Lecture+12+Respiratory+1+2025

Page 1: Respiratory System Overview

Change in volume of thoracic cavity
- Diaphragm plays a key role in changing lung volume
- Breathe involves contraction and relaxation of the diaphragm
- Inhalation (diaphragm contracts) leads to negative pressure in lungs, allowing air intake
- Terminal molecules in the alveoli facilitate gas exchange.

Lungs at rest, with airways open and muscles relaxed, define functional residual capacity (FRC)
Elastic properties of the lung pull inward, while the chest wall exerts an outward pull
Negative pressure in the intrapleural space is a balance of these forces
Pneumothorax: Loss of vacuum leads to lung collapse; chest wall expands
Relaxation leads to inward pressure on the lung, decreasing lung volume.

Lungs rely on inflation to move air; their elasticity causes collapse.
Diaphragm pushes down into the abdominal cavity while intercostal muscles expand ribs outward
Changes in thoracic cavity volume facilitate airflow
Intrathoracic pressure is negative at rest, decreases during inhalation and increases during exhalation
“Loud” breathing characteristics:
- Very negative pressure during inspiration
- Positive during expiration due to forceful contractions
Inhalation is active; exhalation is passive during quiet breathing.

Inhalation: Relaxation of abdominal muscles increases abdominal cavity volume, expanding thoracic cavity.
Exhalation: Contraction of abdominal muscles pushes contents up into thoracic cavity, reducing thoracic cavity volume.

Inspiration: Active process, requires force
Expiration: Passive during quiet breathing
Resistance to air flow is proportional to the radius of airways (1/r^4)
Smaller airways maintained open by elastic fibers; less tension at low volumes leads to reduced airway radius
Impact of smooth muscle tone:
- Bronchodilation: Decreases resistance
- Bronchoconstriction: Increases resistance, affected by inflammation and mucus production.

Total lung capacity consists of several volumes:
- Residual Volume (RV): 1200 mL
- Expiratory Reserve Volume (ERV): 1100 mL
- Tidal Volume (VT): 500 mL
- Inspiratory Reserve Volume (IRV): 3000 mL
Males have a total lung capacity around 5800 mL; females around 4200 mL
Pulmonary capacities sum different volume measurements.

Airways facilitate movement of air to lungs; structural integrity is crucial for function
Changes in airway structure impact airflow dynamics.

Bronchi Structure:
- Trachea divides into primary bronchi which branch into smaller bronchi and bronchioles
- Each cluster of alveoli surrounded by elastic fibers and capillary networks.

Exchange of oxygen (O2) in and carbon dioxide (CO2) out
Simple diffusion is key; factors influencing diffusion:
- O2 less soluble, susceptible to limited diffusion flux
- Alveolar sacs and capillaries are close together to minimize diffusion distance
- Large surface area facilitates efficient gas exchange.

Air in anatomical dead space (e.g., nose, trachea) does not participate in gas exchange
First air inhaled is the last to exit; affects alveolar air.

Tidal Volume (VT) includes both alveolar (VA) and dead space (VD) volumes
Minute Ventilation (VE): Calculated as VT multiplied by breath frequency (f):
- VE = VT * f

Total minute ventilation (VE) comprises both alveolar and dead space ventilation
Alveolar ventilation critically affects blood oxygen and carbon dioxide levels
Example of calculations: total ventilation significantly increases with heavy breathing (5000 mL).