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The Respiratory System

The Respiratory System: An Introduction

Functions of the respiratory system

•Gas exchange between air and circulating blood

•Moving air from the exchange surface of the lungs

•Protection of respiratory surfaces

•Production of sound

•Provision for olfactory sensations


Organization of the respiratory system

•Upper respiratory system

•Nose, nasal cavity, paranasal sinuses, pharynx

•Lower respiratory system

•Larynx, trachea, bronchi, bronchioles, alveoli


The Respiratory tract

•Conducting passageways carrying air to and from the alveoli

•Upper respiratory passages filter and humidify incoming air

•Lower passageways include delicate conduction passages and alveolar exchange surfaces


Respiratory Mucosa

•Respiratory epithelium and underlying connective tissue

•Respiratory membrane, supported by lamina propria, changes along tract

•Lines conducting portion of respiratory tract

•Protected from contamination by respiratory defense system


The Upper Respiratory System

The nose and nasal cavity consists of:

•External nares

•Nasal cavity


•Superior, middle and inferior meatuses

•Hard and soft palates

•Internal nares

•Nasal mucosa


The pharynx

•Shared by the digestive and respiratory systems

•Divided into three sections:

•Nasopharynx – superior portion

•Oropharynx – continuous with the oral cavity

•Laryngopharynx – between the hyoid bone and the esophagus

The Larynx

The larynx

•Air passes through the glottis on the way to the lungs

•Larynx protects the glottis

•Cartilages of the larynx

•Three large cartilages

•Thyroid, cricoid, and epiglottis

•Paired cartilages

•Arytenoids, corniculate, and cuneiform


Folds of the larynx

•Inelastic vestibular folds

•Delicate vocal folds


Sound production

•Air passing through the glottis vibrates the vocal folds producing sound waves

•Pitch depends on conditions of vocal folds





The laryngeal musculature

•Muscles of the neck and pharynx position and stabilize the larynx

•When swallowing,these muscles

•Elevate the larynx

•Bend the epiglottis over the glottis

•Intrinsic muscles control tension on the vocal folds and open the glottis

The Trachea and Primary Bronchi

The trachea

•Extends from the sixth cervical vertebra to the fifth thoracic vertebra

•A tough, flexible tube running from the larynx to the bronchi

•Held open by C-shaped tracheal cartilages in submucosa

•Mucosa is similar to the nasopharynx


The primary bronchi

•Trachea branches in the mediastinum into right and left bronchi

•Bronchi enter the lungs at the hilus

•Root = the connective tissue mass including:


•Pulmonary vessels



The Lungs

Lobes and surfaces of the lungs

•Lobes of the lung are separated by fissures

•Right lung has three lobes

•Left lung has two lobes

•Concavity on medial surface = cardiac notch


The bronchial tree

•System of tubes formed from the primary bronchi and their branches

•Primary bronchi branch into secondary or lobar bronchi

•Secondary bronchus goes to each lobe of the lungs

•Secondary bronchi branch into tertiary bronchi

•Tertiary bronchi supply air to a single bronchopulmonary segment

•Cartilage in walls decrease and smooth muscle increase with branching


The bronchioles

•Ultimately branch into terminal bronchioles

•Delivers air to a single pulmonary lobule

•Terminal bronchiole becomes respiratory bronchioles

•Connective tissue of root branches to form interlobar septa


Alveolar ducts and alveoli

•Respiratory bronchioles end in ducts and sacs

•Respiratory exchange surfaces connected to circulatory system via pulmonary circuit


Respiratory Membrane

•Simple squamous epithelium

•Endothelial cell lining an adjacent capillary

•Fused basal laminae


Cells of the respiratory membrane include

•Septal cells

•Scattered in respiratory membrane

•Produce surfactant

•Alveolar Macrophage

•Patrol epithelium and engulf foreign particles


The blood supply to the lungs

•Conducting portions

•Receive blood from external carotids, thyrocervical, bronchial arteries

•Respiratory exchange surfaces

•receive blood from the arteries of the pulmonary circuit

•are the source of angiotensin-converting enzyme (ACE)

•Pulmonary veins return blood to the left atrium


The pleural cavities and pleural membranes

•Each lung covered by one pleura

•Pleura – serous membranes lining the pleural cavity

•Parietal -  attaches to the walls of the pleural cavity

•Visceral -  adheres to the surface of the lungs

•Pleural fluid – fills and lubricates the space between the pleura

An Overview of Respiratory Physiology

Respiratory physiology is a series of integrated processes

•Internal respiration

•Exchange of gases between interstitial fluid and cells

•External respiration

•Exchange of gases between interstitial fluid and the external environment

•The steps of external respiration include:

•Pulmonary ventilation

•Gas diffusion

•Transport of oxygen and carbon dioxide


Pulmonary Ventilation

Pulmonary Ventilation

•The physical movement of air into and out of the lungs


Air movement

•Movement of air depends upon

•Boyle’s Law

•Pressure and volume inverse relationship

•Volume depends on movement of diaphragm and ribs

•Pressure and airflow to the lungs

•Compliance – an indication of the expandability of the lungs


Pressure changes during inhalation and exhalation

•Relationship between intrapulmonary pressure and atmospheric pressure determines direction of air flow

•Intrapleural pressure maintains pull on lungs

•Pressure in the space between parietal and visceral pleura

Respiratory cycle

•Single cycle of inhalation and exhalation

•Amount of air moved in one cycle = tidal volume


Mechanisms of breathing

•Quiet breathing (eupnea)

•Diaphragm and external and internal intercostals muscles

•Forced breathing (hyperpnea)

•Accessory muscles


Respiratory volumes

•Alveolar volume

•Amount of air reaching the alveoli each minute

•Tidal Volume (VT)

•Amount of air inhaled or exhaled with each breath

•Vital capacity

•Tidal volume plus expiratory and inspiratory reserve volumes

•Residual volume

•Air left in lungs after maximum exhalation


Gas Exchange

The gas laws

•Daltons Law and partial pressure

•Individual gases in a mixture exert pressure proportional to their abundance

•Diffusion between liquid and gases (Henry’s law)

•The amount of gas in solution is directly proportional to their partial pressure


Diffusion and respiratory function

•Gas exchange across respiratory membrane is efficient due to:

•Differences in partial pressure

•Small diffusion distance

•Lipid-soluble gases

•Large surface area of all alveoli

•Coordination of blood flow and airflow


Gas Pickup and Delivery

Blood in peripheral capillaries delivers O2 and absorbs CO2

•Reactions are completely reversible


Oxygen transport

•Carried mainly by RBCs, bound to hemoglobin

•The amount of oxygen hemoglobin can carried is dependent upon:





•Fetal hemoglobin has a higher O2 affinity than adult hemoglobin


Carbon dioxide transport

•7% dissolved in plasma

•70% carried as carbonic acid

•buffer system

•23% bound to hemoglobin


•Plasma transport


Summary of gas transport

•Driven by differences in partial pressure

•Oxygen enters blood at lungs and leaves at tissues

•Carbon dioxide enters at tissues and leaves at lungs


Control of Respiration

Gas absorption/generation balanced by capillary rates of delivery/removal

•Homeostatic mechanisms maintain balance

•Local regulation of gas transport and alveolar function include

•Lung perfusion

•Alveolar capillaries constrict in low oxygen

•Alveolar ventilation

•Bronchioles dilate in high carbon dioxide


Respiratory centers of the brain

•Medullary centers

•Respiratory rhythmicity centers set pace


•Apneustic and pneumotaxic centers


Respiratory reflexes

•Respiratory centers are modified by sensory information including

•Chemoreceptor reflexes

•Level of carbon dioxide

•Baroreceptors reflexes

•Hering-Breuer reflexes

•Prevents overinflation

•Protective reflexes


Voluntary control of respiration

•Regulation of respiratory rate is dependent upon:

•Conscious and unconscious thought

•Emotional state


Changes in the Respiratory System at Birth

Neonatal Respiration

•Upon taking the first breath:

•Inhaled air enters the respiratory passages for the first time

•The bronchial tree and most of the alveoli are inflated

•Subsequent breaths complete inflation of the alveoli

Aging and the Respiratory System

The efficiency of the respiratory system decreases with age as:

•Elastic tissue deteriorates causing lower lung compliance and vital capacity

•Chest movements are restricted by arthritic changes

•Some degree of emphysema normally occurs