Lecture 19Gas exchange and circulation in fishes

Open and closed circulatory systems: fish and insect

Gas exchange: acquiring oxygen and getting rid of carbon dioxideSmaller animals can exchange these gases at the body surface by diffusion and

convection because of a favourable surface to volume ratio.For larger multicellular animals there are special regions that serve for gas

exchange: gills or lungs.Particulate feeders (e.g., amphioxus, sabellid worms) already expose a large surface area to oxygen-rich water; for gas exchange they need only deploy the

circulatory system: capillaries, networks.

Distinction between lung and a gill

• Lung: any specially adapted respiratory chamber whose walls are highly vascularized and form the primary region for exchange of respiratory gases (uptake of oxygen, release of carbon dioxide). A collection of invaginations providing a large surface area for gas exchange with a circulatory system fluid (blood).

• Gill: the same but ‘outpocketed’, an outgrowth of the body wall. The parapodia of Nereis are gills as well as paddles. And they are serviced by efferent and afferent vessels. Efferent: away from parapodium; afferent: toward parapodium. Fish a recessed outpocketing.

There must for larger animals, be a close association with the circulatory system: fine capillaries forming a plexus or network --- increasing the surface area to allow for pickup of oxygen from water (or air) and for the loss of carbon dioxide.

Tracheal gills of a dragonfly nymph

Structural designs for gas exchange

1. Keep diffusion paths short.

2. Circulate the fluid (more important if the fluid is water than air because oxygen has a relatively low solubility in water compared to air) and maintain the steepest possible diffusion gradient

3. Maximize surface area (trade-off with dessication: water loss: important in terrestrial animals)

4. Utilize counter-current flow of the external medium that is the oxygen /carbon dioxide source and the internal medium that is the oxygen/carbon dioxide transport.

Gill ventilation in fishes

Oxford IllustratedScience Encyclopedia

The gills of bony fishes are located in the pharyngeal cavity in a branchial

chamber, covered on the outside by the operculum. Inside the branchial chamber thin epidermis is folded into plates called

lamellae grouped on filaments. The filaments are arranged along a gill arch

(four arches on each side).

Unidirectional flow of water in fishes is generated by a buccal force pump combined with an opercular suction pump. These two pumping actions operate out of phase, to achieve a near continuous flow of water. (Like the phase differences in the cycles of the three muscular fans of Chaetopterus that also improve flow efficiency within its burrow.) The water enters the fish’s mouth, passes through the buccal cavity, then is forced out through gill slits in the gut wall into a branchial cavity containing gills. The branchial cavity is covered with an operculum and the water exits posteriorly between the edges of the operculum and the body. This unidirectional flow of water over the gills is maintained by both types of pump.

There are two ways of passing water by blood: same direction parallel current or in opposite directions; the latter is counter-current . It is far more efficient to use counter-current because diffusion gradients are maintained. Parallel flows will very soon reach saturation and diffusion will stop. The same principle applies to heat exchange.

Circulation systems• Circulatory systems are closed or open.• An example of a closed circulatory system is that of a fish: a two-chambered heart

creates high pressure and relatively high flow rate within vessels, arteries and veins, through which blood circulates.

• An example of an open circulatory system is that of an insect: a muscular dorsally situated tube (heart) pumps fluid anteriorly and sits in the blood it pumps which circulates at low pressure within large body spaces.

• Open systems function to transport gases. In the case of the insect gas exchange is accomplished by a tracheal system.

Highly muscular regions form chambers: hearts