Study Notes: The Respiratory System (Chapter 13) – Essentials of Human Anatomy & Physiology

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The Respiratory System

Functional Anatomy of the Respiratory System

The respiratory system is responsible for gas exchange between the blood and the external environment. Its organs include the nose, pharynx, larynx, trachea, bronchi, and lungs (with alveoli as the site of gas exchange).

Nose: Entry point for air; contains nostrils (nares), nasal cavity, olfactory epithelium, and nasal septum.
Pharynx: Muscular passageway connecting the nasal cavity to the larynx; divided into nasopharynx, oropharynx, and laryngopharynx.
Larynx: Voice box; routes air and food, contains vocal cords, and is made of hyaline cartilage.
Trachea: Windpipe; reinforced with cartilage rings, lined with ciliated mucosa.
Bronchi: Branches from the trachea into the lungs; right bronchus is wider and straighter.
Lungs: Contain alveoli, divided into lobes, covered by pleura.

The Nose

The nose is the only externally visible part of the respiratory system. It is lined with respiratory mucosa, which moistens air, traps foreign particles, and destroys bacteria via lysozyme enzymes. The nasal cavity is divided by the nasal septum, and the palate separates it from the oral cavity.

Conchae: Projections that increase surface area and air turbulence.
Paranasal sinuses: Cavities in surrounding bones; lighten the skull, act as resonance chambers, and produce mucus.

The Pharynx

The pharynx serves as a passageway for both air and food. It contains tonsils, which are clusters of lymphatic tissue that protect against infection.

Regions: Nasopharynx (behind nasal cavity), oropharynx (behind mouth), laryngopharynx (attached to larynx).
Tonsils: Pharyngeal (adenoid), palatine, and lingual tonsils.
Epiglottis: Routes food to the esophagus and air to the trachea.

The Larynx

The larynx, or voice box, is located inferior to the pharynx and is made of eight rigid hyaline cartilages. The thyroid cartilage (Adam’s apple) is the largest. The epiglottis protects the superior opening and routes food and air appropriately.

Vocal folds: Vibrate with expelled air to produce speech.
Glottis: Opening between the vocal cords.

The Trachea

The trachea is a 4-inch-long tube connecting the larynx to the bronchi. Its walls are reinforced with C-shaped rings of cartilage, ensuring a patent airway. The trachealis muscle and ciliated mucosa help move mucus and debris upward.

Goblet cells: Produce mucus.
Cilia: Beat in a superior direction to move contaminated mucus.

The Main Bronchi and Lungs

The main bronchi are formed by the division of the trachea and enter the lungs at the hilum. The lungs occupy the thoracic cavity, except for the mediastinum, and are divided into lobes by fissures (right lung: three lobes; left lung: two lobes).

Pleura: Visceral pleura covers the lung surface; parietal pleura lines the thoracic cavity.
Pleural fluid: Reduces friction during breathing.

The Bronchial (Respiratory) Tree

The bronchial tree is a network of branching passageways that conduct air to and from the respiratory zone. Main bronchi subdivide into smaller branches and bronchioles.

Respiratory zone: Includes respiratory bronchioles, alveolar ducts, alveolar sacs, and alveoli.
Conducting zone: All other passageways.

Respiratory Zone Structures and the Respiratory Membrane

Alveoli are composed of simple squamous epithelial cells and are covered by pulmonary capillaries. The respiratory membrane (air-blood barrier) allows gas exchange by diffusion.

Alveolar pores: Connect neighboring air sacs.
Stroma: Elastic connective tissue.
Alveolar macrophages: "Dust cells" that pick up debris.
Surfactant: Lipid molecule that coats alveolar surfaces and reduces surface tension.

Respiratory Physiology

The respiratory system supplies the body with oxygen and disposes of carbon dioxide. Respiration includes four distinct events:

Pulmonary ventilation: Moving air into and out of the lungs (breathing).
External respiration: Gas exchange between pulmonary blood and alveoli.
Respiratory gas transport: Transport of oxygen and carbon dioxide via the bloodstream.
Internal respiration: Gas exchange between blood and tissue cells.

Mechanics of Breathing

Pulmonary ventilation depends on volume changes in the thoracic cavity, which lead to pressure changes and the flow of gases to equalize pressure.

Inspiration: Diaphragm and external intercostal muscles contract, increasing intrapulmonary volume and decreasing gas pressure.
Expiration: Passive process relying on lung elasticity; intrapulmonary volume decreases and gas pressure increases.
Intrapleural pressure: Always negative, preventing lung collapse.

Nonrespiratory Air Movements

Nonrespiratory movements include cough, sneeze, crying, laughing, hiccup, and yawn. These are caused by reflexes or voluntary actions and help clear the lungs or express emotions.

Movement	Purpose
Cough/Sneeze	Clear lungs of debris
Crying/Laughing	Emotionally induced
Hiccup	Sudden inspiration
Yawn	Deep inspiration

Respiratory Sounds

Respiratory sounds are monitored with a stethoscope. Bronchial sounds are produced by air rushing through large passageways, while vesicular sounds are soft and occur as air fills alveoli.

External Respiration, Gas Transport, and Internal Respiration

Gas exchanges occur by diffusion. External respiration is the exchange between alveoli and pulmonary blood; internal respiration is the exchange between blood and tissue cells. Gases move toward areas of lower concentration.

Oxygen: Loaded into blood from alveoli.
Carbon dioxide: Unloaded from blood into alveoli.

Gas Transport in the Blood

Oxygen is transported mainly attached to hemoglobin (as oxyhemoglobin), with a small amount dissolved in plasma. Carbon dioxide is transported mostly as bicarbonate ions in plasma, with some carried on hemoglobin at different binding sites.

Equation for carbon dioxide transport:
Blood pH: Maintained between 7.35 and 7.45 by buffers such as bicarbonate ion.

Internal Respiration

Internal respiration is the exchange of gases between blood and tissue cells. Carbon dioxide diffuses out of tissue cells to blood (loading), and oxygen diffuses from blood into tissue (unloading).

Control of Respiration

Respiratory rate and depth are controlled by neural and non-neural factors. Neural centers in the medulla and pons set the basic rhythm and smooth out respiratory rate.

Normal rate: 12–15 respirations per minute (eupnea).
Hyperpnea: Increased rate due to extra oxygen needs.
Non-neural factors: Physical (temperature, exercise), volition, emotional, and chemical (CO2 and O2 levels).
CO2: Most important stimulus for breathing; increased levels stimulate breathing.
O2: Detected by peripheral chemoreceptors; important in disease states.
Hyperventilation: Rapid breathing due to acidosis; may lead to alkalosis.
Hypoventilation: Slow breathing due to alkalosis; allows CO2 to accumulate.

Developmental Aspects of the Respiratory System

Respiratory rate changes throughout life, being highest in newborns and decreasing with age. Aging reduces lung elasticity, vital capacity, blood oxygen levels, and the stimulating effects of carbon dioxide. Elderly individuals are more prone to hypoxia, sleep apnea, and respiratory infections.

Newborns: 40–80 respirations per minute
Infants: 30 respirations per minute
Age 5: 25 respirations per minute
Adults: 12–18 respirations per minute

Additional info: Where original content was incomplete, standard anatomical and physiological details were inferred for clarity and completeness.