Overview
This lecture covers the structure, function, and mechanisms of breathing and gas exchange in humans, including respiratory anatomy, physiology, gas transport, and related disorders.
Breathing and Respiration Basics
- Breathing is the physical process of inhaling (inspiration) and exhaling (expiration) air.
- Respiration refers to the cellular use of oxygen to produce energy (ATP) and remove carbon dioxide.
- Breathing and respiration are different: breathing is air movement; respiration is energy production.
Types of Respiration in Animals
- Aquatic animals (e.g., fish) use gills; terrestrial animals (e.g., mammals, birds) use lungs.
- Body structure and habitat determine respiratory organs (e.g., earthworms use moist skin, insects use tracheae).
Human Respiratory System Anatomy
- Air enters via external nostrils, passes through the nasal passage, pharynx, larynx (voice box), trachea (windpipe), and then into lungs.
- The trachea branches into primary, secondary, then tertiary bronchi, bronchioles, and ends in alveoli (site of gas exchange).
- Lungs are protected by double-layered pleura: visceral (inner) and parietal (outer), with pleural fluid between for friction reduction.
- The epiglottis covers the glottis during swallowing to prevent food from entering the trachea.
Mechanics of Breathing
- Inhalation: diaphragm and external intercostal muscles contract, thoracic volume increases, air enters due to lower lung pressure.
- Exhalation: muscles relax, thoracic volume decreases, lung pressure increases, air exits.
- Normal adult breathing rate is 12–16 times per minute; tidal volume (normal breath) is ~500 ml.
Respiratory Volumes and Capacities
- Tidal Volume (TV): normal breath in/out (~500 ml)
- Inspiratory Reserve Volume (IRV): max extra air inhaled (~2.5–3 L)
- Expiratory Reserve Volume (ERV): max extra air exhaled (~1–1.1 L)
- Residual Volume (RV): air remaining after forceful exhale (~1.1–1.2 L)
- Capacities are combinations of these volumes (e.g., Vital Capacity = TV + IRV + ERV).
Gas Exchange and Transport
- Gas exchange occurs in alveoli by diffusion, driven by concentration gradients and thin membranes.
- Oxygen is transported mostly by hemoglobin in RBCs (97%), and a small amount by plasma (3%).
- CO2 is transported as bicarbonate ions (70%), bound to hemoglobin (20–25%), and dissolved in plasma (7%).
- Chloride shift maintains ionic balance during CO2 transport as bicarbonate.
Oxygen-Hemoglobin Dissociation Curve
- The curve is sigmoid-shaped; factors shifting it right (reducing affinity): increased CO2, acidity, temperature, DPGA, exercise ("CADET Right").
- Right shift favors oxygen release; left shift favors binding.
Regulation and Disorders
- Breathing is regulated by centers in the medulla (rhythm), pons (pneumotaxic), and sensors in arteries.
- Disorders include asthma (bronchi inflammation), emphysema (alveoli damage), and occupational diseases (silicosis/asbestosis from dust).
Key Terms & Definitions
- Tidal Volume (TV) — amount of air inhaled/exhaled in a normal breath (~500 ml)
- Residual Volume (RV) — air left in lungs after forceful exhalation (~1.1–1.2 L)
- Alveoli — small lung sacs where gas exchange occurs
- Hemoglobin — blood protein transporting oxygen
- Pleura — double-layered membrane covering lungs
- Chloride Shift — exchange of chloride and bicarbonate ions in RBCs during CO2 transport
- Oxyhemoglobin — hemoglobin bound to oxygen
- Carbaminohemoglobin — hemoglobin bound to CO2
Action Items / Next Steps
- Review and memorize respiratory volumes/capacities and dissociation curve factors.
- Read about circulatory system for the next session.
- Practice labeling diagrams of the respiratory tract and lung volumes.