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Ventilation (breathing)

mechanical process that moves air into and out of the lungs


Where does gas exchange occur?

between blood and lungs

between blood and tissues


Cellular Respiration

oxygen utilization by tissues to make ATP


External respiration

ventilation and gas exchange in lungs


Internal respiration

oxygen utilization and gas exchange in tissues


Gas Exchange in Lungs

Occurs via diffusion

O2 concentration is higher in the lungs than in the blood, so O2 diffuses into blood.

CO2 concentration in the blood is higher than in the lungs, so CO2 diffuses out of blood.


Respiratory System Functions

GAS EXCHANGE between the atmosphere and the blood—brings in O2, eliminates CO2

Homeostatic REGULATION OF BODY PH—via selective retention vs excretion of CO2

PROTECTION from inhaled pathogens and irritating substances—via trapping & either expulsion or phagocytic destruction of potentially harmful substances, pathogens

VOCALIZATION—vibrations created by air passing over vocal cords


Conduction Zone

-->gets air to respiratory zone

primary bronchus
terminal bronchioles


Respiratory Zone

--> site of gas exchange

respiratory bronchioles
alveolar sacs


What is the passage of inspired air?

nasal cavity


larynx (through glottis and vocal cords)


R/L primary bronchi

Secondary bronchi

more branching

terminal bronchioles

respiratory zone (respiratory bronchioles)

terminal alveolar sacs


What does mucus do?

traps small particles


What do cilia do?

move small particles away from lungs by mucus


What is the structure of a lung lobule?

Each cluster of alveoli is surrounded by elastic fibers and a network of capillaries



Air sacs where gas exchange occurs

300 x 10^6 ; provide large surface area (760 ft^2) to increase diffusion rate

Each alveolus: one-cell layer thick

Form clusters at the ends of respiratory bronchioles


What are the two types of alveolar epithelial cells?

Type I: 95−97% total surface area where gas exchange occurs

Type II: secrete pulmonary surfactant and reabsorb sodium and water, preventing fluid buildup


Thoracic Cavity

-Contains the heart, trachea, esophagus, and thymus within the central mediastinum
--> The lungs fill the rest of the cavity.



parietal pleura: lines thoracic wall

visceral pleura: covers the lungs

normally pushed together, with a fluid-filled space between called the intrapleural space (pleural cavity).



a dome-shaped skeletal muscle of respiration that separates the thoracic and abdominal cavities


Physical aspects of ventilation

Air moves from higher to lower pressure.

Pressure differences between the two ends of the conducting zone occur due to changing lung volumes.

Compliance, elasticity, and surface tension are important physical properties of the lungs.


What are the types of pressure?

1. Atmospheric pressure: pressure of air outside the body

2. Intrapulmonary or intraalveolar pressure: pressure in the lungs

3. Intrapleural pressure: pressure within the intrapleural space (between parietal and visceral pleura); contains thin layer of fluid to serve as a lubricant


Pressure differences when breathing

1. Inspiration (inhalation): Intrapulmonary pressure Pressure atmospheric pressure (generally about +3mmHg)


Intrapleural Pressure

LESS than P(intrapulmonary) and P(atmospheric) in both inspiration and expiration

P(intrapulmonary) - P(intrapleural) = P(transpulmonary)

Keeps lungs against thoracic wall, allowing lungs to expand during inspiration



If the sealed pleural cavity is opened to the atmosphere, air flows in. The bond holding the lung to the chest wall is broken, and the lung collapses, creating a pneumothorax
(air in the thorax).


Boyle's Law

↑ lung volume during inspiration ↓’s P(intrapulmonary) to  Air flows in.

↓ lung volume during expiration  --> P(intrapulmonary) > P(atmospheric) --> Air flows out.


Lung compliance

Lungs expand when stretched.

Defined as change in lung volume per change in transpulmonary pressure: ΔV/ΔP

Index of the ease with which the lungs expand under pressure—> high compliance: easily stretched
--> low compliance: requires more force, restrictive lung diseases (e.g. pulmonary fibrosis, surfactant deficiency)


Lung elasticity

Return to initial size after being stretched (recoil)

Lungs have elastin fibers.

Because the lungs are stuck to the thoracic wall, they are always under elastic tension.

Tension increases during inspiration and is reduced by elastic recoil during expiration.


Surface Tension

"force holding fluid molecules together in an air-fluid interface... due to strong attractive force of H bonds between H2O molecules"

Resists distension, promotes collapse of alveolar space

Exerted by fluid secreted on the alveoli

Fluid is absorbed by active transport of Na+ and secreted by active transport of Cl-
--> any imbalance between these can result in viscous fluid that is difficult to clear --> raises the pressure of the alveolar air as it acts to collapse the alveolus

Ex. People with cystic fibrosis have a genetic defect that causes such an imbalance of fluid absorption and secretion


Law of Laplace

Pressure is DIRECTLY proportional to surface tension and INVERSELY proportional to radius of alveolus.

Small alveoli would be at greater risk of collapse without surfactant.




Secreted by type II alveolar cells

Consists of hydrophobic protein and phospholipids

REDUCES surface tension between water molecules by reducing the number of hydrogen bonds between water molecules

More concentrated as alveoli get smaller during expiration to equalize pressure.

Prevents collapse

Allows a residual volume of air to remain in lungs


Respiratory Distress Syndrome

Production of surfactant begins late in fetal life, so premature babies have higher risk for alveolar collapse--respiratory distress syndrome (RDS)
--> treated with surfactant

Similar problem may occur in adults with septic shock
--↓ lung compliance, ↓ surfactant—acute respiratory distress syndrome (ARDS); NOT treatable with surfactant


Muscles of Inspiration: Sternocleidomastoid, Scalenes

used for forced inspiration


Muscles of Inspiration: External Intercostals

raises rib cage during inspiration


Muscles of Inspiration: Parasternal intercostals

works w/external intercostals


Muscles of Inspiration: Diaphragm

contracts in inspiration - lowers --> enlarging thoracic cavity

relaxes in expiration - raises --> thoracic cavity smaller


Muscles of Expiration: Internal Intercostals

lowers rib cage during forced expiration


Muscles of Expiration: External Abdominal Obliques, Internal Abdominal Oblique, Transversus Abdominus, Rectus Abdominus

abdoominal muscles are also used in forced expiration


Quiet Expiration

occurs with the relaxation of the inspiratory muscles (PASSIVE)



Volume of thoracic cavity (and lungs) increases vertically when diaphragm contracts (flattens) and laterally when parasternal and external intercostals raise the ribs.

--Thoracic & lung volume increase --> intrapulmonary pressure decreases --> air in

--occurs when alveolar pressure decreases

1. thoracic cage expands outwards
2. diaphragm drops down- contracts and flattens
--> both of these lead to increased thoracic volume and decreased thoracic (alveolar) pressure



Volume of thoracic cavity (and lungs) decreases vertically when diaphragm relaxes (dome) and laterally when external and parasternal intercostals relax for quiet expiration or internal intercostals contract in forced expiration to lower the ribs.

--Thoracic & lung volume decrease --> intrapulmonary pressure increases --> air out


Regulation of Ventilation

Unlike cardiac muscle, skeletal muscles are NOT spontaneously active, so they must be stimulated by nerve signals.

Rhythmic pattern of contraction & relaxation of breathing muscles arises from a neural network of spontaneously discharging motor neurons from:
1. CEREBRAL CORTEX (voluntary breathing)
2. MEDULLA OBLONGATA and PONS (Involuntary breathing)

Motor neurons-- innervate diaphragm/other breathing muscles; regulated by descending neurons from the brainstem (Medulla & Pons).


Control of Breathing: Medulla

Two rhythmicity centers: excitatory inspiratory neurons vs neurons which inhibit those inspiratory neurons—intrinsic rhythmicity, but influenced by other factors.
1) Involuntary breathing (e.g. at rest)
-intrinsic to medulla
2) Voluntary (“forced,” e.g. exercise)
-input from cerebral cortex


Control of Breathing: Pons

Two resp control centers:
1) apneustic (stimulates inspiratory neurons in medulla)
2) pneumotaxic (antagonizes apneustic to inhibit inspiration)