Question

1. Why should a lungfish be able to meet nearly all its carbon dioxide exchange through the skin while it must use its lungs to handle oxygen exchange?

2. Give an example of (a) a peristaltic heart, (b) a chamber heart, (c) an open circulatory system, and (d) a closed circulatory system.

Answer 1

1) Fish gills are organs that allow fish to breathe underwater. Most fish exchange gases like oxygen and carbon dioxide using gills that are protected under gill covers on both sides of the pharynx (throat). Gills are tissues that are like short threads, protein structures called filaments. These filaments have many functions including the transfer of ions and water, as well as the exchange of oxygen, carbon dioxide, acids and ammonia.] Each filament contains a capillary network that provides a large surface area for exchanging oxygen and carbon dioxide.

Fish exchange gases by pulling oxygen-rich water through their mouths and pumping it over their gills. In some fish, capillary blood flows in the opposite direction to the water, causing counter-current exchange. The gills push the oxygen-poor water out through openings in the sides of the pharynx. Some fish, like sharks and lampreys, possess multiple gill openings. However, bony fish have a single gill opening on each side. This opening is hidden beneath a protective bony cover called the operculum.

Juvenile bichirs have external gills, a very primitive feature that they share with larval amphibians.

Previously, the evolution of gills was thought to have occurred through two diverging lines: gills formed from the endoderm, as seen in jawless fish species, or those form by the ectoderm, as seen in jawed fish. However, recent studies on gill formation of the little skate has shown potential evidence supporting the claim that gills from all current fish species have in fact evolved from a common ancestor.

Breathing with gills Edit

Tuna gills inside the head. The head is oriented snout-down with the view looking towards the mouth.

The red gills detached from the tuna head on the left

Air breathing fish can be divided into obligate air breathers and facultative air breathers. Obligate air breathers, such as the African lungfish, are obligated to breathe air periodically or they suffocate. Facultative air breathers, such as the catfish Hypostomus plecostomus, only breathe air if they need to and can otherwise rely on their gills for oxygen. Most air breathing fish are facultative air breathers that avoid the energetic cost of rising to the surface and the fitness cost of exposure to surface predators.

All basal vertebrates breathe with gills. The gills are carried right behind the head, bordering the posterior margins of a series of openings from the esophagus to the exterior. Each gill is supported by a cartilaginous or bony gill arch.] The gills of vertebrates typically develop in the walls of the pharynx, along a series of gill slits opening to the exterior. Most species employ a counter-current exchange system to enhance the diffusion of substances in and out of the gill, with blood and water flowing in opposite directions to each other

# The classic view of an open circulatory system is based on the image of pseudocoelomic or coelomic fluid bathing the tissues directly; this fluid is circulated throughout the coelom via the actions of the body wall musculature and animal movements. A second and somewhat more robust image of an open system is that of a dorsally located muscular vessel or heart sitting within a hemocoel, pumping hemolymph through anterior and/or posterior aortic vessels. These vessels end abruptly where their contents move into the coelom or other large space where gas, nutrient, and waste exchange take place directly between the cells (tissues) and hemolymph (or lymph—at this point the fluid could be described as extracellular fluid). Hemolymph then moves through venous sinuses or simply through the coelom and into a pericardial sinus, through cardiac ostia and into the heart for recirculation. Indeed both of these views are technically correct, yet convey the idea of a primitive, poorly designed and regulated cardiovascular system that is unable to sustain higher metabolic demands.

Schematics of: (a) a classically defined “open” circulatory system (as seen in may lower invertebrates), (b) a circulatory system that is highly complex with capillary like vessels, a partially lined vasculature yet contains vascular sinuses which classically has been defined as “open” yet should be categorized as an “incompletely closed” circulatory system. (c) A classically defined “closed” circulatory system (as seen in mammals and other higher vertebrates)

Looking at the issue from the other side, our standard view of a closed circulatory system is based on a system where a multichambered muscular heart pumps blood through parallel systemic and pulmonary circuits simultaneously Blood is pumped into major elastic arteries (the aorta and large arteries), which then flows into medium and small smooth muscle-based vessels and then into arterioles, which supply the capillary circulation. At the capillary level, gas, nutrient, and waste exchange take place between blood and tissues across an endothelial layer. Venous blood then returns to the heart via, venules, small and medium veins, and finally back into the heart via the vena cava. In the closed circulatory system at no point does the blood leave the confines of the vascular endothelia and as such there is a clear distinction between blood and lymph

While the descriptions above do represent accurate depictions of the circulatory systems of worm-like invertebrates and mammals, respectively, they do not provide the necessary depth and breadth of information required to understand the subtle yet significant “shades of grey” of the continuum from the invertebrate “open” and vertebrate “closed” circulatory architecture An exhaustive phylogenetic review of cardiovascular morphologies is not necessary to make this point clear. A few well-described examples from specific taxa can be used to illustrate the complexity of the issue and dramatically point out the shortcomings of the existing definitions.

T## he Typical Invertebrate “Open” Circulatory System: The Annelid Blood-Vascular System

Members of the phyla Annelida contain some of the most complex examples of worm-like invertebrates . The segmented annelids have evolved several mechanisms in order to enhance convective transport between internal compartments. The most primitive of these being the development of a coelom and coelomic circulation followed by the development of intracellular iron-based oxygen binding pigments , and the most advanced being a fairly well-developed blood-vascular systemIn the smaller annelids there are few cardio-respiratory adaptations, however; in the larger and/or more active worms, such as the polycheates, a complex vasculature has evolved and in the more active giant Australian earthworm (Oligocheata) a defined heart augments the movement of blood through a well-developed vasculature .