Understanding The Mechanics of Breathing

By February 19, 2021No Comments
mechanics of breathing

Proper breathing can give you more muscular endurance and help in cardio-centric activities like running, swimming and biking, it can also help you recover more quickly during high intensity activities. Learn more about the mechanics of breathing.

The word respiration consists of pulmonary respiration, which refers to breathing and the exchange of oxygen and carbon dioxide in the lungs, and cellular respiration which relates to oxygen utilization and carbon dioxide production by the tissues.

The human respiratory system consists of a group of passages that filter air and transport it into the lungs, where gas exchange occurs within microscopic air sacs called alveoli. The major components of the respiratory system comprise of the nose, nasal cavity, pharynx, trachea, bronchial tree and the lungs.

The air passages of the respiratory system are divided into two functional areas consisting of the conducting zone (trachea, bronchi, terminal bronchioles) and the respiratory zone (respiratory bronchioles, alveolar ducts, alveolar sacs).

Conducting Zone

We breathe through the nose until movement of air in and out of the lungs reaches 20 to 30 litres per minute, by which time the mouth becomes the primary passageway for air, which passes through a valve-like opening called the epiglottis located between the vocal cords. The conducting zone of the respiratory system not only serves as a passageway for air, but also functions to humidify and filter the air as it moves toward the respiratory zone of the lung.

Regardless of the temperature or humidity of the environment, the air that reaches the lung is warmed and is saturated with water vapour. This warming and humidification of air serves to protect body temperature and prevents the delicate lung tissue from drying out. Lung damage due to collection of inhaled air particles are prevented firstly by mucus secreted by the cells of the conducting zone trapping small inhaled particules, which move towards the pharynx through ciliary action, where it can be swallowed or expelled out. Secondly, cells called macrophages that reside in the alveoli engulf particles that reach the alveoli.

Respiratory Zone

Gas exchange in the lungs occurs across about 300 million tiny alveoli. The large number of these structures provides the lung with a large surface area for diffusion, which is the movement of molecules from an area of high concentration to an area of lower concentration. This causes oxygen from the lungs to move into the blood. Moreover, carbon dioxide moves from the blood into the lung and is expired.

Process of Breathing


The diaphragm is the most important muscle of inspiration and is the only skeletal muscle considered essential for life. This thin dome-shaped muscle inserts into the lower ribs. When the diaphragm contracts, it forces the abdominal contents downward and forward. Further the ribs are lifted outward. The outcome causes the lungs to expand. This expansion of the lungs results in a reduction in intrapulmonary pressure below atmospheric, which allows airflow into the lungs. During normal breathing, the diaphragm performs most of the work of inspiration. However, during exercise, accessory muscles of inspiration are called into play. Collectively these muscles assist the diaphragm in increasing the volume of the thorax, which aids in inspiration.


Expiration is passive during normal breathing. No muscular effort is necessary for expiration to occur at rest. This is due to the lungs and chest walls being elastic and tending to return to equilibrium position after expansion during inspiration. During exercise, expiration becomes active. The most important muscles involved in expiration are those found in the abdominal wall, which include the rectus abdominus and the internal oblique. When these muscles contract, the diaphragm is pushed upward and the ribs are pulled downward and inward resulting in expiration.

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Airway Resistance

At any given rate of airflow into the lungs, the pressure difference that must be developed depends on the resistance of the airways. Airflow is increased anytime there is an increase in the pressure gradient across the pulmonary system or if there is a decrease in airway resistance. The most important factor contributing to airway resistance is the diameter of the airway. Airways that are reduced in size due to disease like asthma offer more resistance to flow than healthy, open airways.

Respiratory Muscles and Exercise

The primary task of respiratory muscles, which are skeletal muscles is to act upon the chest wall to move air in and out of the lungs. Muscular exercise results in an increase in pulmonary ventilation and results in an increased workload on respiratory muscles. Prolonged and high intensity exercise can promote respiratory muscle fatigue. However, regular endurance exercise training for marathons increases respiratory muscle oxidative capacity and improves respiratory muscle endurance.

Transport of Oxygen

Oxygen is poorly soluble in plasma, so that less than 2% of oxygen is transported dissolved in plasma. The vast majority of oxygen is bound to haemoglobin, a protein contained within red cells. The quantity of oxygen bound to haemoglobin is dependent on the partial pressure of oxygen in the lung to which blood is exposed. In alveoli at sea level, the partial pressure of oxygen is sufficient to bind oxygen to essentially all available iron sites on the haemoglobin molecule.

Transport of Carbon Dioxide

Transport of carbon dioxide in the blood is considerably more complex. A small portion of carbon dioxide, about 5%, remains unchanged and is transported dissolved in blood. The remainder is found in reversible chemical combinations in red blood cells or plasma. Some carbon dioxide binds to blood proteins, principally haemoglobin, to form a compound known as carbamate. About 88% of carbon dioxide in the blood is in the form of bicarbonate ion. The distribution of these chemicals between the interior of the red blood cell and the surrounding plasma varies greatly, with the red blood cells containing considerably less bicarbonate and more carbamate than the plasma. Less than 10% of the total quantity of carbon dioxide carried in the blood is eliminated during passage through the lungs. Complete elimination would lead to large changes in acidity between arterial and venous blood.

Exercise-Induced Asthma in Runners

Exercise can cause shortness of breath in anyone. Airflow obstruction that occurs because of exercise is exercise- induced asthma (EIA). Coughing is the most common symptom of EIA and may be the only symptom you have. When you exercise, you breathe faster and deeper due to the increased oxygen demands of your body. You usually inhale through your mouth, causing the air to be dryer and cooler than when you breathe through your nose. The dry and/or cold air is the main trigger for airway narrowing (bronchoconstriction). Exercise that exposes you to cold, dry air is more likely to cause asthma symptoms than exercise involving warm and humid air.

Other triggers that can make EIA symptoms worse:

  • Pollution levels
  • High pollen counts
  • Exposure to other irritants, such as smoke and strong fumes
  • A recent cold


Sanjai Banerji

Sanjai Banerji

Started running at the age of 48 in 2008 and has run more than 50 half marathons, marathons and ultra-races in 13 cities in India and abroad. In 2019, he became one of the oldest Indians to run in the top three marathons in Asia (Mumbai, Kuala Lumpur and Singapore). His book, ‘Crossing the Finish Line’ was published in 2019.

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