Download Module IV – Circulation and Gas Exchange

Course: BIOL1040
Topic: Circulation
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Module IV – Circulation and Gas Exchange
GAS EXCHANGE
Respiration occurs in two different ways:
o Cellular respiration – The process of producing energy from glucose.
o Gas exchange – The process of taking up oxygen and discharging carbon dioxide.
The method by which oxygen is inhaled and carbon dioxide is exhaled is simply passive diffusion –
simple diffusion that allows O2 and CO2 to move along its concentration gradient. Passive diffusion
always implies movement from regions of high to low partial pressure.
The concentration of gases can be measured using partial pressure – this is the pressure exerted by a
particular gas in a mixture of gases.
partial pressure = % of gas × pressure of gas
EXAMPLE: Air usually contains 21% oxygen, and at sea level, the atmospheric pressure of oxygen is
760 mmHg. Hence the partial pressure of oxygen is p𝑂2 = 0.21 × 760 = 160 mmHg.
Partial pressure applies to liquids as well – the solubility of a gas in liquids is directly proportional to the
partial pressure of that gas in equilibrium with the liquid.
The respiratory medium is the source of the oxygen – this can either be air or water. Gas exchange
occurs at the respiratory surface of the respiratory organ – this is where oxygen moves in and carbon
dioxide moves out via passive diffusion. As a result, respiratory surfaces tend to be moist, large and thin.
There are many types of respiratory organs:
o Gills in fish are outfoldings of the body that are
suspended in water. They have a very large
surface area to provide more respiratory
surface for gas exchange. A process called
ventilation maintains the partial pressure
gradients of O2 and CO2 across the gill. It is a
closed system.
o Tracheal systems in insects are made up of air
tubes that branch throughout the body. The
largest tubes (tracheae) open to the outside of
the insect’s body surface. The air sacs are
formed from enlarged portions of the tracheae,
and are found near organs that require a large
supply of oxygen. The surfaces of the air sacs
are moist to transfer oxygen. It is an open system.
o Lungs in mammals are localised respiratory organs. Air
is inhaled from the nasal cavity and pharynx, passing
through the larynx, trachea and bronchi to the
bronchioles, which end in microscopic alveoli lined with
thin, moist epithelium. The highly branched structure of
the lungs provides huge surface areas (80 – 100m2) for
gas exchange. Branches of pulmonary arteries
transport oxygen-poor blood to the alveoli, while
branches of pulmonary veins transport oxygen-rich
blood back to the heart.
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The process of ventilation maintains high O2 and low CO2 concentrations at the respiratory surface. The
process that ventilates the lungs is called breathing. There are two main types of breathing:
o An amphibian employs positive pressure breathing, inflating lungs with forced air flow.
 During inhalation, muscles lower the floor of the oral cavity to draw air in, and
subsequently the floor rises to push air down the trachea.
 During exhalation, air is forced back out via compression of the muscular body wall.
o A mammal employs negative pressure breathing – this is the process of pulling rather than
pushing air into the lungs.
 During inhalation,
the diaphragm
contracts and moves
down, expanding the
thoracic/chest cavity,
lowering the air
pressure in the lungs
below that of the air
outside their body. As
gas moves from high
to low partial
pressures, air rushes through the nostrils and mouth down to the alveoli.
 During exhalation, the diaphragm relaxes and moves up, reducing the volume of the
chest cavity, forcing air up and out of the body.
Within the chest cavity, a pleural sac forms a double membrane surrounding the lungs, similar to a fluidfilled balloon surrounding an air filled balloon. The inner membrane adheres to the outside of the lungs,
while the outer membrane adheres to the wall of the chest cavity. The pleural sac contains a thin space
filled with pleural fluid – this sticks the two membranes of the sac together, allowing them to slide
smoothly across each other, but not allowing them to be pulled apart easily.
Negative pressure
must be
maintained –
normally lungs are
inflated such that
they fit snugly
within the ribcage.
This is because
there is an elastic
recoil that pulls the
chest wall
outward. If the
lungs are stabbed,
they collapse, and
the surface area
reduces.
The tidal volume refers to the volume of air inhaled and exhaled with each breath (~ 500 mL).
o The vital capacity refers to tidal volume during maximal inhalation and exhalation (3.4 – 4.8 L).
o The residual volume is air that remains after a maximal exhalation (~ 1.2 L). With age, residual
volume increases at the expense of vital capacity.
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Breathing is regulated by involuntary mechanisms
– these mechanisms ensure that gas exchange is
coordinated with blood circulation and metabolic
demand.
o The stimuli for regulation of respiration
comes not from levels of O2, but rather
levels of CO2.
o Sensors in the carotid arteries and aorta
detect the pH of the blood, which send
signals to the medulla. The medulla itself
also detects the pH of the cerebrospinal
fluid. This is because blood CO2
concentration determines the pH of the
cerebrospinal fluid.
o CO2 diffuses from the blood to the
cerebrospinal fluid, where it reacts with
water to form carbonic acid (H2CO3).
o H2CO3 then dissociates into a bicarbonate
ion (HCO3-) and a hydrogen ion (H+), reducing pH.
𝐶𝑂2 + 𝐻2 𝑂 ⇌ 𝐻2 𝐶𝑂3 ⇌ 𝐻 + + 𝐻𝐶𝑂3 − (decreased pH)
EXAMPLE: During exercise, increased metabolic activity increases the concentration of CO2 in blood as it
is not removed fast enough. This lowers the pH of the blood via the above mechanism. Sensors in blood
vessels (such as the carotid arteries and aorta) detect the decrease in the blood pH, which then signal
the medulla. The medulla itself also detects the decrease in the pH of cerebrospinal fluid.
Subsequently, the medulla signals the rib muscles and diaphragm to increase the rate and depth of
ventilation. This decreases CO2 levels, restoring the pH back to homeostatic levels.
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There are two mechanisms for the transport of large
quantities of oxygen and carbon dioxide:
o PARTIAL PRESSURE – In the lungs, gradients of
partial pressure favour the diffusion of O2 into
the blood and CO2 out of the blood. The
gradients exist due to cellular respiration, which
removes O2 from and adds CO2 to the
surrounding interstitial fluid. This is also known
as Fick’s law of
Inhaled air
diffusion, which
𝑝𝑂2 = 160 mmHg
states that the
𝑝𝐶𝑂2 = 0.2 mmHg
diffusion rate of a gas
across a fluid
Exhaled air
membrane is
𝑝𝑂2 = 120 mmHg
proportional to the
𝑝𝐶𝑂2 = 27 mmHg
difference in partial
pressure, area and
thickness of the membrane.
Partial pressures of O2 and
CO2 can vary in different
parts of the circulatory
system, as well as in inhaled
air compared with exhaled
air. The diagrams adjacent
illustrate the partial
pressures of O2 and CO2 at
varying stages of respiration.
o RESPIRATORY PIGMENTS –
In the rest of the body, O2 is
transported via proteins called respiratory pigments, found in the blood or haemolymph.
Respiratory pigments overcome the low solubility of gases in blood by greatly increasing the
amount of O2 that the blood can carry (from 4.5 mL dissolved O2 per litre of blood to 200 mL
carried by respiratory pigments per litre of blood). Pigments have a distinctive colour, and
usually consist of a protein bound to a metal.
 The blue pigment haemocyanin has copper as its oxygen-binding component, and is
found in arthropods and many molluscs.
 The red pigment haemoglobin has iron as its oxygen-binding component, and is found
in almost all vertebrates and many invertebrates.
Haemoglobin consists of
four subunits (polypeptide
chains) each with a cofactor
called a heme group with an
iron atom at its centre. Each
iron atom is able to bind one
molecule of O2, and so one
haemoglobin molecule can
carry four molecules of O2.
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o
o
When one O2 binds to a subunit, the other subunits change their shapes slightly, making it
easier for more O2 to bind. This phenomenon is called positive cooperativity. The opposite is
also true – when one subunit unloads O2, the other three subunits unload O2 more readily. As
such, even a small change in 𝑝𝑂2 causes haemoglobin to load or unload a large amount of O2.
The production of CO2 lowers the pH of surroundings, which in turn decreases the affinity of
haemoglobin for O2, causing O2 to be unloaded more readily.
Haemoglobin not only transports O2, but CO2 as well – this assists in buffering the blood.
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