Respiratory System Anatomy

Anatomy of the Respiratory System

• Organs are located in the head, neck, and thoracic cavity.
• Includes blood vessels of the pulmonary circuit, rib cage, respiratory muscles, both lungs, and the respiratory tract.
• The respiratory tract consists of hollow passages facilitating gas exchange and has a unique structure.
• The nose and nasal cavity are encased in cranial and facial bones.
• The trachea, or windpipe, is located in the mediastinum.
• The bronchial tree is a collection of branching tubes.
• The pharynx is also known as the throat.
• The larynx is also known as the voice box and is located in the anterior neck.
• The upper respiratory tract includes passageways from the nasal cavity to the larynx.
• The lower respiratory tract includes passages from the trachea to the respiratory tract's terminal structures (alveoli).
• Alveoli are tiny air sacs where where gases are exchanged; arranged in grapelike clusters.
• The lungs are spongy organs in the thoracic cavity, enclosed by the rib cage and diaphragm.
• They contain millions of alveoli and blood vessels embedded in connective tissue with branches of the respiratory tract.

Basic Functions of the Respiratory System

• The respiratory system is classified functionally into conducting and respiratory zones.
• The conducting zone consists of tubes for air travel into (inspired) and out of (expired) the body.
• Air is filtered, warmed, and moistened as it moves through the conducting zone, which includes structures from the nose to the bronchioles.
• The respiratory zone encompasses structures with alveoli, where gas exchange occurs.
• Respiration is a primary function; it provides the body's cells with oxygen and removes carbon dioxide.
• Pulmonary ventilation, or ventilation, is the movement of air in and out of the lungs.
• Pulmonary gas exchange is the movement of gases between the lungs and blood.
• Gas transport is the movement of gases through the blood.
• Tissue gas exchange is the movement of gases between the blood and tissues.
• The system is a mechanism for speech and sound production.
• It contains neurons for the sense of smell.
• Changes in thoracic cavity pressure assist with the flow of venous blood and lymph in the thoracic and abdominopelvic cavities.
• The system is critical in maintaining acid-base balance in extracellular fluid.
• It synthesizes angiotensin-II, which is critical for blood pressure and fluid homeostasis.
• Thoracic cavity pressure can be increased to push the contents of the abdominopelvic cavity out, which can help with defecation, urination, and childbirth.

The Nose and Nasal Cavity

• The nose and nasal cavity serve as the entryway to the respiratory system.
• Inhaled air is warmed and humidified.
• Debris and antibacterial substances are filtered and secreted.
• Olfactory receptors are housed in the structure.
• It enhances resonance of voice.
• The vestibule of the nasal cavity is lined with stratified squamous epithelium and contains bristle-like hairs that prevent large objects from entering and resists mechanical stress.
• Posterior to the vestibule, the epithelium changes to olfactory mucosa and respiratory mucosa.
• Olfactory mucosa, located on the roof of the nasal cavity, houses receptors for smell and has a cribriform plate of ethmoid bone for neuron access.
• The rest of the nasal cavity is lined with pseudostratified ciliated columnar epithelium and goblet cells.
• Air filtration is specialized by the ciliated epithelium and mucus combination
• Foreign particles are trapped in mucus.
• Ciliated cells propel collected debris and mucus toward the posterior nasal cavity and pharynx.

The Pharynx

• The pharynx is the next anatomical segment of the respiratory tract after the nasal cavity.
• It is divided into three anatomical divisions: the nasopharynx, oropharynx, and laryngopharynx.
• The nasopharynx is posterior to the nasal cavity and lined with pseudostratified ciliated columnar epithelium.
• It extends from the posterior nares to the uvula.
• During swallowing, the uvula and soft palate move posteriorly to prevent food from entering the nasopharynx and nasal cavity.
• The oropharynx is posterior to the oral cavity
• It extends from the uvula to the tip of larynx, the epiglottis.
• It is lined with nonkeratinized stratified squamous epithelium for protection and mechanical stress.
• It is a passageway for both air and food.
• The laryngopharynx is the last segment that extends from the hyoid bone to the esophagus; tube that connects oral cavity to stomach.
• It opens anteriorly into the larynx (voice box) and posteriorly into the esophagus.
• It is lined with nonkeratinized stratified squamous epithelium.
• It’s a common passageway for both air and food

The Larynx

• The larynx (voice box) is the next anatomical region that inspired air enters.
• It keeps food and liquids out of the remaining respiratory tract.
• It also houses vocal cords
• The short tube is anterior to the esophagus.
• Vocal cords create the superior and inferior boundary where two types of epithelium are found.
• Stratified squamous nonkeratinized epithelium protects the larynx from mechanical stress superior to vocal cords, where both food and air pass.
• Pseudostratified ciliated columnar epithelium propels mucus and debris up and out of the larynx as one "clears their throat”.
• Vocal cords create both the inferior and superior boundary of two types of epithelia.
• The epiglottis is a flap superior to the glottis.
• It is posterior to the thyroid cartilage.
• Free superior edge usually stands upright; creates opening called glottis, through which air can travel on its way to lungs.
• The inner surface of larynx is made up of folds of mucosa projecting into laryngeal lumen.
• Vestibular folds (false vocal cords) extend from arytenoid cartilages to thyroid cartilage; close off glottis during swallowing.
• They play no role in sound production.
• True vocal cords are inferior to vestibular folds and are attached to both arytenoid cartilages and thyroid cartilage.
• Vocal ligaments are elastic bands at the core of vocal cords that give structure and whitish appearance and vibrate to produce sound when air passes over them.
• The musculature of the larynx controls the length and tension of vocal cords by causing arytenoid and corniculate cartilages to pivot.
• When cartilages rotate inward, they adduct vocal cords, and the glottis narrows.
• When cartilages rotate outward, they abduct vocal cords, and the glottis opens

The Larynx cont.

• Sound loudness is determined by force of airstream; greater force of expiration = louder sound.
• Pitch of sound is largely determined by tension of vocal cords and speed of vibration.
• When cartilages rotate inward, vocal cords adduct, glottis narrows, and higher pitch sound can be produced because vocal ligaments are tense and vibrate more rapidly.
• A lower pitch sound is produced when vocal ligaments are abducted, making them more loose so they vibrate more slowly.
• Adult males have deeper (lower-pitched) voices than females because and their vocal ligaments are longer and thicker as a result of their wider larynx, and thus vibrate more slowly.
• Air movement over vocal cords only produces buzzing sound; actual speech requires coordinated efforts of structures superior to glottis including muscles of pharynx, soft palate, plus tongue and lips

The Trachea

• The trachea (windpipe) is a structure that inspired air flows through on its way to lower respiratory tract.
• It begins in the inferior neck and extends to the mediastinum.
• Hyaline cartilage rings cover the anterior and lateral surfaces of the trachea in a C shape, leaving the posterior surface uncovered.
• Rings are supportive enough to keep trachea open (patent); flexible enough to allow trachea to change in diameter during pulmonary ventilation.
• The posterior surface of the trachea is covered with elastic connective tissue and smooth muscle, which allows the esophagus to expand during swallowing.
• The carina, last tracheal cartilage ring, forms a "hook” that curves down and back to form partial rings that surround first branches of the bronchial tree.
• Carina's mucosa contains sensory receptors that trigger violent cough reflex if foreign materials contact them.
• Mucosa of the trachea, like the inferior larynx, is pseudostratified ciliated squamous epithelium and goblet cells

Bronchial Tree

• Once inhaled air reaches the carina, it enters either the left or right primary bronchus (enters the left lung or right lung at the hilum).
• Inside the lung, each bronchus branches into a bronchial tree, which is a series of progressively smaller tubes that end in alveoli.
• Primary bronchi divide into left and right branches at the beginning of the bronchial tree in the carina.
• Secondary bronchi branch into about 10 smaller tertiary bronchi per lung and continue to branch into smaller and smaller branches.
• As airways divide and get smaller, histology changes significantly; primary bronchi are nearly identical to the trachea, but three changes are evident as the bronchi become smaller.
• Cartilage changes from C-shaped to complete rings to progressively fewer irregular plates.
• The epithelium gradually changes from respiratory epithelium in larger bronchi to columnar cells in smaller bronchi.
• The amount of smooth muscle increases and hyaline cartilage decreases as the bronchi get progressively smaller, enabling the airways to change diameter to control airflow in bronchioles and alveoli.

Bronchial Tree cont.

• Bronchioles are the smallest airways that differ from larger airways.
• It has simple cuboidal epithelium with few cilia, if any, enclosed within a thick ring of smooth muscle but lacks hyaline cartilage.
• The conducting zone of the respiratory tract ends when inspired air reaches terminal bronchioles.
• Terminal bronchioles branch into two or more smaller respiratory bronchioles surrounded by a thin layer of smooth muscle.
• The respiratory zone begins with respiratory bronchioles and alveoli with alveoli budding from walls.
• Each respiratory bronchiole branches into two or more alveolar ducts and also have alveoli attached to their walls.
• Alveolar ducts end in alveolar sacs, which are grapelike clusters of alveoli; inspired air has arrived where gas exchange occurs.

Alveoli and the Respiratory Membrane

• Alveoli are the final destination for inspired air within the respiratory tract; each single, round, thin-walled alveolus has three cell types.
• Type I alveolar cells are squamous cells that account for about 90% of cells in the alveolar wall
• Type I alveolar cells are very thin which provides a structural feature allowing for rapid diffusion of gases across cell membranes.
• Type II alveolar cells are small cuboidal cells that account for about 10% of cells in alveolar walls.
• Type II alveolar cells are responsible for the synthesis of surfactant, which reduces surface tension on alveoli.
• Alveolar macrophages are mobile phagocytes derived from bone marrow that clean up and digest debris that made its way into the alveolus.

Pulmonary Ventilation

• The first process of respiration is breathing (pulmonary ventilation), consisting of two phases: inspiration (inhalation), where air enters the lungs, and expiration (exhalation), where air moves out of the lungs.
• There is a pressure-volume relationship, in which air flows from areas of high to low pressure, providing the driving force for pulmonary ventilation. Pressure gradients (differences in pressure) drive air molecule movement during inspiration and expiration.
• Boyle's law describes the relationship between pressure and volume.
• At constant temperature and number of gas molecules, pressure and volume of gas are inversely related: as volume increases, pressure decreases.
• The process of inspiration and expiration involves volume changes in the thoracic cavity and lungs, creating a pressure gradient that moves air into or out of the lungs.

Pulmonary Ventilation cont.

• Molecules that make up air are subject to the force of gravity.
• Pull of gravity on air creates atmospheric pressure.
• At sea level, atmospheric pressure is about 760 mm Hg, increasing below sea level and decreasing above.
• Intrapulmonary pressure is air pressure within the alveoli that rises and falls with inspiration and expiration, eventually equalizing with atmospheric pressure.
• Intrapleural pressure is the pressure within the pleural cavity, also rising and falling with breathing, remaining about 4 mm Hg less than intrapulmonary pressure due to slight suction effect, and does not equalize with atmospheric pressure.
• At rest between breaths, intrapulmonary pressure equals atmospheric pressure (760 mm Hg), while intrapleural pressure is 4 mm Hg below atmospheric (756 mm Hg). No air flows in or out of lungs.
• For inspiration to occur, increase the volume of lungs which decreases intrapulmonary pressure to about 758 mm Hg; As intrapulmonary pressure decreases, air moves into lungs. The intrapleural pressure remains lower than intrapulmonary pressure.
• Air will continue to move into lungs until the gradient no longer exists when intrapulmonary pressure equals atmospheric pressure. At this point between inspiration and expiration, as there is no pressure gradient to drive its movement, there is no air movement.

Pulmonary Ventilation cont.

• Air moves out of lungs when a pressure gradient is created during expiration; intrapulmonary pressure rises above atmospheric pressure to about 762 mm Hg.
• Lungs do not fully deflate because of the slight suction effect from the intrapleural pressure and outward recoil of the chest wall.
• Both forces oppose elastic recoil of lungs, keeping lungs inflated and most alveoli open at all times.
• Expiration stops when intrapulmonary pressure equals atmospheric pressure.
• If intrapleural pressure increases to levels at or above atmospheric pressure, the lungs immediately collapse when the positive pressure prevents lungs from collapsing, and the added pressure enhances the lungs' elastic recoil.
• Intrapleural pressure can increase due to excess fluid (pleural effusion), air pneumothorax or blood (hemothorax) in cavity.
• The diaphragm is the main inspiratory muscle that divides the thoracic cavity from the abdominopelvic cavity. It is dome-shaped when relaxed (bulges into the thoracic cavity) and flattens during contraction, increasing the height of the thoracic cavity.
• The external intercostals are muscles between ribs. At relaxed state, the external intercostal muscles cause rib cage and lungs to have increased size.
• Normally, expiration is mostly passive and does not utilize muscle contraction. There are two things that happen relaxation occurs: first, the diaphragm returns to its original dome shape and pushes up on the lungs.
• Elastic tissue also recoils when inspiratory muscles relax.
• Recoil and diaphragm relaxation decrease lung volume and raise intrapulmonary pressure above atmospheric pressure, causing air to flow out of the lungs.

Pulmonary Ventilation cont.

• Nonrespiratory movements, not intended for ventilation, include yawns, coughs, and sighs.
• A sigh is a slow, deep inspiration held and followed by a slow expiration. reopens local groups of collapsed alveoli; stimulates release of surfactant
• A yawn is a large sigh that takes the lung volume to inspiratory capacity. When one is tired, the body minimizes alveolar collapse during sleep, which after sleep, opens alveoli that collapsed.
• A sneeze is a deep inspiration followed by large, forceful expiration through the nose (velocity of about 100 mph) to clears foreign or irritating substances.
• A cough is similar to a sneeze except the initial inspiration is small or absent; velocity can be almost 500 mph. Clears the larynx, trachea, or lower airways.