Respiratory System Anatomy

Anatomy of the Respiratory System

• Nasal/oral cavity, pharynx, larynx, trachea, bronchi and branches (bronchioles), and alveoli are the major structures that make up the respiratory system.

Functional Zones of the Respiratory System

• The respiratory system is divided into two functional zones: conducting and respiratory zones.
• Conducting zone: consists of structures that air travels through to reach the respiratory zone (no gas exchange occurs here); includes nasal cavity, pharynx, larynx, trachea, and bronchial tree.
• Respiratory zone: where gas exchange takes place; includes terminal bronchioles, alveolar ducts, alveolar sacs, and alveoli.

Alveoli

• Alveoli are small (~200 μm) sacs connected via alveolar pores, surrounded by capillaries, with thin walls and large surface area, ideal for gas exchange.
• Two types of alveolar cells: Type 1 (~97%) for gas exchange and Type 2 for secreting surfactant.

Pressure and Volume in the Lungs

• Boyle's Law: pressure is inversely related to volume; as volume increases, pressure decreases, and as volume decreases, pressure increases.

Inspiration and Expiration (Pulmonary Ventilation)

• Air moves from high to low pressure; during inhalation, lung pressure is less than atmospheric pressure, and during exhalation, lung pressure is greater than atmospheric pressure.
• Respiratory muscles (diaphragm and external intercostal muscles) contract to increase lung volume during inhalation (active process) and relax to decrease lung volume during exhalation (passive process).

Respiratory Volumes and Capacities

• Respiratory volumes:
  • Tidal volume: volume of air breathed in/out with each breath (average: 500ml)
  • Inspiratory reserve volume: maximum volume of air inhaled above a normal tidal breath (typically: 3000ml)
  • Expiratory reserve volume: maximum volume of air exhaled past a normal tidal breath (typically: 1200ml)
  • Residual volume: volume left in lungs after emptying as much as possible (typically: 1500ml)
• Respiratory capacities:
  • Vital capacity: maximum amount of air that can be moved in and out of the lungs
  • Total lung capacity: volume of air in the lungs upon maximum effort of inspiration

Mechanisms of Gas Exchange

• Gas exchange occurs in two spots: lungs (alveoli, external respiration) and tissues (internal respiration)
• In the lungs, gas is exchanged between the blood and outside air
• Oxygen enters the blood and carbon dioxide is removed from the blood
• pO2 alveoli > pO2 blood = O2 moves from alveoli into blood
• pCO2 alveoli < pCO2 blood = CO2 moves from blood into alveoli
• Gas exchange occurs through simple diffusion down gradients created by differences in gas partial pressure
• Dalton's law: total pressure of a mix of gases is made up of the partial pressure exerted by each gas

Oxygen Transport in the Blood

• Only a small portion (~1.5-2%) of O2 is carried as dissolved O2 in plasma
• Most O2 is carried in red blood cells bound to haemoglobin (Hb)
• Oxygen transport process:
  • Initially dissolves in plasma
  • Diffuses into RBC
  • Reversibly binds to Hb
  • Moves through blood as HbO2
  • Once at tissues, dissociates from Hb
  • Diffuses out of RBC and into tissues

Structure and Function of Haemoglobin

• Made up of 4 subunits, each with a haem group containing iron
• O2 binds to the specific haem group
• Each molecule can bind up to 4 O2 molecules
• Binding of Hb + O2 affected by local pO2
• O2/Hb dissociation curve: describes the relationship between pO2 and the binding of O2/Hb
• Curve is flat at the top, steep at the bottom

Carbon Dioxide Transport in the Blood

• CO2 is more soluble than O2, but still needs alternate transport methods
• Three ways CO2 is transported:
  • Bound to Hb (~20%)
  • In the form of bicarbonate ions (~70%)
  • Dissolved in plasma (~10%)

Regulation of Breathing by Respiratory Centres

• Respiratory centres in the medulla and pons control breathing
• Medulla: contains dorsal respiratory group (DRG) and ventral respiratory group (VRG)
• DRG: maintains normal quiet breathing rhythm
• VRG: innervates accessory respiratory muscles, active during forced breathing
• Pons: contains pontine respiratory group (PRG), modifies the activity of DRG based on input from various receptors/brain regions

Chemoreceptors and Breathing Regulation

• Chemoreceptors: respond to changes in pH/pCO2 in CSF (central) and changes in PCO2, pH and pO2 in the blood (peripheral)
• Chemoreceptors are the most important input for controlling breathing
• pCO2 is the primary determinant of breathing rate
• pCO2 changes directly link to pH changes, which is bad for the body
• pO2 changes are less sensitive, and RBCs/Hb can efficiently bind/deliver O2 even when pO2 levels drop