Chapter 44: Gas exchange and circulation
Chapters shows how gas exchange and circulation works
44.1 The respiratory and circulatory systems
When mitochondria produce ATP via cellular respiration, they consume oxygen and produce carbon
Cells must obtain oxygen and expel carbon dioxide continuously to support continued ATP production
Gas exchange involves four steps (Figure 44.1):
o Ventilation occurs when air or water moves through a specialized gas–exchange organ, such as
lungs or gills.
o Gas exchange takes place as CO and 2 diffus2 between air or water and the blood at the
o The dissolved O and2CO are t2ansported throughout the body along with nutrients, wastes, and
other types of molecules via the circulatory system.
o Where cellular respiration has led to low O le2els and high CO leve2s, O and C2 diffuse2
between blood and cells.
Ventilation and gas exchange are accomplished by the respiratory system (Figure 44.2).
The circulatory system is responsible for moving O , 2O , a2d other materials around the body.
Circulatory systems are classified as open or closed (Figure 44.3).
In a closed circulatory system, the system of vessels is continuous.
In an open circulatory system, some vessels open into portions of tissues.
The transport tissue inside continuous systems is blood.
The transport tissue in open systems is called hemolymph.
Animals with closed circulation have two subsystems, a system of blood vessels, and a system of lymph
Lymph consists of fluid that flows into lymph vessels from the spaces surrounding cells and specialized
2 cicrculatory subsystems are connected because lymph vessels empty into blood vessels.
44.2 Air and water as respiratory media
Gas exchange between the environment and cells is based on diffusion.
Oxygen is high in the environment and low in tissues, while carbon dioxide is high in tissues and low in
Oxygen thus tends to move from the environment into tissues and carbon dioxide tends to move from
tissues to the environment.
How do oxygen and CO beha2e in air?
Partial pressure is the pressure of a particular gas in a mixture of gases.
To calculate the partial pressure of a gas, multiply the percentage composition of that gas in the mixture
by the total pressure exerted by the entire mixture. Oxygen and carbon dioxide diffuse between the environment and cells along their respective partial–
In both air and water, oxygen and carbon dioxide move from regions of high partial pressure to regions
of low partial pressure.
How do oxygen and CO2 behave in water?
The amount of gas that dissolves in water depends on several factors:
the solubility of the gas in water: oxygen had low solubility in water. Due to this low
solubility, blood contains molecules that binds to oxygen and delivers it to tissues.
the temperature of the water: as temp of water increases, amount of gas that dissolves
in it decreases. Warm water = less oxygen
the presence of other solutes: seawater has higher concentration of solutes than fresh
the partial pressure of the gas in contact with the water: increasing the external pp
increases the rate at which a gas diffuses into liquid. If pp in liquid phase exceeds that
in gas phase, gas will boil out of liquid.
Other important aspects of oxygen availability in water include the surface area of the water, the
production of oxygen by photosynthesis, and the depletion of oxygen by organisms using it in cellular
44.3 Organs of gas exchanges
Some animals have organs for gas exchange, others use direct diffusion across the body surface.
Respiratory organs provide a greater surface area for gas exchange, enough to meet the demands of a
large body filled with cells.
Design parametes: law of diffusion
According to Fick's law of diffusion, O and2CO diffus2 in the greatest amounts when three conditions
are met: the surface area for gas exchange is large, the respiratory surface is extremely thin, and the
partial pressure gradient of the gas across the surface is large (Figure 44.5).
How do gills work?
Gills are outgrowths of the body surface used for gas exchange in aquatic animals.
Gills are efficient solutions to the problems posed by water breathing, primarily because they present an
extremely large surface area for oxygen to diffuse across an extremely thin epithelium.
If gills are internal, water must be driven over them by cilia, limbs or other specialized structures. Among invertebrates, the structure of gills is extremely diverse, whereas the gills of bony fish are
similar in structure.
Fish gills are located on both sides of heads and teleosts consist of 4 arches. To move water through
these structures, most fish open and close their mouth and a stiff flap of tissue called operculum hat
Pumping action and operculum creates a pressure gradient that moves water over gills. Some fish swim
through their mouth open called ram ventilation.