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The Respiratory System - Structure And Function

The respiratory system is the system in the human body that enables us to breathe.

The act of breathing includes: inhaling and exhaling air in the body; the absorption of oxygen from the air in order to produce energy; the discharge of carbon dioxide, which is the byproduct of the process.

The parts of the respiratory system


The respiratory system is divided into two parts:

Upper respiratory tract:

This includes the nose, mouth, and the beginning of the trachea (the section that takes air in and lets it out).

Lower respiratory tract:

This includes the trachea, the bronchi, broncheoli and the lungs (the act of breathing takes place in this part of the system).

The organs of the lower respiratory tract are located in the chest cavity. They are delineated and protected by the ribcage, the chest bone (sternum), and the muscles between the ribs and the diaphragm (that constitute a muscular partition between the chest and the abdominal cavity).

The trachea – the tube connecting the throat to the bronchi.

The bronchi – the trachea divides into two bronchi (tubes). One leads to the left lung, the other to the right lung. Inside the lungs each of the bronchi divides into smaller bronchi.

The broncheoli - the bronchi branches off into smaller tubes called broncheoli which end in the pulmonary alveolus.

Pulmonary alveoli – tiny sacs (air sacs) delineated by a single-layer membrane with blood capillaries at the other end.

The exchange of gases takes place through the membrane of the pulmonary alveolus, which always contains air: oxygen (O2) is absorbed from the air into the blood capillaries and the action of the heart circulates it through all the tissues in the body. At the same time, carbon dioxide (CO2) is transmitted from the blood capillaries into the alveoli and then expelled through the bronchi and the upper respiratory tract.

The inner surface of the lungs where the exchange of gases takes place is very large, due to the structure of the air sacs of the alveoli.

The lungs – a pair of organs found in all vertebrates.

The structure of the lungs includes the bronchial tree – air tubes branching off from the bronchi into smaller and smaller air tubes, each one ending in a pulmonary alveolus.


The act of breathing


The act of breathing has two stages – inhalation and exhalation
  • Inhalation – the intake of air into the lungs through expansion of chest volume.
  • Exhalation – the expulsion of air from the lungs through contraction of chest volume.

Inhalation and exhalation involves muscles:
  1. Rib muscles = the muscles between the ribs in the chest.
  2. Diaphragm muscle

Muscle movement – the diaphragm and rib muscles are constantly contracting and relaxing (approximately 16 times per minute), thus causing the chest cavity to increase and decrease.

During inhalation – the muscles contract:

Contraction of the diaphragm muscle – causes the diaphragm to flatten, thus enlarging the chest cavity.

Contraction of the rib muscles – causes the ribs to rise, thus increasing the chest volume.

The chest cavity expands, thus reducing air pressure and causing air to be passively drawn into the lungs. Air passes from the high pressure outside the lungs to the low pressure inside the lungs.

During exhalation – the muscles relax:

The muscles are no longer contracting, they are relaxed.

The diaphragm curves and rises, the ribs descend – and chest volume decreases.

The chest cavity contracts thus increasing air pressure and causing the air in the lungs to be expelled through the upper respiratory tract. Exhalation, too, is passive. Air passes from the high pressure in the lungs to the low pressure in the upper respiratory tract.

Inhalation and exhalation are involuntary and therefore their control requires an effort.

The act of breathing – Illustration & Animation



Source: Merck Manual




Changes in chest volume during inhalation and exhalation – note that it only shows the movement of the diaphragm, not that of the rib muscles.

Source: Wikipedia

The respiratory system- Illustration



Source: Wikipedia


What Do We Measure And How Do We Measure It?


The respiratory airways include the respiratory apertures (mouth and nose), the trachea and a branching system of long, flexible tubes (bronchi) that branch of to shorter and narrower tubes (broncheoli) until they end in sacs called the pulmonary alveoli.

The lungs encompass the entire system of tubes branching out from the main bronchi to the alveoli.

Measuring the functioning of the lungs is a medical tool for diagnosing problems in the respiratory system.

Measurements of lung function


2. Air volume (in liters) – lung capacity

  • Maximum lung volume is known as TLC (total lung capacity). It can be obtained by maximum strenuous inhalation.

The maximum lung volume of a healthy adult is up to 5-6 liters. In children the maximum lung volume is up to 2-3 liters, depending on age. In infants it is up to 600-1000 milliliters.

Note! Differences in lung volume can only be caused by gender, age, and height.

  • Essential air volume is the maximum volume utilized by the lungs for inhalation, also known as VC (vital capacity).
  • Residual volume (RV) is the volume of air remaining in the lungs after strenuous exhalation when the lungs feel completely empty. Residual volume prevents the broncheoli and the alveoli from sticking together. Residual volume is approximately 1.5 liters (adults).
  • The differential between total lung capacity and residual volume is the maximal volume utilized by the lungs in order to breath. It is known as vital capacity (VC). In an adult, the VC is between 3.5 and 4.5 liters.
  • Tidal Volume or VT is the volume of air displaced between normal inspiration and expiration. In a healthy adult the tidal volume is approximately 500 milliliters.
2. Rate of airflow through the respiratory airways (into and out of the lungs).This measures the effectiveness of airflow.

3. Efficiency of diffusion of oxygen from the pulmonary alveoli into the blood (not dealt with in this unit).

TLC (total lung capacity) of children



Examining lung function


The most common, accessible and efficient method of measuring lung function is by means of a spirometer. Its purpose is to diagnose obstructive diseases of the respiratory system. It produces a diagram (graphic depiction) of the volume of air expired in a given time (liter/minute)

The spirometer shows the rate at which air is expelled from the lungs. It measures the total lung capacity up to the residual volume (this test does not show the rate at which oxygen is absorbed).

If the airways are blocked the rate of the airflow of the lungs decreases. This will show on the diagram and thus indicate that there is a problem in the airways.

The most common obstruction stems from excessive phlegm, or from swelling of the inner wall of the air ways.

The most common problem of blockage of the air ways is asthma. people suffering from asthma it take longer to empty the lungs than healthy people. For example, during the first second of exhalation, only half of the vital air capacity in their lungs is expelled as opposed to 90% in healthy people. The rest is exhaled much later.

A spirometer examination takes only a few seconds. It is completely safe but there is a need for the patient to cooperate in order to obtain accurate results.

Stages of the examination:

  1. The patient is asked to inhale as deeply as possible.
  2. The patient is asked to exhale strenuously into the spirometer.
  3. The patient is asked to continue to expel air for a few seconds, despite the strong urge to breathe in.
  4. The test is repeated twice or three times.

Respiratory rate


Children in the upper classes of elementary school breathe about 20 times per minute.

Every breath causes an inhalation of approximately 7 milliliters of air volume per kilogram of body weight.

A child who weighs 30 kilos inhales approximately 210 milliliters of air volume (210X30). In other words, in the duration of a minute some 4200 milliliters of air volume enters and be expelled from the lungs.

Athletes breathe slightly deeper and slower. With every breath they inhale approximately 10 milliliters of air per kilogram. Thus an athletic child who weighs 30 kilos will only breathe 15 times in the duration space of a minute. Each inhalation will require some 300 milliliters of air volume. In the space of a minute 4500 milliliters of air volume will enter and be expelled from the his lungs. We can deduce from this that athletes ventilate their airways in a much more efficient way.

When we are under strain we breathe faster and more deeply. Since the lungs contain a reserve of air, we do not become tired because lack of air (oxygen) is causing respiratory restriction, but because of strain and tiredness in our respiratory and heart muscles.

When we are under emotional stress (before an exam, in distress, or feeling very frightened) we breathe faster, but our breathing is shallower. For example, under stress we inhale 30 times per minute but at a rate of only 4 milliliters per kilo. In other words, overall only 3600 milliliters per minute are passing through our airways, so we feel “short of breath.”

During severe asthma attacks, the breathing of asthma patients is shallower and at a higher rate. Their breathing is thus not very efficient.
Date Created: 05/05/13
Date Updated: 06/05/13