51. Alveolar air, alveolar ventilation, dead spaces. Function of the respiratory passageways

Last updated on October 6, 2020 at 12:46

Breathing patterns

Eupnoea refers to the normal breathing which occurs during rest. The tidal volume is around 500 mL and the respiratory frequency is 14 – 18/min.

Tachypnoea refers to abnormally rapid breathing, with a respiratory frequency > 18/min. It can by physiological, for example during exercise.

Hyperpnoea refers to abnormally high tidal volume, with or without increased respiratory frequency. It can also be physiological, for example during exercise.

Bradypnoea refers to abnormally slow breathing.

Hypopnoea refers to abnormally low tidal volume, with or without decreased respiratory frequency.

Apnoea refers to a stop in breathing when the lungs are empty, after an expiration.

Apneusis refers to a stop in breathing when the lungs are full, after an inspiration.

Hyperventilation refers to ventilation which is too excessive for the metabolic demand on the body. As a result, the pCO2 will decrease.

Hypoventilation refers to ventilation which is too little for the metabolic demand of the body. As a result, the pCO2 will increase.

Dyspnoea refers to shortness of breath.

Kussmaul breathing is a breathing pattern specific for acidosis, especially ketoacidosis.

Alveolar air and dead space

The minute volume (MV) or minute ventilation refers to how much air enters the respiratory system each minute. It is equal to the tidal volume times the respiratory rate per minute. MV = TV x breaths/min. It’s normally around 6 L/min.

The whole point of the lungs is to continually renew air in the gas exchange area of the lungs, where gases are exchanged with the blood. Some of the air we inhale never reaches the gas exchange areas but rather fills respiratory passages where gas exchange does not occur, like the nose, pharynx and trachea. This air is called dead space air because it is not useful for gas exchange. During expiration the dead space air is expired first, before any of the air from the alveoli is exhaled. The dead space air is about 150 mL.

The alveolar ventilation refers to how much air enters the gas exchange areas per minute. It is equal to the respiratory rate times the tidal volume minus the dead space volume. Alveolar ventilation = (TV – dead space) x breath/min. It’s normally around 4200 mL/min.

Function of the respiratory passageways

The different parts of the respiratory passageways have different functions.

The upper airways, especially the nose, influence the temperature and humidity of the air. They humidify dry air and warm cool air. They also filter the air for particles. Only particles <2 µm get into the alveoli. The epiglottis separates food and air and prevents food from entering the trachea. The larynx allows for phonation, the production of sounds.

The mucosa of the respiratory passageways is kinociliated. If any small particles enter the airways these kinocilia will move the particles upwards and out.

The respiratory passageways must also keep themselves open to allow easy passage of air. Many parts of the respiratory systems have cartilage rings around them to prevent them from collapsing, including the trachea and the bronchi. The trachea and bronchi also have strong smooth muscle in their walls.

The bronchioli don’t have cartilage and are kept open with the help of strong smooth muscle in their walls.

Airway resistance

The airway resistance depends mostly on the diameter of the airways. It is lowest in the smaller bronchioli and highest in the bronchi and trachea.

Like vascular smooth muscle the bronchial smooth muscle can contract or relax to vary its lumen. This is called bronchodilation and bronchoconstriction. During bronchoconstriction the lumen becomes smaller, which increases the airway resistance.

Bronchodilation is stimulated by circulating catecholamines binding to beta-2 receptors in the bronchial smooth muscle during sympathetic activation.

Bronchoconstriction is stimulated by:

  • Parasympathetic activation, which releases acetylcholine which binds to type 3 muscarinic acetylcholine receptors
  • Histamine

Abnormally high airway resistance is typical for obstructive lung diseases, and is often caused by bronchoconstriction, swollen mucous membranes and accumulation of mucus in the airways.


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50. Mechanics of respiration (functions of respiratory muscles, compliance, intrathoracic pressures, respiratory volumes)

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52. Gaseous exchange in the lungs and tissues

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