Functional Organization of Respiratory System

Introduction

The respiratory system is functionally organized into conducting and respiratory zones that work together to ensure efficient gas exchange. While the conducting passages transport, warm, humidify, and filter inspired air, the distal respiratory region serves as the primary site for oxygen and carbon dioxide exchange.

General Concept

• Respiration is the physiological process by which oxygen is delivered from the atmosphere to body tissues and carbon dioxide is removed from tissues to the external environment.
• The process of respiration occurs in four sequential stages.
• The first stage, pulmonary ventilation, involves movement of air between the atmosphere and the alveoli through inspiration and expiration.
• The second stage is gas transport, during which oxygen and carbon dioxide are carried between the lungs and tissues by the blood.
• The third stage, tissue gas exchange, involves diffusion of oxygen from systemic capillaries into tissues and movement of carbon dioxide in the opposite direction.
• The fourth stage, cellular respiration, occurs within mitochondria, where oxygen is utilized for oxidative metabolism to generate energy in the form of adenosine triphosphate, with carbon dioxide produced as a by-product.
• Oxygen and carbon dioxide cross biological membranes mainly by simple diffusion along their partial pressure gradients.
• In unicellular organisms, diffusion alone is sufficient for gas exchange.
• In multicellular organisms, diffusion is inadequate for cells located deep within the body; therefore, specialized respiratory structures have evolved, including tracheal systems in insects, gills in aquatic animals, and lungs in terrestrial vertebrates.

Components of Respiratory Systems

• The respiratory system extends from the nose to the terminal alveoli within the lungs.
• Anatomically, it is divided into the upper respiratory tract and the lower respiratory tract.
• The upper respiratory tract includes the nose, paranasal sinuses, pharynx, and larynx up to the level of the vocal cords.
• The lower respiratory tract comprises the trachea, bronchi, bronchioles, and alveoli, where gas exchange occurs.
• The respiratory apparatus also includes the thoracic cage, diaphragm, intercostal muscles, and other respiratory muscles, which generate the mechanical movements necessary for ventilation.

• The upper respiratory tract comprises the nose, paranasal sinuses, pharynx, and larynx.

Upper Respiratory Tract

• Its primary function is to warm, humidify, and filter inspired air before it reaches the lower airways.

Nose

• The nose contains olfactory epithelium, which is responsible for the sense of smell.
• It effectively filters inhaled particles larger than approximately 10 μm in diameter.
• Nasal secretions contain immunoglobulins, interferons, and other antimicrobial substances that help protect against infection.
• Although the nasal cavity has a relatively small volume, the presence of nasal turbinates greatly increases its surface area.
• The nose contributes nearly 50% of the total airway resistance in the respiratory system.
• Nasal resistance increases during conditions such as viral upper respiratory infections because of mucosal edema and increased secretions.

Paranasal Sinuses

• The major paranasal sinuses are the maxillary, frontal, ethmoidal, and sphenoidal sinuses.
• These air-filled cavities communicate with the nasal cavity and may become involved in nasal infections.
• Paranasal sinuses enhance resonance of the voice and contribute to normal speech quality.
• They reduce the overall weight of the skull, facilitating maintenance of an upright posture.
• The air-filled sinuses also help absorb and dissipate forces during facial trauma, thereby offering partial protection to adjacent structures.

Pharynx

• The pharynx is divided into three regions: nasopharynx, oropharynx, and laryngopharynx.
• The nasopharynx lies posterior to the nasal cavity and extends from the posterior nasal apertures to the soft palate.
• The openings of the auditory (Eustachian) tubes communicate with the nasopharynx and help equalize middle ear pressure.
• The oropharynx extends from the soft palate to the level of the hyoid bone and serves as a common pathway for air and food.
• The laryngopharynx extends from the hyoid bone to the esophagus and directs food toward the digestive tract.
• Infections of the upper respiratory tract commonly involve the nasopharynx.

Larynx and Glottis

• The larynx consists of the epiglottis, arytenoid cartilages, and vocal cords.
• During swallowing, the epiglottis and arytenoids protect the lower airways by preventing aspiration of food and fluids.
• Intrinsic muscles of the larynx are supplied predominantly by branches of the vagus nerve.
• During inspiration, contraction of laryngeal abductor muscles widens the glottis and reduces airway resistance.
• Reflex contraction of the adductor muscles closes the glottis during deglutition, preventing entry of food into the respiratory tract.
• Laryngeal inflammation, especially in children, can markedly increase airway resistance because of airway narrowing.
• In unconscious or anesthetized individuals, impaired glottic closure predisposes to aspiration pneumonia.
• Paralysis of laryngeal adductors increases the risk of aspiration, whereas paralysis of abductors may produce inspiratory stridor.

Lower Respiratory Tract

The lower respiratory tract consists of the tracheobronchial tree and the lungs.

Airway Tree

• The trachea divides into right and left main bronchi, which enter the lungs and undergo repeated branching to form progressively smaller bronchi and bronchioles.
• This branching network is collectively termed the tracheobronchial tree.
• Airway diameter decreases with each successive division, increasing the total cross-sectional area of the distal airways.
• The trachea and bronchi contain cartilaginous support that maintains airway patency and prevents collapse during respiration.
• These larger airways are lined by ciliated epithelium containing mucous and serous glands that contribute to mucociliary clearance.
• Cilia extend up to the respiratory bronchioles, whereas submucosal glands are absent in bronchioles and terminal bronchioles.
• Cartilage is absent distal to the bronchi; therefore, bronchioles depend on surrounding lung tissue and smooth muscle tone to remain patent.
• The proportion of smooth muscle increases in smaller airways and is greatest in terminal bronchioles, allowing effective regulation of airway resistance through bronchoconstriction and bronchodilation.

Bronchial Tone

• Bronchial tone refers to the baseline degree of contraction of bronchial smooth muscle and plays an important role in maintaining airway caliber and regulating airflow.
• Bronchial tone is primarily controlled by the autonomic nervous system but is also influenced by several local mediators and hormones.
• Important modulators include inflammatory mediators, cytokines, adenosine, histamine, substance P, and vasoactive intestinal peptide.
• Sympathetic stimulation acting mainly through β₂-adrenergic receptors produces bronchodilation, whereas parasympathetic vagal stimulation causes bronchoconstriction.
• Airway irritation by allergens, pollutants, or chemical agents often triggers reflex bronchoconstriction mediated by vagal cholinergic pathways.
• Inhalation of cold air may also induce bronchoconstriction in susceptible individuals.
• Some individuals develop exercise-induced bronchoconstriction, particularly during exercise in cold and dry environments.
• Bronchial tone exhibits a circadian rhythm, with maximal airway narrowing typically occurring during the early morning hours, which may worsen nocturnal asthma symptoms.

• The trachea represents generation 0, while the main bronchi constitute generation 1 of the airway tree.
• With successive branching, the airways become progressively smaller, shorter, and narrower.
• The number of cilia, mucus-secreting cells, and submucosal glands gradually decreases toward the distal airways.
• Cartilage progressively diminishes and is absent in bronchioles.
• In contrast, the relative proportion of smooth muscle increases and is greatest in terminal bronchioles.

• Mucus forms a protective lining over the airway mucosa and traps inhaled particles, microorganisms, and other foreign materials.
• Cilia beat in a coordinated manner toward the pharynx, transporting mucus and entrapped particles out of the lower respiratory tract through the mucociliary escalator.
• Excess mucus is either swallowed or expectorated.
• Cartilage supports the larger airways and prevents their collapse during breathing. Cartilage is present in the trachea and bronchi but is absent beyond the bronchi.

Branching of Airways

• The airway tree undergoes approximately 23 generations of branching from the trachea to the alveoli.
• Airways are functionally divided into conducting and respiratory zones.
• The conducting zone transports air to distal lung regions without participating in gas exchange.
• The respiratory zone, comprising respiratory bronchioles, alveolar ducts, and alveoli, is the principal site of oxygen and carbon dioxide exchange.

Conducting Zone

• The conducting zone extends from the trachea to the terminal bronchioles and includes approximately the first 16 generations of the airway tree.
• These airways do not participate directly in gas exchange because alveoli are absent in this region.

Arrangement and Functions

• The conducting airways warm inspired air to near body temperature before it reaches the alveoli.
• They humidify inhaled air, thereby preventing drying of the respiratory epithelium.
• The branching pattern of the tracheobronchial tree ensures efficient and uniform distribution of air to the distal regions of the lungs.
• The conducting zone also forms an important component of the nonspecific defense system by removing inhaled particles, microorganisms, and harmful substances through mucociliary clearance.

Structural Specialization

• Substantial amounts of cartilage are present in the larger airways, particularly in the proximal generations, to maintain airway patency during changes in intrathoracic pressure.
• Cartilage gradually decreases in amount and is present only up to the bronchi.
• Airways containing cartilage are termed bronchi, whereas bronchioles lack cartilaginous support.
• The smallest conducting airways are the terminal bronchioles.
• Although bronchioles lack cartilage, their patency is maintained by elastic fibers of the surrounding lung parenchyma and radial traction exerted by the lung tissue.
• During expiration, especially forced expiration, bronchioles are more susceptible to dynamic collapse.
• Since gas exchange does not occur in the conducting zone, the volume of air contained within these airways constitutes the anatomic dead space of the respiratory system.

Blood Supply

• The conducting airways receive their nutritional blood supply from the bronchial circulation, which arises mainly from the bronchial arteries originating from the descending thoracic aorta.
• Venous drainage from the bronchial circulation occurs predominantly through the pulmonary veins, with a smaller portion draining into the azygos system.

Nerve Supply

• The bronchi and bronchioles are innervated by the autonomic nervous system.
• Parasympathetic stimulation through the vagus nerve activates muscarinic receptors and produces bronchoconstriction.
• Sympathetic stimulation acting via β₂-adrenergic receptors causes bronchodilation and reduces airway resistance.
• Activation of β₂ receptors also enhances bronchial gland secretion, whereas α-adrenergic stimulation tends to reduce secretion.
• In addition, nonadrenergic noncholinergic inhibitory nerves are present in the airways and contribute to bronchodilation, largely through mediators such as vasoactive intestinal peptide.

Respiratory Zone

The respiratory zone comprises the distal seven generations of the airway tree and is the principal site of gas exchange.

Arrangement and Functions

• It includes respiratory bronchioles, alveolar ducts, alveolar sacs, and alveoli.
• Alveoli begin to appear from approximately the seventeenth generation of airways.
• Bronchioles containing alveoli in their walls are termed respiratory bronchioles.
• Successive airway generations contain progressively greater numbers of alveoli, thereby increasing the surface area available for gas exchange.
• Alveolar ducts arise distally and terminate in clusters of alveolar sacs.

Blood Supply

• The respiratory zone receives its blood supply from the pulmonary circulation.
• A dense capillary network surrounds the alveoli and covers nearly 70–80% of the alveolar surface area.
• The close apposition of alveolar air and capillary blood forms the alveolar-capillary membrane, facilitating rapid diffusion of oxygen and carbon dioxide.

Alveolus

• The alveolus is the structural and functional unit of pulmonary gas exchange.
• Adult lungs contain approximately 300 million alveoli.
• Individual alveoli measure about 75–300 μm in diameter.
• The combined alveolar surface area in adults ranges from approximately 50 to 100 m², providing an extensive interface for gaseous exchange.
• The alveolar wall is lined by two major epithelial cell types: Type I and Type II pneumocytes.
• Type I pneumocytes are thin squamous cells that cover about 95% of the alveolar surface and are primarily responsible for gas diffusion.
• Type II pneumocytes synthesize and secrete pulmonary surfactant, which reduces alveolar surface tension and prevents alveolar collapse.
• Type II cells also serve as progenitor cells for alveolar epithelial repair following injury.
• Alveolar macrophages provide an important defense mechanism by phagocytosing inhaled particles and microorganisms.
• Mast cells, lymphocytes, plasma cells, and neuroendocrine cells are also present and participate in immune and allergic responses within the lungs.

Lungs

• The lungs are the principal organs of gas exchange and are divided into lobes enclosed within the thoracic cavity.
• Each lung is covered by a double-layered serous membrane called the pleura.

Pleura

• The parietal pleura lines the thoracic wall, diaphragm, and mediastinum and possesses a rich blood supply.
• The visceral pleura closely adheres to the surface of the lungs.
• The narrow space between these layers is known as the pleural cavity.
• Normally, the pleural cavity contains approximately 10–20 milliliters of pleural fluid.
• Pleural fluid is derived mainly from filtration of plasma across pleural capillaries.
• This thin layer of fluid acts as a lubricant, allowing the lungs to move smoothly against the thoracic wall during breathing.
• Pleural fluid also facilitates expansion and recoil of the lungs during respiration while minimizing friction and providing mechanical protection to the lung surface.

Lobes of Lungs

• The lungs are divided into lobes: the right lung has three lobes, whereas the left lung has two lobes to accommodate the heart.
• The right lung contributes slightly more than half of the total pulmonary volume and function.
• Lung tissue consists of the airway tree and the vascular tree, both embedded within elastic connective tissue known as the lung parenchyma.
• The vascular tree comprises pulmonary arteries, veins, and an extensive capillary network.

Bronchopulmonary Segments

• A bronchopulmonary segment is a discrete anatomical and functional unit supplied by a segmental (tertiary) bronchus and its accompanying arterial branch.
• The right lung typically contains ten bronchopulmonary segments, whereas the left lung usually contains eight to ten segments because some segments may fuse.
• Each segment is separated from adjacent segments by connective tissue septa, allowing relative independence.
• Because of this anatomical arrangement, pathological processes may initially remain confined to a single segment.
• Segmental anatomy is clinically important because individual bronchopulmonary segments can be surgically removed with minimal effect on neighboring segments.

Functions of Respiratory System

The primary function of the lungs is gas exchange, whereby oxygen is transferred from inspired air to the blood and carbon dioxide is eliminated from the body.

Respiratory Functions

• Efficient gas exchange across the alveolar-capillary membrane ensures adequate oxygen delivery to tissues and removal of metabolic carbon dioxide.
• The lungs also play a crucial role in maintaining acid–base balance.
• By regulating the excretion of carbon dioxide through ventilation, the respiratory system influences blood hydrogen ion concentration and plasma bicarbonate levels.
• Alterations in alveolar ventilation can therefore compensate for metabolic disturbances and help maintain normal arterial pH.

Non-respiratory Functions

• In addition to gas exchange, the lungs perform several important non-respiratory functions essential for homeostasis.
• The pulmonary circulation acts as a blood reservoir, containing approximately 500 milliliters of blood at rest because of its high vascular compliance. This reservoir helps maintain adequate filling of the left ventricle.
• The pulmonary capillary bed filters small thrombi, fat droplets, and air bubbles from venous blood, thereby preventing their entry into the systemic circulation. Large emboli, however, may obstruct pulmonary vessels and produce pulmonary embolism.
• The lungs possess important biochemical functions by metabolizing or inactivating many circulating substances. Bradykinin, serotonin, and several prostaglandins are largely removed during passage through the pulmonary circulation.
• Pulmonary vascular endothelial cells contain abundant angiotensin-converting enzyme, which converts angiotensin I into angiotensin II, an important regulator of vascular tone and blood pressure.
• The lungs also synthesize and release biologically active substances, including prostaglandins, histamine, and kallikrein, which exert local and systemic effects.
• Effective respiration is necessary for olfaction, as airflow transports odorant molecules to the olfactory epithelium.
• Conducting airways process inspired air by filtering, warming, and humidifying it. These mechanisms protect the delicate alveolar surfaces from desiccation, thermal injury, and harmful particles.
• The respiratory system contributes significantly to host defense. Particles larger than 10 μm are trapped by nasal hairs, whereas particles measuring 2–10 μm are removed by the mucociliary apparatus of conducting airways.
• Particles smaller than 2 μm can reach the alveoli, where they are engulfed by alveolar macrophages. These cells provide an important component of innate immunity.
• Secretory immunoglobulin A in airway secretions inhibits microbial adherence and helps maintain mucosal integrity.
• Pulmonary epithelial cells also release protective mediators that limit tissue injury during inflammation.
• The lungs possess local fibrinolytic activity and contribute to the dissolution of small intravascular clots.
• Pulmonary cells synthesize surfactant, which reduces alveolar surface tension and prevents alveolar collapse.
• Airflow through the larynx and upper airways is essential for normal speech and voice production. Nasal obstruction commonly alters voice quality.
• The respiratory tract also serves as an effective route for administration of inhalational anesthetic agents and other aerosolized medications because of its large absorptive surface area.

Symbols used in Respiratory Physiology

• Standardized symbols are widely used in respiratory physiology to represent physiological variables and simplify data interpretation.
• A denotes alveolar values, whereas a refers to systemic arterial values.
• aw indicates airway measurements.
• B represents barometric pressure, and P denotes pressure.
• C signifies concentration or content of a substance in blood or other fluids.
• D represents diffusion capacity, and R indicates resistance.
• E and I denote expired and inspired gases, respectively.
• F represents fractional concentration, whereas f indicates respiratory frequency.
• Q denotes blood flow, V indicates gas volume, and V̇ represents ventilation per unit time.
• S refers to hemoglobin saturation, v denotes systemic venous blood, and v̄ indicates mixed venous blood.