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Anatomy & Physiology (Open + Free)
Sy lla bu s
Unit 10:: The Respiratory System
Introduction to the
Respiratory Sy stem
Module 39 /
Respiratory Structures
and Functions
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Mor e
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Respiratory Lev els of
Organization
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Macromolecules of the Respiratory System
Describe how the structure of
these m acrom olecules allow the
structures of the respiratory
sy stem to perform their
functions.
Mucus
Mucus is a slimy substance secreted by mucus membranes throughout the respiratory system and other
organ systems. Mucus formation in the nasal passages and upper respiratory system, helps to moisten the air
and to trap microorganisms and particles. Throughout the entire respiratory system, mucus helps humidify
and buffer the cells that are in direct contact with air. Other organ systems including in the digestive,
urogenital, visual, and auditory systems use mucus production to protect epithelial cells. While the molecular
content varies between organ systems, in general, mucus contains primarily water with enzymes,
immunoglobulins (and other immune proteins) salts, and high molecular weight glycoproteins called mucins.
The glycosylations on the proteins attract large amounts of water. Consequently mucus serves as a means to
maintain local levels of hydration.
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Surfactant
At the gas-liquid interface of the alveoli cell membranes, surfactants found in the liquid surface layer lower
surface tension. Surface tension arises when water molecules hydrogen bond with each other. The hydrogen
bonding of water molecules makes it hard to “pull” the water molecules apart, which must to happen for the
alveoli to expand. If you have ever tried to pull to 2 wet glass slides apart you have experienced surface
tension created by the hydrogen binding of water molecules. The molecules in the surfactant interrupt these
hydrogen bonds, making it much easier to expand the alveoli during inhalation. The composition of surfactant
is 80% phospholipids with the remaining fraction made up of cholesterol and proteins. The alveoli themselves
are so small that without the surfactant, the surface tension created by the water on the internal surface of
the alveoli would force them to collapse during exhalation.
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EXAMPLE
Respiratory Distress Syndrome of the Newborn Infants.
Respiratory distress syndrome of newborn infants (RDS) most commonly affects premature infants
whose lungs have not developed fully, children born by caesarean section, and children of diabetic
mothers. The lungs of these infants cannot make sufficient quantities of surfactant, making it very difficult
for the affected infants’ lungs to expand properly to breathe. Most cases of RDS occur in infants born
before 28 weeks gestation as the cells of the lungs responsible for surfactant production, called type II
alveolar cells, are generally not very active until later in pregnancy.
Neonates with RDS struggle to breathe, leading to poorly oxygenated blood and cyanosis (appearance
of blue skin). Additional symptoms generally include apnea (periods of breathing cessation) or rapid,
shallow breathing. Laboratory procedures can be done to determine the level of fetal lung maturity.
Treatment for RDS often involves administration of a higher fraction of inhaled oxygen (above the normal
21%), or the use of a ventilator. Artificial surfactant can be given, but is still considered experimental. If a
premature birth is likely, the expectant mother may be given a steroid such as cortisol to promote
maturation of the fetal lungs before birth. Prognosis is dependent upon the severity of the disorder.
Children often worsen within the first few days after birth, but then usually recover with appropriate
treatment.
Hemoglobin
The protein used to carry oxygen by nearly all vertebrates is hemoglobin. It functions by binding oxygen in
the capillaries of the lungs and carrying it inside red blood cells. When the blood enters capillaries in tissues
with a low partial pressure of oxygen, the oxygen will leave the hemoglobin molecule and diffuse into the
tissue. One gram of hemoglobin can carry 1.34 ml of oxygen under normal conditions. There are normally
about 15 grams of hemoglobin in each deciliter of blood meaning that each deciliter of blood has the potential
to carry about 20 ml of oxygen. This is much greater than the 0.3 ml of oxygen that a deciliter of blood can
carry dissolved in plasma.
Hemoglobin is a complex compound containing two alpha and beta protein chain pairs and four heme
molecules that contain iron in its ferrous (Fe+3) state. Each heme can bind one oxygen molecule, so a
hemoglobin, because it contains 4 heme groups, can bind up to 4 oxygen molecules. When a hemoglobin
molecule is fully bound (saturated) with oxygen, we consider it to be an oxyhemoglobin. When it is not fully
saturated, it is typically referred to as deoxyhemoglobin, even though it may still have oxygen bound to some
of the heme groups. Note that even though an individual hemoglobin molecule can only be 0%, 25%, 50%,
75% or 100% saturated, we have nearly 300 million hemoglobin molecules in each red blood cell. Collectively
they can have any saturation level between 0% and 100%.
Hem oglobin m olecule. This work by Cenv eo is licensed under a Creativ e Com m ons Attribution 3 .0 United States
(http://creativ ecom m ons.org/licenses/by /3 .0/us/).
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When oxygen binds to one heme group the affinity of the other binding sites changes so that the other three
heme groups more easily bind each additional oxygen. Similarly, one heme group losing its oxygen makes it
easier for the other three to lose theirs. This characteristic of heme facilitates loading and unloading of oxygen
and helps to explain how oxygen can be so quickly attached and detached from hemoglobin as it passes
through the appropriate capillaries.
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