CHAPTER TEN: THE RESPIRATORY SYSTEM
I. Introduction to the Respiratory System A. Functional Overview of the Respiratory System: The purpose of the respiratory system is to move air in and out of the body, allowing for gas exchange between the atmosphere and the blood. Specifically, we must get oxygen (O2) into the body and carbon dioxide (CO2) out of the body. Cells of the body use oxygen for glucose metabolism. CO2 is a waste product of glucose metabolism and will cause increased blood acidity if not released from the body. There are 4 basic processes that accomplish the task of the respiratory system.
Pulmonary ventilation: the movement of air into and out of the lungs -
breathing. External respiration: diffusion of oxygen from the lungs into the blood and
diffusion of carbon dioxide from the blood into the lungs. Transport of respiratory gases: O2 is carried by hemoglobin in erythrocytes.
CO2 is mostly carried dissolved in the blood in the form of bicarbonate (HCO3). Internal respiration: the movement of oxygen from the blood to the tissue cells
and the movement of carbon dioxide from the tissues into the blood.
More about these 4 processes will be discussed below.
B. Anatomy Overview of the Respiratory System The organs of the respiratory system include the nose, nasal cavity, paranasal sinuses, pharynx, larynx, trachea, bronchi and their smaller branches, bronchioles, and the lungs, which contain alveoli, where gas exchange occurs. The respiratory tract is a continuous passageway that air follows from the nose to the alveoli in lungs. The upper respiratory tract includes the:
Nasal cavity - entrance and exit area for air
Pharynx - passageway for air and food The lower respiratory tract includes:
Larynx - houses the vocal cords for speech Trachea - an air passageway beginning at the sternal notch Bronchi - two branches off of the trachea, one to each lung Bronchioles - smaller continuations of the bronchi Alveoli - where gas exchange occurs between blood and air
The lungs are the primary organs of the respiratory system. Lungs are made of air sacs
(alveoli) and the tissues between the alveoli (interstitial tissue). Exchange of gases
between blood and air takes place in the alveoli. The tubes of the respiratory tract
penetrate the fleshy substance of the lungs.
The diaphragm is the primary muscle of respiration. Contraction of the diaphragm
produces inhalation and expansion of the lungs, while relaxation of the diaphragm
allows exhalation as the lungs recoil.
There are protective structures along the respiratory tract to prevent injury and provide
defense mechanisms should dirt or disease-causing microorganisms (pathogens) enter
The protective structures include:
Ciliated mucous membrane lines the nasal cavity and much of the lower
respiratory tract below the vocal folds of the larynx. Mucus is a thick sticky fluid
that cleans air by trapping particles such as dust and bacteria. It also warms and
moistens air entering the lungs. Cilia are microscopic hair-like fibers lining the
epithelium. Cilia have waving motions to constantly move the mucous downward
in the nasal cavity and upward in the lower respiratory tract. Anything trapped in
the mucous sheet is eventually swept out of the larynx into the esophagus and
swallowed. Diffuse lymphatic tissue and macrophages are in much of the respiratory
epithelium to fight pathogens that may enter with air flow. The alveoli also have
large numbers of macrophage cells that crawl over the internal surface looking
for pathogens. Cartilage and bone surround much of the respiratory tract to ensure the airway
doesn't collapse during breathing.
II. Organs of the Respiratory System A. Nasal Cavity
The nasal cavity is lined with ciliated glandular epithelium. It is located below the cranium and above the mouth. Air enters the nasal cavity through exterior openings called the nostrils or external nares. The external nares are separated from each other by a wall of cartilage called the nasal septum. Air leaves the nasal cavity and enters the throat through two openings called the internal nares. There are several other structures associated with the nasal cavity:
The nasal cavity is made of a roof formed by the ethmoid and sphenoid bones,
and floor formed by the palate. As air enters each nasal cavity it passes through the nasal conchae, 3 scroll-like turbinate bones where the air is warmed and moistened. The roof of the nasal cavity has olfactory epithelium. Olfactory neuron receptors start here and travel through the cribriform plate to become Cranial Nerve I (Olfactory Nerve).
A ciliated pseudostratified respiratory mucosa lines the rest of the nasal cavity.
The nasal cavity is surrounded by the paranasal sinuses, mucous lined cavities
in the frontal, ethmoid, sphenoid and maxilla bones. The sinuses are resonating chambers for speech. They also provide additional warming and moistening of air. Nasal mucus helps trap dirt, bacteria and other debris.
The nasolacrimal duct empties into the nasal cavity providing a drainage
pathway for the lacrimal fluid that washes the eye.
B. Pharynx (Throat) The pharynx has three areas:
Nasopharynx - located behind the nasal cavity, the uppermost part of the throat.
There are some tonsils here as well as the opening for the Eustachian tube, a passageway connecting to the middle ear for equalization of air pressure.
Oropharynx - located posterior to the oral cavity and is the common
passageway for food and air. Some tonsils are located here. Laryngopharynx - located posterior to the larynx (voice box) and a passageway
for both food an air. Its inferior portion divides into the esophagus posteriorly and
the larynx anteriorly.
C. Larynx (Voice Box) The larynx is located in the front of the pharynx and superior to the trachea. It is made mostly of several pieces of cartilage that particularly provide anterior protection for the
larynx. A ridge in the anterior cartilage is known commonly as the Adam's apple. The larynx also connects to the hyoid by membrane and muscle. These muscles elevate the larynx during swallowing. The larynx has 3 main functions:
It is an open airway for air to be conducted between the pharynx and the trachea.
It helps route food into the proper channel via the epiglottis ("above the glottis").
The epiglottis is a flap of cartilage covered with stratified squamous epithelium. The flap closes off the larynx during the swallowing reflex, thereby directing food and liquid into the esophagus.
It houses the vocal cords, made of elastic connective tissue covered with
mucous membrane. When air moves past the cords they vibrate, producing sound. The midline opening between the cords is called the glottis. When the cords are abducted, noiseless breathing occurs. When they are adducted, sound (speech) can be produced.
D. Trachea (Windpipe)
The trachea begins inferior to the larynx, runs anterior to the esophagus and divides at T4/T5 into two tubes called bronchi (one to each lung). The trachea functions as a passageway for air and is the "trunk" of the bronchial tree (see below).
Structurally, the trachea is a membranous tube of dense regular fibrous connective tissue and smooth muscle. It is reinforced with C-shaped rings of cartilage to support the airway. The posterior portion of each cartilaginous ring (the open part of the "C") is composed of smooth muscle allowing the esophagus to expand into the trachea as food travels down the esophagus to the stomach. This muscle also contracts to narrow the trachea and help expel air and mucus when coughing.
The internal lining of the trachea is made of ciliated glandular epithelium. Cilia in the trachea move mucus upwards towards the larynx (cilia in the upper respiratory system will move mucus downwards).
By the time air reaches the end of the trachea it is warmed, cleansed of most impurities, and saturated with water vapor.
The bronchi or bronchial tubes are a series of air tubes branching from the trachea and forming continuous smaller branches to form what is called the bronchial tree (or tracheobronchial tree). It is called this because it looks like an upside down tree with many smaller branches coming off of fewer larger branches which eventually derive from only the two branches off of the single trachea (the trunk of the tree). Eventually, as the tubes of the bronchial tree get smaller and smaller, they lead to the air sacs of the lungs, called alveoli. As the bronchial tubes leave the trachea they still have C-rings, but the cartilage rings get progressively smaller and are replaced by smooth muscle until the smallest air tubes (called bronchioles) are just cuboidal epithelium and smooth muscle only, with no cartilage and no mucosa. The various branches of the bronchial tree are classified from largest to smallest as:
Primary bronchi - the two branches from the trachea (right and left primary
bronchus) entering each respective lung for further branching. Secondary bronchi - branch off to discrete sections of the lungs called lobes
(two lobes on the left and three on the right). Tertiary bronchi - branches into tubes to different sections (called segments) of
the lobes. Bronchioles - tiny tubes that branch into terminal bronchioles that lead into end
tubes called alveolar ducts. Alveolar ducts - tiniest tubes connecting directly to the individual air sacs of the
lungs. Alveolar sacs - air sacs of simple squamous epithelium. They contain alveoli.
The sacs are like a cluster of grapes, while the alveoli are the individual grapes.
Alveoli are the site of gas exchange between air and blood.
The blood supply for the bronchial tree consists of pulmonary blood vessels branching alongside the bronchial tubing of the airways to ultimately become capillary beds surrounding the alveoli. The walls of both alveoli and capillaries are made of simple squamous epithelium, providing good permeability for the diffusion of O2 and CO2.
F. Lungs The lungs are the organs of gas exchange and the site of external respiration. They take up most of the thoracic cavity, from just above the diaphragm and extending to above the clavicle.
Each lung is enclosed within a serous membrane sac called the pleura. It consists of two layers with a cavity between (pleural cavity). The cavity is filled with serous fluid
(pleural fluid) to create a negative pressure space that allows the lungs to expand when the thorax expands. (Think of trying to separate two pieces of glass with a small amount of water between them: they slide over each other easily but it is very hard to pull them apart.) Each lung is filled with bronchi, alveolar sacs and blood vessels. The right lung has three distinct areas called lobes. The left lung has two lobes. Lobes are further divided into segments. Still, the lungs are mostly air spaces. The supportive connective tissue of the lungs (stroma) is mostly elastic connective tissue, which makes the lungs very spongy and springy. A healthy lung is very elastic and can stretch and recoil with ease. As noted above, there are three phases of respiration:
Pulmonary ventilation involves the inhalation (or inspiration) and exhalation (or
expiration) of air into and out of the alveoli. Inhalation involves the active contraction of the diaphragm, the external intercostals and other respiratory muscles. As these muscles contract, the volume of the lungs increases and air is drawn into the lungs. During exhalation the diaphragm relaxes, and the volume of the lungs decreases as the passive elastic recoil of the lungs forces air out of the lungs. Ventilation occurs about 16 to 20 times a minute in an adult at rest.
External respiration is the exchange of gases occurring between the alveoli and
the pulmonary blood capillaries. This occurs by passive diffusion. There is a higher concentration of O2 in the inhaled air in the alveolar sacs than in the alveolar blood capillaries. There is a higher concentration of CO2 in the blood than in the inhaled air. Therefore, O2 enters the blood and CO2 leaves the blood.
Internal respiration is the exchange of gases between capillaries of the body
and the individual cells of the surrounding tissues. This occurs through passive diffusion. There is a higher concentration of O2 in the blood capillaries than in the tissues and a higher concentration of CO2 in the tissues than in the blood of the capillaries. Therefore, in the tissues oxygen moves from the blood into the tissue space and carbon dioxide moves from the tissue space into the blood. Remember, blood in the systemic arteries is oxygenated and blood in the systemic veins is deoxygenated.
III. Muscles of Respiration and Nervous System Control A. Muscles of Respiration
The muscles involved in ventilation are presented in the chart below.
QUIET, AT REST BREATHING
Diaphragm External intercostals
ACTIVE, LABORED BREATHING
Diaphragm External intercostals Sternocleidomastoid Scalenes Pectoralis minor Pectoralis major
Passive elastic recoil of the lungs and thoracic wall
Internal intercostals Abdominals
B. Nervous System Control of Respiration The nervous system sets the automatic rate and depth of breathing but also gives voluntary control of respiration. We can breathe automatically but we can also override the reflex control of breathing in certain circumstances and exert very precise control over respiratory function. The medullary rhythmicity center sets and controls the basic rhythm of respiration. It receives constant input from chemoreceptors in blood vessel walls via chemoreflexes (such as the aortic sinus reflex and carotid sinus reflex).
Breathing centers in the pons also affect breathing rate and depth during speech, sleep and exercise. The cerebral cortex allows voluntary control of breathing for things such as speech, holding the breath, breathing meditation, emotional control, singing, etc. Recall that the phrenic nerve (arising from the cervical plexus) innervates the diaphragm.
The sympathetic nervous system causes bronchodilation.
The parasympathetic nervous system causes bronchoconstriction.
IV. Respiratory Volumes and Capacities Respiratory volumes involve measuring the amount of air in different situations of ventilation. For example:
Tidal air volume is the amount of air inhaled and exhaled in a normal breath at
rest (about 500mL).
Respiratory capacities indicate the maximum amount of air in the lungs in different situations. For example:
Vital capacity is the maximum amount of air that can be exhaled after a
maximal inspiration (4800mL). It represents the total volume of air a person can exchange during complete deep breathing without forcing.
Total lung capacity is the maximum amount of air in the lungs after a maximal
inspiration (6000mL). Total lung capacity is how much air is inspired plus what is already "resting" in the lungs. There is always some air left in the lungs (the air sacs and tubes never collapse).
There are other volumes and capacities that are measured besides the two above. These measurements can be used to help diagnose respiratory diseases and to monitor the effectiveness of treatment.