842
UNIT 4
Maintenance of the Body
22
causing the baby to take its first breath. Te alveoli inflate and
begin to function in gas exchange, but it is nearly two weeks
before the lungs are fully inflated.
Homeostatic Imbalance
22.16
Important birth defects of the respiratory system include
clef
palate
(described in Chapter 7) and cystic fibrosis.
Cystic fi-
brosis (CF)
, the most common lethal genetic disease in North
America, strikes in one out of every 2400 births. In CF, abnor-
mally viscous mucus clogs the respiratory passages, providing
a breeding ground for airborne bacteria and predisposing the
child to respiratory infections.
At the root of CF is a faulty gene that codes for the
CFTR
(cystic fibrosis transmembrane conductance regulator)
pro-
tein
. Te normal CF±R protein works as a membrane channel
to control Cl
2
flow in and out of cells. In one common mu-
tation, CF±R lacks a critical amino acid and so does not fold
correctly. As a result, it gets “stuck” in the endoplasmic reticu-
lum, is marked for degradation, and never reaches the plasma
membrane to perform its normal role. Consequently, less Cl
2
is secreted and less water follows, resulting in the thick mucus
typical of CF.
Tis thick mucus forms a perfect harbor for bacterial in-
fection. By early adulthood, 80% of patients are colonized by
Pseudomonas aeruginosa
, which causes chronic inflammation
and triggers the disabled cells to churn out a thick sludge of ab-
normal mucus. Repeated cycles of infection and inflammation
eventually result in extensive tissue damage that can be treated
only by a lung transplant.
Defects in the CF±R protein can affect other organ systems,
too. Te ducts of the pancreas become clogged with secretions,
impairing food digestion. Obstructed reproductive ducts render
97% of males with CF infertile. Characteristically, sweat glands
of CF patients produce extremely salty perspiration.
Conventional therapy for CF has included mucus-dissolving
drugs, “clapping” the chest to loosen the thick mucus, and anti-
biotics to prevent infection. Te goal of CF research is to restore
normal salt and water movements by (1) introducing normal
CF±R genes into respiratory tract mucosa cells, (2) prodding
another channel protein to take over the duties of transporting
Cl
2
, and (3) developing techniques to free the CF±R protein
from the ER. A novel and surprisingly simple approach involves
inhaling hypertonic saline droplets. Tis draws water into the
mucus, making it more liquid. Alone or in combination, these
therapies provide new hope to patients with CF.
Te respiratory rate is highest in newborn infants (40–80
breaths per minute). At five years of age it is around 25 per
minute, and in adults it is between 12 and 18 per minute. In old
age, the rate o²en increases again.
At birth, only about one-sixth of the final number of alveoli
are present. Te lungs continue to mature and form more alve-
oli until young adulthood. However, if a person begins smoking
in the early teens, the lungs never completely mature, and those
additional alveoli are lost forever.
In infants, the ribs take a nearly horizontal course. For this
reason, infants rely almost entirely on descent of the diaphragm
to increase thoracic volume for inspiration. By the second year,
the ribs are more obliquely positioned, and the adult form of
breathing is established.
Te maximum amount of oxygen we can use during aerobic
metabolism,
.
V
O
2
max
, declines about 9% per decade in inactive
people beginning in their mid-20s. In those who remain active,
.
V
O
2
max
still declines but much less. As we age, the thoracic wall
becomes more rigid and the lungs gradually lose their elastic-
ity, decreasing the ability to ventilate the lungs. Vital capacity
declines by about one-third by age 70. Blood O
2
levels decline
slightly, and many old people tend to become hypoxic during
sleep and exhibit
sleep apnea
(they stop breathing temporarily
during sleep).
Frontonasal elevation
Olfactory placode
Stomodeum
(future mouth)
Future mouth
Eye
Trachea
Bronchial buds
Pharynx
Foregut
Olfactory
placode
Esophagus
Liver
Laryngotracheal
bud
(a) 4 weeks:
anterior superficial view
of the embryo’s head
(b) 5 weeks:
left lateral view of the developing lower
respiratory passageway mucosae
Figure 22.28
Embryonic development of the respiratory system.
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