Medical Learning Library

ACLS Guide to the Human Respiratory System

The respiratory system is a vital part of the human body for it supplies oxygen to the different parts of the body which is necessary for survival. As the body’s breathing mechanism, it is composed of the nose, mouth, trachea ( or wind pipe ) which collectively serve as the airway, a pair of lungs and a muscle for respiration, the diaphragm. The respiratory system begins with the nose where air is inhaled and then carried through the nasal cavity into the pharynx. Once air passes through the pharynx, it goes into the trachea, and from there, enters into the lungs.

About the Human Respiratory System

Each cell in your entire body needs oxygen in order to move, build, reproduce and to turn food into energy. The respiratory system supplies oxygen to all parts of your body. Every time you inhale, your blood circulates the air all over your body. On an average, you are breathing 13 pints of air every minute. Also for the respiratory system to function, there are four essential requirements needed:

  1. The respiratory system surface must be moist.

  2. The surface must be thin-walled so diffusion of air can rapidly occur.

  3. It must have a source of oxygen.

  4. The respiratory surfaces must be in contact with the transport system that carries oxygen.

Parts and Functions of the Human Respiratory System

The human respiratory system is divided into the upper and lower respiratory tracts:

Upper Respiratory Tract

  1. The nose serves as the primary organ that air passes through. The nose is part of the conducting zone in which it purifies the air to get rid of dust and other impurities, maintains the right temperature of the air if it is cold outside and humidifies the air if it is dry.     

  2. The nasal cavity is a large air-filled area located behind and above the nose in the middle part of the face. It conditions the air so that it can pass through all areas of the respiratory tract.

  3. The pharynx is a part of the throat right behind the mouth and nasal cavity. The human pharynx serves as an air passage and has three sections: nasopharynx (area of the upper throat that lies behind the nose), oropharynx (part of the pharynx that is located between the soft palate and the hyoid bone) and laryngopharynx (the lower part of the pharynx). The pharynx is part of both the digestive system and the respiratory system; it is also important in vocalization. 

  4. The epiglottis is actually an extension of a connective tissue made of elastic cartilage and located at the base of the tongue. It closes the laryngeal inlet during swallowing.

  5. The larynx, commonly called the voice box, is an organ in the neck which has to do with breathing and production of sounds. The larynx also protects the trachea from food dislodgement or obstruction which may lead to aspiration.

Lower Respiratory Tract

  1. Trachea or windpipe is a tube connecting the pharynx or larynx to the lungs. It allows the passage of air and is lined with cells that produce mucus.

  2. Lungs serve as the essential respiration organ in humans and are located near the backbone on either side of the heart. Their vital function is to carry oxygen from the atmosphere into the bloodstream and to give off carbon dioxide from the bloodstream into the atmosphere.

  3. Bronchi (singular: bronchus) are airway passages in the respiratory tract that transport air into the lungs. 

  4. Bronchioles are bronchus branches which are smaller tubes that carry air into the lungs.

Breathing Lung Mechanics

  1. Breathing is the movement of the air in and out of the lungs. The process is automatic and comes from the respiratory control center of the body located in the medulla oblongata section of the brain. The normal breathing rate of an adult is 12 to 20 breaths per minute.

  2. The pathway of breathing starts with the air being inhaled through the nose then passes through the nasopharynx to the oral cavity and then through the glottis. From here, the air enters the trachea into the right and left bronchi and next to the alveoli (tiny air sacs in the lungs). The alveoli are where the actual gas exchange takes place. 

  3. Ventilation is the interchange of air between the lungs and the atmosphere so that oxygen replaces carbon dioxide in the alveoli.

  4. The diaphragm muscle is pulled downward to the abdomen causing the diaphragm to flatten and at the same time enlarging the chest cavity. As the intercostal muscles (located between the ribs) contract, the rib cage is lifted thereby enlarging the chest cavity. The enlargement of the chest cavity decreases the air pressure of the alveoli causing the air from outside to rush into the nose or mouth to equalize the pressure.

  5. In expiration or exhalation, the diaphragm and intercostal muscles relax. The diaphragm muscle resumes its dome-like shape (moves upward) while the intercostal muscles relax. The rib cage then returns to its original position (moves down and inward). These actions raise the air pressure in the alveoli and expel air out of the lungs through the airway and out of the nose or mouth.

  6. Control of respiration, also known as the control of ventilation, refers to the physical mechanics involved in controlling physiological ventilation. Exchange of gas controls the rate of respiration since the most important function of breathing is the exchange of oxygen and carbon dioxide. Therefore, the control of respiration is centered on how well the lungs function.

Homeostasis and Gas Exchange

Homeostasis is an ideal condition or state of equilibrium in which all systems of the body are operating and working harmoniously to accomplish all its needs. Homeostasis in respiration is under the control of gas exchange and regulated by blood pH. It is in the lungs and alveoli where gas exchange takes place, specifically between the alveoli and pulmonary blood capillaries. The process called diffusion is the exchange of air resulting in the increase of oxygen concentration and decrease of C02.

External respiration, commonly called breathing, is the exchange of gas between the air in the alveoli and blood within the pulmonary capillaries at a normal rate of 12-20 breaths per minute. Internal respiration is the exchange of gas at the cellular level.

Lung Capacity    

Lung capacity refers to the amount of air that a person’s lungs can hold at any given time. Human lungs can hold approximately four to six liters of air but only about a third of this capacity is used during normal activities. Males on average have a higher lung capacity than females and those who live in higher altitudes have a bigger lung capacity than people living at sea level due to the air being less dense at higher altitudes. Total lung capacity is measured by adding together the Tidal Volume, Expiratory Reserve Volume, Inspiratory Reserve Volume, and Residual Volume. All of these values are measured with the use of a spirometer.

Problems Associated with Respiratory Tract and Breathing

Respiratory tract infections are caused by more than 200 different viruses and may include bacteria and fungus as other causative agents. Respiratory tract infections are classified into upper respiratory tract infections (URTI) and lower respiratory tract infections (LRTI). LRTIs tend to be more serious than URTIs and include infections such as pneumonia, bronchitis and influenza.

 Here are some problems related to respiratory tract and breathing:

  1. Asthma affects the air passage towards the lungs which causes difficulty in breathing. These airways are quite sensitive and they can become inflamed and swollen. When the muscles are swollen, they are tightened and the condition is worsened by the production of excessive mucus. Consequently, airflow to and from the lungs is reduced.

  2. Difficulty in breathing, shortness of breath and breathlessness or dyspnea are all related to respiration but can come from a variety of causes. You can experience shortness of breath performing strenuous activities or it can be due to some environmental conditions like being at a high altitude or in a very warm or cold temperature. If these are not the causes, then shortness of breath may be a symptom of some type of medical problem.

  3. Pneumonia is a lung infection that is caused by either bacteria or a virus. You will run a fever, have coughing spells and experience difficulty in breathing. The duration of the illness can be 2 to 3 weeks and can be treated at home with the doctor’s advice.

  4. Emphysema is a chronic and long-term disease of the lungs which causes difficulty in breathing due to limitations in inhaling and exhaling air. There are multiple causes of emphysema but smoking is by far the most common.

  5. Bronchitis is a respiratory disease caused by the inflammation of the mucous membrane in the lung’s bronchial passages. Due to the growth and thickening of the irritated membrane, the airways in the lungs are obstructed. The results are coughing, expelling of thick phlegm and breathlessness. It comes in two forms: acute, which lasts less than 6 weeks, and chronic, which can recur frequently within 2 years.

For more information about respiratory tract infections, refer to the links below:

Written by and last updated Mar 27, 2017

Last reviewed by on Sep 11, 2016