Respiratory System Anatomy and Physiology

Oxygenation drives most of what you assess at the bedside: respiratory rate, breath sounds, SpO2, ABGs. Know the airway from nose to alveolus and the four events of respiration, and abnormal findings will make sense instead of being memorized in a vacuum.

Functions of the Respiratory System

The respiratory system does five jobs:

1. Oxygen supplier. Keep the body constantly supplied with oxygen.
2. Elimination. Clear carbon dioxide.
3. Gas exchange. Run gas exchange between blood and the external environment.
4. Passageway. Carry air to and from the lungs.
5. Humidifier. Purify, humidify, and warm incoming air.

Anatomy of the Respiratory System

The organs are the nose, pharynx, larynx, trachea, bronchi and their smaller branches, and the lungs, which hold the alveoli.

The Nose

The nose is the only externally visible part of the system.

• Nostrils. Air enters through the nostrils (nares).
• Nasal cavity. The interior of the nose, divided by a midline nasal septum.
• Olfactory receptors. Smell receptors sit in the mucosa of the slitlike superior nasal cavity, just beneath the ethmoid bone.
• Respiratory mucosa. The rest of the lining rests on a rich network of thin-walled veins that warm the passing air.
• Mucus. Sticky mucus from the mucosal glands moistens air and traps bacteria and debris, and lysozyme enzymes in the mucus destroy bacteria chemically.
• Ciliated cells. Cilia move the sheet of contaminated mucus posteriorly to the throat, where it is swallowed and digested.
• Conchae. Three mucosa-covered projections on the lateral walls that increase mucosal surface area and air turbulence.
• Palate. Separates the nasal cavity from the oral cavity. The bone-supported anterior part is the hard palate, the unsupported posterior part the soft palate.
• Paranasal sinuses. A ring of sinuses in the frontal, sphenoid, ethmoid, and maxillary bones. They lighten the skull and act as a resonance chamber for speech.

Pharynx

• Size. A muscular passageway about 13 cm (5 inches) long.
• Function. Commonly called the throat. It serves as a shared passageway for food and air.
• Portions. Air enters the superior nasopharynx from the nasal cavity, then descends through the oropharynx and laryngopharynx to the larynx.
• Pharyngotympanic tube. Drains the middle ear and opens into the nasopharynx.
• Pharyngeal tonsil. The adenoid, high in the nasopharynx.
• Palatine tonsils. In the oropharynx at the end of the soft palate.
• Lingual tonsils. At the base of the tongue.

Larynx

The larynx (voice box) routes air and food into the right channels and plays a role in speech.

• Structure. Inferior to the pharynx, formed by eight rigid hyaline cartilages and the spoon-shaped elastic epiglottis.
• Thyroid cartilage. The largest hyaline cartilage, shield-shaped, protruding anteriorly as the Adam's apple.
• Epiglottis. The "guardian of the airways," protecting the superior opening of the larynx.
• Vocal folds. A pair of mucous-membrane folds (true vocal cords) that vibrate with expelled air to produce speech.
• Glottis. The slitlike passageway between the vocal folds.

Trachea

• Length. Air from the larynx travels down the trachea (windpipe), 10 to 12 cm or about 4 inches, to the level of the fifth thoracic vertebra, roughly midchest.
• Structure. Fairly rigid, with walls reinforced by C-shaped rings of hyaline cartilage. The open parts of the rings abut the esophagus and let it expand when swallowing, while the solid parts keep the trachea patent despite breathing pressure changes.
• Cilia. Ciliated mucosa beats opposite to incoming air, propelling dust-laden mucus up toward the throat to be swallowed or spat out.

Main Bronchi

• Structure. The right and left main (primary) bronchi form when the trachea divides.
• Location. Each runs obliquely before entering the medial depression of its lung.
• Size. The right main bronchus is wider, shorter, and straighter than the left.

Lungs

• Location. The lungs fill the thoracic cavity except for the central mediastinum, which holds the heart, great vessels, bronchi, esophagus, and other organs.
• Apex. The narrow superior tip of each lung, just deep to the clavicle.
• Base. The broad lung area resting on the diaphragm.
• Division. Fissures divide each lung into lobes: the left lung has two lobes, the right has three.
• Pleura. A visceral serosa (pulmonary, or visceral, pleura) covers each lung; the parietal pleura lines the thoracic cavity walls.
• Pleural fluid. A slippery serous secretion that lets the lungs glide over the thorax wall and makes the pleural layers cling together.
• Pleural space. The lungs are held tight to the thorax wall, so the pleural space is more potential than actual.
• Bronchioles. The smallest conducting passageways.
• Alveoli. Terminal bronchioles lead to respiratory-zone conduits that end in alveoli (air sacs).
• Respiratory zone. Respiratory bronchioles, alveolar ducts, alveolar sacs, and alveoli. The only site of gas exchange.
• Conducting zone structures. All other passages, serving as conduits to and from the respiratory zone.
• Stroma. The rest of the lung, mostly elastic connective tissue that lets the lungs recoil passively on exhalation.

The Respiratory Membrane

• Wall structure. Alveolar walls are largely a single thin layer of squamous epithelial cells.
• Alveolar pores. Connect neighboring air sacs and give air an alternate route to alveoli whose feeder bronchioles are blocked.
• Respiratory membrane. The alveolar and capillary walls, their fused basement membranes, and occasional elastic fibers form the air-blood barrier, with air on one side and blood on the other.
• Alveolar macrophages. "Dust cells" that wander the alveoli picking up bacteria, carbon particles, and debris.
• Cuboidal cells. Scattered among the epithelial cells, they produce surfactant, a lipid that coats the alveolar surfaces and is critical to lung function.

Physiology of the Respiratory System

The system's main job is to supply oxygen and remove carbon dioxide. That takes four events, collectively called respiration.

Respiration

• Pulmonary ventilation. Air moves into and out of the lungs to refresh the air sacs. This is breathing.
• External respiration. Gas exchange between pulmonary blood and alveoli.
• Respiratory gas transport. Oxygen and carbon dioxide move to and from the lungs and tissue cells via the bloodstream.
• Internal respiration. Gas exchange between blood and tissue cells at the systemic capillaries.

Mechanics of Breathing

• Rule. Volume changes cause pressure changes, which drive gas flow to equalize pressure.
• Inspiration. Air flows in. The chest expands laterally, the rib cage elevates, the diaphragm flattens. The lungs stretch to a larger volume, intrapulmonary pressure falls, and air flows in.
• Expiration. Air flows out. The chest narrows, the rib cage descends, the diaphragm domes upward. The lungs recoil to a smaller volume, intrapulmonary pressure rises, and air flows out.
• Intrapulmonary volume. The volume within the lungs.
• Intrapleural pressure. Pressure in the pleural space, always negative. This is the major factor preventing lung collapse.
• Nonrespiratory air movements. Mostly reflex, some voluntary: coughing, sneezing, crying, laughing, hiccups, yawning.

Respiratory Volumes and Capacities

• Tidal volume. Quiet breathing moves about 500 ml of air in and out per breath.
• Inspiratory reserve volume. Air taken in forcibly over tidal volume, normally between 2100 ml and 3200 ml.
• Expiratory reserve volume. Air forcibly exhaled after a tidal expiration, about 1200 ml.
• Residual volume. About 1200 ml remains after the most strenuous expiration and cannot be voluntarily expelled. It keeps gas exchange going between breaths and keeps the alveoli inflated.
• Vital capacity. Total exchangeable air, about 4800 ml in healthy young men. It is the sum of tidal volume, inspiratory reserve volume, and expiratory reserve volume.
• Dead space volume. Air that stays in conducting passages and never reaches the alveoli, about 150 ml during a normal tidal breath.
• Functional volume. Air that actually reaches the respiratory zone and joins gas exchange, about 350 ml.
• Spirometer. Measures respiratory capacities by reading exhaled air volumes on an indicator.

Respiratory Sounds

• Bronchial sounds. Made by air rushing through the large passages (trachea and bronchi).
• Vesicular breathing sounds. Soft, muffled-breeze sounds as air fills the alveoli.

External Respiration, Gas Transport, and Internal Respiration

• External respiration. Pulmonary gas exchange: oxygen loaded, carbon dioxide unloaded from the blood.
• Internal respiration. Systemic capillary gas exchange: oxygen unloaded, carbon dioxide loaded into the blood.
• Gas transport. Oxygen travels two ways: most binds hemoglobin in RBCs as oxyhemoglobin, a small amount dissolves in plasma. Carbon dioxide travels mainly as bicarbonate ion in plasma, with a smaller amount (between 20 to 30 percent of transported carbon dioxide) bound to hemoglobin inside RBCs.

Control of Respiration

Neural Regulation

• Phrenic and intercostal nerves. Regulate the respiratory muscles, the diaphragm, and external intercostals.
• Medulla and pons. The main rhythm and depth centers. The medulla sets the basic rhythm with a self-exciting inspiratory pacemaker and an expiratory center that rhythmically inhibits it. The pons smooths out that basic rhythm.
• Eupnea. Normal respiratory rate, held at 12 to 15 respirations/minute.
• Hyperpnea. Deeper, more vigorous breathing during exercise as brain centers fire more impulses to the respiratory muscles.

Non-neural Factors Influencing Rate and Depth

• Physical factors. Talking, coughing, and exercise change rate and depth, and increased body temperature speeds breathing.
• Volition. Voluntary control is limited. The respiratory centers override the cortex when blood oxygen drops or blood pH falls.
• Emotional factors. Emotional stimuli act through the hypothalamus to change rate and depth.
• Chemical factors. The strongest drivers. Rising carbon dioxide and falling blood pH are the most important stimuli for faster, deeper breathing. Falling oxygen matters only when levels get dangerously low.
• Hyperventilation. Blows off more carbon dioxide and lowers carbonic acid, returning blood pH to normal when acids accumulate.
• Hypoventilation. Slow or shallow breathing lets carbon dioxide build up, returning blood pH to normal when blood turns slightly alkaline.

Age-Related Changes in the Respiratory System

Respiratory efficiency drops with age. Older patients cannot ramp up oxygen intake to meet higher demand, so dyspnea on exertion is common. Weaker expiratory muscles cut cough efficiency and leave more residual air in the lungs. Teach smoking cessation, infection prevention through handwashing, and current influenza and pneumonia vaccinations.