Revision: Class 11 >> Breathing and Exchange of Gases NEET (UG) Breathing and Exchange of Gases

Difficulty or labored breathing, often described as shortness of breath.

It is a type of reflex whose stimulus is in the nasal passage which causes spasmodic contraction of expiratory muscles that forcefully expel the air through the nasal passage.

It is a type of reflex whose stimulus is any foreign particle, resulting from deep inspiration followed by strong expiration, which forcefully expels the air through the mouth.

The trachea is commonly called a windpipe. It is a tube supported by cartilaginous rings that connect the pharynx and larynx to the lungs, allowing the passage of air. The trachea divides into right and left bronchi and enters the lungs.

The process of conversion of glucose molecules in food into energy-rich molecules, carbon dioxide and water with the help of oxygen is known as respiration.

Eupnea is the medical and physiological term for normal, unlabored, and quiet breathing in a healthy individual at rest. It represents an efficient respiratory state where the body maximizes oxygen intake while minimizing muscular effort.

Apnea is defined as the temporary cessation of breathing, marked by the absence of respiratory muscle movement and airflow.

The exchange of gases through moist skin and blood capillaries underneath is called cutaneous respiration.

• Living organisms need energy for life processes. Complex organic compounds (potential energy) must be converted into a usable form (ATP) through respiration.
• Respiration is a biochemical process of oxidation of organic compounds in an orderly manner to release chemical energy as ATP.
• Respiratory organs are structures or systems in organisms that facilitate the exchange of gases, primarily oxygen and carbon dioxide, between the organism and its environment.
• Equation - C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + 38 ATP
• Gaseous Exchange - Respiration involves the exchange of gases (O₂ in, CO₂ out) between the organism and the environment. The site where this exchange occurs is called the respiratory surface.

• Two Parts - Upper (nasal cavities, pharynx, throat) | Lower (larynx, trachea, bronchi, bronchioles, lungs).
• Nasal Cavity & Pharynx - The nasal cavity is divided into 2 chambers by the mesethmoid cartilage. Pharynx: Nasopharynx → Oropharynx (common food & air) → Laryngopharynx.
• Larynx & Trachea - Glottis covered by epiglottis (prevents food entry). Trachea held by 16–20 C-shaped cartilage rings.
• Lungs & Alveoli - Right = 3 lobes, Left = 2 lobes, covered by pleural membranes. Alveoli = site of O₂/CO₂ exchange.
• Path of Air - Nasal cavity → Pharynx → Larynx → Trachea → Bronchi → Bronchioles → Alveoli (exchange) → reverse for CO₂.
• Gas Transport - O₂ carried by haemoglobin (RBCs), CO₂ in dissolved form in plasma.
• Other Organisms - Plants: stomata, Fish: gills (breathe faster, less O₂ in water), Terrestrial animals: lungs.

• Breathing - Physical process of gaseous exchange between the atmosphere and the lungs involving the thoracic cage, ribs, sternum, intercostal muscles and diaphragm.
• Two Phases - Breathing has two phases: Inspiration (air in) and Expiration (air out).
• Inspiration (Active) - External intercostal muscles and diaphragm contract. Ribs & sternum move up and outward, diaphragm flattens downward → thoracic volume increases → lung pressure decreases → air rushes in.
• Expiration (Passive) - Intercostal muscles and diaphragm relax. Ribs & sternum move down and inward, diaphragm arches upward (dome-shaped) → thoracic volume decreases → lung pressure increases → air is expelled out.
• Key Difference - Inspiration = active (needs muscle contraction) | Expiration = passive (muscles simply relax).

• Exchange of gases (O₂ & CO₂) between alveolar air and blood via simple diffusion, driven by pressure gradients.
• O₂ Exchange - O₂ diffuses from alveoli (pO₂ = 104 mmHg) → blood (pO₂ = 40 mmHg), raising blood pO₂ to 95 mmHg.
• CO₂ Exchange - CO₂ diffuses from blood (pCO₂ = 45 mmHg) → alveoli (pCO₂ = 40 mmHg), lowering blood pCO₂ to 40 mmHg.
• Driving Force - Exchange occurs by simple diffusion from high pressure → low pressure (no energy needed).

• Meaning - O₂ from blood is delivered to cells/tissues, and CO₂ from cells passes into the blood.
• O₂ Transport - 97% as oxyhaemoglobin (HbO₂) via RBCs, 3% dissolved in plasma. One Hb molecule has 4 Fe²⁺ ions, each binding one O₂: Hb + 4O₂ → Hb(O₂)₄
• Bohr Effect - Rise in CO₂ / lower pH / higher temperature → reduces Hb-O₂ affinity (curve shifts right) → O₂ released to tissues.
• Haldane Effect - Binding of O₂ with Hb displaces CO₂ from blood (curve shifts left, higher Hb-O₂ affinity).
• CO₂ Transport - 70% as bicarbonate ions (HCO₃⁻) in plasma | 23% as carbaminohaemoglobin | 7% dissolved in plasma.
• Chloride Shift (Hamburger's Phenomenon) - When CO₂ enters blood, Cl⁻ moves into RBCs (Na⁺ stays behind). When CO₂ leaves, Cl⁻ moves back out. This alternate Cl⁻ movement maintains electrical balance.

• Tidal Volume (TV) - Volume of air inhaled or exhaled during normal breathing - 500 mL.
• Inspiratory Reserve Volume (IRV) - Extra air inhaled by forceful inspiration - 2500 to 3000 mL.
• Expiratory Reserve Volume (ERV) - Extra air exhaled by forceful expiration - 1000 to 1100 mL.
• Residual Volume (RV) - Air remaining in lungs even after forceful expiration - 1100 to 1200 mL.
• Inspiratory Capacity (IC) = TV + IRV, Expiratory Capacity (EC) = TV + ERV, Functional Residual Capacity (FRC) = ERV + RV (AIPMT 2010)
• Vital Capacity (VC) = ERV + TV + IRV - Maximum air inhaled after forced expiration or exhaled after forced inspiration.
• Total Lung Capacity (TLC) = RV + ERV + TV + IRV = VC + RV - Total air in lungs after forced inspiration.

• Gas exchange occurs between alveoli, blood, and tissues by simple diffusion based on pressure and concentration gradients.
• Partial pressure is the pressure exerted by an individual gas in a mixture, represented as pO₂ for oxygen and pCO₂ for carbon dioxide.
• O₂ moves from alveoli (pO₂ = 104) → blood → tissues (pO₂ = 40); CO₂ moves in the opposite direction - from tissues (pCO₂ = 45) → blood → alveoli (pCO₂ = 40).
• CO₂ is 20-25 times more soluble than O₂, so it diffuses much more easily through the diffusion membrane.
• The diffusion membrane has 3 layers: thin squamous epithelium of alveoli, endothelium of alveolar capillaries, and the basement membrane between them. Total thickness is less than 1 mm.
• Alveoli are the primary sites of gas exchange. Solubility of gases and the thickness of membranes also affect the rate of diffusion.
• Negative intrapleural pressure (pressure in the pleural cavity, lower than atmospheric pressure) is the key factor that prevents collapse of the lungs.

• O₂ transport - 97% by RBCs (as oxyhaemoglobin) and 3% dissolved in plasma. CO₂ transport - 20-25% by RBCs (as carbaminohaemoglobin), 70% as bicarbonate, and 7% dissolved in plasma.
• Haemoglobin is a red-coloured, iron-containing pigment in RBCs. Each haemoglobin molecule can carry 4 molecules of O₂.
• The Oxygen Dissociation Curve is a sigmoid curve plotting % saturation of haemoglobin with O₂ against pO₂.
• In alveoli - high pO₂, low pCO₂, low H⁺, lower temperature → favourable for oxyhaemoglobin formation. In tissues - low pO₂, high pCO₂, high H⁺, higher temperature → favourable for dissociation of O₂ from oxyhaemoglobin.
• Every 100 mL of oxygenated blood delivers 5 mL of O₂ to tissues; every 100 mL of deoxygenated blood delivers 4 mL of CO₂ to alveoli.
• Carbonic anhydrase enzyme (present in RBCs) facilitates: CO₂ + H₂O ⇌ H₂CO₃ ⇌ HCO₃⁻ + H⁺ in tissues. This reaction reverses in alveoli (low pCO₂), releasing CO₂.
• High pCO₂ and low pO₂ in tissues promote CO₂ binding to haemoglobin; low pCO₂ and high pO₂ in alveoli promote dissociation of CO₂ from carbaminohaemoglobin.

• The respiratory rhythm centre is in the medulla of the hindbrain - it controls the normal breathing rate.
• Pneumotaxic centre in the pons modulates the rhythm centre and adjusts the duration of inspiration.
• Chemosensitive area (near rhythm centre) is sensitive to CO₂ and H⁺ ions - increased levels signal the body to eliminate them.
• Receptors in the aortic arch and carotid artery detect CO₂ and H⁺ changes and send signals to the rhythm centre for correction.
• O₂ plays an insignificant role in regulating respiration - CO₂ and H⁺ are the main regulators.