The main objective of the lung is to match the ventilation of the air with the perfusion of blood through the lungs.

V = Ventilation- air going into the lung. Approx 4 litres.

Q = Perfusion- blood going around the lung. Approx 5 litres.

Ventilation/perfusion should be approx 0.9. This is otherwise known as the Ventilation Quotient.

Model of 3 Zones of the Lungs

A useful concept to understand. The lung is divided into three zones in this model.

Lung zonesZone 1 is the apex of the lung or the highest most part in the upright patient.

  • As this area is above the heart the blood pressure will be lower relative to the vessels below. That means that the blood vessels will be narrower than those below.
  • The reduced venous pressure will mean that the alveoli are bigger than those below.
  • As a consequence the ventilation will be much greater than the perfusion.

Zone 2 is the middle part of the lung.

  • Zone two tends to be the zone where the blood vessels and the alveoli are at their maximal sizes.
  • As a consequence the ventilation and perfusion will be balanced.

Zone 3 is the base of the lung, or the lowest part in the upright patient.

  • The blood supply is very distended in zone 3 because of the effects of gravity. They are below the heart, so are subject to a slightly higher pressure than those above them.
  • This increased venous pressure will also make the alveoli smaller as it acts on them, relative to those above.
  • As a consequence the perfusion will be greater than the ventilation.

A very important point to know here is that at any given time over half of our lung is not being utilised. Potentially our lungs can hold up to 6 litres of air, but normally is only holding just under 3 litres.

This means that a large proportion of out alveoli are collapsed. If we were perfusing all of those collapsed alveoli we would have a lot of our lung blood volume not being effectively ventilated. So how does the body compensate for this?

The body has a process called Hypoxic Vasoconstriction. Anytime a region is poorly ventilated and has low oxygen levels the muscles around the blood vessels in the area concerned constrict, thereby limiting the blood supply to the region.

So the blood flow to Zone 3 is actually reduced compared to Zone 2. This means that Zone 3 is neither perfused or ventilated well under normal circumstances. When we take a deep breath then we will utilise Zone 3 more fully.

6 Ways To Be Better With a BVM
Mechanical Ventilation- Physiologic Effects
Anatomy of Adult ETT
Mechanical Ventilation- Terminology
Mechanical Ventilation- Modes of Ventilation I
Mechanical Ventilation- Modes of Ventilation II
Mechanical Ventilation- Pressure/Volume/Flow Loops
Mechanical Ventilation- Peak Pressure and Plateau Pressure
PEEP (Positive End Expiratory Pressure)
Increase the rate or the tidal volume
Ventilation Screen- What do some of those numbers mean?
Phase Variables
I:E ratios and total cycle time.
Why do we ventilate?.
Volume controlled ventilation and compliance.
Lung compliance.
How do I describe how my patient is being ventilated?
Pressure/Volume loop
AC versus SIMV
A-a gradient
Pressure Support
Pressure Support Ventilation Curves
Pressure/Volume/Flow Curves
Ventilator Induced Lung Injury
Trigger, Limit and Cycle
Ventilator Associated Pnuemonia


Originally posted 2015-04-02 07:00:55. Republished by Blog Post Promoter

Tagged with →  
Share →

One Response to Mechanical Ventilation- Ventilation/Perfusion

  1. Jl says:

    I just want to ask if that explanation is also applief during inpiration with apex has a greater degree of ventilation than the bases… but then theres a source that contradicts your explanation it states that apex has a greater transpulmonary pressure than the bases therefor the apex hold more volume after expiration including the force of gravity however during inspiration the apex expands less due to the presence of alveoli at end expiration therefore the alveoli at the bases is inflated and ventilated more than the apex during inspiration and the anatomical struction pf the thorax can sum to it….

Leave a Reply

Your email address will not be published. Required fields are marked *