8.1: Background Information

Upper Respiratory Tract

All of the organs and other structures of the upper respiratory tract are involved in the conduction or the movement of air into and out of the body. Upper respiratory tract organs provide a route for air to move between the outside atmosphere and the lungs. They also clean, humidity, and warm the incoming air. However, no gas exchange occurs in these organs.

Nasal Cavity

The nasal cavity is a large, air-filled space in the skull above and behind the nose in the middle of the face. It is a continuation of the two nostrils. As inhaled air flows through the nasal cavity, it is warmed and humidified. Hairs in the nose help trap larger foreign particles in the air before they go deeper into the respiratory tract. In addition to its respiratory functions, the nasal cavity also contains chemoreceptors that are needed for the sense of smell and that contribute importantly to the sense of taste.

Pharynx

The pharynx is a tube-like structure that connects the nasal cavity and the back of the mouth to other structures lower in the throat, including the larynx. The pharynx has dual functions: both air and food (or other swallowed substances) pass through it, so it is part of both the respiratory and digestive systems. Air passes from the nasal cavity through the pharynx to the larynx (as well as in the opposite direction). Food passes from the mouth through the pharynx to the esophagus.

Larynx

The larynx, a cartilaginous structure inferior to the laryngopharynx connects the pharynx and trachea and helps to conduct air through the respiratory tract. Three large cartilage pieces—the thyroid cartilage (anterior), epiglottis (superior), and cricoid cartilage (inferior)—form the major structure of the larynx. The thyroid cartilage is the largest piece of cartilage that makes up the larynx. The thyroid cartilage consists of the laryngeal prominence, or “Adam’s apple,” which tends to be more prominent in males. The thick cricoid cartilage forms a ring, with a wide posterior region and a thinner anterior region. Three smaller, paired cartilages—the arytenoids, corniculates, and cuneiforms—attach to the epiglottis and the vocal cords and muscle that help move the vocal cords to produce speech.

A very important function of the larynx is protecting the trachea from aspirated food. When swallowing occurs, the backward motion of the tongue forces a flap called the epiglottis, a flexible piece of elastic cartilage, attached to the thyroid cartilage covers the opening of the trachea. When in the “closed” position, the unattached end of the epiglottis rests on the glottis. The glottis is composed of the vestibular folds, the true vocal cords, and the space between these folds. to close over the entrance to the larynx. This prevents swallowed material from entering the larynx and moving deeper into the respiratory tract. If swallowed material does start to enter the larynx, it irritates the larynx and stimulates a strong cough reflex. This generally expels the material out of the larynx and into the throat.

Figure \(\PageIndex{2}\) contains two labeled diagrams of the larynx (voice box).

The respiratory system produces sound by passing air through the upper airways and the mouth. Most of the sound people make is caused when air passing through the larynx causes the vibration of two flaps of tissue called the vocal cords (vocal folds). The sound gets louder when more air flows through the larynx. Different muscle contractions in the larynx control the position and tension of the vocal cords and thus alter the pitch of sounds. The sounds made by the vocal cords are modified by the other upper airways, especially the nasal passages and the mouth. By changing the shape of these passages and moving the tongue, a person can create a multitude of sounds and form words. All the actions that produce and modify sounds from the respiratory system are controlled by the nervous system.

Lower Respiratory Tract

The trachea and other passages of the lower respiratory tract conduct air between the upper respiratory tract and the lungs. These passages form an inverted tree-like shape (Figure 3), with repeated branching as they move deeper into the lungs. All told, there are an astonishing 2,414 kilometres (1,500 miles) of airways conducting air through the human respiratory tract! It is only in the lungs, however, that gas exchange occurs between the air and the bloodstream.

Figure \(\PageIndex{3}\) shows the main air passages involved in breathing within the lungs. It traces the airway from the larynx at the top down through the trachea, where firm cartilage helps keep the passage open. From there, the airway divides into many smaller branches that spread throughout the lungs, forming a complex network that allows air to move efficiently to all regions for breathing.

Trachea

The trachea, or windpipe, is the widest passageway in the respiratory tract. It is about 2.5 cm (1 inch) wide and 10-15 cm (4-6 inch) long. It is formed by rings of cartilage, which make it relatively strong and resilient. The trachea connects the larynx to the lungs for the passage of air through the respiratory tract. The trachea branches at the bottom to form two bronchial tree.

Bronchi and Bronchioles

There are two main bronchial tubes, or bronchi (singular, bronchus), called the right and left bronchi. The bronchi carry air between the trachea and lungs. Each bronchus branches into smaller, secondary bronchi; and secondary bronchi branch into still smaller tertiary bronchi. The smallest bronchi branch into very small tubules called bronchioles. The tiniest bronchioles end in alveolar ducts, which terminate in clusters of minuscule air sacs, called alveoli (singular, alveolus), in the lungs. The respiratory zone begins where the terminal bronchioles join a respiratory bronchiole, the smallest type of bronchiole (Figure 4), which then leads to an alveolar duct, opening into a cluster of alveoli

Lungs

The lungs are the largest organs of the respiratory tract. They are pyramid shaped, paired organs connected to the trachea, suspended within the pleural cavity of the thorax. The lungs are surrounded by two thin membranes called pleura, which secrete fluid that allows the lungs to move freely within the pleural cavity. This is necessary so the lungs can expand and contract during breathing. In Figure 4, you can see that each of the two lungs is divided into sections. These are called lobes, and they are separated from each other by connective tissues. The right lung is larger and contains three lobes. The left lung is smaller and contains only two lobes. The smaller left lung allows room for the heart, which is just left of the center of the chest.

Figure \(\PageIndex{4}\) shows the right and left lungs with the main airway and branching bronchi.

Figure \(\PageIndex{5}\) shows an alveolar cluster at the end of a small airway.

These tiny air sacs are the functional units of the lungs where gas exchange takes place. The two lungs may contain as many as 700 million alveoli, providing a huge total surface area for gas exchange to take place. In fact, alveoli in the two lungs provide as much surface area as half a tennis court! Each time you breathe in, the alveoli fill with air, making the lungs expand. O2 in the air inside the alveoli is absorbed by the blood in the mesh-like network of tiny capillaries that surrounds each alveolus. The blood in these capillaries also releases CO2 into the air inside the alveoli. Each time you breathe out, air leaves the alveoli and rushes into the outside atmosphere, carrying waste gases with it.

Control of Breathing

Breathing is one of the few vital bodily functions that can be controlled consciously as well as unconsciously. Think about using your breath to blow up a balloon. You take a long, deep breath, and then you exhale the air as forcibly as you can into the balloon. Both the inhalation and exhalation are consciously controlled.

Conscious Control of Breathing

You can control your breathing by holding your breath, slowing your breathing, or hyperventilating, which is breathing more quickly and shallowly than necessary. You can also exhale or inhale more forcefully or deeply than usual. Conscious control of breathing is common in many activities besides blowing up balloons, including swimming, speech training, singing, playing many different musical instruments, and doing yoga, to name just a few. There are limits on the conscious control of breathing. For example, it is not possible for a healthy person to voluntarily stop breathing indefinitely. Before long, there is an irrepressible urge to breathe. If you were able to stop breathing for a long enough time, you would lose consciousness. The same thing would happen if you were to hyperventilate for too long. Once you lose consciousness so you can no longer exert conscious control over your breathing, involuntary control of breathing takes over.

Spirometry: Lung volumes and Lung Capacities

Pulmonary function is tested to detect lung diseases or to monitor the progress of a disease. An instrument called a spirometer is used to measure the volume of air that moves air into and out of the lungs, and the process of taking the measurements is called spirometry. Respiratory capacity (pulmonary capacity) is the sum of two or more volumes. Factors such as age, sex, body build, and physical conditioning have an influence on lung volumes and capacities.

Figure \(\PageIndex{8}\) features a spirometer, a medical device prominently used in pulmonary function tests to measure the volume of air a person can exhale from their lungs. The process involves the patient exhaling into a mouthpiece connected to a dial, which records the airflow and volume. This test is crucial for healthcare professionals to diagnose and monitor lung conditions such as asthma and Chronic Obstructive Pulmonary Disease (COPD). 

Spirometry tells a clinician whether the lungs are functioning within the normal range. Using the graph generated from spirometry, the clinician can make several measurements:

• Tidal volume (TV): the volume of air displaced in the lungs during normal (tidal) inhalation and exhalation. This is the amount of air exchanged by breathing under normal, or resting, conditions. Average: Adult male = 0.5 L, Adult female = 0.5 L
• Inspiratory reserve volume (IRV): the additional amount of air that can inflate the lungs after a tidal inhalation. Average: Adult male = 3.3 L, Adult female = 1.9 L
• Inspiratory capacity (IC): the total amount of air that can be inhaled into the lungs (TV+IRV). A forceful inhalation can determine the IC with spirometry. Average: Adult male = 3.8 L, Adult female = 2.4 L
• Expiratory reserve volume (ERV): the amount of air that can be forcefully exhaled from lungs after a tidal exhalation. Average: Adult male = 1.1 L, Adult female = 0.7 L
• Vital capacity (VC): the maximum amount of air that can be expelled from the lungs after a maximal inhalation. VC = IRV + TV + ERV (see graph). The subject forcefully inhales and then forcefully exhales as much air as possible to determine the VC with spirometry. The vital capacity varies with height, gender and age of the subject. Average: Adult male = 4.8 L , Adult female = 3.1 L

Other important volumes or capacities that cannot be determined directly by spirometry include:

• Residual volume (RV): the amount of air left in the lungs after a forceful exhalation. This volume cannot be directly determined by spirometry because the amount cannot be expelled through the spirometer mouthpiece for measurement. This is the amount of air that always remains in the lungs, keeping the lungs from “collapsing.” Average: Adult male = 1.2 L, Adult female = 1.1 L.
• Functional residual capacity (FRC): the amount of air remaining in the lungs after a tidal exhalation. FRC is unknown in spirometry because the residual volume cannot be directly measured. Average: Adult male = 2.4 L, Adult female = 1.8 L.
• Total lung capacity (TLC): the total air capacity of the lungs. TLC = VC + RV. TLC is unknown in spirometry because the residual volume cannot be directly measured. Average: Adult male= 6.0 L, Adult female = 4.2 L.

Figure \(\PageIndex{9}\) shows lung volumes and capacities using two side-by-side diagrams.

• On the left (panel a), the vertical axis represents air volume in milliliters, from 0 to about 6000 mL. A black wavy line shows normal breathing within a small range called tidal volume. Above tidal volume is inspiratory reserve volume, the extra air inhaled with a deep breath. Below tidal volume is expiratory reserve volume, the extra air exhaled after a normal exhalation. At the bottom is residual volume, the air remaining after maximal exhalation that cannot be breathed out.
• On the right (panel b), stacked blocks show how volumes combine into capacities. Inspiratory capacity equals tidal volume plus inspiratory reserve volume. Functional residual capacity equals expiratory reserve volume plus residual volume. Vital capacity equals inspiratory reserve volume plus tidal volume plus expiratory reserve volume. Total lung capacity equals vital capacity plus residual volume (all volumes combined).

These graph shows combinations such as vital capacity, inspiratory capacity, and total lung capacity, helping to understand the full spectrum of lung function. 

Rate and Depth of Breathing Determine the Efficiency of Breathing

• Total pulmonary ventilation = the volume of air moved into/out of the lungs each minute. There is a formula to calculate total pulmonary ventilation, it is Respiratory Rate (breaths/min) times Tidal Volume (TV), which has the units ml/breath. This gives a measure of effectiveness of ventilation.
• Typically the resting adult Respiratory Rate (RR) is from 12-20 breaths per minute.
• Total Pulmonary Ventilation = RR x TV. If RR =12 breaths per minute and TV = 500 ml per breath, then …Total Pulmonary Ventilation = 12 breaths/min x 500 ml/breath = 6,000 ml/min, or 6.0 L/min