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Biology 12
Respiratory System
Respiratory System
We have seen how the mitochondria in the cell use oxygen to carry out cellular respiration, and we have
seen how the body gets oxygen to the cells via the circulatory system. We have also learned how the
circulatory system transports the carbon dioxide created as waste by cellular respiration in what we have
been calling deoxygenated blood. We will now look at how the body gets the oxygen in the surrounding air
of the earth’s atmosphere to the circulatory system, and how it gets rid of the carbon dioxide.
The earth’s atmosphere is made up of several gases; however, found in higher concentrations are the gases
nitrogen (78%), oxygen (21%), argon (1%), and carbon dioxide (0.04%). Although nitrogen is important for
building biological molecules, it is the oxygen in the air that we use directly for cellular respiration.
There are four processes to get the oxygen in the atmosphere to help create ATP (energy for our cells) in
the mitochondria of our cells.
1. Breathing – getting oxygen to the lungs by inhalation, and expelling carbon dioxide as waste by
exhalation.
2. External Respiration – the exchange of oxygen and carbon dioxide between the blood and the air in
the lungs. Oxygen enters the blood, while carbon dioxide exits the blood.
3. Internal Respiration – oxygen and carbon dioxide are exchanged between the circulatory system at
the ________________________ and the ________________________.
4. Cellular Respiration (not covered in this unit) – once the oxygen is in the tissues the cells of the
tissues can carry out cellular respiration with oxygen entering the process and carbon dioxide exiting
as a waste.
Respiratory Structures
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Biology 12
Respiratory System
The mechanism of breathing basically brings air in and out of our body past several structures on its route
from nose to the lungs and back out again.
As air makes this return trip it brings oxygen deep into the lungs, and brings carbon dioxide out.
Air enters through the nose and the mouth; the former being the major entryway.
o the nose has nostrils, which are lined with hairs and a mucosal cell layer; both act to trap
and filter the air that is entering the lungs.
o there is also a number of blood vessels in the nose which house white blood cells which offer
further protection from foreign entities.
We have seen the pharynx when we studied the ____________________ system.
o common passageway for air and ______________.
o the ________________ covers the top of the trachea when we are eating to prevent food
from entering our airway; it remains open when we are breathing.
the larynx is our voice box
o found below the epiglottis
o contains the vocal cords which control the pitch of sounds according to how taut they are.
o
below the larynx is the trachea
o the trachea connects the pharynx to the smaller passageways that branch into the lungs
o it is a hard, cylindrical structure that is made up of C-shaped rings of cartilage. The
esophagus can be found in the open part of the “C”.
o the shape and hardness prevent the structure from collapsing.
the bronchi are smaller tubes that branch from the bottom of the trachea
o these are also cartilaginous
o
the bronchi branch into even smaller passageways called bronchioles
o the branching of the bronchioles forms the bronchiole tree
bronchioles finally end at the alveoli
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Biology 12
Respiratory System
Alveoli
these are blind, sac-like ending the passageway for air
millions of alveoli are found in the adult lung, which provides a large amount of _____________________
for the diffusion of gases (____________________ respiration).
alveoli are specialized four ways
1. Walls of the alveoli are one cell thick which helps with ___________________ of gases.
2. Lipoproteins on the inner surface which maintains surface tension, preventing them
from collapsing during exhalation.
3. Stretch receptors are nerve endings sensitive to stretch. When the alveoli are fully
stretched (i.e. during inhalation) these nerves send impulses to the medulla oblongata
which triggers exhalation
4. Pulmonary capillaries highly vascularize (i.e. provide lots of blood vessels) the alveolar
surfaces, ensuring maximum diffusion of gasses, and helps ensure that alveoli do not dry
out. Moisture is required for the diffusion of the gases.
Breathing: Inspiration and Expiration
the respiratory centre is located in the medulla oblongata of the brain
the medulla oblongata is sensitive to the carbon dioxide and hydrogen ions in the blood plasma; both are
waste products from cellular metabolism
when the concentrations of these waste products gets too high, the medulla oblongata sends nerve
impulses to the muscular diaphragm and the intercostals muscles (inter = between + costal = ribs)
the impulse stimulates the muscles to contract
o contraction of the diaphragm moves it down, and the contraction of the intercostals moves
them out and up
o the combined effect of the contraction of these muscles increases the volume in the thoracic
cavity; this creates a negative pressure (vacuum effect) in the cavity.
o air is drawn from the higher pressure outside through the trachea to the lower pressure
inside (due to the increased volume)
o ATP is required for this process; therefore it is considered an “active” process.
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Biology 12
Respiratory System
FYI – Hiccups
A hiccup is a contraction of the diaphragm that is usually rhythmic. This is an involuntary action (does not
occur by conscious choice of individual) involving a reflex arc (i.e. signal does not pass directly to the brain,
similar to the “knee-jerk” reflex). Holding your breath does increase the amount of carbon dioxide in your
blood, which triggers the medulla oblongata to send a message to the diaphragm to contract. This might
trick the diaphragm out of the rhythmic contractions of a hiccup.
the surfaces of the lungs are covered with a pleural membrane, as well as the inside of the thoracic
cavity; therefore, there are two membranes surrounding the lungs
o their function is to help the lungs slide over the body wall easily, and seals off the thoracic
cavity.
o pneumothorax is caused when a lung collapses due to a puncture of the pleural membrane.
 it is not the lung that is punctured, but simply the pressure from the atmosphere
entering through the puncture and pushing in on the lung.
o pleurisy is an infection of the pleura causing inflammation and severe sharp pain.
the stretch receptors of the alveoli detect when they are open (stretched) and signal the medulla
oblongata to stop the contraction of the diaphragm and intercostals muscles
when the diaphragm relaxes it bows upwards, and when the intercostals muscles relax gravity pulls them
back down and in.
o these actions decrease the volume of the thoracic cavity, and increase the pressure on the
lungs
o this causes exhalation.
chemoreceptors in the carotid bodies (in the carotid arteries) and the aortic bodies (in the aorta) are
sensitive to levels of oxygen in the blood.
o if concentrations have decreased these bodies communicate with the medulla oblongata and
the rate of breathing increases; and vice versa for increased levels of oxygen.
o this is a second mechanism to control the amount of breathing needed by the respiratory
system.
The Conditioning of Inhaled Air
air drawn into the air passageways needs to be conditioned for clean and proper gas exchange to occur.
this happens in three ways
1. Air is cleaned – cleaning by the nose hairs and mucous, and then by cilia found along the
walls of the trachea and bronchi
cilia are microscopic protein filaments
constantly in motion and move particles in the mucous back up the airway to the
pharynx to be coughed out
2. Air is warmed – the air moving along the passageway is in close contact with tissues that
are around the body’s temperature of 37C
3. Air is moistened – the air gets saturated with water as it passes over the mucous-lined
passageways.
helps keep the lungs from drying out
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Biology 12
Respiratory System
The Gas Exchange
to exchange the gases between the atmosphere and the tissues it happens in two ways: external
respiration and internal respiration
External Respiration
diffusion of oxygen into the pulmonary capillaries and the diffusion of carbon dioxide (with some water)
into the alveoli to be exhaled
the approximate conditions in the blood at the alveoli are 37C with a pH of 7.4
haemoglobin under these conditions is free to bind with oxygen
haemoglobin has four binding spots for oxygen, and 99% of these sites will be occupied by oxygen when
leaving the alveoli and entering the vascular system.
oxygen (O2) + haemoglobin (Hb) = oxyhaemoglobin (HbO2)
haemoglobin will carry oxygen to tissues throughout the body for internal respiration.
Internal Respiration
at the tissues the temperature is higher at 38C, and slightly more acidic at pH 7.35
hemoglobin in these conditions readily releases oxygen as blood enters the capillary beds
when oxygen has been released into the plasma it __________________ across the capillary beds and
into the _____________________.
without oxygen at its binding site hemoglobin is free as a transport molecule (Hb).
remember at the venule end of the capillary bed water along with wastes enter the bloodstream.
carbonic anhydrase is an enzyme found in the red blood cell (and kidney cells) that works at an optimum
temperature and pH found in the capillary beds of the tissues.
o carbonic anhydrase catalyses the reaction between carbon dioxide and water in the plasma
o this reaction is one of the fastest known enzymatic reactions
o the reaction is summarized below
carbonic
anhydrase
CO2
carbon dioxide
+
H2O
water
↔
[H2CO3]
carbonic acid
↔
HCO31bicarbonate ion
+
H1+
hydrogen ion
 bicarbonate ion: most widely known buffers in
the human body (re: buffers _______________
coming from the stomach entering the ____________________)
 hydrogen ion: binds to hemoglobin and is transported as reduced haemoglobin (HHb)
o hydrogen ions act to reduce the pH in plasma, so Hb operate as an effective buffer
of blood
 carbon dioxide mostly enters the reaction above, but some will be taken up by Hb forming
carbaminohaemoglobin (HbCO2)
Once the blood has reached the venule side of the capillary bed it is ready to the pulmonary circuit.
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Biology 12
Blood returning to the alveolar capillaries has the following characteristics
Respiratory System
o blood is carrying bicarbonate ions (______________) and a small amount of carbon dioxide
o Haemoglobin is either carrying carbon dioxide (carbaminohaemoglobin) or hydroden ions
(HHb)
o remember the temperature is slightly __________ and the pH is slightly ___________ at the
lungs, which creates an ideal environment for the release of CO2 and H+ from haemoglobin
o the enzyme carbonic anhydrase catalyses the reverse reaction it did before
CO2
carbon dioxide
+
H2O
water
↔
[H2CO3]
carbonic acid
↔
HCO31bicarbonate ion
+
H1+
hydrogen ion
o the carbon dioxide released from this reaction, the HbCO2 and along with the small amount
that is dissolved in the plasma diffuses into the alveoli and is exhaled into the atmosphere as
CO2
haemoglobin as Hb is now free again to bond with _______________ and start the process over again.
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