Notes on Breathing and Gas Exchange

Difference between Breathing and Respiration

• Breathing

  • i) It is a physical process

  • ii) No energy is released

  • iii) Enzymes are not involved

  • iv) Modes of breathing vary among organisms; respiration is the chemical process that occurs in all organisms

• Respiration

  • i) It is a chemical process

  • ii) Energy is released during the breakdown of nutrients

  • iii) Enzymes are involved in the metabolic steps

  • iv) Involves continuous supply of O₂ and removal of CO₂ from the body

• Breathing vs Respiration: quick summary

  • Breathing = physical inhalation/exhalation of air

  • Respiration = biochemical oxidation of nutrients to release energy

Steps involved in respiration

• External respiration

  • Exchange of gases between the lungs and the atmosphere

• Internal respiration

  • Exchange of gases between the lungs and blood cells

• Cellular respiration

  • Utilization of oxygen by cells and oxidation of nutrients to release energy

Types of respiration on the basis of oxygen availability

• Aerobic respiration

  • Requires O₂

  • Site: Cytoplasm + mitochondria

  • Energy released: Complete oxidation

  • End products: $CO<em>2 + H</em>2O$

• Anaerobic respiration

  • Does not require O₂

  • Site: Cytoplasm only

  • Energy released: Incomplete oxidation

  • End products: Ethanol and lactic acid

  • Organisms/contexts: some microorganisms (yeast, microbes); occurs in muscle under O₂ deficit

Types of respiration on the basis of organs

• Lower invertebrates (e.g., sponges, coelenterates, flatworms, hydra)

  • Body surface for gas exchange (cutaneous/transparent diffusion surfaces)

• Annelids (earthworms)

  • Cutaneous/moist surface

• Arthropods/Insects

  • Tracheal system for gas exchange

• Amphibians

  • Skin, tegument, buccal cavity participate in gas exchange (and often lungs as well)

• Fish

  • Gills

• Molluscs

  • Gills

• Aquatic arthropods

  • Gills

• Terrestrial vertebrates

  • Lungs (pulmonary respiration); some amphibians use both skin and lungs

Types of respiration on the basis of respiratory organ

• Cutaneous (moist skin)

  • Examples: Frog and some annelids

• Pulmonary (lungs)

  • Terrestrial vertebrates: reptiles, birds, mammals; some amphibians

• Branchial (gills)

  • Fish, molluscs, aquatic arthropods

• (Notes: Some amphibians use skin + lungs; the respiratory surface adapts to the environment)

Human respiratory system

• Respiratory zones

  • Conducting zone: Nose to terminal bronchioles (air transport, conditioning, filtration)

  • Exchange zone: Respiratory bronchioles, alveolar ducts, alveolar sacs (gas exchange)

• Conducting zone functions

  • Carries air to the exchange part

  • Filters dust, humidifies and warms air to body temperature

• Anatomy from nostrils to alveoli

  • Nostrils → Nasal chamber → Nasopharynx → Pharynx (oropharynx) → Larynx → Trachea

  • Trachea bordered by C-shaped hyaline cartilage rings

  • Primary, secondary, tertiary bronchi → bronchioles → terminal bronchioles

  • Alveolar ducts → alveolar sacs → alveoli (site of gas exchange)

• Supporting structures

  • Epiglottis covers glottis during swallowing to prevent food entry

  • Pleural membranes: Parietal (outer) and Visceral (pulmonary) pleura with a pleural cavity in between

  • Lungs located in thoracic cavity; protected by the sternum and vertebral column; separated from abdominal cavity by the diaphragm

• Lung anatomy specifics

  • Lungs are lobed: Right lung typically has superior, middle, and inferior lobes; left lung has superior and inferior lobes (cardiac notch and oblique fissure are present)

  • Pleural membranes envelop lungs; visceral pleura adheres to lung surface; parietal pleura lines the thoracic cavity

• Bronchial tree features

  • Trachea → primary bronchi → secondary bronchi → tertiary bronchi → smaller bronchioles → terminal bronchioles

  • C-shaped cartilage rings in the trachea; cartilage decreases toward terminal bronchioles

• Epithelium and surface features

  • Nasal chamber lined by mucous membrane; ciliated epithelium and goblet cells trap and move particles

• Diaphragm and thoracic mechanics

  • Diaphragm contracts (flattens) during inspiration, increasing thoracic volume; relaxes during expiration

Mechanism of breathing

• Primary muscles

  • Diaphragm and external intercostal muscles govern normal breathing

• Accessory muscles (strengthening breaths)

  • Abdominal muscles and internal intercostals (during forced breathing)

• Pressure-gradient mechanism

  • Inhalation: thoracic volume increases, intra-pulmonary pressure decreases; air flows into lungs

  • Exhalation: thoracic volume decreases, intra-pulmonary pressure increases; air flows out of lungs

• Negative-pressure breathing in humans

  • The pressure in the lungs is less than atmospheric pressure during inhalation

• Inhalation details

  • Diaphragm contracts → dome-shaped diaphragm becomes flattened

  • Thoracic volume increases in anterio-posterior and dorso-ventral axes

• Exhalation details

  • Diaphragm relaxes → dome-shaped again

  • Ribs move downward and backward; sternum moves downward

  • Thoracic volume decreases along both axes; air is expelled

Breathing rates and lung volumes (spirometry)

• Breathing rate

  • Typical adult: 12–16 breaths per minute

• One breathing cycle

  • Inhalation (about 2 s) + Exhalation (about 3 s)

• Spirometry and volumes

  • Tidal Volume (TV): $TV \,\approx \,500\ \text{mL}$

  • Inspiratory Reserve Volume (IRV): $IRV \,\approx \,2500{-}3000\ \text{mL}$

  • Expiratory Reserve Volume (ERV): $ERV \,\approx \ 1000{-}1100\ \text{mL}$

  • Residual Volume (RV): $RV \,\approx \ 1100{-}1200\ \text{mL}$

  • Inspiratory Capacity (IC): $IC = IRV + TV \,\approx \ 3000{-}3500\ \text{mL}$

  • Functional Residual Capacity (FRC): $FRC = ERV + RV$

  • Total Lung Capacity (TLC): $TLC = TV + IRV + ERV + RV$

• Inhalation vs exhalation

  • Inhalation is an active process (requires energy)

  • Exhalation is typically a passive process

• Typical values (example calculations)

  • MV (Minute Ventilation) = breath rate × TV; with 12–16 breaths/min and TV ≈ 500 mL,

  • MV ≈ 6000–8000 mL/min

  • Alveolar ventilation Va ≈ (TV − VD) × rate, with anatomical dead space VD ≈ 150 mL

  • Va ≈ (500 − 150) × 12–16 ≈ 4200–5600 mL/min

Gas exchange and transport of gases

• Alveolar gas tensions (typical)

  • Alveolar PO₂ ≈ $104\ \text{mmHg}$, PCO₂ ≈ $40\ \text{mmHg}$

  • Systemic arterial blood PO₂ ≈ $95\ \text{mmHg}$, PCO₂ ≈ $40\ \text{mmHg}$

  • Systemic venous blood PO₂ ≈ $40\ \text{mmHg}$, PCO₂ ≈ $45\ \text{mmHg}$

• Transport of O₂

  • About 3% is dissolved in plasma

  • About 97% is bound to hemoglobin (Hb)

  • Oxyhemoglobin formation:

  • $ext{Hb} + O<em>2 ightleftharpoons ext{HbO}</em>2$

• Transport of CO₂

  • CO₂ is transported by: dissolved in plasma, bound to hemoglobin (carbaminohemoglobin), and as bicarbonate (HCO₃⁻) in plasma

  • Most CO₂ is converted to bicarbonate in red blood cells via carbonic anhydrase:

  • $CO<em>2 + H</em>2O \leftrightarrow HCO_3^- + H^+$

  • Chloride shift maintains electrochemical balance (Cl⁻ in, HCO₃⁻ out) across the RBC membrane

• Hemoglobin and myoglobin

  • Hemoglobin (Hb): tetramer with 2 α and 2 β subunits; binds up to ~4 O₂ molecules per Hb

  • Oxygen capacity: approx. $20\ \text{mL O}_2/100\ \text{mL blood}$ when fully saturated

  • Myoglobin: single polypeptide, stores O₂ in muscle, facilitates diffusion to mitochondria

  • Fetal hemoglobin (HbF): higher O₂ affinity than adult Hb (facilitates transfer of O₂ from maternal to fetal blood)

  • Carboxyhemoglobin (HbCO): CO binds Hb with high affinity, reducing O₂ transport

• Oxygen dissociation curve

  • Hb saturation increases with rising PO₂ following a sigmoidal curve

  • P50 (PO₂ at 50% saturation) ~ around 26–27 mmHg in adults (typical value; varies with conditions)

  • Left shift: higher O₂ affinity (easier loading in lungs, harder release in tissues)

  • Right shift: lower O₂ affinity (easier release in tissues, harder loading in lungs)

  • Factors causing shifts (Bohr-related):

  • Left shift: lower CO₂, higher pH, lower temperature, decreased 2,3-BPG

  • Right shift: higher CO₂, lower pH, higher temperature, higher 2,3-BPG

• Gases and solubility

  • CO₂ is more soluble in plasma than O₂ (approximately 20–25× more soluble)

  • Gas solubility is influenced by partial pressure and pH (affects Hb binding)

Regulation of respiration

• Neural regulation

  • Medulla oblongata houses the respiratory rhythm center (RRC) – main control center

  • Pons modulates the duration and intensity of inhalation

  • Signals travel via phrenic nerves to the diaphragm and intercostal nerves to intercostal muscles

• Chemical regulation

  • Central chemoreceptors (in medulla) respond to changes in H⁺ concentration in CSF (driven by CO₂ levels in blood)

  • Peripheral chemoreceptors

  • Carotid bodies (in left/right carotid arteries) and aortic bodies detect changes in CO₂, H⁺, and O₂ levels

  • They send signals to modulate breathing rate and depth

  • There is a relatively small direct role of O₂ partial pressure in regulating breathing under normal conditions; CO₂/H⁺ is the primary driver

• Integration and effectors

  • Chemosensitive receptors influence the respiratory rate and depth to maintain stable blood gas levels

  • The diaphragm and intercostal muscles execute the motor response

Diffusion and gas exchange at the alveolar membrane

• Mechanism

  • Gas exchange occurs by diffusion down partial pressure gradients across the alveolar and capillary membranes

• Influencing factors

  • Partial pressures of the gases (driving forces)

  • Solubility of the gases

  • Thickness of the respiratory membrane

• Alveolar structures relevant to gas exchange

  • Type I pneumocytes (thin cells) form the gas exchange surface

  • Type II pneumocytes (secrete surfactant) reduce surface tension and prevent alveolar collapse

  • Basement membrane and endothelial lining capillaries

Respiratory pigments and oxygen transport (summaries)

• Hemoglobin (Hb)

  • Tetramer with 2 α and 2 β subunits

  • Binds O₂ reversibly to form oxyhemoglobin (HbO₂)

  • Normal adult Hb carries about $20\ \,\text{mL O}_2/100\ \,\text{mL blood}$ when fully saturated

• Myoglobin

  • Monomeric oxygen-binding protein in muscle tissue; stores O₂ to facilitate diffusion to mitochondria

• Fetal Hb (HbF)

  • Higher affinity for O₂ than maternal Hb, aiding placental transfer

• Carboxyhemoglobin (HbCO)

  • CO binds Hb with high affinity, reducing O₂ delivery

Alveolar gas tensions and gas exchange in circulation

• Alveolar gas tensions vs systemic blood

  • Alveolar PO₂ ≈ $104\ \,\text{mmHg}$; Alveolar PCO₂ ≈ $40\ \,\text{mmHg}$

  • Systemic arterial PO₂ ≈ $95\ \,\text{mmHg}$; PCO₂ ≈ $40\ \,\text{mmHg}$

  • Systemic venous PO₂ ≈ $40\ \,\text{mmHg}$; PCO₂ ≈ $45\ \,\text{mmHg}$

• Diffusion direction

  • O₂ moves from alveolar air (higher PO₂) to blood (lower PO₂ in tissues at rest)

  • CO₂ moves from blood (higher PCO₂) to alveolar air (lower PCO₂)

Practical measurements and values

• Dead space volume (anatomical dead space)

  • ≈ $150\ \text{mL}$ per breath

• Minute ventilation and alveolar ventilation (examples)

  • MV = f × TV

  • Va = (TV − VD) × f, with VD ≈ $150\ \text{mL}$

  • Example: f = 12–16 min⁻¹, TV = 500 mL → MV ≈ 6000–8000 mL/min; Va ≈ 4200–5600 mL/min

• Respiratory quotient (RQ)

  • $RQ = \frac{\dot{V}<em>{CO</em>2}}{\dot{V}<em>{O</em>2}}$

  • Typical values around 0.8 in a resting person (noted as a ratio of CO₂ produced to O₂ consumed)

• Oxygen carrying capacity and oxygen delivery during exercise

  • At rest: Hb can deliver around 5 mL O₂ per 100 mL blood

  • During strenuous exercise: delivery can rise to about 15 mL O₂ per 100 mL blood

Quick reference: common abbreviations and relations

• TLC = TV + IRV + ERV + RV

• VC = IRV + TV + ERV

• FRC = ERV + RV

• IC = IRV + TV

• Va ∝ (TV − VD) × f

• MV = f × TV

• RQ = CO₂ produced / O₂ consumed

Where the parts fit in real-world relevance

• Understanding breathing vs respiration helps differentiate mechanical ventilation from cellular energy production.

• Gas exchange principles explain how oxygen reaches tissues and why CO₂ must be expelled to maintain pH balance.

• Regulation of breathing demonstrates how the body automatically maintains homeostasis under changing activity levels and environmental conditions.

• Pulmonary function tests (spirometry) use the volumes and capacities listed to diagnose and monitor respiratory diseases.

• Gas transport mechanisms (Hb, HbO₂ dissociation curve, and CO₂ carriage as bicarbonate) link lung function to tissue metabolism and exercise physiology.

• The neural and chemical regulation of respiration shows how feedback systems adapt breathing to metabolic needs and environmental CO₂/O₂ levels.