Macromolecules of the Respiratory System
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.
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 happen for the alveoli to expand.
If you have ever tried to pull two wet glass slides apart, you have
experienced surface tension created by the hydrogen bonding 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.
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+2) 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%.
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.