The Respiratory System


Fish and some amphibians respire primarily with gills. Gills are comprised of highly vascular structures, with their outer surfaces thrown into thin folds or narrow filaments to increase the surface area for the exchange of oxygen between the water and the blood.

These gill structures develop entirely from the pharyngeal arch complex. Blood is supplied through the aortic arches that pass through each pharyngeal arch. The aortic arches branch to form the afferent branchial arteries and the capillary network in the gills. Oxygenated blood comes from the gills through efferent branchial arteries which supply the dosal aorta; the vessel which is the primay supply of oxygenated blood to the body.

Typically, pharyngeal movements bring water into the pharynx through either the mouth or a modified first pharyngeal opening called a spiracle. The mouth and/or spiracle is then closed as the pharynx contracts to push the water out through the gill openings. The gill membranes cover the surfaces of the gills and collect oxygen from the expelled water.


The typical pharyngeal arch has a surface of gill filaments on both its anterior wall and posterior wall. The gill surface on the anterior wall is called the post-trematic demibranch. The gill surface on the posterior wall of the arch is the pre-trematic demibranch. The terms pre-trematic and post-trematic are assigned according to their position in respect to the flow of water. Pre-trematic means in front of the water current, and post-trematic means posterior to the water current.


In each pharyngeal arch there is an artery, a nerve, and a skeletal member (branchial arch cartilage or bone). Extending from the cartilage or bone of the gill arch are thinner skeletal supports called gill rays, which radiate into the soft tissue of the gill septum between the demibranchs, and also short stout skeletal projections called gill rakers that form short, blunt pointed, finger-like projections at the internal gill opening.

One phylogentic trend seen in the fishes is the decrease in the body of the gill septum from agnathans and elasmobranchs to other fish. Agnathans and elasmobranchs have a gill septum which extends beyond the gill filament surfaces to the outside plate of skin which separates the gill openings. In holocephalans the gills are covered by an operculum and the septum which separates the pre-trematic from the post-trematic demibranch is shortened. In the advanced bony fish the septum is lacking. The two demibranchs extend free of any septum into the gill cavity beneath the operculum.

Some aquatic salamanders, aquatic larvaeof other amphibian species, and a few larval fish species have external gills. External gills are of two types, those that grow from internal gill surfaces and those which grow from external gill surfaces. Salamander gills grow from the external surface of the pharyngeal arch, as do those of larval polypterid and lungfishes. Those of most other fishes grow from internal surfaces and extend through the gill opening to be visible on the outside.













Overview of Respiratory System

Human Nasal Passages

Human Pharynx Regions

Human Larynx and Trachea

Human Bronchi and Lungs

Muscles of Human Respiration

Lower Respiratory Development

Fish Lungs/Air Bladders

Tetrapod Lungs

Lung Ventilation Systems

Gills of Vertebrate Animals

Things I Should Know