An Introduction to the Invertebrates, Second Edition - tiera.ru

TRANSPORT SYSTEMS113Oxygen dissociation curvesGraphs showing oxygen dissociation (see diagram) express the relationshipbetween the amount of oxygen available and the amounttaken up by the blood.They are informative about the haemoglobins in different animals or in differentcircumstances. The characteristic sigmoid curve may be shifted either to theleft, when oxygen can more easily be taken up, or to the right, when oxygen ismore easilygiven off inthe tissues.The‘Bohr shift’exemplifies these two positionsof the curve within most animals: in diagram (a) curve A represents conditionsat the respiratory surface and the shift to the right (to curve B) is caused by thehigher CO 2 concentration (causing greater acidity) at the tissues, enabling moreoxygen to be given off from the haemoglobin molecule. Animals living in placeswith low oxygen tensions, where ease of oxygen uptake is paramount, havecurves well to the left see diagram (b), curve C, for the polychaete Arenicolain its burrow.Other factors affecting the balance of advantage between ease ofuptake and ease of release of oxygen are temperature (see curves D and E forthe earthworm Lumbricus) and high altitude, where there is less total oxygendue to low atmospheric pressure and the curve moves to the left.Countercurrent systemsWhere the direction of flow is opposite in closely apposed channels,countercurrent systems increase the uptake of oxygen at respiratory surfaces.With countercurrent flow there is a concentration gradient of oxygen (or anyother substance, or heat) along the whole surface in contact, allowing verymuch more diffusion than can occur in parallel (‘co-current’) flow, where thereis a high initial gradient progressively decreasing see diagrams (c) and (d).(c) Co-current and(d) countercurrent flow of waterand blood at a gill, showinggradients of oxygen content asthe water passes along the gill.The transport systems of different invertebratesNo transport systemNo transport system occurs in sponges or in most cnidarians, where everycell is close to the ambient medium. None is present in platyhelminths, acoelomorphs,gnathostomula, gastrotrichs, nematodes (where fluid under high pressureis moved in the body cavity), nematomorphs, rotifers, acanthocephala,kinorhynchs, priapulids (though there are cells containing haemerythrin in thebody cavity), entoprocts, sipuncula, bryozoa or chaetognaths.