Respiratory system of vertebrates: Notes for the TYBSc course USZ0601Sem VI of University of Mumbai

Notes: Zoology- VI Semester, University of Mumbai, India.

Prof. S. D. Rathod, Associate Professor in Zoology, B. N. Bandodkar College of Science, Thane -400605

Comparative Anatomy of Chordates

Respiratory System of Vertebrates

[Skin, gills of cartilaginous & bony fish, lungs in vertebrates]

Skin (cutaneous) respiration in vertebrates:

Respiration through the skin can take place in air, water, or both

Most important among amphibians (especially the family Plethodontidae)

Adult, terrestrial salamanders of the family Plethodontidae, however, rely solely on

cutaneous respiration, as they lack lungs and gills. Species that rely mainly on cutaneous

respiration are typically long, cylindrical in shape, with thin epidermal layers laden with

dense capillary beds that pickup O2 and expel

CO2. Cutaneous respiration is the absorption of

oxygen, and disposal of carbon dioxide, through

the skin e.g. Plethodon spp. The long, cylindrical

shape creates a high surface area to volume ratio,

which enhances the amount of oxygen diffused.

To further promote cutaneous respiration, these

salamanders also have slow metabolisms, costal

grooves that increase the surface area, and the

ability to withstand oxygen debt through anaerobic

glycolysis (energy metabolism). All of these

features make for the successful utilization of

cutaneous respiration in most amphibians. Aquatic

species Giant Salamanders also possess large folds of skin that increase the oxygen-

absorbing surface area, thus increasing the oxygen intake.

Plethodon spp. Frog Salamander

Gills in cartilaginous and bony fish:

If we look at the pharyngeal region of a vertebrate embryo we find that the wall of the pharynx

develops pouches that go out. The lining of these pouches is endodermal tissue. Between the

pouches we have a piece of tissue that is the visceral arch. It is made of lateral plate mesoderm.



The outer covering of the pharynx is made from ectoderm. The ectoderm bulges in a bit to meet

the pouches. This forms a groove. The endoderm from the pouch induces the ectoderm to form

the groove. Vertebrates have 7 or fewer pouches on each side laterally supported by visceral

arches. Inside the visceral arch we find one of the aortic arches. This short vessel connects the

ventral aorta to the dorsal aorta. There are also nerves in the arches too. As we go up the

taxonomic ladder the number decreases.

The 1st visceral arch gives rise to the jaw of all vertebrates except Agnathans. The 1st closing

plate is lost during development in gilled fish and becomes the tympanum of tetrapods. The 1st

pharyngeal pouch becomes a gill chamber (Agnatha), a spiracle (other fish) or the middle ear

cavity (tetrapods).

The 2nd visceral arch becomes part of the jaw support or moves into the middle ear (columella).

The other visceral arches support gills in fish and in tetrapods they help support the tongue, and

becomes parts of the trachea and larynx. In fishes the gills are derived from the visceral arches.

The mesodermal arch is surrounded by ectoderm on the outer surface and endoderm on the other

surfaces. The arch starts to develop bone or cartilage skeletal structures and muscles. The gill

filaments sit close together and so water has to flow over the filaments to get to the external gill

chamber.

Water flows in with high oxygen content. It comes in contact with blood that has lower oxygen

content. By diffusion oxygen goes from higher -> lower concentration.

Countercurrent exchange - works well if the water is well aerated. Water holds less oxygen

than air, but fish Hb binds to oxygen better than tetrapod Hb.

1. Pouched gills in Agnatha:

6 - 15 pairs of gill pouches

Pouches connected to pharynx by afferent branchial (or gill) ducts & to exterior

by efferent branchial (or gill) ducts

Water flows from mouth to gill pouch and exits via an external pore.

The latter occurs when the mouth is involved in feeding. Muscular contraction

expels water.

2. Septal gills in Cartilaginous fishes:

5 ‘naked’ gill slits

Anterior & posterior walls of the 1st 4 gill chambers have a gill surface

(hemibranch). Posterior wall of last (5th) chamber has no hemibranch.

Interbranchial septum lies between 2 hemibranchs of a gill arch

Gill rakers protrude from gill cartilage & ‘guard’ entrance into gill chamber



2 hemibranchs + septum & associated cartilage, blood vessels, muscles, & nerves

= holobranch

3. Opercular gills in Bony fishes (teleosts):

Usually have 5 gill slits

Operculum projects backward over gill chambers

Interbranchial septa are very short or absent

Lungs in vertebrates:

Swim bladder & origin of lungs -

The ancient archetype of the lung was the

swimbladder in fishes. The swimbladder is

homologous in position and structure with

the lungs of the higher vertebrate animals.

Most vertebrates develop an out-pocketing

of pharynx or esophagus that becomes one

or a pair of sacs (swim bladders or lungs)

filled with gases derived directly or

indirectly from the atmosphere. Similarities

between swim bladders & lungs indicate

they are the same organs. Vertebrates

without swim bladders or lungs include

cyclostomes, cartilaginous fish, and a few

teleosts (e.g., flounders and other bottom-

dwellers).



Swim bladders:

May be paired or unpaired. During development, a pneumatic duct that usually connects to the

esophagus is formed. The duct remains open (physostomous) in bowfins and lungfish, but

closes off (physoclistous) in most teleosts (see above fig.). Serve primarily as a hydrostatic

organ (regulating a fish's specific gravity). Gain gas by way of a 'red body' (or red gland); gas is

resorbed via the oval body on posterior part of bladder.

Lungs:

Higher iertebrate respiratory organs include the lung. Lungs develop from the pharynx. Lungs

arise in the embryo as an endodermal diverticulum from the ventral wall of the pharynx. The

diverticulum soon divides into two parts, which form right and left lungs. A windpipe or trachea

connects the lungs with the pharynx. Anterior part of the trachea is modified into the larynx. The

larynx communicates with the pharynx by a slit like opening the glottis. The laryrnx functions as

sound producing organ in tetrapods except in birds. The birds have their sound-producing organ

known as 'syrinx'. The trachea bifurcates into two branchi. Each primary branchus further

divided inside the lungs as secondary bronchi, tertiary bronchi and bronchides. The bronchides

are connected to the alveoli.

Amphibian lungs

2 simple, long, spindle shaped, semitransparent, elastic, delicate, thin walled and sac

like structures.

Internal lining may be smooth or have simple sacculations or pockets.

Air exchanged via positive-pressure ventilation. In the Amphibia, however, there are

not even ribs developed, or, if they exist at all, they are such mere rudiments.

The lungs communicate with the buccal cavity through the laryngeotracheal chamber. The laryngeotracheal chamber opens in the floor of the buccal cavity by means of

glottis. A pair of lungs is located in the anterior region of coelom on the dorsolateral

sides of the heart. The outer surface of the lungs is lined by visceral peritoneum.

The inner surface of the lungs is projected in the lumen as large number of irregular

and radially arranged folds. The space between two consecutive folds of the inner

surface of the lungs forms an alveolus.

The Alveoli greatly increase the inner respiratory surface of the lungs.



Reptilian lungs

Simple sacs in Sphenodon & snakes

Lizards, crocodilians, & turtles - lining is septate, with lots of chambers & sub-

chambers. The lungs of Reptiles are two capacious membranous sacs occupying a

considerable portion of the visceral cavity, which, as there is no diaphragm as yet

developed, cannot properly be divided into thorax and abdomen, as it is in

Mammalians. From the internal surface of the walls of each lung membranous septa

project inwards, so as partially to divide the interior of the organ into numerous

polygonal cells, which are themselves subdivided into smaller compartments in a

similar manner. This structure is well seen in the lung of the Tortoise.

Air exchanged via positive-pressure ventilation as well developed ribs assist in

respiration.



Avian lungs

Modified from those of reptiles:

Air sacs (diverticula of lungs) extensively distributed throughout most of the body.

Functionally, these 9 air sacs can be divided into anterior sacs (interclavicular, cervicals,

& anterior thoracics) & posterior sacs (posterior thoracics & abdominals). Air sacs have

very thin walls with few blood vessels. So, they do not play a direct role in gas exchange.

Rather, they act as a 'bellows' to ventilate the lungs. Arrangement of air ducts in lungs ----

> no passageway is a dead-end.

Air flow through lungs (parabronchi) is unidirectional. Parabronchial lungs of birds are

subdivided into large numbers of extremely small alveoli or air capillaries. The avian

respiratory system is partitioned heterogeneously, so the functions of ventilation and gas

exchange are separate in the air sacs and the parabronchial lung, respectively. Air sacs act

as bellows to ventilate the tube-like parabronchi.

Mammalian lungs:

Multi-chambered & usually divided into lobes about 1-6 lobes. Sometimes right lung has

more lobes than left lung and hence becomes asymmetric (e.g. rabbit, human).

Respiratory system in mammals starts with nose, leading into the pharynx and is drawn

into the larynx and then the trachea. The epiglottis is found within the larynx. This

structure prevents food and drink passing into the respiratory system. When swallowing,

the larynx is pulled up and the epiglottis flaps back to block the entrance of the larynx.

The larynx is connected with trachea of lung. The trachea contains C-shaped cartilage

rings which prevent the tube collapsing due to the change of pressure. It divides into 2

tubes with smaller diameter called bronchi. The bronchi further divide into bronchioles.

The bronchioles terminate with alveoli (100µm in diameter) which are the site of gas

exchange.



The pulmonary alveoli, thereby the lungs, just like the lungs of the Carnivora of the

mammals, may have 3-5 hundred million pulmonary alveoli.

Air flow is bidirectional: From nose, the air passes into the pharynx ↔larynx ↔trachea

↔ primary bronchi ↔ secondary bronchi ↔ tertiary bronchi ↔ bronchioles ↔ alveoli

Air exchanged via negative pressure ventilation, with pressures changing due to

contraction & relaxation of diaphragm & intercostal muscles. The greater the partial

pressure of O2 in alveolar air the more O2 will dissolves in blood (Henry's Law).

The compound evolution of the lung can illustrate this case (Fig. below).

The ancient archetype of the lung was the swimbladder in fishes (Fig A),

"The swimbladder is homologous in position and structure with the lungs

of the higher vertebrate animals". The lung of the amphibians is very

simple, being merely a pair of sacs with thin walls, such as the lungs of

salamander (Cynops orientalis) the lungs of the frog (Rana) (Fig. C) and

the toad (Bufo bufo gargarizans) (Fig. D). The evolution from the lung of

the salamander through that of the frog (Rana) up to the lung of the toad

(Bufo bufo gargarizans) can be accomplished with only a few grades of

compound. The lung of the reptiles (Fig. E) is relatively more developed.

Many small lacunae are separated out inside the lung and the gas exchange

area is thus enlarged. The cells of the reptiles further develop toward new

individuals and further undergo multi-grade compound and following

gastrulation, thereby forming the dense spongy lung of the birds with

millions of alveoli (Fig. F). The pulmonary alveoli, thereby the lungs, just

like the lungs of the Carnivora of the mammals, may have 3-5 hundred

million pulmonary alveoli.

Prepared by

Mr. S. D. Rathod.

Respiratory system of vertebrates: Notes for the TYBSc course USZ0601Sem VI of University of Mumbai

Education

Transcript of Respiratory system of vertebrates: Notes for the TYBSc course USZ0601Sem VI of University of Mumbai