Pulmonary Structure and Function for First Aid

This blog post is designed to provide supplemental information to candidates learning about choking emergencies. The information posted here will not be covered in Red Cross first aid and / or CPR training as it is extensive in detail and information. The material is here for candidates wanting to look further into the breathing structure of the human body.


I. Anatomy of the Respiratory System
Respiratory system consists of nose, pharynx, larynx, trachea, bronchi, and lungs. Bronchi – primary, secondary, and tertiary bronchi —> terminal and respiratory bronchioles —> alveolar ducts —> alveoli.
With branching, supportive cartilage is gradually replaced by smooth muscle. Contraction and relaxation of this smooth muscle constricts or dilates the bronchioles –> major effects on airway resistance. The conducting airways lead inspired air to the alveoli. Volume of conducting airways = anatomic dead space (VD) – 150 ml.

Alveoli – small, thin walled sacs that have capillary beds in their walls; site of gas molecule (O2 & CO2) exchange between air and blood; there are millions of alveoli
Respiratory membrane – alveolar-capillary membranes that separate the air molecules in the alveoli from the blood in the capillaries – average thickness is 0.6 micrometers. The respiratory membrane has a very large
surface area – 70 square meters in the normal adult – size of tennis court. Lungs – contain conducting airways, alveoli, blood vessels, elastic tissue.

II. Mechanics of Breathing
Molecules move from areas of high pressure or concentration to areas of low pressure or concentration. Boyle’s Law – the pressure of a gas is inversely proportional to its volume. The movement of air into and out of the lungs results from a pressure difference between the pulmonary air and the atmosphere.

Inspiration – active process – diaphragm descends and external intercostal muscles contract thus increasing the volume of the thoracic cavity —> decreased pressure in thoracic cavity causing a one or two mm Hg drop in
intra-alveolar pressure at rest compared to the outside atmospheric pressure —> air molecules move through the respiratory tubes into the lungs from the atmosphere following the pressure gradient. Inspiratory muscles, when they work their hardest, can produce a negative pressure as great as -30 mm Hg below atmospheric pressure within the alveoli.

Expiration – passive process at rest. Secondary muscles, such as abdominal muscles become involved in exercise. Forced expiration can produce intra-alveolar pressure as great as +50 mm
Hg above atmospheric pressure. During exercise, mouth breathing tends to replace nasal breathing – less resistance to airflow. Air that enters the respiratory passages via either the nose or the mouth is quickly saturated with water vapor and warmed to body temperature, 37 degrees centigrade, even under conditions when very cold air is inspired. Compliance – the amount of volume change in the lung for a given change in alveolar pressure.
III. Lung Volumes

Normal values at rest:

  • Minute ventilation (VE ) – 6 – 8 liters/min.
  • Tidal volume (VT) – 500 ml per inspiration or expiration
  • Breathing frequency (FR) – 12 – 16 breaths per minute
  • Expiratory reserve volume (ERV) – approximately 25% of vital capacity(VC)
  • Inspiratory capacity (IC) – approximately 75 % of vital capacity (VC)
  • Total lung capacity = vital capacity plus residual volume

In maximal aerobic exercise, breathing frequency can increase up to 60 breaths per minute and tidal volume can increase up to 50% of vital capacity.
Alveolar ventilation(VA) – the volume of air that reaches the alveoli per minute. This value is very important because this is the only air that participates in gas exchange with the blood.
VA = (VT X FR ) – (VD X FR)
= 500 ml X 12 – 150 ml X 12
= 6000 ml – 1800 ml
= 4200 ml/min.

Most volumes and capacities decrease when a person lies down and
increase when standing. Reasons:

  1. Abdominal contents push up against diaphragm
  2. There is an increase in intrapulmonary blood volume in the horizontal position which decreases the space available for pulmonary air.

IV. Pulmonary Disorders
Pulmonary function test norms are usually based on sex, age, and height. It is important to know the size and make-up of the population used to construct the norms. Problems with pulmonary function norms:

  • don’t consider the “size” of the subject, particularly the chest size
  • would be better to use sitting height rather than standing height

Chronic pulmonary dysfunctions can be divided into two categories:
1. Obstructive disorders – blockage or narrowing of the airways causing increased airway resistance – asthma, bronchitis, emphysema. Bronchiolar obstruction can result from inflammation and edema, smooth
muscle constriction, or bronchiolar secretion. Very difficult to move air rapidly in and out of lungs –> decreased FEV1.0, FEV1.0/VC much less than 80%, decreased MBC
Airways collapse during expiration before normal amount of air is emptied from the lung – air trapping —> increased FRC, RV, TLC

2. Restrictive disorders – no problem with the airways but there is damage to the lung tissue – loss of elasticity and compliance – limited expansion of the lung – pulmonary fibrosis, pneumonia.
All lung volumes are reduced – VC, RV, FRC, TLC – because the lung tissue is stiff and can’t be expanded very far. FEV1.0 and MBC are reduced but FEV1.0/VC ratio is frequently 90% or greater.
Pulmonary function tests must be interpreted in relation to a patient’s medical history, occupational history, smoking habits, and a chest X-ray. V. Ventilation During Incremental Exercise

During exercise, minute ventilation increases linearly with increasing exercise intensity and oxygen uptake until approximately 60% of VO2 max. In untrained subjects and 75-80% of VO2 max. in endurance athletes.

Ventilatory threshold – the point at which minute ventilation increases disproportionately with oxygen consumption during graded exercise. For a given individual, the exercise intensity at the ventilatory threshold is
similar to the exercise intensity at the lactate threshold, the point at which lactic acid begins to accumulate in the blood. Prior to this exercise intensity, aerobic metabolism matches the energy requirement of the active muscles and no blood lactate accumulates because lactate production equals lactate disappearance.

For a given work rate, arm or upper body exercise causes a greater minute ventilation than leg exercise – example of arm cycle ergometry versus leg cycle ergometry.

For more information on breathing emergencies and how to treat and react to other first aid scenario enroll in a standard first aid and CPR course. Training centres are located in Vancouver, Richmond, Surrey, Coquitlam and Burnaby. Material posted in this page is for information purposes only.