Gill Respiration

Why are gills more efficient?

Unidirectional, less diffused oxygen, slower diffusion rate, more variable partial pressure/dissolved oxygen.

If they are so efficient, why don't we use them?

Since water both holds less oxygen and diffuses oxygen less slowly, the respiratory structures of aquatic animals in general must be more efficient.

Why do gills fail on land?

1) it requires moist surfaces for gas exchange-these gills are often exposed and prone to drying out.

2) there is no support for the gills, the respiratory structures stick together and then the exchange medium cannot interact with the surfaces.

Partial pressure of oxygen is constant at sea level throughout the world. 21% oxygen. It is well-mixed.

This is not true in aquatic systems-

Aquatic respiration-

Slower diffusion rate

Lower levels of dissolved oxygen

Greater variation in partial pressure of oxygen and other dissolved gases

For these reasons gills are more efficient respiratory structures than lungs to minimize hypoxia (too little oxygen)

1) Unidirectional ventilation

2) Larger proportional respiratory surfaces

3) Cross current and countercurrent blood flow within gills

Most internal gills develop from the pharynx.

The pharynx is that area lying between the buccal cavity and the esophagus.

Internal Gill structure is very similar in all fishes

20 kg fish has about 9.2 square meters of gill surface.

44 lb fish has 60 square feet of gill surface.

1 micrometer thick separation between the environment and the capillary. Each capillary is barely wider than a single red blood cell.

Generalized Gill Structure

Gill Septa-separates individual gill bars-channels water more efficiently

Each Gill bar has two filaments that straddle a gill septa

One side of a gill filament=demibranch

Both sides of a gill bar=holobranch

Gill Ray passes through septum and separates each gill opening

Each demibranch has secondary lamellae (parallel branches)

Gill rakers-prevent food particles from entering the gill chambers

Afferent branchial artery enters each gill bar

Efferent branchial artery exits each gill bar

Internal Gills

Pouched Gills-Agnathans

External pore or common duct (not a slit)

Fifteen to five pouches and associated pores

Septal gills-Elasmobranchs (plate gills)

Gill septa and gill slits arranged parallel

Gill septa longer than gill filaments

First gill slit reduced to spiracle

Larger openings into pharynx than in

pouched gills

Four holobranchs plus one demibranch

Opercular Gills-Bony fishes

Gill septa reduced and shorter than gill

filaments

Bony operculum present-protects filaments

Operculum contributes to ventilation of gills

Holobranchs reduced to three or less

Ventilation of Internal Gills

Pouched Gills-Tidal respiration

Water actively expelled from pouches by muscular contraction. Visceral skeleton allows cartilage to rebound and create suction-allowing water to re-enter the pouches.

Septal Gills-

Suction Phase

Pharynx contracts and expels water out of gill slits and mouth

Recoil from contractions creates buccal cavity suction. Water rushes in

branchial muscles enlarge creates suction. Water rushes in

Opercular Gill Respiration

Suction Phase

1) Mouth and operculum are closed and compressed

2) Buccal cavity expands, opercular cavity expands

3) Mouth opens-water rushes into buccal cavity then opercular cavity.

Force Phase

4) Mouth closes, operculum opens

5) Buccal cavity compresses and operculum remains open

6) Buccal cavity compresses and opercular cavity compresses

Ram ventilation-moving through the water with an open gape to ventilate the lungs. Only works while moving forward.

Evolution of lungs

More than 20 genera of fishes are habitual air-breathers. There are some species of fish that will drown if they are prevented from gulping air periodically.

In drought situations, gills are detrimental because they dry out easily. Oxygen intake, not carbon dioxide expulsion is a limiting factor. In other words, gills can continue to rid the body of CO2 even if they are not useful in taking up oxygen.

Lungs are any paired or unpaired structures that are derived from the gut tube and are filled with air and function primarily in respiration.

Internal organs filled with gas but are not respiratory structures are called gas bladders.

Gas bladders only occur in bony fishes

The respiratory system of amniotes develops from a single ventral evagination of the gut tube right behind the pharynx.

The early evagination quickly buds off into two

Two types of gas bladders/lungs

ON BOARD

Physostomous (bladder mouth)-have a pneumatic duct attached near the pharynx

Physoclistous (bladder closed)-a gas bladder lacking a pneumatic duct-more specialized fish

Gas bladders make up between 4 and 11% of the body mass of fish.

Long, short, curved, straight, simple, or partitioned into two or three distinct parts

Usually lies above the center of gravity-allows the fish to maintain a vertical position without expending muscular energy.

Other fish have to use their fins to prevent rolling if the gas bladder is underneath the center of gravity (or swim upside down).

Gas is usually secreted into the gas bladder in Physoclistous bladders by GAS GLANDS

Underneath each gas gland is a rete mirabile (marvelous net) of capillaries all oriented in the same direction. Efferent and Afferent capillaries exchange gas at a maximum gradient (through countercurrents) to maintain maximum gas pressure.

But the oxygen content may reach 1000 times that of the surrounding water and the partial pressure of oxygen may be 200 atm.

Some deep sea fishes have no gas bladder, others do.

Fish that tend to remain at a constant level or inhabit shallow streams often lack a gas bladder or have one reduced in size.

Flounders and Halibut have no gas bladder.

How do gas bladders differ from lungs?

Gas bladders are usually stiuated dorsal to the digestive tract whereas lungs are ventral

Gas bladders are single, whereas lungs are usually paired.

For gas bladders used as lungs, how are they ventilated?

Gas bladder breathing is usually done using tidal ventilation or pulse ventilation.

In this case, for exhalation, the sphincter muscle surrounding the glottis relaxes and brings swallowed air into the buccal cavity where it is expelled or vented through the operculum.

Inhalation-takes in air into mouth.

Closes mouth and compresses air to force it into gas bladder/lung

Movement of the air may be helped by compression of the outside water environment.

A few fish use unidirectional breathing when they use a vascularized stomach for respiration. In this case, the fish passes the air all the way through the digestive tract.

Amphibians use the same mechanism to get air into the lungs, but the buccal cavity muscles are stronger.

Types of Ventilation Pumps

Aquatic Gills

1) Dual Pump-unidirectional

Suction phase

Force phase

2) Ram Ventilation-unidirectional

3) Branchial muscle respiration

Used in external gills

Aquatic gas bladder/lung

1) Pulse pump-bidirectional (tidal)

Inhalation-buccal cavity compression

Exhalation-sphincter relaxation, expel air

2) Stomach ventilation-unidirectional

Inhalation-buccal cavity into vascularized

stomach

Exhalation-pass gas through rectum

Terrestrial Lung Ventilation

Aspiration pump-bidirectional

(birds, reptiles, mammals)

Inhalation-intercostals expand rib cage

Diaphragm drops-creates suction

Exhalation-intercostals contract rib cage

Diaphragm is raised-pushes air out.

Gas Bladder Function

Change buoyancy in water

Respiration

Sound production

Sound or pressure reception

Terrestrial Lung structure

Elastic bag

Surfactants coating the passages

Gas Bladder Function

1) Hydrostatic function

Change buoyancy in water

Physostomous (bladder mouth)-

pneumatic duct attached near the pharynx

Physoclistous (bladder closed)-

lacking a pneumatic duct

2) Respiration

Lined with blood vessels

3) Sound production-

resonating chamber-teeth grinding

Belching, drumming

4) Sound or pressure reception

Swim bladder vibrates and transmits sound to inner ear

Connected to ear (sacculus and lagena) by Weberian ossicles

Terrestrial Lung structure

Glottis

Used to moderate food and air transport

Trachea

Cartilaginous tube

Bronchi

Primary bifurcations of trachea into each lung

Bronchioles (secondary, tertiary etc)

Branching of bronchi into lobes of lung

Alveoli and Faveoli

Elastic air bags with surfactants

Alveoli-(mammals) round respiratory

compartments at terminus of bronchioles

Faveoli-(non-mammals) divided septa

branching from central lumen structure

Breathing Mechanisms within select Vertebrates

Amphibians

1) Closes mouth and nares, elevate bottom of mouth

2) Air is forced into lungs

3) Mouth and nares open, thorax compressed-air is forced out of lungs

Reptiles

Posterior portion of snake lungs may serve the same function as a diaphragm-bellows

Crocodilians use a diaphragm from the abdomen and pull the diaphragm back…piston-like. Connective tissue attaches the liver to the diaphragm

Mammals

Diaphragm acts directly. Forms a tight seal between abdominal and thoracic cavitities-only the vena cava, aorta, and esophagous pass through it.

Locomotion may cause breathing to cycle with strides.

Abdominal viscera may be used as part of the pump.

External intercostals-inhalation

Internal intercostals-exhalation-or through gravity

10 times the respiratory surfaces of amphibians-higher metabolic rate

Bird Respiration

Trachea

Parabronchi

One way branches

Air capillaries

Nine air sacs are connected to lungs

Interclavicular

2 cervicals

2 anterior thoracic

2 posterior thoracic

2 abdominal

Air is drawn into Parabronchi and posterior air sacs

Two cycles of ventilation to remove air in respiratory system.

Trachea branches into posterior thoracic and abdominal air sacs first

Spent air enters anterior air sacs before being exhaled.