Gases are transported through the body's respiratory system primarily through two mechanisms: bulk flow (convection) and molecular diffusion.
Understanding how gases like oxygen and carbon dioxide are transported is fundamental to breathing and overall body function. This process involves movement through different parts of the respiratory system, each utilizing a specific method.
Based on the reference, the transport of gases, specifically during ventilation, happens in two main stages:
1. Convection or Bulk Flow
Initially, gases move through the larger airways, like the trachea and bronchi, via convection or bulk flow. This is similar to how air is pushed through a tube. Pressure differences created by breathing move large volumes of gas quickly through the conducting airways.
- Mechanism: Bulk movement of gas due to pressure gradients.
- Location: Primarily in the conducting zone of the airways (nose, pharynx, larynx, trachea, bronchi, bronchioles).
- Purpose: Efficiently moves gas over longer distances from the atmosphere towards the lungs' gas exchange areas.
2. Molecular Diffusion
Once the gases reach the smaller areas, like the alveoli (tiny air sacs in the lungs) and pulmonary capillaries (small blood vessels surrounding the alveoli), the movement changes. Gas molecules move individually from an area where they are in high concentration to an area where they are in low concentration. This process, known as molecular diffusion, allows gases to cross the thin barriers between the alveoli and the blood.
- Mechanism: Passive movement of individual gas molecules down their concentration or partial pressure gradients.
- Location: Across the respiratory membrane between the alveoli and the pulmonary capillaries.
- Purpose: Enables the actual exchange of oxygen into the blood and carbon dioxide out of the blood.
Therefore, ventilation or gas transport involves the movement of gas by convection or bulk flow through the conducting airways and then by molecular diffusion into the alveoli and pulmonary capillaries.
This two-stage process ensures that fresh air reaches the deep parts of the lungs where gas exchange can occur efficiently with the bloodstream.
