Respiratory Systems:
Physics & Strategies
Respiratory Systems

1.
2.
Purpose:
Gas Exchange
Receive oxygen from environment
Remove carbon dioxide from blood
Respiration

Respiration = the entire sequence of
events that results in the exchange of
oxygen and carbon dioxide between the
external environment of an animal and
the mitochondria within its cells.
Mitochondrial Respiration

Mitochondria consume O2 and act as sinks.

Mitochondria deplete local [oxygen].

An oxygen gradient is formed from the
outside of the cell to the mitochondrion.

Oxygen moves into the mitochondria and
carbon dioxide moves out.
The Pathway for Oxygen
Respiratory Strategies

3 main respiratory strategies:
1. Circulate external medium though the body.
2. Diffusion of gases across all or most of the
body surface, accompanied by transport of
gases in an internal circulatory system.
3. Diffusion across specialized respiratory
surface accompanied by circulatory
transport.
Circulating External Medium

Sponges, cnidarians, terrestrial arthropods.
Cutaneous Respiration

Diffusion of gases through skin.

Most aquatic inverts, terrestrial annelid
worms, some vertebrates such as
salamanders and frogs.
Cutaneous Respiration

Limitations:
◦ Very thin skin: minimize diffusion distance and
maximize diffusion rate
◦ Thin barrier must remain moist: enables dissolved
O2 to diffuse into the cell

Usually confined to aquatic or very moist
terrestrial habitats.

Surface area of the skin is usually quite
limited.
Exception: Lake Titicaca Frog
Exception: Adult Male Hairy Frog
Respiratory Strategies

As organisms get larger surface area to
volume ratio decreases which:
◦ Limits the surface area available for diffusion.
◦ Increases the distances within the animal that
oxygen must diffuse across
◦ Increasing the time needed for diffusion.
Respiratory Strategies

Larger animals: diffusion alone is insufficient.

Many animals have specialized respiratory
organs (ex. gills or lungs) with large surface
areas which they use for external gas
exchange
Respiratory Strategies

Animals with internal gills or lungs often
move external medium by bulk flow
across their respiratory surface.

This process is called ventilation.
Respiratory Strategies

In animals which ventilate a respiratory
surface, respiration is divided into 4 steps:
1. Bulk flow of medium across respiratory
surface
2. Diffusion across this surface
3. Bulk flow (gas transport) in the circulatory
system.
4. Diffusion into the tissues.
Physics of Respiratory Systems

The rate of diffusion will be greatest when:
(1) the ability of substance to diffuse is high
(2) the area of the membrane is large
(3) the energy gradient is large
(4) and the diffusion distance is small.

Consequently …
◦ Gas exchange surfaces are typically thin,
often fragile, and have large surface areas.
Specialized Respiratory Surfaces

Confine their gas exchange to a small
region of the body.

Increase the surface area of this region.

Respiratory surface is moist and thin, with
a large surface area.

Body covered with thick protective layer.
Specialized Respiratory Surfaces

Major Classifications:
◦ Gills
◦ Lungs

Often ventilated – move external medium
across these surfaces by bulk flow.
Gills

Originate as out-pocketings (evaginations)
of the body surface.

Can be external or located within a
respiratory cavity protected by a flap or
other covering.

Most common in water.
Lungs

Originate as infoldings (invaginations) of
the body surface

Forming an internal body cavity that
contains an external medium.

Most common in air
Oxygen Content In Air v. In Water
Oxygen Content In Air v. In Water

Molar [oxygen] (sea level at 20C):
◦ in air = 9mM,
◦ in water = >0.3 mM.

IMPORTANT IMPLICATIONS for the
respiratory strategies of an organism!

To obtain the same amount of oxygen, an
animal that uses water as a respiratory
medium must move 30x more fluid across
its respiratory surface.
Implications

Solubility of oxygen in water decreases
with increasing temperatures, resulting in
large decreases in [oxygen].

For fish, body temperature and oxygen
demand increase with increasing
temperature = CHALLENGE!
Implications

Solubility of gases decreases with
increasing ion concentration in a fluid.

So solubility of O2 in seawater is 20% less
than in freshwater at the same temp.

CHALLENGE for marine species!