RESPIRATION IN WATER • Many small organisms obtain oxygen by diffusion through their body surface , without having any special respiratory organs and without circulating blood . • Larger and more complex animals often have specialized surfaces for gas exchange and also a blood system to transport oxygen more rapidly than diffusion alone can provide.
Animals without specialized respiratory organs • The simplest geometrical shape of an organism is a sphere. • A sphere has the smallest possible surface corresponding to a given volume ; any deviation from the spherical shape gives a relative enlargement of the surface area. • If we assume that a spherical organism is to be supplied with oxygen by diffusion through the surface and into every part of the body , the longest diffusion distance is from that surface to the center.
According to an equation developed by E.Newton Harvey(1928), we can calculate the necessary oxygen tension at the surface, sufficient to supply the entire organism with oxygen by diffusion. • FO2 = VO2 . r2 / 6K
FO2 = VO2 . r2 / 6K Where, • F=concentration of oxygen at surface expressed in fraction of an atmosphere(atm.). • r = the radius of the sphere (Cm). • V=the rate of oxygen consumption (Cm3 Oxygen .Cm-3 tissue. Min-1 minute.). • K = a diffusion constant of oxygen in water ( Cm2 .atm-1. Min-1.).
Fatm. = Cm3.Cm-3.Min.-1.Cm2 6(Cm2.atm-1 .Min-1). • NOW • Suppose an animal have 1 Cm. body radius , oxygen consumption of 0.001 ml(Cm3 ) oxygen/g(Cm3 )/Min. and a diffusion constant 11x 10-6 Cm2/atm./Min. (the same as connective tissue and many other animal tissues).So , the required oxygen concentration at the surface necessary to supply the entire organism to the center by diffusion is 15 atm.
FO2 = 0.001 X 1/ 6 x11x 10-6 =15 atm. • From this result ,it is clearly that such organism can not supplied by diffusion alone with such postulated metabolic rate. • Again , let us consider a small organism of a radius 1 mm(0.1 Cm.).By calculation , the required oxygen concentration at the surface is 0.15 atm.
Taking in consideration, a well-aerated water is in equilibrium with the atmosphere(aircontains 0.21 atm . oxygen),such an organism could obtain enough oxygen by diffusion only and would be quite feasible. • Therefore , according to such an calculation , one can observe the following:- - Organisms that are supplied with oxygen by diffusion only (e.g. Protozoa , flatworms)are
mostly quite small( less than 1mm.) or have very low metabolic rates(as Jellyfish). • Jellyfish , a large organism, has a very low average rate of oxygen consumption. In addition, the actively metabolizing cells are located along the surface ,where the diffusion distances are relatively short.
In conclusion , an organism that deviates from the spherical shape has larger relative surface and shorter diffusion distance than a sphere.
Animals with respiratory organs. • If the respiratory surface is turned out,forming an evagination , the resulting organ is usually called a GILL. Secondarily the , the gills may be enclosed in a cavity , such as in FISH ,but this does not change the fact that gills fundamentally are EVAGINATION.
If the general body surface is turned in , or invaginated,the resulting hollow is called a LUNG. The term lung is used whether the respiratory medium is water or air. • Insects have a special form of respiratory system . Small openings on an insect’s body surface connect to a system of tubes(tracheae) that branch and lead to all parts of the body . In general , gills mostly serve for aquatic breathing and lungs for breathing in air.
There are some exceptions. 1-Sea cucumbers have water lungs in which most of the gas exchange seems to take place. Respiration is carried on through the cloaca. Connected to the cloaca are two long-branched tubes ,respiratory trees . The muscular cloaca pumps water in and out of the tree , which serves as both a respiratory and excretory organ.
2-Gills may also be modified for use in air, but on the whole they are rather unsuited for atmospheric respiration. For example, most fish when taken out of water rapidly become asphyxiated ,although there is far more oxygen in air than in water . The reason is that in water the weight of gills is well supported, but in air the gills can not support their own weight . This is because an effective respiratory organ requires:-
1- a large surface and • 2- a thin cuticle (surface). Both these demands make it difficult to provide the mechanical rigidity for support of a gill in air. Furthermore, in air the surfaces of the fish gill tend to stick together because of surface adhesion . Therefore, the surface area exposed to water ,severely impeding oxygen uptake.
Ventilation of gills • Various mechanical devices are used to increase the flow of water over the gill's surface. There can be two means of increasing the flow of water over the gill surface: 1- By movement of gills as seen in small organisms. Some aquatic insect larvae use this method, but this is not a very efficient and practical method. The force needed to overcome resistance to movement is great and the energy needed to move the gill also increases correspondingly. The mechanical strength of the gill in most larger animals would also need to be increased.
2 -By moving water over the gill. This is achieved by ciliary action as in gills of mussels and clams. Movement of water due to a mechanical pump is a device used by fishes and crabs. The locomotion of many aquatic animals helps to circulate the water. Many pelagic fish, especially tuna, swim rapidly through the water and create a rapid flow over the gills.
So ,moving water over the respiratory surface is a much more feasible solution. The movement may be achieved by ciliaryaction, as in protozoans and in the gills of mussels and clams. Sponges move water through their ostia and the action of flagella. Moving the water with a mechanical pump like device is more common. Fish and crabs, for example, move water over their gills in this way. As a matter of principle , it is less expensive to move water slowly over a large surface than to move water fast over a smaller surface.
For some animals their own locomotion contributes to the movement of water. This is true of many pelagic fish; the large , fast-swimming tunas have practically immobile gill covers and obtain the required high water flow over the gills by swimming rapidly through the water. They probably cannot survive if kept from swimming forward , and when these fish are maintained in captivity it is common to keep them in large circular tanks so they can keep moving without meeting obstacles ( Fig.).
Note that gill surface area must be large enough to provide adequate gas exchange . Highly active fish have the largest relative gill areas . The fast swimming mackerel’s gill surface area, expressed per unit body weight, is some 50 times as high as the sluggish, bottom-living goosefish’s. For the gas exchange to be adequate , a high rate of water flow and close contact between the water and the gill are necessary. This is achieved by the anatomical structure of the gillapparatus.
Fish gills consist of several major gill arches on each side. • From each gill arch extended two rows of gill filaments(primary gill filaments). • Each filament carries densely packed , flat lamellae in rows. (secondary lamellae )Gas exchange takes place in these lamellae as water flows between them in one direction and blood within them in the opposite direction. • This mechanism, called countercurrent flow, is highly efficient in extracting oxygen from water, whose oxygen content is lower than air.
In life the gills filaments form a beautiful series of a delicate pink colour because they are very profusely supplied with blood. This supply comes from the ventral aorta and up each of the branchial arches there passes an efferent branchial artery. From these arteries a branch leads to each of the many filaments and in each filament the small afferent blood vessel divides repeatedly to supply each of the secondary lamellae.
Cartilaginous fish • Sharks and rays typically have five pairs of gill slits that open directly to the outside of the body, though some more primitive sharks have six or seven pairs. Adjacent slits are separated by a cartilaginous gill arch from which projects a long sheet-like septum, partly supported by a further piece of cartilage called the gill ray. The individual lamellae of the gills lie on either side of the septum. The base of the arch may also support gill rakers, small projecting elements that help to filter food from the water.
A smaller opening, the spiracle, lies in the back of the first gill slit. This bears a small pseudobranch that resembles a gill in structure, but only receives blood already oxygenated by the true gills. The spiracle is thought to be homologous to the ear opening in higher vertebrates.
Most sharks rely on ram ventilation, forcing water into the mouth and over the gills by rapidly swimming forward. In slow-moving or bottom-dwelling species, especially among skates and rays, the spiracle may be enlarged, and the fish breathes by sucking water through this opening, instead of through the mouth.
Water pumping. To move water over the gills ,bony(teleost) fish use a combined pumping action of the mouth and the operculum covers ,aided by suitable valves to control the flow. The system actually consists of a double set of pumps(Fig.). 1-The oral cavity , the volume of which can be enlarged by lowering the jaw and especially the floor of the mouth. 2- The opercular cavity, its volume can be increased by movements of opercular covers.
Pressure recordings made in the mouth and opercular cavities during the respiratory cycle shows that: 1- The pressure changes are synchronized with the movements of the mouth and the opercula. 2-The difference between the pressures in the mouth(oral) and in the opercular cavities remains positive almost throughout the cycle and provides the pressure that drives water through the gills
Ram Ventilation Some fish are unable to breath by water pumping device. Fish biologist known that large tunas cannot be kept alive in captivity unless they can swim continuously; this can be arranged by keeping them in large ring-shaped tanks. Where they can cruise without stopping. The fish swims with its mouth partly open , there are no visible breathing movements, and water flows continuously over the gills ;this is called ram ventilation.
Ram ventilation is not restricted to large ,fast –swimming pelagic fish. Many fish breathe pumping at low speed and change to ram ventilation at higher speeds(Fig.1.11). • The change to ram ventilation dose not mean that the gills are ventilated for free , but it means that the work of breathing is transferred from the muscle of the opercular pumps to the swimming muscle of the body and tail. The open mouth causes increased drag, and this has to paid for in increased muscular work . However , the continuous flow during ram ventilation is more economical in energy than opercular pumping at the high rats required for fast swimming.