Bioch.of Respiration & PH Regulation SA, 2015(Pre U)
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Transcript of Bioch.of Respiration & PH Regulation SA, 2015(Pre U)
1
BIOCHEMISTRY OF RESPIRATION & pH REGULATION
SADIAH ACHMAD
DEPARTMENT OF BIOCHEMISTRY FACULTY OF MEDICINE
UNISBA
Sadiah Achmad
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RESPIRATION
Respiration is the interchange of 2 gases : O2 & CO2, between the body & the atmosphere. The respiratory system consists of :
• A gases interchange system : organized by the lungs• A gases transport system : organized by the blood
O2 : atmosphere → alveoli → alveolar capillary → blood circulation → cells CO2 : cells → blood circulation → alveolar capillary → alveoli → atmosphere
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The gases interchange system Function of the lungs :
• Interchange gases (O2 & CO2) between the alveolar capillary blood & the atmosphere• Keep the blood pH at its normal level
Gas exchanges between alveoli & alveolar capillaries by : simple diffusion
Alveolar capillary
pO2 : 40 mmHg pO2 : 100 mmHgpCO2 : 45 mmHg pCO2 : 40 mmHg
Alveoli : pO2 : 100 mmHg pCO2 : 40 mmHg.
Atmosphere : pO2 : 160 mmHgpCO2 : 0,2 mmHg
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The gases transport system Transport of O2
O2 is not soluble enough in plasma (only about 2,3 ml O2 can be dissolved in 1 L of plasma at 38◦C ) → O2 need a carrier to meet the body’s needs : hemoglobine (Hb)
• O2 dissolved in plasma : 1-1.5 %
• O2 bound to Hb : 98.5 %
Hb performs 2 major transport functions : 1. transport of O2 from resp. organ to periph.tissues 2. transport of CO2 & H+ from periph. tissues to resp. organ
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Hb is a tetramer of polypeptides
Hb adult : Hb A, Hb A2, Hb F
Hb A consists of 2 subunits & 2 subunits . Hb A2 consists of 2 subunits α & 2 subunits δ .
Hb F consists of 2 subunits α & 2 subunits γ . Each subunit contains 1 heme group with a central Fe2+ in the heme pocket .
1 subunit heme binds 1 O2 mol 1 mol Hb binds 4O2 molecules.
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100
80
60
40
20
0 20 40 60 80 100
Partial pressure of oxygen (mm Hg)
Reduced bloodreturning from tissues
Oxygenated bloodleaving the lungs
Perc
ent s
atur
atio
nOxygen binding curve of hemoglobin A
26
pO2 periph. cap P50pO2 alv. cappO2 ven.bl.
50
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O2 binding curve of Hb in normal blood : a sigmoid curve
- 98% saturated in the lungs
- 33% saturated in working muscle
50% of total Hb is saturated with O2 at pO2 : 26 mmHg
P50 HbA = 26 mmHg expressing O2 affinity of Hb
P50 Hb F = 20 mm Hg this difference permits Hb F in the
fetal blood to extract O2 from Hb A of placental blood.
Post partum : Hb F is unsuitable because of its high O2
affinity, makes HbF delivers less O2 to the tissues
deliver 65% of O2
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The cooperative effect of Hb The 4 subunits of Hb generates a “cooperative effect” in
binding O2 (cooperative binding kinetics of Hb)
- The binding of the first O2 to each Hb mol. (sub units)
enhances the binding of subsequent O2 molecules
- Hb binds O2 weakly at low O2 tension & tightly at high
O2 tension
- Hb binds maximal quantity of O2 at the respiratory organ
& delivers maximal quantity of O2 at peripheral tissues
The cooperativity effect happens because Hb exist in 2 forms :
T form and R form → Quaternary structure
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Quaternary structure of Hb : Hb exists in 2 different forms
deoxy Hb : T (tense) form oxy Hb : R (relax) form
Binding of O2 is accompanied by conformational changes of Hb
C- terminal residues of subunits are held together by salt bonds
1.O2 binding rupture of salt bonds between C–terminal residues of all 4 subunits - binding of O2 to 1 subunit of T - form local conformation changes weakens the association between subunits rupture of bonds - increasing PO2 more molecules are converted to R-form
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2. Upon O2 binding Fe2+ of deoxy Hb moves into the plane of the heme - ring
His. F8 & its associated residues are pulled along with Fe2+
Fe
N
HCCH
N
F helix
Steric repulsion
Porphyrinplane
His F8
C N
HC CHN
F helix
O O
+O2
Fe
C
T form of Hb R form of Hb
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Effect of BPG (2,3 biphosphoglycerate)
RBC contain high levels of BPG, nearly equimolar with Hb
In peripheral tissues, O2 shortage ↑ accumulation of BPG
1 mol of BPG is bound per Hb tetramer in the central cavity of
Hb formed by all 4 subunits
The cavity is sufficient for BPG, only when the space between
chains is wide : Hb in T - form (deoxy Hb)
The cavity is lined with (+) charged groups (i.e. N – terminal
amino groups Val NA 1, Lys EF 6, His H 21 of chains) that
could firmly bind 1 mol of (-) charged BPG by salt bridges
BPG stabilizes deoxy Hb by cross linking its chains
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BPG binds more weakly to Hb F : H21 residue of chains is
Ser. rather than His. (serine is a neutral amino acid)
- can not form salt bridges with BPG
- less effect on stabilization of deoxy Hb
- Hb F has a higher affinity for O2 than HbA
Concentration of BPG in RBC in tissue hypoxia (anemia,
cardiopulmonary insuf, high altitude )
- enhance formation of deoxy Hb at low PO2
- Hb deliver more O2 to the tissues
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Physical factors affecting O2 binding1. temperature
- high temperature weakens O2 affinity of Hb enhance
unloading of O2
- below normal temperature : the binding is tighter
In fever or excercised muscle (elevated temp.) : additional
O2 is needed to support high metabolic rate.
In a hypothermic conditions : Hb’s diminished ability to
release O2 is compensated by :
- O2 utilization within the body
- solubility of O2 in plasma
- solubility of CO2 acidifies the blood
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2. pH
H+ concentration influences the O2 binding of Hb
- low pH weakens O2 affinity of Hb enhancing O2
delivery
- high pH O2 delivery
Decreased in pH is often associated with O2 demand.
Increased metabolic rate production of CO2 & lactic
acid pH ↓ → O2 is released to support the
metabolism
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Transport of CO2
CO2 transport is closely tied to Hb
acid – base balance
CO2 in blood is present in 3 major forms :
- dissolved CO2 : 5.5 %
- bicarbonate ion (HCO3-) : 89 %
- carbamino Hb : 5.5 %
Each is present both in arterial blood & venous blood
Net transport to the lung for excretion : concentration
difference between arterial & venous blood
Venous blood contains only 10% more than total CO2
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• Bicarbonate formation CO2, a metabolic product, enters the bloodstream & diffuses into RBC generates H+, most come from formation of HCO3
-
. This reaction is catalyzed by carbonic anhydrase.
CO2 + H2O H2C03 H+ + HC03-
Carbonic anhydrase : - found abundantly inside RBC - a very active enzyme : reaction
reaches equilibrium within 1 sec
- contains Zn
Most of H+ generated during HCO3- production is handled by
buffer action and / or other processes
carbonic anhydrase Spontaneous
rapid
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Carbamino - Hb formation, also generates H+
CO2 forms carbamino substances with amino groups of any
proteins within RBC, mostly Hb
Deoxy - Hb forms carbamino - Hb more readily than oxy Hb
Oxygenation causes release of CO2 in carbamino Hb
Carb. Hb formation occurs only with uncharged aliphatic
amino – groups, not with charged form R - NH3+
H HR – N + CO2 R – N + H+
H C – O ll carbamino
O substancecov. bond
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- N–terminal amino groups of globin chains are the
principle sites of carbamination
If block chemically by cyanate carbamino formation
does not occur
- N–terminal amino group of -globin is also the binding site
for BPG CO2 competes with BPG in binding Hb mol.
CO2 ↑ : effect of BPG
BPG ↑: the ability of Hb to form carbamino-Hb
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Two processes regulate [ H+ ] derived from CO2 transport
1. Buffering Hb carrying O2
carrying CO2 as carb. Hb handling H+ produced by CO2 transport :
- buffering- isohydric mechanism
Hb buffer capacity is provided by its ionizable groups : - 4 N - terminal amino groups
- imidazole side chains of His. res (38 His. per Hb tetramer)
The buffer system handles 60% of H+ produced by CO2 transport : buffered by Hb : 50%
other buffers : 10% - HCO3
- / CO2
- organic - P in RBC - plasma proteins
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2. Isohydric mechanism
The remainder of H+ (40%) arising from CO2 is taken
up by Hb : isohydric carriage of CO2
This system takes up H+ ions with no change in blood
pH, through the operation of Bohr Effect
Hb binds 2 protons for every 4 O2 molecules released.
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2H2O + 2CO2 CO2
2H2CO3
2HCO3–
+ 2H+
Hb.4O2
Hb.2H+ + 4O2 O2
Hb.2H+ + 4O2 O2
Hb.4O2
2HCO3- + 2H+
2H2CO3
2H2O + 2CO2 CO2
HCO3–
Cl– Cl–
Cl– Cl–
HCO3–
exhaled toatmosphere
to peripheraltissues
generated bythe Krebs cycle
ArteryVeinVenous retum
Chloride shift
pulmonarycapillaries
extra pulmonarycapillaries
From atmosphere
The Bohr effect
Carb. anh.
carb. anh.
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pH REGULATIONMaintenance of relatively constant blood pH value is essential
for health, since changes in blood pH will affect intracell pH
alter : - protein conformation
- enzyme activity, metabolism
- equilibria of reactions that consume / generate H+
(oxidation - reduction reaction)
Maintenance of a constant blood pH is, in part, achieved by :
- The buffer system in the blood control short - term
changes in blood pH.
- for long term changes : balancing proton (H+) loss &
gain by the lung & the kidney
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pH value of blood may be affected by :
- malfunctioning of the blood buffer system or
- disturbance of acid-base balance due to diseases
e.g. - kidney disease or
- altered breathing frequency
( hypo / hyperventilation)
Normal arterial plasma pH : 7.40 0.05
- pH < 7.35 acidosis
- pH > 7.45 alkalosis
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BUFFER SYSTEM
3 major body water components : - plasma : within circulating system - interstitial fluid : fluid that bathes cells - intracellular fluidComposition : - plasma : - major cation : Na+
- small amounts : K+, Ca2+, Mg2+
- dominant anions : HCO3-, Cl-
- small amount anion : protein, HPO42-, SO4
2-
- mixture organic anions - interstitial fluid : similar with plasma, but contain less protein plasma & interst. fluid extracell. fluid - intracell fluid : - major cation : K+
- major anions : - organic phosphates - protein
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The buffer system in the blood• Blood plasma is a mixed buffer system :
H2CO3 / HCO3- , H2PO4
- / HPO42- , H protein / protein-
* Major buffer of plasma: H2CO3 / HCO3- system
- an open system: pCO2 is adjusted to meet the body,s
need by increasing / decreasing respiration.
- effective in controlling pH changes caused by other
than pCO2 changes
* Major buffer inside RBC : Hb buffer system
(HHb/Hb) → also responsible for buffering pCO2 changes
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A buffer system consists of : - a weak acid : H A
- its conjugate base : A-
e.g. Acetic acid / acetate-, NH4+
/ NH3, H2PO4- / HPO4
2-
The weak acid might be neutral, (+) charged, (-) charged
The conjugate base : 1 less (+) charge or 1 more (-) charge
than its weak acid
The Henderson - Hesselbalch equation :
pH = pKa + log
direct relationship between pH & ratio [conj. base] / [acid]
Ratio ↑ → pH ↑ or Ratio ↓ → pH ↓
[conj. base ] [acid]
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[HCO3-] / [CO2] buffer system :
Blood pH : 7,4
p.K.H2CO3 : 6,1 (1.3 = log 20)
In order to keep the blood pH at its normal level (7.4) →
[HCO3-] / [CO2] ratio must be 20/1
Every changes in [HCO3-] or [CO2] changes the ratio
changes pH body compensation is needed to
normalize blood pH.
7,4 = 6,1 + log 20/1
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ACID – BASE BALANCE & ITS MAINTENANCE
• Acidosis : excess acid or def. of alkali in the body
• Alkalosis : excess alkali or def. of acid in the body
There exist mechanisms where the body normally rids itself of
excess acid or alkali
• Metabolism in individuals produces large amounts of acids
• Major acid: H2CO3 (CO2) volatile: excreted by the lungs
Disorder of the lungs respiratory acidosis or alkalosis
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Respiratory acidosis : hypoventilation CO2 accumulates
within the blood → shift direction of the reaction :
H2O + CO2 ↔ H2CO3 ↔ H+ + HCO3- → to the right
Hypoventilation occurs when depth or rate of respiration ↓
- airway obstruction
- neuromuscular disorders
- diseases of CNS
- chronic obstr lung disease → chronic resp acidosis
- inhalation of gas with high pCO2 → resp. acidosis
Acute resp. acidosis
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Respiratory alkalosis : excess of breathing → CO2 will be
exhaled abundantly. Direction of reaction will shift to the left
H2O + CO2 ↔ H2CO3 ↔ H+ + HCO3-
Hyperventilation : - anxiety : most common cause
- CNS injury involving resp. center
- salicylate poisoning
- fever
- artificial ventilation
High altitude alv. pCO2 chronic resp. alkalosis
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Metabolic acidosis
The body metabolism produces many nonvolatile (fixed) acids
- protein metabolism → H+ + SO42-
- hydrolysis of phosphate-esters phosphoric acid
- metabolism - lactic acid
- acetoacetic acid
- -hydr. butyric acid
- administration of : NH4Cl / Arg-HCl / Lys-HCl → urea + HCl
produced in excess accumulation acidosis
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- ingestion of salicylate, methyl alcohol, ethylene glycol
production of strong organic acid
accumulation of nonvolatile acids metabolic acidosis
- abnormal loss of base (HCO3-)
- renal tubular acidosis : abnormal amount of HCO3-
escape from blood into urine
- severe diarrhea fecal excretion of HCO3-
→ HCO3 -
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Metabolic alkalosis
- intake excess of alkali / salt of organic acid :
- NaHCO3
- Na – lactate
- abnormal loss of acids : vomiting, gastric lavage
- rapid loss of body water : diuresis temporary [HCO3-]
in blood
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Causes of acid – base imbalance
Acidosis :
Respiratory : alveolar hypoventilation
Metabolic : - H+ overproduction
- HCO3- overexcretion
Alkalosis :
Respiratory : alveolar hyperventilation
Metabolic : - alkali ingestion
- H+ overexcretion
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Normal
Acidosis- Respiratory- Metabolic
Alkalosis- Respiratory- Metabolic
Blood pH
7,4
Urine pH
6 –7
[HCO3- ]/
[H2CO3]
20/1
20 / > 1< 20 / 1
20 / < 1>20 / 1
Cause
HypoventilationH+ production orHCO3
- excretion
HyperventilationAlkali ingestion or H+ excretion
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(1)
(10)
(<1)
(20)
(>1)
(20)
(1)
(20)
(1)
(>20)
(+) (-) (+) (-) (+) (-) (+) (-) (+) (-)Normal Resp.
acidResp.alkal
Met.acid
Metalkal
= H+
= HCO3-
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Role of kidney in the acid–base balance system
Excess of nonvolatile (fixed) acids & bicarbonates are excreted by the kidney pH urine varies between 4.4 - 8.0 Daily urine volume : 1,2 L
• Formation of urine Fundamental functional unit of kidney : the nephron :
- filter the blood - modify the glomerular filtrate to become urine
• Glomerulus : capillaries enclosed by glomerular capsule Water & low molecular weight solutes (inorganic ions, urea, sugars, amino acids pass the capillaries membrane into the capsular space ultra filtrate
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• Proximal convoluted tubule :
- most of water & solutes are reabsorbed tubular
fluid continues through the loop of Henle
• Distal convoluted tubule :
- further reabsorption of solutes
- secretion
• Collecting tubule :
- additional concentration urine :
Urine contains 1% or less of water & solutes of
glomerular filtrate
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41Nephron
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• The Kidney regulates acid - base balance by :- controlling bicarbonate (HCO3
-) reabsorption
- secreting acid (H+)
Both processes depend on the formation of H+ & HCO3- from
CO2 & H2O within the tubular cells
- H+ formed is actively secreted into tubular fluid in
exchange with Na+
- Na+ uptake transport by the tubule :
- partly passive
- partly active via antiport systems which reabsorb Na+
in exchange with H+,K+ or NH4+
- Na+ reabsorbed in exchange with H+ regeneration of
NaHCO3 within the tubular cell out of the cell plasma
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Fates of excreted H+
1. React with HCO3- H2CO3 ↔ H2O + CO2
CO2 will be absorbed from tubular fluid & reassociated
with H2O to form H2CO3. The dissociated HCO3- is
transported back to the plasma: reabsorption of NaHCO3.
Tubular fluid becomes depleted of HCO3- pH drops
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2. Buffered by HPO42- / H2PO4
- buffer system
H2PO4- is not reabsorbed passes out in the urine
represents net excretion of H+
In diabetic ketoacidosis : plasma acetoacetate & -hydroxy
butyrate & pass the glomerulus into tubular fluid → could
serve as buffers Effect of buffering : - excrete acid (H+)
- regenerate bicarbonate The amount of acid excreted could be measured → titratable acidity of urine : 1/3-1/2 of normal daily acid excretion
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3. Neutralization by NH3 H++ NH3 → NH4
+ elimination of NH4+ contributes
to net acid excretion
Tubular cells produce NH3 from amino acids, particularly glutamine ( tubular cells contain a lot of glutaminase)
NH4+ is the major urinary acid : ½ - 2/3 of daily acid excretion
This mechanism is more important in acidosis : - acid can be excreted without lowering pH of urine (formation of titratable acidity causes decrease in pH urine) - ammonia can be excreted in an enormous amount - spares body stores of Na+ & K+
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Compensatory mechanisms 4 primary types of acid – base imbalance
- resp. acidosis : caused by plasma pCO2
- resp. alkalosis : caused by plasma pCO2
- metab. acidosis : - addition of strong organic /
inorganic acid
- loss of HCO3-
plasma [HCO3-]
- metab. alkalosis : - loss of acid
- ingestion of alkali
plasma HCO3-
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Principles of acid-base imbalance compensation
Abnormal condition directly altered 1 term of [HCO3-] / [CO2]
If one of the term of [HCO3-] / [CO2] ratio is altered plasma pH can be readjusted back toward normal by compensatory alteration of the other term.
e.g ketoacidosis plasma [HCO3-]
Compensation : decreasing plasma [CO2] so that the ratio &
therefore pH is readjusted back toward normal.
Compensation does not have to return the HCO3- & CO2 blood
levels toward normal
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• If [HCO3-] changes to restore the [HCO3
-] / [CO2] ratio
is to change pCO2 in the same direction
• If primary change is pCO2 to restore the original ratio is
to alter [HCO3-] in the same direction
Specific compensatory processes• Acute resp. acidosis (eg breathing gas mixture containing
high level of CO2) pCO2 - plasma pH
- [HCO3-]
compensation : - renal excretion of H + & bicarbonate
reabsorption [HCO3-] , although its
level is already above normal
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• Acute resp. alkalosis pCO2 rapidly - pH
- [HCO3-]
Compensation : - renal excretion of [HCO3-] →
- plasma [HCO3-] →
- plasma pH toward normal• Metab. acidosis : 2 compensatory mechanisms in dealing with the excess acid
1. Renal H+ excretion : slow, inadequate to return [HCO3-]
& pH to normal
2. Respiratory compensation : hyperventilation pCO2
Compensation of metab. acidosis involves :
- pCO2
- small in [HCO3-]
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• Metab. alkalosis
- primary defect : plasma [HCO3-]
- immediate physiologic response : hypoventilation,
followed by renal excretion of HCO3-
Hypoventilation - pCO2
- small in [HCO3-]
Respiratory response to metabolic acid-base imbalance
is a rapid response
Sadiah Achmad