Respiratory System: Anatomy and Physiology Study Guide

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Respiratory System Anatomy

Respiratory Mucosa

The respiratory mucosa is a specialized lining found throughout much of the respiratory tract. It consists of a ciliated pseudostratified columnar epithelium with goblet cells that secrete mucus. This mucosa functions to trap inhaled particles and pathogens, humidify incoming air, and facilitate the movement of debris out of the respiratory passages.

Goblet cells produce mucus to trap dust and microbes.
Cilia move the mucus toward the pharynx for removal.

Mucus Escalator

The mucus escalator refers to the coordinated movement of cilia in the respiratory tract that propels mucus (and trapped particles) upward toward the throat, where it can be swallowed or expectorated. This mechanism is essential for keeping the lower respiratory passages clear of debris and pathogens.

Warming and Moistening of Inspired Air

The nose warms and moistens inspired air through its rich vascular supply and the presence of mucus. The nasal conchae increase surface area, causing air to swirl and come into contact with the warm, moist mucosa, which helps condition the air before it reaches the lungs.

Regions of the Pharynx and Their Epithelium

Nasopharynx: Lined with ciliated pseudostratified columnar epithelium.
Oropharynx: Lined with stratified squamous epithelium for protection against abrasion.
Laryngopharynx: Also lined with stratified squamous epithelium.

Larynx Structure

The larynx is a cartilaginous structure that connects the pharynx to the trachea. It contains the vocal cords and is involved in sound production and protecting the lower airways during swallowing.

Major cartilages: thyroid, cricoid, arytenoid, and epiglottis.
Vocal folds (true vocal cords) and vestibular folds (false vocal cords).

Tracheal Wall

The tracheal wall consists of several layers:

Mucosa: Ciliated pseudostratified columnar epithelium with goblet cells.
Submucosa: Connective tissue with seromucous glands.
Hyaline cartilage rings: Provide structural support and keep the airway open.
Adventitia: Outermost connective tissue layer.

Bronchial Tree and Airflow Pathway

Air passes through the following structures:

Nasal cavity
Pharynx
Larynx
Trachea
Primary bronchi (right and left)
Secondary (lobar) bronchi
Tertiary (segmental) bronchi
Bronchioles
Terminal bronchioles
Respiratory bronchioles
Alveolar ducts
Alveolar sacs
Alveoli

Anatomical Differences Between Right and Left Lungs

Right lung: Three lobes (superior, middle, inferior), shorter and broader.
Left lung: Two lobes (superior, inferior), narrower due to the cardiac notch.

Layers for Gas Diffusion (External Respiration)

During external respiration, an air molecule must diffuse through:

Alveolar epithelium (simple squamous cells)
Fused basement membrane
Capillary endothelium

Serous Membrane of the Lungs

The pleura is the serous membrane of the lungs, consisting of:

Visceral pleura: Covers the lung surface.
Parietal pleura: Lines the thoracic cavity.
Pleural cavity: Space between the layers, filled with pleural fluid to reduce friction.

Respiratory System Physiology

Functions of the Respiratory System

Gas exchange (O2 in, CO2 out)
Regulation of blood pH
Voice production
Olfaction (smell)
Protection from inhaled pathogens and particles

Key Definitions

Pulmonary ventilation: Movement of air into and out of the lungs.
External respiration: Gas exchange between alveoli and blood.
Internal respiration: Gas exchange between blood and tissues.

Pressure Terms

Atmospheric pressure (Patm): Pressure exerted by air outside the body (typically 760 mm Hg at sea level).
Pulmonic (intrapulmonary) pressure (Ppul): Pressure within the alveoli.
Intrapleural pressure (Pip): Pressure within the pleural cavity, usually slightly less than atmospheric pressure.

Boyle’s Law and Pulmonary Ventilation

Boyle’s Law states that the pressure of a gas is inversely proportional to its volume at constant temperature:

As lung volume increases, pressure decreases, causing air to flow in (inhalation).
As lung volume decreases, pressure increases, causing air to flow out (exhalation).

Movement of Gases During Ventilation

Oxygen moves from the alveoli (high partial pressure) into the blood (low partial pressure).
Carbon dioxide moves from the blood (high partial pressure) into the alveoli (low partial pressure).

Muscles of Breathing

Inspiration: Diaphragm contracts (moves down), external intercostals contract (lift ribs).
Expiration: Usually passive; during forced expiration, internal intercostals and abdominal muscles contract.

Normal vs. Forced Exhalation

Normal (quiet) exhalation: Passive process due to lung recoil.
Forced exhalation: Active process involving contraction of abdominal and internal intercostal muscles.

Respiratory Volumes

Tidal volume (TV): Volume of air inhaled or exhaled in a normal breath (~500 mL).
Inspiratory reserve volume (IRV): Additional air that can be inhaled after a normal inhalation.
Expiratory reserve volume (ERV): Additional air that can be exhaled after a normal exhalation.
Vital capacity (VC): Maximum amount of air that can be exhaled after a maximal inhalation.

Dalton’s Law and Partial Pressure

Dalton’s Law states that the total pressure of a gas mixture is the sum of the partial pressures of each individual gas:

Partial pressure: The pressure exerted by a single gas in a mixture.

Gas Concentrations in Air

Oxygen (O2): ~21%
Carbon dioxide (CO2): ~0.04%
Nitrogen (N2): ~78%

Henry’s Law and Gas Solubility

Henry’s Law states that the amount of gas that dissolves in a liquid is proportional to its partial pressure and its solubility coefficient:

Where C is the concentration of dissolved gas, k is the solubility constant, and P is the partial pressure.
Gases with higher solubility dissolve more readily in liquids.

Example: When a soda can is opened after shaking, the pressure above the liquid drops suddenly, causing dissolved CO2 to come out of solution and form bubbles (foam).

Gas Exchange During Respiration

External respiration: O2 diffuses from alveoli to blood; CO2 diffuses from blood to alveoli.
Internal respiration: O2 diffuses from blood to tissues; CO2 diffuses from tissues to blood.

Oxygen Transport in Blood

Most O2 is transported bound to hemoglobin (Hb) in red blood cells.
A small amount is dissolved in plasma.

Hemoglobin Saturation and Factors Affecting It

At 100 mm Hg (normal arterial PO2), Hb is nearly 100% saturated.
At 60 mm Hg, Hb saturation drops to about 90% (see oxygen-hemoglobin dissociation curve).
Decreased pH (increased acidity) reduces Hb’s affinity for O2 (Bohr effect).
Fetal Hb has a higher affinity for O2 than adult Hb, allowing efficient transfer of O2 from mother to fetus.

Carbon Dioxide Transport in Blood

As bicarbonate ions (HCO3-) in plasma (majority).
Bound to hemoglobin as carbaminohemoglobin.
Dissolved in plasma.

Chemical Reactions Producing Bicarbonate Ions

CO2 reacts with water in red blood cells to form carbonic acid, which dissociates into bicarbonate and hydrogen ions:

Tracing Gas Movement by Pressure Gradients

Oxygen: Air (high PO2) → alveoli → blood (lower PO2) → tissues (lowest PO2).
Carbon dioxide: Tissues (high PCO2) → blood → alveoli (lowest PCO2) → air.

Neural Control of Respiration

Respiratory centers in the medulla oblongata and pons regulate the rate and depth of breathing.
Chemoreceptors in the brainstem and arteries monitor CO2, O2, and pH levels.
Voluntary control is possible via the cerebral cortex, but overridden by homeostatic needs.