16 THE RESPIRATORY SYSTEM: PULMONARY VENTILATION
Overview of Respiratory Function
Internal respiration (cellular respiration): refers to the use of oxygen within mitochondria to generate ATP by
oxidative phosphorylation, and the production of carbon dioxide as a waste product.
External respiration: refers to the exchange of oxygen and carbon dioxide between the atmosphere and body
tissues, which involves both the respiratory and circulatory systems. Encompasses 4 processes:
1. Pulmonary ventilation: the movement of air into the lungs (inspiration) and out of the lungs (expiration)
by bulk flow.
2. Exchange of oxygen and carbon dioxide between lung air spaces and blood by diffusion.
3. Transporation of oxyen and carbon dioxide between the lungs and blood by diffusion.
4. Transportation of oxygen and carbon dioxide between the lungs and body
Respiratory system also contributes to the regulation of acid-base balance in the blood, enables vocalization,
participates in defense against pathogens and foreign particles in the airways, provides a route for water and
heat losses (expiration/inspiration), enhances venous return (respiratory pump), and activates certain plasma
proteins as they pass through the pulmonary circulation.
Anatomy of the Respiratory System
Each lung is divided into lobes: right lung – 3 lobes, left lung – 2 lobes.
Air gets into and out of the lungs by way of the upper airways and a network of tubes forming a system of
passageways called the respiratory tract.
Air enters the nasal cavity and/or the oral cavity, both leading to the pharynx (a muscular tube that serves as a
common passageway for both air and food).
Food enters the esophasgus (a muscular tube leading to the stomach); air enters the larynx.
Can be functionally divided into a conducting zone and respiratory zone.
Conducting zone: upper part of the respiratory tract; functions in conducting air from the larynx to the lungs.
Respiratory zone: lowermost part of the respiratory tract; contains the sites of gas exchange within the lungs.
o Starts with the larynx (a tube held open by cartilage in its walls).
o Glottis (opening of the larynx) is covered by an epiglottis (flap of tissue), which during swallowing is
forced down over the glottis and prevents food or water from entering the larynx.
o Trachea is a tube (2.5 cm in diameter, 10 cm long) that runs parallel with and anterior to the esophagus.
o The front and sides of the wall contain 15-20 C-shaped bands of cartilage for structural rigidity. Without
them, the decline in air pressure that occurs in the trachea during inspiration would collapse it and cut
off the flow of air.
o Trachea then divides into left and right bronchi that conduct air to each lung. Cartilage forms rings
around the whole bronchus.
o Bronchi divide into secondary bronchi (3 for the right, 2 for the left). Cartilage is less abundant.
o Secondary bronchus divides into smaller tertiary bronchi, which in turn divide into successively smaller
o The extensive branching ultimately results in approximately 8 million tubules, the smallest less than
0.5mm in diameter.
o Bronchioles: tubules less than 1mm in diameter. No cartilage, capable of collapsing. To prevent
collapsing, walls of bronchioles contain elastic fibers.
o Bronchioles divide into terminal bronchioles, final smallest component.
o Conducting zone holds approx. 150mL of air and is considered dead space because the air does not
participate in gas exchange with blood.
o Epithelium lining of the larynx and trachea contains goblet cells and ciliated cells.
o Goblet cells: secrete a viscous fluid (mucus), which coats the airways and traps foreign particles. 16 THE RESPIRATORY SYSTEM: PULMONARY VENTILATION
o Cilia: beat in a whiplike fashion to propel the mucus containing the trapped particles up toward the
glottis and then into the pharynx, where mucus is swallowed.
o Mucus escalator: prevents mucus from accumulating in the airways and clears trapped foreign matter.
o At levels below the bronchioles phagocytic cells (macrophages) engulf foreign matter in the interstitial
space and on the surface of the epithelium.
o Smooth muscle is sparse in the trachea and bronchi but increases in abundance as the airways become
smaller. Lack of cartilage and presence of smooth muscle enables the airways to change their diameter
to alter resistance in airflow.
o Site of gas exchanges, maximizes surface area and minimizes thickness.
o Respiratory bronchioles terminate in alveolar ducts, which lead to alveoli, the primary structures where
gas exchange occurs.
o Most alveoli occur in clusters called alveolar sacs. Alveolar pores connect adjacent alveoli. Wall of the
alveolus consists of single layer of epithelial cells (type I alveolar cells) overlying a basement membrane.
o The capillary and the alveolar wall form a barrier called the respiratory membrane that separates air
from blood (0.2um).
o Approx. 300 million alveoli in the lungs have a total surface area of approx. 100sq meters.
o Also have type II alveolar cells and alveolar macrophages, which engulf foreign particles and pathogens.
Macrophages roam freely by amoeboid movements. Dead macrophages go on the mucus escalator.
STRUCTURES OF THE THROACIC CAVITY
Chest wall composes of rib cage (12 pairs of ribs), sternum (breast bone), thoracic vertebrae, and associated
muscles and connective tissue.
Muscles responsible for breathing are the internal intercostals and external intercostals, located between the
ribs, and the dome-shaped diaphragm, which seals off the lower end of the chest wall and separates the thoracic
and abdominal cavities.
The interior surface of the chest wall and the exterior surface of the lungs are lined by pleura, which is composed
of a layer of epithelial cells and connective tissue.
A separate pleural sac surrounds each lung. The side of the pleural sac attached to the lung tissue is called the
visceral pleura. The side of the pleural sac attached to the chest wall is called the parietal pleura.
Between the two pleura is a thin compartment called the intrapleural space, which is filled with a small volume
(15mL) of intrapleural fluid.
Forces of the Pulmonary Ventilation
Inspiration occurs when the pressure in the alveoli is less than the pressure in the atmosphere. Expiration
occurs when the pressure in the alveoli exceeds the pressure in the atmosphere.
4 primary pressures: atmospheric pressure, intra-alveolar pressure, intrapleural pressure, and transpulmonary
The volume of air in the lungs between breaths is called the functional residual capacity (FRC). When the lungs
are at FRC, all forces are balanced.
Atmospheric pressure (P atm: pressure of the outside air. Normally at 760mmHg. At higher altitudes, P atm
Intra-alveolar pressure (P alvpressure of air within the alveoli. At rest it’s equal tatm and thus is 0mmHg.
Varies during phases of ventilation. Difference between P atm and Palvdrives ventilation. When P atm> Palv
negative (inspiration), Patm< Palv= positive (expiration).
Intrapleural pressure (P ip pressure inside the pleural space. Contains fluid not air. At rest, -4mmHg. Varies with
the phase of ventilation. Always less than P and is always negative during normal breathing because opposing
forces exerted by the chest wall and lungs tned to pull the parietal and visceral pleura apart.
Transpulmonary pressure: difference between the P alv– Pip Measure of the distending force across the lungs;
increase in transpulmonary pressure creates a larger distending pressure across the lungs and the alveoli
expand. 16 THE RESPIRATORY SYSTEM: PULMONARY VENTILATION
Lungs and chest wall are both elastic. They tend to recoil.
At rest, chest wall is compressed and tends to recoil outward. When lungs are stretched, tend to recoil inward.
Intrapleural fluid keeps the parietal pleura and the visceral pleura from pulling apart due to surface tension.
The chest wall pulls outward on the intrapleural space while the lungs pull inward creating a negative
intrapleural pressure that opposes the separation and thus opposes the outward and inward recoil forces fo the
chest wall and lungs, respectively.
At rest, all breathing muscles are relaxed; volume of air is called functional residual capacity. No pressure
gradient to drive air movement.
To maintain the negative intrapleural pressure, pleural sac must be airtight. If broken, the negative intrapleural
pressure is lost as its equilibrates with the atmostpheric pressure. Lungs recoils and collapse while the chest
wall recoils and expands (pneumothorax).
A spontaneous pneumothorax occurs if disease damages the wall of the pleura adjacent to a bronchus or
alveolus such that air from inside the lungs enters the intrapleural space.
MECHANICS OF BREATHING
The relationship between pressure and volume follows Boyle’s Law, which states that for a given quantity of any
gas in an airtight container, the pressure is inversely related to volume of the container.
Flow euals the pressure gradient divided by the resistance. Air flow into tand out of thelungs also cocurs by bulk
flow, with the rate of flow determined by a pressure gradient (Patm– Palvand resistance: Flow = (Patm– Palv R.
Because P atmis constant, changes in alveolar pressure determine the direction of air movement.
DETERMINANTS OF INTRA-ALVEOLAR PRESSURE
o Determined by quantity (moles) of air molecules in the alveoli, and the volume of alveoli themselves.
o at the start of inspiration, lungs expand as a result of contraction of the inspiratory muscles, and the
expansion increases the volume of the alveoli, thereby lowering intra-alveolar pressure.
o Reduction in intra-alveolar pressure creates a pressure gradient that draws air into the lungs.
o Air flows into the alveoli during inspiration, the number of air molecules in the alveoli increases, so