Chapter 15: The Respiratory System – Structure, Function, and Regulation

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Introduction to the Respiratory System

Overview

The respiratory system is essential for supplying oxygen to body cells and removing carbon dioxide, a waste product of metabolism. It works closely with the cardiovascular system to maintain homeostasis and support cellular activities such as growth, defense, and division.

Oxygen is obtained from the air via diffusion across lung surfaces and transported by the cardiovascular system.
Carbon dioxide is returned to the lungs for elimination.

Functions of the Respiratory System

Provides a large surface area for gas exchange between air and blood.
Moves air to and from the lungs.
Protects respiratory surfaces from environmental hazards.
Produces sounds for communication.
Participates in the sense of smell (olfaction).

Components of the Respiratory System

Conducting and Respiratory Portions

Conducting portion: Nasal cavity to terminal bronchioles; conducts air into the lungs.
Respiratory portion: Respiratory bronchioles and alveoli; site of gas exchange.

Alveoli

Air-filled sacs where all gas exchange occurs.

Respiratory Epithelium

Changes along the tract: ciliated columnar epithelium in larger airways; simple squamous in alveoli.

Respiratory Defense System

Mucous cells/glands: Produce mucus to trap debris.
Cilia: Move mucus toward the pharynx ("mucus escalator").
Filtration: Nasal hairs filter large particles.
Alveolar macrophages: Engulf small particles in the lungs.

Alveolar Epithelium

Simple squamous cells for efficient gas exchange.
Contains specialized cells (type I and II pneumocytes).

Anatomy of the Respiratory Tract

The Nose

Air enters through nostrils (external nares) into the nasal vestibule.
Nasal hairs: First filtration system.
Nasal septum: Divides cavity; made of cartilage, ethmoid, and vomer bones.
Olfactory region: Superior portion for smell.

The Pharynx

Commonly called the "throat"; shared by respiratory and digestive systems.
Divided into nasopharynx, oropharynx, and laryngopharynx.

The Larynx

Formed by three large cartilages: thyroid (Adam's apple), cricoid, and epiglottis.
Epiglottis: Prevents food from entering the trachea.
Vocal folds (cords): Produce sound (phonation); modified by articulation.

The Trachea

"Windpipe"; extends from cricoid cartilage to the mediastinum, where it branches into bronchi.
Supported by 15–20 C-shaped hyaline cartilage rings (open posteriorly for esophagus contact).
Histology: ciliated mucosa and submucosa with mucous glands.

The Bronchi and Bronchioles

Primary bronchi: Right and left; right is wider and more vertical (more likely to be obstructed).
Secondary (lobar) bronchi: One per lung lobe (3 right, 2 left).
Tertiary (segmental) bronchi: Supply bronchopulmonary segments.
Bronchioles: No cartilage; dominated by smooth muscle; can constrict or dilate.

Alveolar Ducts and Alveoli

Respiratory bronchioles connect to alveoli via alveolar ducts ending in alveolar sacs.
Alveoli are surrounded by capillaries and elastic fibers.

Alveolar Epithelium and Surfactant

Type I pneumocytes: Thin, squamous cells for gas exchange.
Type II pneumocytes: Produce surfactant (reduces surface tension, prevents collapse).
Alveolar macrophages: "Dust cells" that remove debris.

The Respiratory Membrane

Composed of three layers:
- Simple squamous epithelium of alveolus
- Fused basal laminae
- Endothelial cells of adjacent capillary
Extremely thin to allow rapid gas diffusion.

The Lungs and Pleural Cavities

Right lung: 3 lobes (superior, middle, inferior); left lung: 2 lobes (superior, inferior).
Lobes separated by fissures (horizontal and oblique).
Each lung is in a pleural cavity, lined by parietal (outer) and visceral (inner) pleura.
Pleural fluid lubricates and reduces friction.

Gas Exchange (Respiration)

External and Internal Respiration

External respiration: Exchange of O2 and CO2 with the environment.
Internal respiration: Cellular uptake of O2 and production of CO2.

Three Processes of External Respiration

Pulmonary ventilation (breathing)
Gas diffusion across membranes and capillaries
Transport of O2 and CO2 in blood

Pulmonary Ventilation

Physical movement of air in and out of the lungs.
Driven by pressure differences (air flows from high to low pressure).
Respiratory cycle: inspiration (inhalation) and expiration (exhalation).
Volume changes in the thoracic cavity (via diaphragm and rib cage) alter pressure.

Compliance

Indicator of lung expandability.
Low compliance: more force needed (labored breathing).
High compliance: less force needed (easy breathing).
Affected by lung tissue structure, surfactant levels, and thoracic mobility.

Modes of Breathing

Quiet breathing (eupnea): Active inhalation, passive exhalation.
Diaphragmatic breathing: Deep breathing, dominated by diaphragm.
Costal breathing: Shallow breathing, dominated by rib cage.

Lung Volumes and Capacities

Respiratory rate: Breaths per minute.
Tidal volume: Volume of air per breath (about 500 mL).
Pulmonary function tests measure rates and volumes.

Gas Exchange and Partial Pressures

Gas exchange depends on partial pressures and diffusion.
Atmospheric air composition: N2 (78.6%), O2 (20.9%), H2O (0.5%), CO2 (0.04%).
Dalton's Law: Total pressure is the sum of partial pressures of individual gases.
Partial pressure of O2 (PO2) and CO2 (PCO2) drive diffusion in lungs and tissues.

Gas Exchange in Lungs and Tissues

Blood entering lungs: low PO2, high PCO2.
O2 diffuses into blood; CO2 diffuses out.
In tissues: O2 diffuses out of blood; CO2 diffuses in.

Gas Transport in Blood

Oxygen Transport

Most O2 is bound to hemoglobin (Hb) in red blood cells.
Each Hb molecule binds up to four O2 molecules.
Binding is reversible and affected by PO2, pH, and temperature.

Carbon Dioxide Transport

CO2 is transported in three forms:
- As bicarbonate ions (HCO3–) – 70%
- Bound to hemoglobin (carbaminohemoglobin) – 23%
- Dissolved in plasma – 7%
Bicarbonate formation involves the enzyme carbonic anhydrase and the chloride shift.

Control of Respiration

Neural Regulation

Controlled by centers in the medulla oblongata and pons.
Involuntary centers regulate respiratory muscles in response to sensory input.
Voluntary centers in the cerebral cortex can override automatic control.

Respiratory Centers

Respiratory rhythmicity centers (medulla): Set basic pace.
Dorsal respiratory group (DRG): Controls inspiration (quiet and forced breathing).
Ventral respiratory group (VRG): Controls forced inspiration and expiration.

Respiratory Reflexes

Chemoreceptors: Respond to changes in PCO2, PO2, and pH.
Baroreceptors: Detect blood pressure changes; affect respiratory rate.
Stretch receptors: Respond to lung volume changes (Hering–Breuer reflexes).
Irritant receptors: Trigger protective reflexes (e.g., coughing).

Hering–Breuer Reflexes

Inflation reflex: Prevents overexpansion of lungs.
Deflation reflex: Stimulates inspiration when lungs deflate.

Chemoreceptor Reflexes

Monitored by cranial nerves IX (glossopharyngeal) and X (vagus), and central receptors in the medulla.
Hypercapnia: Increased arterial PCO2 stimulates increased breathing rate.

Higher Center Control

Strong emotions and anticipation can alter breathing via the hypothalamus and autonomic nervous system.
Exercise increases respiratory rate and cardiac output.

Developmental and Age-Related Changes

Respiratory Changes at Birth

Before birth: Lungs are collapsed; oxygen supplied via placenta.
At birth: First breath inflates lungs, changes circulatory patterns (closes foramen ovale and ductus arteriosus).

Aging and the Respiratory System

Elastic tissues deteriorate; compliance decreases.
Vital capacity and respiratory minute volume decline.
Emphysema risk increases with age and irritant exposure.

Integration with Other Systems

Respiratory and cardiovascular systems coordinate to maintain O2 and CO2 balance.
Homeostasis depends on efficient gas exchange, perfusion, and neural regulation.

Key Terms and Definitions

Term	Definition
Alveoli	Microscopic air sacs in the lungs where gas exchange occurs.
Epiglottis	Elastic cartilage flap that prevents food from entering the trachea during swallowing.
Pulmonary embolism	Blockage of a pulmonary artery by a blood clot or other material.
Diaphragm	Primary muscle of respiration; separates thoracic and abdominal cavities.
Surfactant	Lipid-protein secretion from type II pneumocytes that reduces alveolar surface tension.
Tidal volume	Volume of air moved in or out of the lungs during a normal breath (~500 mL).
Pharynx	Muscular tube (throat) shared by respiratory and digestive systems.
Atelectasis	Collapse of part or all of a lung, reducing gas exchange.
Vital capacity	Maximum amount of air exhaled after a maximal inhalation.
Trachea	Windpipe; conducts air from larynx to bronchi.
Larynx	Voice box; contains vocal cords and connects pharynx to trachea.
Pneumothorax	Presence of air in the pleural cavity, causing lung collapse.
Compliance	Measure of the lung's ability to stretch and expand.

Key Equations

Dalton's Law of Partial Pressures:
Oxygen Transport (Hemoglobin Saturation):
Carbon Dioxide Transport (Bicarbonate Formation):

Summary Table: Forms of Gas Transport in Blood

Gas	Main Transport Form	Percentage
Oxygen (O2)	Bound to hemoglobin	98.5%
Oxygen (O2)	Dissolved in plasma	1.5%
Carbon dioxide (CO2)	As bicarbonate ion (HCO3–)	70%
Carbon dioxide (CO2)	Bound to hemoglobin (carbaminohemoglobin)	23%
Carbon dioxide (CO2)	Dissolved in plasma	7%

Example: Clinical Application

Pulmonary embolism: A blood clot blocking a pulmonary artery can cause sudden shortness of breath and decreased oxygenation, requiring immediate medical attention.
Atelectasis: Collapse of alveoli can occur after surgery or due to blockage, reducing gas exchange efficiency.

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