Course: BIOL1040 Produced by cyrionTM. © 2014. All rights reserved. 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 fluid-

filled 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 + 𝐻2𝑂 ⇌ 𝐻2𝐶𝑂3 ⇌ 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

diffusion, which

states that the

diffusion rate of a gas

across a fluid

membrane is

proportional to the

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.

Inhaled air

𝑝𝑂2 = 160 mmHg

𝑝𝐶𝑂2 = 0.2 mmHg

Exhaled air

𝑝𝑂2 = 120 mmHg

𝑝𝐶𝑂2 = 27 mmHg

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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.

o 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.