🫁 Breathing & Exchange of Gases

Breathing (Pulmonary Ventilation) involves atmospheric air intake into pulmonary alveoli and $C{O}_2$-rich alveolar air expiration driven by pressure gradients generated by the diaphragm and intercostal muscles.

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1. 📊 Spirometric Lung Volumes & Capacities

  Volume (mL)
   6000 ┬───────────────────────────────────────────────────────────── Total Lung Capacity (TLC: 5800 mL)
        │                      /───\
        │                     /     \ <-- Inspiratory Capacity (IC: 3500 mL)
   3000 ┼───────────────/───\       \───────
        │    Tidal     /     \     / \
   2500 ┼─── Volume ──/       \───/   \───── Vital Capacity (VC: 4600 mL)
        │   (500 mL)                   \
   1200 ┼───────────────────────────────\─── Expiratory Reserve Volume (ERV: 1100 mL)
        │                                 ── Residual Volume (RV: 1200 mL, Cannot be measured by Spirometer!)
      0 ┴────────────────────────────────────

Spirometric Parameter	Standard Normal Value	Mathematical Composition & Clinical Definition
Tidal Volume (TV)	$500\text{mL}$	Volume inspired or expired during a normal relaxed breath ($6000-8000\text{mL/min}$).
Inspiratory Reserve (IRV)	$2500-3000\text{mL}$	Additional air volume inhaled by forcible inspiration over TV.
Expiratory Reserve (ERV)	$1000-1100\text{mL}$	Additional air volume expelled by forcible expiration after normal expiration.
Residual Volume (RV)	$1100-1200\text{mL}$	Volume remaining in lungs even after maximal forcible expiration.
Vital Capacity (VC)	$4000-4600\text{mL}$	$\mathbf{VC}\mathbf{=}\mathbf{TV}\mathbf{+}\mathbf{IRV}\mathbf{+}\mathbf{ERV}$ (Maximal air volume exhaled after maximal inhalation).
Functional Residual Capacity (FRC)	$2100-2300\text{mL}$	$\mathbf{FRC}\mathbf{=}\mathbf{ERV}\mathbf{+}\mathbf{RV}$ (Air volume remaining in lungs after normal expiration).
Total Lung Capacity (TLC)	$5800-6000\text{mL}$	$\mathbf{TLC}\mathbf{=}\mathbf{VC}\mathbf{+}\mathbf{RV}\mathbf{=}\mathbf{TV}\mathbf{+}\mathbf{IRV}\mathbf{+}\mathbf{ERV}\mathbf{+}\mathbf{RV}$.

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2. 🩸 Gas Transport & The Oxygen-Hemoglobin Dissociation Curve

$$\text{Hemoglobin Binding: }{\mathbf{Hb}}_{\mathbf{4}}\mathbf{+}\mathbf{4}{\mathbf{O}}_{\mathbf{2}}\rightleftharpoons {\mathbf{Hb}}_{\mathbf{4}}\mathbf{(}{\mathbf{O}}_{\mathbf{2}}{\mathbf{)}}_{\mathbf{4}}\mathbf{(}\mathbf{1}\text{g }\mathbf{Hb}\text{ binds }\mathbf{1.34}\text{mL }{\mathbf{O}}_{\mathbf{2}}\mathbf{)}$$

  % Hb Saturation
   100 ┬                             /───────
       │                           /
       │            Left Shift ◄─ / ─► Right Shift (Bohr Effect)
    50 ┼─────────────────────── / :  (High pCO2, High H+, High Temp, High 2,3-DPG)
       │                      /   :
       │                    /     :
     0 ┴───────────────────┴──────┴──────────► pO2 (mmHg)
                           P50 ≈ 27 mmHg

Parameter	Shift to the LEFT (Higher $O_2$ Affinity / Alveoli)	Shift to the RIGHT (Lower Affinity / Tissues: Bohr Effect)
Partial Pressure $p{O}_2$	High ($104\text{mmHg}$)	Low ($40\text{mmHg}$)
Partial Pressure $pC{O}_2$	Low ($40\text{mmHg}$)	High ($45\text{mmHg}$)
Hydrogen Ions ($H^{+}$ / pH)	Low $H^{+}$ (Alkaline $\text{pH}=7.4$)	High $H^{+}$ (Acidic pH)
Temperature	Low Temperature	High Temperature (Metabolically active tissue)
Physiological Role	Promotes $O_2$ loading in pulmonary capillaries	Promotes $O_2$ unloading / delivery to working tissues

2.2 Carbon Dioxide ($C{O}_2$) Transport Forms

1. As Bicarbonate ions ($HC{O}_3^{-}$): $\mathbf{70}\text{\%}$ via RBC Carbonic Anhydrase ($C{O}_2+H_{2}O\rightleftharpoons H_{2}C{O}_3\rightleftharpoons H^{+}+HC{O}_3^{-}$). Involves Chloride Shift (Hamburger Phenomenon): $C{l}^{-}$ moves into RBC as $HC{O}_3^{-}$ exits into plasma.
2. As Carbamino-hemoglobin ($Hb-C{O}_2$): $\mathbf{20}\mathbf{-}\mathbf{25}\text{\%}$.
3. In Dissolved form in plasma: $\mathbf{7}\text{\%}$.