gas exchange in fish 3.1.1
Bony fish
Gills
the fish equivalent to lungs
fish would have problems obtaining O2 with lungs because
O2 diffuses more slowly through water than air
water has a higher density than air so its harder to move over an exchange surface during ventilation
O2 is less soluble in water compared to air, so the O2 concentration found in water is far less than air
use gills to absorb O2 dissolved in water & release CO2
gill are found just behind the head
when a fish breathes it opens its mouth to let in water, shortly after a pair of openings appear behind the head & water flows out of the body
Gill structure
operculum
bony plate that covers gills in bony fish
each gill consists of 2 rows of gill filaments (primary lamellae) attached to a gill arch
filaments
very thin & their surface is folded into many secondary lamella (or gill plates) - provides large SA
capillaries
carry deoxygenated blood close to surface of secondary lamellae where gas exchange takes place - maintains steep concentration gradient
afferent artery
brings deoxygenated blood from the heart
efferent artery
takes oxygenated blood from gills to the body tissues for respiration
gill adaptations for efficient gaseous exchange
primary & secondary lamellae
provide large SA
rich blood supply
maintains steep concentration gradient
lamella are made up of thin layers of epithelial cells
short diffusion distance
ventilation
moves water rich in O2 over gills, maintains steep concentration gradient
Counter current flow
blood flows along gill arch & out along filaments - the blood flows through the capillaries in the opposite direction to flow of water over lamellae - counter current flow
counter current flow absorbs the maximum amount of O2 from the water along the whole length f the lamellae
counter-current flow mechanism
in the gills the flow of blood is in the opposite direction to the flow of water across the gills - meaning that blood is continually meeting fresh water with a higher % saturation of O2
this way the concentration gradient is maintained across the gill lamellae & O2 continues to diffuse into the blood
Parallel flow mechanism
if blood & water flowed in the same direction, diffusion would only occur until equilibrium was achieved - less efficient the blood would only be able to reach a max of 50% O2 saturation
Ventilation in bony fish
bony fish can keep water flowing over their gill by using a buccal-opercular pump
during inhalation the volume of the buccal (mouth) cavity increases (floor of mouth moves down) - reduces the pressure of the buccal cavity, so water enters
when buccal cavity is full the mouth shuts & muscular contractions reduce its volume, increasing the pressure - pushes water over the gills
movement of the operculum are coordinated with the movements of the buccal cavity
as water is pushed from the buccal cavity the operculum moves outwards, this reduces the pressure in the opercular cavity (space under the operculum) helping water flow through the gills
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6) buccal cavity floor is raised
7) pressure inside buccal cavity is now higher than in opercular cavity
5) opercular cavity expands
8) water moves from buccal cavity over gills into opercular cavity
4) water rushes into the mouth down the pressure gradient
9) mouth is now closed & operculum opens
3) this increases volume & decreases pressure of buccal cavity compared to outside
10) sides of opercular cavity move inwards, increasing pressure
2) buccal cavity floor is lowered
11) water rushes out of fish through operculum
1) mouth opens (operculum is closed)
why can't fish survive out of water?
their gills collapse, the SA exposed to air is therefore very small - insufficient to allow gaseous exchange, eventually the gills will dry out
sharks
no operculum or buccal pump
without the buccal pump or operculum they cannot pump water over their gills so they rely on moving to pass water over their gills
if they stop moving then the flow will stop & they won't be able to obtain O2 from the water
