How can a fish, which
is underwater, breathe if there is no air? When we go under water,
we have to bring air with us to survive. Whales and dolphins have
lungs that store air from the surface. Fish do not have lungs,
they rarely ever venture into the air, so how do they survive?
We all know it has something to do with gills, but what exactly?
The water surrounding a fish contains
a small percentage of dissolved oxygen. In the surface waters there
can be about five ppm of oxygen in the water. This is much less
than the 210,000 ppm of oxygen in the air, so the fish must use
a special system for concentrating the oxygen in the water to meet
their physiological needs. Here it comes again, a counter current
exchange system, similar to the one found in the fish's swim bladder
and in the tuna's muscles.
circulation of blood in fish is simple. The heart only has two chambers,
in contrast to our heart, which has four. This is because the fish
heart only pumps blood in one direction. The blood enters the heart
through a vein and exits through an artery on its way to the gills.
In the gills, the blood picks up oxygen from the surrounding water
and leaves the gills in arteries, which go to the body. The oxygen
is used in the body and goes back to the heart. This is a very simple
closed-circle circulatory system.
The gills: The gills are composed
of a gill arch (which gives the gill rigid support), gill filaments
(always paired), and secondary lamellae (where gas exchange takes
The blood flows through the gill
filaments and secondary lamellae in the opposite direction from
the water passing the gills. This is a couter current system and
is very important for getting all of the available oxygen out of
the water and into the blood.
If the blood flowed in the same direction
as the water passing it, then the blood would only be able to get
half of the available oxygen from the water. The blood and water
would reach equilibrium in oxygen content and diffusion would no
longer take place.
By having the blood flow in the opposite
direction, the gradient is always such that the water has more available
oxygen than the blood, and oxygen diffusion continues to the place
after the blood has acquired more than 50% of the water's oxygen
content. The counter current exchange system gives fish 80-90% efficiency
in acquiring oxygen.
do fish ventilate (or move water through) their gills? Fish must
pass new water over their gills
continuously to keep a supply of oxygenated water available for
diffusion. Fishes use two different methods for keeping a continuous
supply of new water available, one is very simple and the other
--Ram Ventilation: Swim through the
water and open your mouth (such as a shark would). Very simple,
but the fish must swim continuously in order to breathe, not so
--Normal Ventilation: Occurs by the
fish taking in water through the mouth. The mouth closes, forcing
the water back over the gill filaments and out through the gill
How do fish gills work?
Fish gills are a pretty complex structure, and are very well
adapted to getting oxygen out of water. Gills are made up of filaments
red things) attached to a rigid gill arch. The arches are hollow
and have arteries inside them that contain blood low in oxygen.
branch into smaller arterioles that run inside the filaments. Each
flat filament has many tiny folds on it (called lamellae) to increase
area. In fast moving fish, the surface area of the gills may be
ten times that of the actual animal. Tiny capillaries branch off
of the arterioles
and carry the blood close to the inner surface of the lamellae.
Because the oxygen concentration is less in the blood than in the
over the gills, the oxygen from the water naturally diffuses into
There is an adaptation that fish have to maximize the flow of oxygen
into the blood called countercurrent exchange. This is when the
water flowing over the lamellae is in the opposite direction as
the blood flowing
through the capillaries. In this way, the concentration of oxygen
in the blood as it moves through the capillaries is always lower
water, and oxygen will diffuse over the whole length of the lamellae.
Once the blood is fully oxygenated from it's trip through the gills,
it is pumped back into the body and used by the fish for energy, filling
the float bladder, and for nearly all of the metabolic processes in the
Fish extract oxygen from the water using their gills in a manner
somewhat similar to how land animals extract oxygen from the air
using lungs. In both cases, oxygen diffuses into the blood through
permeable membrane. Of course, there are some major design differences
between gill and lungs - it would take a comparative physiology
class to cover everything, but basically, the lungs of mammals
are like a balloon
with a single opening. Air goes in, and then goes out again through
the same opening (bi-directional). Oxygen is transferred from the
the blood across a thin membrane in the lungs, in the tiny, surface
area enhancing pockets called alveoli . Gills, on the other had,
water flowing past them constantly in one direction (unidirectional).
Most fish take advantage of this by having a "counter current" blood
system where the blood in the gills travels in the opposite direction
to the water flow. This allows for up to up to 80% efficiency in
getting the oxygen from the water to the blood, much better than what
accomplish. This efficiency is also helped by the very fine structure
of the gills, which greatly increases their surface area.
Fish have very these very efficient gills because of the fact that water
contains about one-thirtieth as much oxygen per volume as the atmosphere
above it. To put that in perspective, if we were somehow able to "breath" water,
we would need to take about 450 "breaths" per minute just to
get enough oxygen into our lungs!
People have been thinking about how humans could "breath" underwater
for quite some time (just think of the military implications!) For a
review of this topic, see:
Kylstra J. 1982, Liquid breathing and artificial gills, in P. Bennett
and D. Elliott, The Physiology and Medicine of Diving, Bailliere and
For an interesting experiment in which dogs did just fine breathing
a liquid other than water, see:
Model, J., C. Hood, E. Kuck and B. Ruiz, 1971. Oxygenation by ventilation
with flourocarbon liquid FX-80.l Anesthesiology 34:312-320.
* - Sections in Research background have been taken from http://www.geocities.com/aquarium_fish/how_fish_breathe.htm and