
loop of Henle.
 For each HCO3− reabsorbed, an H+ must be
secreted.
 About 80% to 90% of the HCO3− reabsorption
(and H+ secretion) occurs in the proximal tubule,
so only a small amount of HCO3− flows into the
distal tubules and collecting ducts.
 In the thick ascending loop of Henle, another 10%
of the filtered HCO3− is reabsorbed, and the
remainder of the reabsorption takes place in the
distal tubules and collecting ducts.

37.  The filtrate arriving at the DCT & CT is low
in HCO3-.
 The distal segments of the nephron are
characterised by the presence of
“intercalated cells” capable of actively
secreting H+ through H+-ATPase and
H+-K+ ATPase present on their apical
membrane (Type-A intercalated cells).
 Only a limited number of H+ can be
excreted in its free form in urine.
 Lowest possible urine pH=4.5 → ≈ 0.04
mmol/L of free H+.

38. The extra H+ secreted will need to be buffered
in the tubular lumen
 2 main non-bicarbonate buffers in the tubule
 Phosphate buffer system ( filtered)
H2PO4- ↔ HPO4+ H
 Ammonia buffer system ( synthesizes)
NH4+ ↔ NH3 + H+

39.  The phosphate buffer system is composed of
HPO4=.
 Both become concentrated in the tubular
fluid because water is normally reabsorbed
to a greater extent than phosphate by the
renal tubules.
 Therefore, although phosphate is not an
important extracellular fluid buffer, it is
much more effective as a buffer in the
tubular fluid.
The phosphate buffer
system

40. The phosphate buffer system

41.  Renal tubular cells, especially PCT, are
capable of generating ammonium (NH4+)
“ammoniagenesis” which is then excreted in
urine carrying with it H+.
 The rate of ammoniagenesis can be modified
according to the needs of the body.
 Quantitatively, the ammonia buffer system is
more important than the phosphate buffer
system for H+ excretion in urine.
 It is the most important system in case of
acidosis.

43. To excrete acid:
1.Freely filter HCO3-
2.Reabsorb the majority of
filtered HCO3-
3.Reabsorb some additional
HCO3-
4.Secrete H+ (titrate filtered
bases, i.e. HPO4) and secrete
NH4+
5.Excrete acidic urine containing
NH4+
To excrete base:
1.Freely filter HCO3-
2.Reabsorb the majority of filtered
HCO3-
3.Reabsorb some additional HCO3-
4.Secrete some HCO3-
5.Excrete alkaline urine containing
HCO3-
1.
4.
2.
3.
5.

48. 1)Note whether the pH is low (acidosis) or high
(alkalosis)
2) Decide which value, pCO2 or HCO3- , is
outside the normal range and could be the
cause of the problem. If the cause is a change
in pCO2, the problem is respiratory. If the
cause is HCO3- the problem is metabolic.
If PCO2>45 = Respiratory acidosis
If PCO2<35= Respiratory alkalosis
If HCO3-< 22= Metabolic acidosis.
If HCO3-> 26 = metabolic alkalosis.

49.  The difference between diarrhea and vomiting
:
In diarrhea : cause metabolic acidosis due to
loss of bicarbonate from intestine so the PH
will decrease.
In vomiting : cause metabolic alkalosis due
to loss of HCL so the PH will increase .