COMPARATIVE HISTOLOGY OF RESPIRATION

FISH - Gill Respiration predominates (aquatic respiration)
- Teleosts may also use (air breathing):

1) Swim Bladder - mainly used as a hydrostatic organ, assumes an accessory
respiratory function in a few fish
2) Highly vascularized anterior stomach or oral cavity
3) Vascularized skin

- Primitive Actinopterygians (ray-finned fish) and most Sarcopterygians (lobe-finned
fish) possess lungs in addition to gills for respiration.
SEE HANDOUT FOR REVIEW OF RESPIRATORY STRUCTURES IN FISH

- In vertebrate embryos, there are 5 branchial arches; in fishes 4 are involved in support
of the gills (#1 ---> jaws)

TELEOST GILL ARCH STRUCTURE
- Blood flow proceeds from Afferent Artery ---> Capillary Net (in Gill Lamella) --->
Efferent Artery (Vein)
- Exchange of gases occurs in capillary network in gill lamellae; water and blood flow in
opposite directions over lamellae = Countercurrent Exchange

SEE HANDOUT FOR REVIEW OF COUNTERCURRENT EXCHANGE

- Rakers = cartilaginous or bony structures attached to gill arch and directed
anteriorly; act like shutters to control flow of water over gills; several muscles are
associated with gill arch - regulate flow over gills by controlling position of rakers

1) Resting Condition = adductor muscles pull individual filaments together so
separate filaments stay separated
2) Active Condition = abductor muscles pull individual filaments apart so separate
filaments come together, water passes over gills more slowly so that oxygen
extraction efficiency increases

- Simple squamous epithelium lines gill lamellae. Diffusion barrier consists of 2 thin
simple squamous epithelial layers plus a shared basal lamina between them (same
situation as in lung).


AMPHIBIANS AND REPTILES - Lungs predominate
- Respiratory system similar to mammalian system. Lungs with bidirectional flow.
Differences include a lesser number of tube divisions and much less surface area
available for gas exchange. They can get away with less surface area since less
oxygen is required for metabolism than in endothermic organisms.

Cutaneous Respiration in Amphibians
- Requires moist skin, thus amphibians are restricted to environments with relatively
high humidities
- Epidermis consists of a thin stratified squamous. The underlying dermis is highly
vascularized.
- Diffusion is slower than in lung due to thicker diffusion barrier
- Most amphibians utilize cutaneous respiration to some degree. Usually more
important for getting rid of CO2 than for loading oxygen. However, it is important for
both in some groups, particularly the lungless salamanders, which use cutaneous
respiration for all their gas exchange.

External Gills in Aquatic and Larval Salamanders
- External Gills are essentially modifications of integument.
- 2 cell/basal lamina complex is diffusion barrier (as in lungs and internal gills)


FOR CIRCULATION IN EXTERNAL GILLS SEE HANDOUT

BIRD LUNGS
- Respiratory System of birds contains a complicated Air Sac System, Lungs are
spongy, rather than sac-like.
- Air sac system allows unidirectional flow of air through the lungs; gas exchange takes
place in air capillaries that open to a tube (parabronchus)
through which air flows in one direction.
- Blood flow in the bird lung is via a Crosscurrent Mechanism relative to air
flow. Functions similar to a countercurrent mechanism to provide more effective
oxygenation than with bidirectional flow. This makes birds more effective at extracting
oxygen from air and explains their success at high altitudes.





To Lecture 19