The Respiratory System
Henry’s law states that the amount of gas that will dissolve in a
liquid is proportional to the partial pressure of that gas. Other
important factors are the solubility of the gas in the liquid and the
temperature of the liquid.
Respiratory System; Topic: Gas Exchange, pp. 1–6.
Composition of Alveolar Gas
Alveolar gas contains more carbon dioxide and water vapor and
considerably less oxygen than atmospheric air.
External respiration is the process of gas exchange that
occurs in the lungs. Oxygen enters the pulmonary capillaries;
carbon dioxide leaves the blood and enters the alveoli. Factors
inﬂuencing this process include the partial pressure gradients,
the thickness of the respiratory membrane, surface area available,
and ventilation-perfusion coupling (matching alveolar ventilation
with pulmonary perfusion).
Internal respiration is the gas exchange that occurs between the
systemic capillaries and the tissues. Carbon dioxide enters the
blood, and oxygen leaves the blood and enters the tissues.
Respiratory System; Topic: Gas Exchange, pp. 6–11, 15–16.
Transport of Respiratory Gases by Blood
Molecular oxygen is carried bound to hemoglobin in the red
blood cells. Te amount of oxygen bound to hemoglobin depends
on the P
of blood, blood pH, the presence of BPG,
and temperature. A small amount of oxygen gas is transported
dissolved in plasma.
Hypoxia occurs when inadequate amounts of oxygen are
delivered to body tissues. When this occurs, the skin and
mucosae may become cyanotic.
Carbon Dioxide Transport
is transported in the blood dissolved in plasma, chemically
bound to hemoglobin, and (primarily) as bicarbonate ions in
plasma. Loading and unloading of O
lowers blood pH; depletion of CO
blood raises blood pH.
Respiratory System; Topic: Gas Transport, pp. 1–15.
Control of Respiration
Medullary respiratory centers are the ventral and dorsal
respiratory groups. Te ventral respiratory group is likely
responsible for the rhythmicity of breathing.
Te pontine respiratory centers inﬂuence the activity of the
medullary respiratory centers.
Factors Inﬂuencing Breathing Rate and Depth
Important chemical factors modifying baseline respiratory rate
and depth are arterial levels of CO
, and O
An increasing arterial P
level (hypercapnia) is the most
powerful respiratory stimulant. It acts (via release of H
tissue) on central chemoreceptors to cause a reﬂexive increase in
the rate and depth of breathing.
cavity; normally it is negative relative to intrapulmonary
Respiratory System; Topic: Pulmonary Ventilation, pp. 7–9.
Gases travel from an area of higher pressure to an area of lower
Inspiration occurs when the diaphragm and external intercostal
muscles contract, increasing the dimensions (and volume) of the
thorax. As the intrapulmonary pressure drops, air rushes into the
lungs until the intrapulmonary and atmospheric pressures are
Expiration is largely passive, occurring as the inspiratory muscles
relax and the lungs recoil. When intrapulmonary pressure
exceeds atmospheric pressure, gases ﬂow from the lungs.
Respiratory System; Topic: Pulmonary Ventilation, pp. 3–6, 11–13.
Physical Factors Inﬂuencing Pulmonary Ventilation
Friction in the air passageways causes resistance, which decreases
air passage and causes breathing movements to become more
strenuous. Te greatest resistance to air ﬂow occurs in the
Surface tension of alveolar ﬂuid acts to reduce alveolar size and
collapse the alveoli. Surfactant reduces this tendency.
Premature infants have problems keeping their lungs inﬂated
owing to the lack of surfactant in their alveoli, resulting in infant
respiratory distress syndrome (IRDS). Surfactant formation
begins late in fetal development.
±otal respiratory compliance depends on elasticity of lung tissue
and ﬂexibility of the bony thorax. When compliance is impaired,
inspiration becomes more diﬃcult.
Respiratory System; Topic: Pulmonary Ventilation, pp. 14–18.
Respiratory Volumes and Pulmonary Function Tests
Te four respiratory volumes are tidal, inspiratory reserve,
expiratory reserve, and residual. Te four respiratory capacities
are vital, functional residual, inspiratory, and total lung.
Spirometry measures respiratory volumes and capacities.
Anatomical dead space is the air-ﬁlled volume (about 150 ml) of
the conducting passageways. If alveoli become nonfunctional in
gas exchange, their volume is added to the anatomical dead space,
and the sum is the total dead space.
Te FVC and FEV tests, which determine the rate at which VC
air can be expelled, are particularly valuable in distinguishing
between obstructive and restrictive disease.
Alveolar ventilation is the best index of ventilation eﬃciency
because it accounts for anatomical dead space.
Nonrespiratory Air Movements
Nonrespiratory air movements are voluntary or reﬂex actions that
clear the respiratory passageways or express emotions.
Gas Exchanges Between the Blood, Lungs,
Basic Properties of Gases
Dalton’s law states that each gas in a mixture of gases exerts
pressure in proportion to its percentage in the total mixture.