O₂ & CO₂ TRANSPORT — LOGIC-BASED MASTER NOTE

1️⃣ Why O₂ and CO₂ move at all (Driving force)

Core principle

• Gases move down their partial pressure gradient

High partial pressure → Low partial pressure

In lungs

• PO₂
• Highest: Alveoli (after inspiration)
• Lowest: Deoxygenated pulmonary capillary blood

→ O₂ diffuses alveoli → blood

• PCO₂
• Highest: Venous blood
• Lowest: Alveoli

→ CO₂ diffuses blood → alveoli → expired

✔ Diffusion alone is not enough to meet tissue demand

2️⃣ Why diffusion alone is insufficient

Key problem

• O₂ and CO₂ have low solubility in plasma

Physiologic solution

• O₂: ~99% binds hemoglobin
• CO₂: ~94.5% enters reversible chemical reactions

Result

• Hemoglobin ↑ O₂ carrying capacity 70-fold
• CO₂ reactions ↑ CO₂ carrying capacity 17-fold

3️⃣ Oxygen delivery to tissues (Definition & determinants)

Definition

Oxygen delivery (DO₂)

= Cardiac Output × Arterial O₂ content

Depends on TWO systems

• Respiratory: O₂ entry + gas exchange
• Cardiovascular: Blood flow to tissues

Tissue O₂ delivery depends on

1. Amount of O₂ entering lungs
2. Adequacy of pulmonary gas exchange
3. Blood flow to the tissue
4. Blood’s O₂ carrying capacity

Blood flow to tissue depends on

• Cardiac output
• Degree of local vascular constriction

O₂ content of blood depends on

• Dissolved O₂
• Hemoglobin concentration
• Hemoglobin affinity for O₂

4️⃣ Hemoglobin structure → O₂ binding ability

Structure

• Protein with 4 subunits
• Normal adult Hb = 2 α + 2 β chains
• Each chain has 1 heme
• Each heme contains ferrous (Fe²⁺) iron

Binding

• Each Fe²⁺ binds 1 O₂ molecule
• Total = 4 O₂ per hemoglobin

✔ Iron stays ferrous → oxygenation, not oxidation

5️⃣ Stepwise oxygenation reaction

Hb reacts sequentially:

• Hb₄ + O₂ ⇌ Hb₄O₂
• Hb₄O₂ + O₂ ⇌ Hb₄O₄
• Hb₄O₄ + O₂ ⇌ Hb₄O₆
• Hb₄O₆ + O₂ ⇌ Hb₄O₈

Speed

• Oxygenation: < 0.01 s
• Deoxygenation: equally rapid

6️⃣ Tense–Relaxed (T–R) transition → cooperative binding

Deoxyhemoglobin

• T (tense) state
• Subunits tightly bound
• Low O₂ affinity

First O₂ binding

• Breaks inter-subunit bonds
• Switches to R (relaxed) state
• Exposes more binding sites

Result

• 500-fold increase in O₂ affinity
• Each bound O₂ increases affinity for next one

In tissues

• Reverse process → O₂ released

✔ Hb switches T ↔ R ~10⁸ times during RBC lifespan

7️⃣ Oxygen–hemoglobin dissociation curve

What it shows

• Y-axis: % Hb saturation (SaO₂)
• X-axis: PO₂

Shape

• Sigmoid
• Due to cooperative binding (T–R transition)

Important implication

• Low PO₂ region = steep slope
• Small ↓ PO₂ → large ↓ saturation → easy O₂ unloading

1️⃣3️⃣ Factors shifting Hb–O₂ affinity

Three major factors

1. pH
2. Temperature
3. 2,3-DPG

1️⃣4️⃣ P50 (Affinity index)

• P50 = PO₂ at 50% saturation
• ↑ P50 → ↓ affinity
• ↓ P50 → ↑ affinity

1️⃣5️⃣ pH & Temperature effects

Right shift (↓ affinity, ↑ P50)

• ↓ pH (acidosis)(H+ increase)
• ↑ temperature

Left shift (↑ affinity, ↓ P50)

• ↑ pH (alkalosis)(H+ decrease)
• ↓ temperature

1️⃣6️⃣ Bohr effect

Definition

• ↓ pH (acidosis)→ ↓ Hb affinity for O₂

Mechanism

• DeoxyHb binds H⁺ more than oxyHb
• ↑ CO₂ → ↓ pH → right shift

Contribution

• Tissue unsaturation mainly due to ↓ PO₂
• Extra 1–2% due to ↑ PCO₂ (Bohr effect)

1️⃣7️⃣ Role of 2,3-DPG

Source

• Produced in RBCs via Embden–Meyerhof glycolysis
• From 3-phosphoglyceraldehyde

Action

• Highly charged anion
• Binds β chains of deoxyHb
• 1 mol deoxyHb binds 1 mol 2,3-DPG

Reaction

HbO₂ + 2,3-DPG ⇌ Hb–2,3-DPG + O₂

↑ 2,3-DPG → ↑ O₂ release → right shift

1️⃣8️⃣ Factors affecting 2,3-DPG

Decrease

• Acidosis (inhibits glycolysis)

Increase

• Thyroid hormones
• Growth hormone
• Androgens
• Exercise (within ~60 min, not always in trained athletes)
• Pregnancy

1️⃣9️⃣ Exercise & O₂ unloading

During exercise:

• ↑ temperature
• ↑ CO₂
• ↓ pH
• ↑ 2,3-DPG
• ↓ tissue PO₂

Result

• ↑ P50
• Steep part of curve used
• Large O₂ unloading per PO₂ drop
• Right shift curve

2️⃣0️⃣ Myoglobin vs Hemoglobin

Myoglobin

• Found in skeletal muscle
• Binds 1 O₂ only
• No cooperative binding
• Curve = rectangular hyperbola
• Higher O₂ affinity (left-shifted)

Functional significance

• Facilitates O₂ transfer from Hb → muscle
• Releases O₂ only at very low PO₂
• Important during:
• Sustained contraction
• Reduced blood flow
• Exercise

✔ Highest in muscles specialized for sustained activity

CARBON DIOXIDE TRANSPORT

I. WHY CO₂ IS EASY TO TRANSPORT

High solubility

• CO₂ is ~20× more soluble than O₂ in blood.
• 👉 Therefore, at equal partial pressures, much more CO₂ than O₂ is present as dissolved gas.
• This high solubility underlies all downstream transport mechanisms.

II. MOLECULAR FATE OF CO₂ IN BLOOD (STEP-BY-STEP)

Once CO₂ diffuses from tissues → blood → red blood cells (RBCs), it has three main fates.

1️⃣ CO₂ → Bicarbonate (MAJOR PATHWAY)

Inside RBC

• CO₂ + H₂O⟶ (carbonic anhydrase) ⟶H₂CO₃ (carbonic acid)
• H₂CO₃ rapidly dissociates:
• H₂CO₃ ⟶ H⁺ + HCO₃⁻

Handling of products

• H⁺
• Buffered mainly by hemoglobin
• HCO₃⁻
• Accumulates inside RBC → must be exported

👉 This pathway accounts for ~70% of total CO₂ transport.

2️⃣ CO₂ → Carbamino Compounds

• CO₂ reacts with amino (–NH₂) groups of:
• Hemoglobin
• Other plasma proteins

Reaction

CO₂ + R–NH₂ ⇌ R–NH–COOH

• Forms carbamino-CO₂
• About 11% of CO₂ is transported this way.

3️⃣ CO₂ Remains Dissolved

• Because of high solubility:
• A small fraction remains physically dissolved
• Important for:
• Setting PCO₂
• Driving diffusion gradients

III. HALDANE EFFECT (CRITICAL CONCEPT)

Key observation

• Deoxyhemoglobin:
• Binds H⁺ more strongly
• Forms carbamino compounds more readily
• Oxyhemoglobin:
• Has reduced affinity for CO₂ and H⁺

Definition

Haldane effect = Increased ability of deoxygenated hemoglobin to bind and carry CO₂.

Physiological consequences

• In tissues
• O₂ unloads → Hb becomes deoxyHb
• CO₂ uptake ↑
• In lungs
• O₂ binds Hb
• CO₂ affinity ↓ → CO₂ released

👉 Therefore:

• Venous blood carries more CO₂ than arterial blood
• CO₂ loading in tissues and unloading in lungs is optimized

IV. CHLORIDE SHIFT (HAMBURGER PHENOMENON)

Why it happens

• Large amounts of HCO₃⁻ are formed inside RBC
• RBC cannot accumulate excess negative charge

Mechanism

• ~70% of HCO₃⁻ exits RBC → plasma
• In exchange:
• Cl⁻ enters RBC

Transporter

• Anion Exchanger 1 (AE1)
• Also called Band 3
• Major RBC membrane protein

Consequences

• Venous RBCs:
• Higher Cl⁻ content
• Higher osmolarity
• Chloride shift:
• Very rapid
• Essentially complete within 1 second

V. RBC VOLUME & HEMATOCRIT CHANGES

Osmotic effect

• For each CO₂ molecule added:
• One osmotically active particle appears:
• HCO₃⁻ or Cl⁻
• Water enters RBC → cell swelling

Result

• Venous hematocrit ~3% higher than arterial
• Also aided by lymphatic return of arterial plasma

In lungs

• Reverse chloride shift
• RBCs shrink back to normal size

VI. SPATIAL DISTRIBUTION OF CO₂ IN BLOOD

Arterial blood (per 100 mL)

• Total CO₂ ≈ 49 mL
• Dissolved: 2.6 mL
• Carbamino: 2.6 mL
• Bicarbonate: 43.8 mL

CO₂ added in tissues (per 100 mL)

• 3.7 mL added
• Dissolved: 0.4 mL
• Carbamino: 0.8 mL
• Bicarbonate: 2.5 mL

pH change

• Arterial pH: 7.40
• Venous pH: 7.36

In lungs

• Entire 3.7 mL CO₂ released
• At rest:
• ~200 mL CO₂/min excreted
• Over 24 hours:
• Equivalent to >12,500 mEq of H⁺

VII. ACID–BASE BALANCE & GAS TRANSPORT

Main acid source

• Cellular metabolism → CO₂ → H₂CO₃ → H⁺

Handling

• Lungs
• Excrete CO₂ (major load)
• Kidneys
• Excrete residual H⁺
• Regulate plasma HCO₃⁻

VIII. BUFFER SYSTEMS IN BLOOD

Three major buffers

1. Plasma proteins
2. Hemoglobin
3. Carbonic acid–bicarbonate system (MOST IMPORTANT)

1️⃣ Protein buffers

• Carboxyl (–COOH) and amino (–NH₂) groups dissociate
• Moderate buffering capacity

2️⃣ Hemoglobin buffer (VERY POWERFUL)

Why Hb is special

• Contains 38 histidine residues
• Imidazole groups buffer H⁺ efficiently
• Hb concentration is very high

👉 Buffering capacity = ~6× plasma proteins

Oxygenation matters

• DeoxyHb
• Weaker acid
• Better buffer
• OxyHb
• Stronger acid
• Buffers less

(Titration curves show this clearly)

3️⃣ Carbonic Acid–Bicarbonate System

Core reaction

H₂CO₃ ⇌ H⁺ + HCO₃⁻

Henderson–Hasselbalch

• Ideal:

pH = pK + log([HCO₃⁻]/[H₂CO₃])

• Physiologic (using CO₂):

pH = 6.10 + log([HCO₃⁻]/[CO₂])

• Clinical form:

pH = 6.10 + log([HCO₃⁻]/(0.0301 × PCO₂))

Measurements

• pH → electrode
• PCO₂ → electrode
• HCO₃⁻ → calculated

IX. WHY THIS SYSTEM IS SO EFFECTIVE

1️⃣ Open system

• CO₂ controlled by ventilation
• Kidneys control HCO₃⁻

2️⃣ Carbonic anhydrase localization

• Present in RBC
• Absent in plasma
• Reaction spatially controlled

3️⃣ Hemoglobin assistance

• Binds H⁺
• Allows HCO₃⁻ to move into plasma

Net effect

• Massive acid loads tolerated
• pH maintained near normal

Without CO₂ removal:

• pH could drop from 7.4 → 6.0With compensation:
• pH only falls to ~7.2–7.3

X. TABLE 35–2 — FATE OF CO₂ (COMPLETE)

In Plasma

1. Dissolved CO₂
2. Carbamino compounds with plasma proteins
3. Hydration → H⁺ buffered → HCO₃⁻ in plasma

In Red Blood Cells

1. Dissolved CO₂
2. Carbamino-Hb
3. Hydration → H⁺ buffered → 70% HCO₃⁻ enters plasma
4. Cl⁻ shifts in → osmolarity ↑ → cell swelling

FINAL INTEGRATION (EXAM GOLD)

• CO₂ transport is inseparable from acid–base balance
• RBC + hemoglobin + carbonic anhydrase form a functional unit
• Lungs and kidneys act as dual regulators
• Haldane effect + chloride shift make CO₂ transport efficient and directional

ACIDOSIS, ALKALOSIS & HYPOXIA

I. NORMAL pH & DEFINITIONS

Normal values

• Arterial plasma pH ≈ 7.40
• Venous plasma pH slightly lower (due to higher CO₂)

Definitions (technical)

• Acidosis → arterial pH < 7.40
• Alkalosis → arterial pH > 7.40

⚠️ In practice:

• Variations of ±0.05 pH units occur without harm

II. CLASSIFICATION OF ACID–BASE DISORDERS

There are four primary categories:

1. Respiratory acidosis
2. Respiratory alkalosis
3. Metabolic acidosis
4. Metabolic alkalosis

👉 These disorders may occur in combination

III. RESPIRATORY ACIDOSIS

Primary problem

• ↑ Arterial PCO₂ (> 40 mm Hg)

Cause

• Hypoventilation
• CO₂ retention

Mechanism

• Retained CO₂:
• CO₂ ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻
• ↑ CO₂ → ↑ H₂CO₃ → ↑ H⁺
• Plasma HCO₃⁻ rises, but pH falls

Graphical representation

• Shift on HCO₃⁻ vs pH curve
• Initial change reflects uncompensated respiratory acidosis

Key determinant

• Degree of pH change depends on:
• Blood buffering capacity

IV. RESPIRATORY ALKALOSIS

Primary problem

• ↓ Arterial PCO₂ (< 35 mm Hg)

Cause

• Hyperventilation
• Excess CO₂ removal

Mechanism

• ↓ CO₂:
• Drives reaction backward
• ↓ H₂CO₃ → ↓ H⁺
• Result:
• ↑ pH

Initial state

• Changes before compensation =Uncompensated respiratory alkalosis

V. METABOLIC ACIDOSIS

Primary problem

• Addition of strong acids
• OR failure to excrete acids

Example

• Aspirin overdose
• Organic or inorganic acids added

Mechanism

• ↑ H⁺ → HCO₃⁻ consumed
• H₂CO₃ formed → converted to:
• CO₂ + H₂O
• CO₂ is rapidly excreted by lungs

Key distinction

• PCO₂ does NOT change initially
• Shift occurs along an isobar line

State

• Uncompensated metabolic acidosis

VI. METABOLIC ALKALOSIS

Primary problem

• Loss of H⁺ or addition of alkali

Common cause

• Vomiting
• Loss of gastric acid

Mechanism

• ↓ H⁺ → ↑ pH
• Occurs along isobar line
• Initial state = uncompensated metabolic alkalosis

VII. COMPENSATION MECHANISMS

Uncompensated states are rare due to powerful compensation.

Two main compensatory systems

1. Respiratory compensation
2. Renal compensation

VIII. RESPIRATORY COMPENSATION (FAST)

Target

• Correct metabolic acid–base disorders

In metabolic acidosis

• Ventilation ↑
• PCO₂ ↓ (e.g., 40 → 20 mm Hg)
• ↓ CO₂ → ↑ pH toward normal

In metabolic alkalosis

• Ventilation ↓
• PCO₂ ↑
• pH ↓ toward normal

⚠️ Important note:

• Compensation starts immediately
• Large pH swings shown in graphs do not occur in reality

IX. RENAL COMPENSATION (SLOW BUT COMPLETE)

Required for full compensation of:

• Respiratory disorders
• Persistent metabolic disorders

Renal response to ACIDOSIS

Tubular cell mechanisms

• Carbonic anhydrase present
• CO₂ + H₂O → H⁺ + HCO₃⁻

Actions

• H⁺ actively secreted into tubule (Na⁺/H⁺ exchange)
• HCO₃⁻ reabsorbed into blood

👉 For each H⁺ secreted:

• 1 Na⁺ + 1 HCO₃⁻ added to blood

Effect

• Plasma pH moves toward normal
• Seen as shift from acute → chronic respiratory acidosis

Renal response to ALKALOSIS

Actions

• ↓ H⁺ secretion
• ↓ HCO₃⁻ reabsorption

Effect

• Excess HCO₃⁻ lost
• pH decreases toward normal
• Seen as shift from acute → chronic respiratory alkalosis

X. CLINICAL EVALUATION OF ACID–BASE STATUS

Essential measurements

• pH
• PCO₂
• HCO₃⁻ (calculated)

Measurement tools

• pH electrode
• CO₂ electrode

Arterial vs venous differences

• Venous PCO₂ ≈ 8 mm Hg higher
• Venous pH ≈ 0.03–0.04 lower
• Venous HCO₃⁻ ≈ 2 mmol/L higher

👉 Free-flowing venous blood is acceptable clinically if adjusted.

XI. ANION GAP (METABOLIC ACIDOSIS TOOL)

Definition

• Difference between:
• Unmeasured cations
• Unmeasured anions

Normal value

• ≈ 12 mEq/L

Major contributors

• Proteins (albumin)
• HPO₄²⁻
• SO₄²⁻
• Organic acids

Increased anion gap

• ↓ K⁺, Ca²⁺, Mg²⁺
• ↑ plasma proteins
• Accumulation of:
• Lactate
• Ketones
• Foreign anions

👉 Seen in:

• Ketoacidosis
• Lactic acidosis

Decreased anion gap

• ↑ cations
• ↓ albumin

XII. HYPOXIA — DEFINITION

Hypoxia

• O₂ deficiency at tissue level

⚠️ Preferred over “anoxia”:

• True absence of O₂ is rare

XIII. FOUR TYPES OF HYPOXIA

1️⃣ Hypoxemic (Hypoxic) Hypoxia

• ↓ arterial PO₂

2️⃣ Anemic Hypoxia

• Normal PO₂
• ↓ hemoglobin concentration

3️⃣ Ischemic (Stagnant) Hypoxia

• Normal PO₂ & Hb
• ↓ blood flow

4️⃣ Histotoxic Hypoxia

• Adequate O₂ delivery
• Cells cannot utilize O₂
• Due to toxins (e.g., cyanide)

XIV. CELLULAR EFFECTS OF HYPOXIA

Hypoxia-inducible factors (HIFs)

Normal O₂

• HIF-α ubiquitinated → destroyed

Hypoxia

• HIF-α + HIF-β dimerize
• Activate genes for:
• Angiogenesis
• Erythropoietin
• Adaptive proteins

XV. EFFECTS OF HYPOXIA ON THE BRAIN

Brain affected first

Severe hypoxia

• Inspired PO₂ < 20 mm Hg
• Loss of consciousness: 10–20 s
• Death: 4–5 min

Moderate hypoxia

• Mental changes:
• Impaired judgment
• Drowsiness
• Disorientation
• Loss of time sense
• Headache
• Autonomic effects:
• Tachycardia
• Hypertension (severe)
• GI symptoms:
• Anorexia
• Nausea
• Vomiting

Ventilatory response

• Carotid chemoreceptors stimulated
• Ventilation ↑ with severity of hypoxia

XVI. RESPIRATORY STIMULATION & BREATHING TERMS

Dyspnea

• Conscious sensation of difficult breathing

Hyperpnea

• ↑ rate or depth of breathing
• Independent of subjective discomfort

Tachypnea

• Rapid, shallow breathing

Normal perception

• Breathing unnoticed until ventilation doubles
• Discomfort when ventilation 3–4 × normal

Causes of dyspnea

• Hypercapnia (major)
• Hypoxia (lesser)
• ↑ work of breathing

FINAL INTEGRATED EXAM LOGIC

• Acid–base balance is governed by:
• CO₂ (lungs)
• HCO₃⁻ (kidneys)
• Compensation is:
• Fast → respiratory
• Complete → renal
• Hypoxia classification depends on:
• PO₂
• Hb
• Blood flow
• Cellular utilization

HYPOXEMIA, OTHER FORMS OF HYPOXIA, CO₂ DISTURBANCES & OXYGEN THERAPY — COMPLETE LOGIC NOTE

I. HYPOXEMIA — CORE DEFINITION

Hypoxemia

• Defined as reduced arterial PO₂
• It is the most common clinical form of hypoxia

Seen in

• Normal individuals at high altitude
• Respiratory diseases:
• Pneumonia
• Pulmonary fibrosis
• COPD
• ARDS
• V/Q mismatch
• Shunts

II. EFFECTS OF DECREASED BAROMETRIC PRESSURE (ALTITUDE PHYSIOLOGY)

Key principle

• Air composition remains constant
• Total barometric pressure ↓ with altitude
• Therefore:
• Inspired PO₂ ↓
• Alveolar PO₂ ↓
• Arterial PO₂ ↓ → hypoxemia

Altitude milestones

III. BREATHING 100% O₂ AT HIGH ALTITUDE

Limiting factors

• Water vapor pressure in alveoli = 47 mm Hg (constant)
• Normal alveolar PCO₂ = 40 mm Hg

Lowest barometric pressure allowing normal alveolar PO₂ (100 mm Hg):

• 187 mm Hg
• Corresponds to ~10,400 m (34,000 ft)

Extreme altitude physiology

Key conclusion

• 100% O₂ helps only up to a limit
• Ultimately total atmospheric pressure becomes limiting

Artificial environments

• Pressurized suits / cabins with O₂ + CO₂ removal → survival at any altitude or space vacuum

IV. ACCLIMATIZATION TO HIGH ALTITUDE

Definition

• Gradual physiological adaptations improving altitude tolerance

1. O₂–Hemoglobin dissociation curve changes

• Hyperventilation → respiratory alkalosis
• Shifts curve left
• ↑ RBC 2,3-DPG
• Shifts curve right
• Net effect → slight ↑ P50
• ↓ Hb-O₂ affinity
• ↑ tissue O₂ delivery

⚠️ Limitation:

• At very low arterial PO₂, ↓ affinity also impairs O₂ loading in lungs

2. Ventilatory adaptation

• Initial ventilatory response is small
• Alkalosis blunts hypoxic drive
• Over ~4 days:
• Active H⁺ transport into CSF
• ± brain lactic acidosis
• ↓ CSF pH → ↑ hypoxic ventilatory response
• After 4 days:
• Ventilation declines slowly
• May take years to approach baseline (if ever)

3. Erythropoietin & RBC mass

• EPO rises immediately
• Begins to fall after ~4 days as ventilation improves PO₂
• RBC increase begins in 2–3 days
• Polycythemia maintained at altitude

4. Tissue-level adaptations

• ↑ Mitochondrial number
• ↑ Myoglobin
• ↑ Cytochrome oxidase

Real-world proof

• Permanent human habitation at >5500 m
• Natives:
• Barrel-chested
• Polycythemic
• Low alveolar PO₂
• Otherwise physiologically normal

V. DELAYED EFFECTS OF HIGH ALTITUDE (CLINICAL BOX 35–4)

1. Acute Mountain Sickness

• Onset: 8–24 h
• Duration: 4–8 days
• Symptoms:
• Headache
• Irritability
• Insomnia
• Breathlessness
• Nausea, vomiting
• Likely mechanism:
• Cerebral arteriolar dilation
• ↑ capillary pressure
• Cerebral edema

2. High-Altitude Cerebral Edema (HACE)

• Progression of mountain sickness
• Severe brain swelling
• Features:
• Ataxia
• Disorientation
• Coma
• Death from herniation

3. High-Altitude Pulmonary Edema (HAPE)

• Patchy pulmonary edema
• Cause:
• Hypoxic pulmonary vasoconstriction
• Uneven muscularization of pulmonary arteries
• Capillary stress failure

Treatment

• Immediate descent
• Acetazolamide
• Carbonic anhydrase inhibition
• ↑ ventilation
• ↓ CSF formation
• Glucocorticoids for cerebral edema
• O₂, hyperbaric chamber for HAPE
• Nifedipine → ↓ pulmonary artery pressure

VI. DISEASES CAUSING HYPOXEMIA

Three broad mechanisms

1. Gas exchange failure
• Alveolar-capillary block
• V/Q mismatch
2. Venous-to-arterial shunts
3. Respiratory pump failure

VII. VENOUS-TO-ARTERIAL SHUNTS

Mechanism

• Unoxygenated venous blood bypasses lungs
• Dilutes arterial blood

Examples

• Cyanotic congenital heart disease
• Interatrial septal defect(ASD)

Key point

• 100% O₂ ineffective
• Shunted blood never reaches alveoli

VIII. VENTILATION–PERFUSION (V/Q) IMBALANCE

Most common cause of hypoxemia

Mechanism

• Underventilated but perfused alveoli act like shunts
• Overventilated alveoli cannot compensate fully

Why O₂ saturation falls

• Hb already near-maximally saturated
• Extra PO₂ adds little
• Desaturation from low-V/Q units dominates

CO₂ behavior

• Arterial CO₂ often normal
• Overventilated regions eliminate extra CO₂

IX. OTHER FORMS OF HYPOXIA

A. ANEMIC HYPOXIA

• ↓ Hb concentration
• PO₂ normal
• Resting hypoxia mild due to ↑ 2,3-DPG
• Severe limitation during exercise

B. CARBON MONOXIDE POISONING

Mechanism

• CO + Hb → Carboxyhemoglobin (COHb)
• CO affinity for Hb = 210× O₂
• COHb:
• Cannot bind O₂
• Dissociates slowly
• Remaining Hb-O₂ curve shifts left
• ↓ O₂ unloading

Key comparisons

• 50% Hb loss (anemia) → moderate function
• 50% COHb → severe incapacitation

Threshold

• Progressive COHb formation when alveolar PCO > 0.4 mm Hg

Symptoms

• Headache
• Nausea
• No respiratory stimulation (PO₂ normal)
• Cherry-red skin
• Death at 70–80% COHb

Chronic exposure

• Progressive brain damage
• Parkinsonism-like features

Treatment

• Remove source
• Ventilate
• 100% O₂
• Hyperbaric O₂

C. ISCHEMIC (STAGNANT) HYPOXIA

• Reduced blood flow
• Seen in:
• Shock
• Heart failure
• Kidney, heart, liver injury
• ARDS after prolonged circulatory collapse

D. HISTOTOXIC HYPOXIA

• Cells cannot utilize O₂
• Common cause: Cyanide
• Inhibits cytochrome oxidase

Treatment

• Nitrites / methylene blue
• Form methemoglobin → cyanmethemoglobin
• Hyperbaric O₂ adjunct

X. OXYGEN TREATMENT OF HYPOXIA

Limited benefit in

• Anemic hypoxia
• Histotoxic hypoxia
• Ischemic hypoxia
• Right-to-left shunts

Highly beneficial in

• V/Q mismatch
• Diffusion defects
• COPD
• Chronic hypoxemia

Evidence

• <100% O₂ for 24 h/day × 2 years
• ↓ mortality in COPD

XI. HYPERCAPNIA

Definition

• CO₂ retention

Effects

• Mild → ↑ ventilation
• Severe →
• Confusion
• Coma
• CNS depression
• Respiratory acidosis

Renal compensation

• ↑ HCO₃⁻ reabsorption
• Partial pH correction

Causes

• V/Q mismatch
• Pump failure
• ↑ CO₂ production:
• Fever: +13% CO₂ per 1°C
• High carbohydrate intake

XII. HYPOCAPNIA

Cause

• Hyperventilation

Acute effects

• PCO₂ ↓ to ~15 mm Hg
• PO₂ ↑ to 120–140 mm Hg

Chronic effects

• Cerebral vasoconstriction
• ↓ cerebral blood flow (≥30%)
• Dizziness, paresthesias

Acid–base effects

• Respiratory alkalosis (pH 7.5–7.6)
• ↓ HCO₃⁻ (renal compensation)

Calcium effects

• Total Ca unchanged
• ↓ ionized Ca²⁺
• Tetany:
• Carpopedal spasm
• Positive Chvostek sign

XIII. OXYGEN TOXICITY & HYPERBARIC O₂ (CLINICAL BOX 35–5)

Mechanism

• Reactive oxygen species:
• Superoxide
• H₂O₂

High O₂ (8+ h)

• Tracheobronchial irritation
• Chest pain
• Cough

Neonates

• Bronchopulmonary dysplasia
• Retinopathy of prematurity
• Preventable with antioxidants (vitamin E)

Hyperbaric O₂

• 2–3 atm → PaO₂ > 2000 mm Hg
• Tissue PO₂ ≈ 400 mm Hg
• Safe ≤ 5 h

Indications

• CO poisoning
• Decompression sickness
• Air embolism
• Gas gangrene
• Severe anemia
• Radiation injury
• Non-healing wounds
• Grafts/flaps with poor circulation

Critical caution

• In severe hypercapnia:(COPD)
• Hypoxic drive maintains breathing
• O₂ administration may abolish respiration
• O₂ therapy must be carefully titrated

ABG ANALYSIS — COMPLETE LOGIC SYSTEM

NORMAL VALUES (lock these)

• pH: 7.35 – 7.45
• PaCO₂: 35 – 45 mmHg
• HCO₃⁻: 22 – 26 mmol/L
• Anion Gap (AG): 8–12 mmol/L
• Base Excess (BE): –2 to +2 mmol/L

STEP 1 — pH (Direction of Disorder)

• pH < 7.35 → ACIDOSIS
• pH > 7.45 → ALKALOSIS

STEP 2 — Primary Cause (Respiratory vs Metabolic)

STEP 3 — Compensation (Opposite System Reacts)

• Respiratory disorder → renal compensation (HCO₃⁻)
• Metabolic disorder → respiratory compensation (PaCO₂)

Compensation status

• Only primary abnormal + pH abnormal→ Uncompensated
• Both abnormal + pH abnormal → Partially compensated
• Both abnormal + pH normal → Fully compensated

STEP 4 — ANION GAP (ONLY FOR METABOLIC ACIDOSIS)

When to calculate AG

👉 ONLY if metabolic acidosis is present

Formula

Anion Gap = Na⁺ − (Cl⁻ + HCO₃⁻)

(K⁺ usually ignored in exams)

INTERPRETATION OF ANION GAP

1️⃣ High Anion Gap Metabolic Acidosis (HAGMA)

AG > 12

Cause = addition of acids

Classic causes (high-yield):

• DKA
• Lactic acidosis
• Renal failure (uremia)
• Toxins (methanol, ethylene glycol, salicylates – late)

🧠 Logic:

Acid → H⁺ buffered by HCO₃⁻ → HCO₃⁻ falls → unmeasured anions rise

2️⃣ Normal Anion Gap Metabolic Acidosis (NAGMA)

AG 8–12

Cause = loss of bicarbonate

Classic causes:

• Diarrhea
• Renal tubular acidosis
• Ureteric diversion

🧠 Logic:

Lost HCO₃⁻ replaced by Cl⁻ → hyperchloremic acidosis

EXAM PEARL

• All metabolic acidosis ≠ same
• AG tells you the CAUSE, not the severity

STEP 5 — BASE EXCESS (METABOLIC STATUS MARKER)

What is Base Excess?

Amount of acid or base needed to return blood pH to 7.40 at normal PaCO₂.

👉 It reflects pure metabolic component, independent of lungs.

BASE EXCESS INTERPRETATION

WHY BASE EXCESS IS IMPORTANT (EXAM LOGIC)

• Confirms metabolic disturbance
• Helpful when pH looks “normal” due to compensation
• Used heavily in ICU, sepsis, trauma, neonates

Examples

• Respiratory acidosis with BE normal → acute respiratory problem
• Normal pH + BE –8 → fully compensated metabolic acidosis

PUTTING IT ALL TOGETHER (MASTER FLOW)

1. pH → acidosis or alkalosis
2. PaCO₂ vs HCO₃⁻ → respiratory or metabolic
3. Check compensation → none / partial / full
4. If metabolic acidosis → calculate AG
5. Check BE → confirms metabolic load

ULTRA-HIGH-YIELD MEMORY LOCK

• ROME
• Respiratory = Opposite
• Metabolic = Equal
• AG = WHY
• BE = HOW MUCH metabolic problem

Fetomaternal Oxygen Transfer — Bohr effect + Haldane effect (clear logic)

🔴 First: understand the placental setting (exam foundation)

At the placenta:

• Maternal and fetal blood do NOT mix
• Gas exchange occurs by diffusion across the placental membrane
• Direction of diffusion depends on:
• Partial pressure gradients
• Hemoglobin behavior (Bohr + Haldane effects)
• Fetal hemoglobin properties (HbF)

1️⃣ Bohr Effect — OXYGEN TRANSFER MECHANISM

Definition (must know)

Increase in CO₂ or decrease in pH reduces hemoglobin’s affinity for oxygen

This shifts the O₂ dissociation curve to the RIGHT.

🔹 What happens at the placenta?

Step-by-step

1. Fetal blood arrives at placenta
• High CO₂
• Low pH
• Low O₂
2. CO₂ diffuses from fetus → mother
• Because fetal PCO₂ > maternal PCO₂
3. Consequences:
• Fetal blood loses CO₂
• pH rises
• HbF curve shifts LEFT
• HbF binds O₂ more strongly
• Maternal blood gains CO₂
• pH falls
• Maternal Hb curve shifts RIGHT
• Maternal Hb releases O₂

🔑 Why this is called DOUBLE Bohr effect

• Opposite Bohr shifts occur simultaneously
• Maternal side → right shift → O₂ unloading
• Fetal side → left shift → O₂ loading

🎯 Result

➡️ Oxygen transfer from mother to fetus is greatly enhanced

📌 Exam line

The Bohr effect at the placenta facilitates oxygen unloading from maternal hemoglobin and increased oxygen binding by fetal hemoglobin.

2️⃣ Haldane Effect — CARBON DIOXIDE HANDLING

Definition (core concept)

Deoxygenated hemoglobin can carry more CO₂ than oxygenated hemoglobin

This applies to:

• Carbamino compounds
• Hydrogen ion buffering

🔹 What happens at the placenta?

Maternal side

1. Maternal hemoglobin binds oxygen
2. Oxygenated Hb:
• Loses affinity for CO₂
• Releases CO₂ into maternal plasma
3. CO₂ is carried away to maternal lungs

Fetal side

1. Fetal hemoglobin releases oxygen
2. Deoxygenated Hb:
• Gains affinity for CO₂
• Picks up CO₂ from fetal tissues

🎯 Result

➡️ Efficient removal of fetal CO₂ into maternal circulation

📌 Exam line

The Haldane effect promotes transfer of carbon dioxide from the fetus to the mother by reducing CO₂ binding in oxygenated maternal hemoglobin.

3️⃣ HOW Bohr & Haldane effects WORK TOGETHER (critical)

Combined logic

• Bohr effect:
• Modifies oxygen affinity
• Drives O₂ from mother → fetus
• Haldane effect:
• Modifies CO₂ carrying capacity
• Drives CO₂ from fetus → mother

🔁 Gas exchange loop

4️⃣ Why fetal hemoglobin (HbF) makes this EVEN STRONGER

HbF:

• Has higher affinity for O₂
• Binds 2,3-BPG poorly
• O₂ dissociation curve is left-shifted

➡️ Even at low placental PO₂, fetal blood loads oxygen effectively.

📌 Ultimate exam synthesis line

Fetomaternal oxygen transfer is optimized by a double Bohr effect at the placenta, reinforced by the Haldane effect and the intrinsically higher oxygen affinity of fetal hemoglobin.

5️⃣ One-look comparison table (exam gold)

🧠 Single-sentence MCQ lock

At the placenta, fetal loss of CO₂ and maternal gain of CO₂ produce a double Bohr effect that favors oxygen transfer to the fetus, while the Haldane effect facilitates carbon dioxide transfer to the mother.