Respiratory System: Anatomy and Physiology Study Guide

Respiratory System

Respiratory Anatomy

The respiratory system is responsible for the exchange of gases between the body and the environment. Its anatomy is specialized to facilitate the movement, conditioning, and exchange of air.

• Respiratory Mucosa: The respiratory mucosa is a mucous membrane lining most of the respiratory tract. It consists of ciliated pseudostratified columnar epithelium with goblet cells that secrete mucus, which traps dust, microbes, and other particles.

• Mucus Escalator: The 'mucus escalator' refers to the coordinated movement of cilia on epithelial cells, which transport mucus (and trapped particles) upward toward the pharynx, where it can be swallowed or expelled.

• Warming and Moistening Air: The nose contains a rich blood supply and moist mucosa. As air passes through the nasal cavity, it is warmed by blood vessels and humidified by mucus and serous secretions.

• Regions of the Pharynx:

  • Nasopharynx: Lined by pseudostratified ciliated columnar epithelium.

  • Oropharynx: Lined by stratified squamous epithelium.

  • Laryngopharynx: Lined by stratified squamous epithelium.

• Larynx Structure: The larynx is composed of cartilage (thyroid, cricoid, arytenoid), ligaments, and muscles. It houses the vocal cords and functions as a passageway for air and a voice-producing organ.

• Tracheal Wall: The trachea has a mucosa (ciliated epithelium), submucosa (connective tissue), hyaline cartilage rings (C-shaped), and an adventitia (outer connective tissue layer).

• Bronchial Tree Pathway: Air passes from the trachea → primary bronchi → secondary (lobar) bronchi → tertiary (segmental) bronchi → bronchioles → terminal bronchioles → respiratory bronchioles → alveolar ducts → alveoli.

• Right vs. Left Lungs: The right lung has three lobes (superior, middle, inferior) and is shorter and wider; the left lung has two lobes (superior, inferior) and a cardiac notch for the heart.

• Respiratory Membrane: For external respiration, an air molecule diffuses through: alveolar epithelium, fused basement membrane, and capillary endothelium.

• Serous Membrane of Lungs: The pleura consists of two layers: the visceral pleura (covers lungs) and parietal pleura (lines thoracic cavity), with pleural fluid in between to reduce friction.

Respiratory Physiology

The physiology of the respiratory system involves the mechanics of breathing, gas exchange, and transport of gases in the blood.

• Functions of the Respiratory System:

  1. Gas exchange (O2 in, CO2 out)

  2. Regulation of blood pH

  3. Voice production

  4. Olfaction (smell)

  5. Protection (filtering, warming, humidifying air)

• Key Definitions:

  • Pulmonary Ventilation: Movement of air into and out of the lungs (breathing).

  • External Respiration: Gas exchange between alveoli and blood.

  • Internal Respiration: Gas exchange between blood and tissues.

• Pressures in Respiration:

  • Atmospheric Pressure (Patm): Pressure exerted by air outside the body.

  • Pulmonic (Intrapulmonary) Pressure (Ppul): Pressure within the alveoli.

  • Intrapleural Pressure (Pip): Pressure within the pleural cavity, usually less than Ppul.

• Boyle’s Law: At constant temperature, the pressure of a gas is inversely proportional to its volume. Equation: $P_1V_1 = P_2V_2$ Application: During inspiration, lung volume increases, pressure decreases, and air flows in. During expiration, volume decreases, pressure increases, and air flows out.

• Gas Movements: Oxygen diffuses from alveoli (high partial pressure) to blood (low partial pressure); carbon dioxide diffuses from blood to alveoli.

• Muscles of Breathing: The diaphragm and external intercostals contract during inspiration, increasing thoracic volume. Forced breathing uses accessory muscles.

• Normal vs. Forced Exhalation: Normal (quiet) exhalation is passive (muscle relaxation); forced exhalation is active (abdominal and internal intercostal muscles contract).

• Respiratory Volumes:

  • Tidal Volume (TV): Volume of air inhaled or exhaled in a normal breath (~500 mL).

  • Inspiratory Reserve Volume (IRV): Additional air inhaled after a normal inspiration.

  • Expiratory Reserve Volume (ERV): Additional air exhaled after a normal expiration.

  • Vital Capacity (VC): Maximum amount of air exhaled after maximum inhalation. Equation: $VC = TV + IRV + ERV$

• Dalton’s Law and Partial Pressure: Each gas in a mixture exerts its own pressure (partial pressure) independently. Equation: $P_{total} = P_1 + P_2 + P_3 + \ldots$

• Gas Concentrations in Air:

  • Oxygen (O2): ~21%

  • Carbon Dioxide (CO2): ~0.04%

  • Nitrogen (N2): ~78%

• Henry’s Law: The amount of gas dissolved in a liquid is proportional to its partial pressure and solubility. Equation: $C = kP$ Where C = concentration, k = solubility constant, P = partial pressure. Example: Soda foams when opened after shaking because CO2 comes out of solution as pressure is released.

• Gas Exchange:

  • External Respiration: O2 moves from alveoli to blood; CO2 moves from blood to alveoli.

  • Internal Respiration: O2 moves from blood to tissues; CO2 moves from tissues to blood.

• Oxygen Transport: Most O2 is carried by hemoglobin (Hb) in red blood cells. Hb saturation depends on partial pressure of O2 (see oxygen-hemoglobin dissociation curve).

• Effect of pH on Hb Saturation: Decreased pH (increased acidity) reduces Hb's affinity for O2 (Bohr effect), causing more O2 to be released to tissues.

• Fetal vs. Adult Hb: Fetal hemoglobin has a higher affinity for O2 than adult hemoglobin, facilitating O2 transfer from mother to fetus.

• CO2 Transport: CO2 is transported in three forms:

  1. Dissolved in plasma (~7%)

  2. Bound to hemoglobin as carbaminohemoglobin (~23%)

  3. As bicarbonate ions (HCO3-) (~70%)

• Bicarbonate Formation: CO2 reacts with water in red blood cells to form carbonic acid, which dissociates into bicarbonate and hydrogen ions. Equation: $CO_2 + H_2O \leftrightarrow H_2CO_3 \leftrightarrow H^+ + HCO_3^-$

• CO2 Pathway: CO2 diffuses from tissues to blood, is transported to lungs, diffuses into alveoli, and is exhaled. Each step is driven by partial pressure gradients.

• Neural Control of Respiration: The respiratory centers in the medulla oblongata and pons regulate the rate and depth of breathing, responding to chemical and neural signals.

Example Table: Respiratory Volumes

Volume	Definition	Typical Value (mL)
Tidal Volume (TV)	Air moved in/out during normal breath	~500
Inspiratory Reserve Volume (IRV)	Extra air inhaled after normal inspiration	~3100
Expiratory Reserve Volume (ERV)	Extra air exhaled after normal expiration	~1200
Vital Capacity (VC)	Maximum air exhaled after maximum inhalation	~4800

Additional info: Some explanations and values were inferred and expanded for academic completeness.