Anatomy and Physiology of the Respiratory System: Study Notes

Study Guide - Smart Notes

Tailored notes based on your materials, expanded with key definitions, examples, and context.

Introduction to the Respiratory System

Overview

The respiratory system works in conjunction with the circulatory system to provide oxygen to the body and remove carbon dioxide. It is essential for gas exchange, which sustains cellular metabolism and homeostasis.

Primary function: Exchange of oxygen (O2) and carbon dioxide (CO2).
Collaboration: Works closely with the circulatory system to transport gases.

Objectives of Respiratory System Study

Trace the passageway of air from the nostrils to the lungs.
Describe the location, structure, and function of the nose, paranasal sinuses, pharynx, larynx, and trachea.
Identify protective mechanisms of the respiratory system.
Distinguish between conducting and respiratory zone structures.
Describe the gross and microscopic anatomy of the lungs and pleura.
Explain the muscles of respiration and mechanics of breathing, including pressure relationships in the thoracic cavity.
Discuss control of respiration: neural mechanisms, factors influencing breathing rate and depth, and adjustments due to exercise and altitude.
Apply Boyle’s, Dalton’s, and Henry’s laws to pulmonary ventilation and gas exchange.
Describe transport of gases in the respiratory and circulatory systems.
Compare atmospheric and alveolar air composition.
Recognize age-related changes in the respiratory system.

Major Structures of the Respiratory System

Nose and Nasal Cavity

The nose is the only externally visible part of the respiratory system. It provides a passageway for air, contains receptors for smell, filters, moistens, and warms incoming air, and serves as a resonating chamber for voice.

External nose: Shaped by bones and cartilage; opens to the exterior and is divided by the nasal septum.
Nasal cavity: Located posterior to the external nose; roof formed by ethmoid and sphenoid bones, floor by the palate (hard and soft).
Nasal vestibule: Lined with skin, sebaceous glands, and hair follicles to filter dust and pollen.
Mucosa types: Olfactory mucosa (smell receptors) and respiratory mucosa (seromucous glands).
Nasal conchae: Mucosa-covered projections that filter, heat, and moisten air during inhalation and reclaim heat and moisture during exhalation.

Paranasal Sinuses

Paranasal sinuses are air-filled spaces in the frontal, sphenoid, ethmoid, and maxillary bones. They lighten the skull and help warm and moisten air.

Pharynx

The pharynx is a funnel-shaped tube connecting the nasal cavity and mouth to the larynx and esophagus. It serves as a passageway for both air and food.

Nasopharynx: Air passageway; closed off during swallowing by the soft palate.
Oropharynx: Posterior to the oral cavity; passageway for food and air; lined with protective squamous epithelium.
Laryngopharynx: Passageway for food and air; lined with stratified squamous epithelium.

Larynx (Voice Box)

The larynx extends from the third to sixth cervical vertebra, attaches to the hyoid bone, and opens into the laryngopharynx. It is continuous with the trachea and consists of nine cartilages.

Thyroid cartilage: Shield-shaped; forms the Adam's apple.
Epiglottis: Elastic cartilage; acts as a switching mechanism to route air and food.
Functions: Maintains an open airway, routes air and food, and produces voice.

Trachea

The trachea (windpipe) descends from the larynx into the mediastinum and divides into two main bronchi. Its wall consists of mucosa, submucosa, and adventitia layers, supported by C-shaped rings of hyaline cartilage.

Mucosa: Stratified epithelium with cilia to propel debris-laden mucus toward the pharynx.
Submucosa: Connective tissue with seromucous glands.
Adventitia: Outermost layer covering cartilage rings.
Carina: Point where trachea branches into main bronchi.

Functional Zones of the Respiratory System

Conducting Zone

The conducting zone includes all respiratory passageways from the nose to the terminal bronchioles. It provides passageways for air, cleanses, humidifies, and warms incoming air.

Respiratory Zone

The respiratory zone is the actual site of gas exchange and consists of respiratory bronchioles, alveolar ducts, and alveoli.

Alveoli: Microscopic air sacs where gas exchange occurs by simple diffusion across the respiratory membrane.
Respiratory membrane: Formed by alveolar and capillary walls; allows O2 to pass into blood and CO2 to leave blood.
Lung surfactant: Reduces surface tension, preventing alveolar collapse.

Lungs and Pleura

Lungs

The lungs are cone-shaped organs occupying most of the thoracic cavity. The apex is near the clavicle, and the base rests on the diaphragm.

Left lung: Two lobes
Right lung: Three lobes
Blood supply: Pulmonary arteries deliver deoxygenated blood; pulmonary veins return oxygenated blood; bronchial arteries supply lung tissue.

Pleura

The pleura are serous membranes surrounding the lungs.

Pulmonary (visceral) pleura: Covers lung surface.
Parietal pleura: Lines thoracic cavity walls.
Pleural fluid: Fills space between layers, allowing smooth gliding during breathing.

Respiratory Muscles and Mechanics of Breathing

Respiratory Muscles

Movement of air into and out of the lungs is brought about by changes in thoracic cavity size, primarily due to the activity of respiratory muscles.

Diaphragm: Contracts and descends during inspiration, increasing thoracic cavity volume; relaxes during expiration.
Intercostal muscles: Elevate ribs during inspiration, further expanding the thoracic cavity.
Normal respiration: Active inspiration followed by passive expiration.

Mechanics of Breathing

Breathing involves inspiration (inhalation) and expiration (exhalation), driven by pressure differences between atmospheric and intrapulmonary pressures.

Atmospheric pressure: Pressure of air outside the body.
Intrapulmonary pressure: Pressure within the lungs.
Intrapleural pressure: Pressure within the pleural cavity.

Air flows into the lungs when intrapulmonary pressure drops below atmospheric pressure and flows out when it rises above atmospheric pressure.

Gas Laws Relevant to Respiration

Boyle's Law

Boyle's Law describes the inverse relationship between the pressure (P) and volume (V) of a gas:

As lung volume increases, pressure decreases, allowing air to flow in.
As lung volume decreases, pressure increases, forcing air out.

Dalton's Law of Partial Pressure

Dalton's Law states that the total pressure of a mixture of gases equals the sum of the partial pressures of individual gases:

Each gas in air (O2, N2, CO2) contributes to total atmospheric pressure according to its concentration.

Henry's Law

Henry's Law states that the amount of dissolved gas in a liquid is proportional to its partial pressure above the liquid:

Where C is the concentration of dissolved gas, k is the solubility constant, and P is the partial pressure.
Application: Predicts how gases dissolve in alveoli and bloodstream during gas exchange.

Gas Exchange and Transport

External Respiration

External respiration is the exchange of gases between alveoli and pulmonary capillaries. Oxygen binds to hemoglobin in red blood cells, while carbon dioxide is released from blood into alveoli.

Factors influencing external respiration: Partial pressure gradients, gas solubility, thickness and surface area of respiratory membrane, ventilation-perfusion coupling.

Transport of Gases

Oxygen: Mostly carried bound to hemoglobin; a small amount is dissolved in plasma.
Carbon dioxide: Transported dissolved in plasma, as bicarbonate ions (), and bound to hemoglobin.

Control of Respiration

Neural Mechanisms

Respiratory control centers are located in the medulla oblongata and pons.

Dorsal respiratory group (DRG): Sets basic respiratory rate; stimulates inspiratory muscles.
Ventral respiratory group (VRG): Active during increased respiratory drive; contributes to both inspiration and expiration.
Pontine respiratory group (PRG): Modulates activity of medullary centers.

Factors Influencing Breathing Rate and Depth

Exercise: Increases ventilation due to psychological stimuli, muscle activation, and excitatory impulses.
Altitude: Decreased atmospheric pressure lowers arterial PO2 and hemoglobin saturation; kidneys produce more erythropoietin to stimulate RBC production.

Respiratory Volumes and Capacities

Tidal volume (TV): Volume of air moved in and out during normal breathing (~500 ml).
Inspiratory reserve volume (IRV): Air that can be forcibly inspired beyond tidal volume (2100–3200 ml).
Expiratory reserve volume (ERV): Air that can be forcibly expired beyond tidal volume (1000–1200 ml).
Residual volume (RV): Air remaining in lungs after forced expiration.
Inspiratory capacity (IC): Total air that can be inspired after normal expiration.
Functional residual capacity (FRC): Air remaining after normal expiration.
Vital capacity (VC): Total exchangeable air.
Total lung capacity (TLC): Sum of all lung volumes.

Dead Space

Anatomical dead space: Volume of conducting passages (~150 ml).
Functional dead space: Alveoli not participating in gas exchange due to collapse or obstruction.
Physiological dead space: Sum of anatomical and alveolar dead spaces.

Factors Influencing Respiration

Airway resistance: Friction in respiratory passages.
Alveolar surface tension: Resists lung expansion; surfactant lowers surface tension.
Lung compliance: Stretchiness of lung tissue and thoracic cage.

Composition of Air

Atmospheric and alveolar air differ in gas concentrations. Alveolar air contains more CO2 and water vapor, and less O2 than atmospheric air due to continuous gas exchange and humidification.

Age-Related Changes in the Respiratory System

By 28 weeks gestation, the respiratory system is developed but still premature.
With age, the thorax becomes more rigid, lungs lose elasticity, and vital capacity declines.
Sleep apnea and reduced effectiveness of protective mechanisms become more common.

References

Marieb, E. N. and Hoehn, K., 2023. Human Anatomy & Physiology. 12th ed.