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(2008). high rate of oxygen uptake for respiration/energy release; ), Keys to the Trematoda, Vol. What are 3 ways that exchange surfaces are adapted to their function? P_1 & V_1 & T_1 & P_2 & V_2 & T_2 \\ In some cases, the openings may be fused together, effectively forming an operculum. the large surface area of the blood capillaries in each gill filament. \end{array} Their kind of gill respiration is shared by the "fishes" because it was present in their common ancestor and lost in the other living vertebrates. P111.21atm721torr5.51atmV11.58L141mL0.879LT112.2C135K22.1CP21.54atm801torrV2152mL1.05LT232.3C38.3C, Circle the BEST answer. As , EL NORTE is a melodrama divided into three acts. There is a one way flow of water across the gills. [13], 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. In the hagfish, the pouches connect with the pharynx internally. Leaves. A cubic meter of air contains about 250 grams of oxygen at STP. (Just keep swimming) State 4 ways that gas exchange in fish is efficient. Suggest one advantage to a fish of this one-way flow of water over its gills. Heart decompensation. If you have any suggestions and queries you can contact us on the below details. This opening is hidden beneath a protective bony cover called the operculum. Summary. Why is large surface area important for gas exchange? 631-734). Leeches, Lice and Lampreys. However, recent studies on gill formation of the little skate (Leucoraja erinacea) has shown potential evidence supporting the claim that gills from all current fish species have in fact evolved from a common ancestor. The oxygen content of water is much lower compared to air, so fish have special adaptations which enable them to make the most of the available oxygen. The earthworms skin is kept moist by a slimy mucus produced by epithilial cells. Countercurrent exchange means the flow of water over the gills is in the opposite direction to the flow of blood through the capillaries in the lamellae. Even as the blood reaches the end of the lamella and is 80% or so saturated with oxygen, it is flowing past water which is at the beginning of the lamella and is 90 or 100% saturated. In R. A. Bray, D. I. Gibson & A. Jones (Eds. Anemia. Then would one team of nine horses succeed if the other team were replaced with a strong tree? So that maximum oxygen can enter the blood at the gils and maximum carbon dioxide can leave by diffusion. (2). The gills push the oxygen-poor water out through openings in the sides of the pharynx. In his writing, Alexander covers a wide range of topics, from cutting-edge medical research and technology to environmental science and space exploration. The cookies is used to store the user consent for the cookies in the category "Necessary". 1.1.10 Biochemical Tests: Sugars & Starch, 1.1.11 Finding the Concentration of Glucose, 1.3.7 The Molecular Structure of Haemoglobin, 1.3.8 The Molecular Structure of Collagen, 1.4.4 Required Practical: Measuring Enzyme Activity, 1.4.5 Maths Skill: Drawing a Graph for Enzyme Rate Experiments, 1.4.6 Maths Skill: Using a Tangent to Find Initial Rate of Reaction, 1.4.7 Limiting Factors Affecting Enzymes: Temperature, 1.4.8 Limiting Factors Affecting Enzymes: pH, 1.4.10 Limiting Factors Affecting Enzymes: Enzyme Concentration, 1.4.11 Limiting Factors Affecting Enzymes: Substrate Concentration, 1.4.12 Limiting Factors Affecting Enzymes: Inhibitors, 1.4.13 Models & Functions of Enzyme Action, 1.4.14 Practical Skill: Controlling Variables & Calculating Uncertainty, 1.5 Nucleic Acids: Structure & DNA Replication, 1.5.2 Nucleotide Structure & the Phosphodiester Bond, 1.5.6 The Origins of Research on the Genetic Code, 1.5.8 The Process of Semi-Conservative Replication, 1.5.9 Calculating the Frequency of Nucleotide Bases, 2.2.2 Microscopy & Drawing Scientific Diagrams, 2.2.6 Cell Fractionation & Ultracentrifugation, 2.2.7 Scientific Research into Cell Organelles, 2.3 Cell Division in Eukaryotic & Prokaryotic Cells, 2.3.7 Uncontrolled Cell Division & Cancer, 2.4.2 Components of Cell Surface Membranes, 2.4.8 Comparing Osmosis in Animal & Plant Cells, 2.4.13 Factors Affecting Membrane Fluidity, 2.5.5 The Role of Antigen-Presenting Cells, 2.6 Vaccines, Disease & Monoclonal Antibodies, 2.6.6 Ethical Issues with Vaccines & Monoclonal Antibodies, 3.2.3 Looking at the Gas Exchange under the Microscope, 3.2.11 Correlations & Causal Relationships - The Lungs, 3.4.7 Animal Adaptations For Their Environment, 3.5.8 Interpreting Data on the Cardiovascular System, 3.5.9 Correlations & Causal Relationships - The Heart, 3.5.10 Required Practical: Dissecting Mass Transport Systems, 4.2.6 Nucleic Acid & Amino Acid Sequence Comparison, 4.3 Genetic Diversity: Mutations & Meiosis, 4.3.5 Meiosis: Sources of Genetic Variation, 4.3.7 The Outcomes & Processes of Mitosis & Meiosis, 4.4.2 Maths Skill: Using Logarithms When Investigating Bacteria, 4.4.4 Directional & Stabilising Selection, 4.6.7 Quantitative Investigations of Variation, 4.6.9 Genetic Relationships Between Organisms, 5. In addition, loss of elastic tissue from the walls of the destroyed alveoli causes the lungs to expand within the chest cage. The oxygen content of water is much lower compared to air, so fish have special adaptations which enable them to make the most of the available oxygen. In this video, Head of Biology Mr May carries out a fish head dissection and explains how the gills of a fish are adapted for efficient gas exchange. 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. The structure of the gills (filaments, lamellae) as well as the counter current mechanism in fish for maintaining a large concentration gradient. The respiratory system. per mm of gill length (1). Also covered by the video includes how the fish draws water into its mouth and over its gills. The arrangement of water flowing past the gills in the opposite direction to the blood (called countercurrent flow) means that they can extract oxygen at 3 times the rate a human can. [7], The gill arches of bony fish typically have no septum, so that the gills alone project from the arch, supported by individual gill rays. [8] Water is 777 times more dense than air and is 100 times more viscous. Fish gills have similar adaptations to the alveoli in lungs Suggest how fish gills are adapted for gas exchange DO NOT WRITE IN TUS ARBA DO NOT WRITE IN THE AREA WRITE THIS AREA 17 (b) A person keeps small fish in a glass container called an aquarium The diagram shows an aquarium. Some amphibians retain the external larval gills in adulthood, the complex internal gill system as seen in fish apparently being irrevocably lost very early in the evolution of tetrapods. Gills in fish Exchange of gases in fish is very efficient because of: the large surface area of the gills. In some fish, capillary blood flows in the . What are the 4 main functions of the lungs? They also contain elastic fibres which expand to allow air in and recoil to help force out air. Out of these, the cookies that are categorized as necessary are stored on your browser as they are essential for the working of basic functionalities of the website. The alveoli are where the lungs and the blood exchange oxygen and carbon dioxide during the process of breathing in and breathing out. The cookie is set by the GDPR Cookie Consent plugin and is used to store whether or not user has consented to the use of cookies. On this Wikipedia the language links are at the top of the page across from the article title. The effect of this is that the blood flowing in the capillaries always encounters water with a higher oxygen concentration, allowing diffusion to occur all the way along the lamellae. Lra has a particular interest in the area of infectious disease and epidemiology, and enjoys creating original educational materials that develop confidence and facilitate learning. Mudskippers breathe by absorbing oxygen across the skin (similar to frogs). Seawater contains more osmolytes than the fish's internal fluids, so marine fishes naturally lose water through their gills via osmosis. Explain 2 ways in which the structure of fish gills is adapted for efficient gas exchange. The rows of gill filaments have many protrusions called gill lamellae. This website uses cookies to improve your experience while you navigate through the website. Things World-Class Poker Players Can Teach Business Managers. Do not penalise for confusion between two 2 Increases diffusion/makes diffusion efficient; At the most extreme, some air-breathing fish are able to survive in damp burrows for weeks without water, entering a state of aestivation (summertime hibernation) until water returns. Because of this reason large amount of energy is required to move the gill. Decreased PO2 levels are associated with: Decreased oxygen levels in the inhaled air. A gill is a respiratory organ found in many aquatic organisms that extracts dissolved oxygen from water and excretes carbon dioxide. [3], Air breathing fish can be divided into obligate air breathers and facultative air breathers. Most fish exchange gases like oxygen and carbon dioxide using gills that are protected under gill covers (operculum) on both sides of the pharynx (throat). Lungs are organs that are adapted for breathing air, and they are not found in fish or other aquatic animals. 1. One-to-one online tuition can be a great way to brush up on your Biology knowledge. \hline 11.21 \mathrm{~atm} & 1.58 \mathrm{~L} & 12.2{ }^{\circ} \mathrm{C} & 1.54 \mathrm{~atm} &- & 32.3{ }^{\circ} \mathrm{C} \\ [7], Chimaeras differ from other cartilagenous fish, having lost both the spiracle and the fifth gill slit. It is spherical in shape and has a diameter of 20m. a Fantasy the efficient ventilation of the gills with water - there is a counter current flow of water and blood The moving blood and ventilated gill surfaces mean that gases exchanged are continually. The water that passes over the gill lamellae flows in the opposite direction to the blood within the gill lamellae. Instead, the gills are contained in spherical pouches, with a circular opening to the outside. 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. This bears a small pseudobranch that resembles a gill in structure, but only receives blood already oxygenated by the true gills. Lra graduated from Oxford University in Biological Sciences and has now been a science tutor working in the UK for several years. Each gill consists of many fine gill lamellae, supported by a bony gill bar. 8 study hacks, 3 revision templates, 6 revision techniques, 10 exam and self-care tips. [8] The use of sac-like lungs to remove oxygen from water would not be efficient enough to sustain life. The large muscles of the body actually do most of the work, but the fins help with balance and turning. describe the structure of the components of the fish gas exchange system, including the filaments and lamellae, describe how fish gills are adapted for efficient gas exchange, describe the countercurrent flow of blood and oxygenated water in fish gills, explain the advantages of the countercurrent flow for efficient gas exchange. Image showing the structure of fish gills and the counter-current system within gills. (2). 3. In some species cutaneous respiration accounts for 5 to 40 percent of the total respiration, depending on temperature. [7], A smaller opening, the spiracle, lies in the back of the first gill slit. These further increase the surface area, and because they are thin, ensure that the diffusion distance between the blood, in the lamellae, and the water is small. Advertisement cookies are used to provide visitors with relevant ads and marketing campaigns. exchange/diffusion across body surface/skin; This extra space on the left leaves room for your heart. Then the blood moves through the fish's body to . Gills in fish Exchange of gases in fish is very efficient because of: the large surface area of the gills. Write short notes on Educational Excursion. Why must gaseous exchange structures hvave all these requirements? Fish exchange gases by pulling oxygen-rich water through their mouths and pumping it over their gills. The difference in pressure across membranes. (assume the number of moles of gas to be constant): P1V1T1P2V2T211.21atm1.58L12.2C1.54atm32.3C721torr141mL135K801torr152mL5.51atm0.879L22.1C1.05L38.3C\begin{array}{ccccccc} 4. Also co. As a result the gills can extract over 80% of the oxygen available in the water. The gills are a fish's gas exchange system. The blood vessels are in very close proximity to the lamellae, allowing a short diffusion pathway. Do NOT follow this link or you will be banned from the site! The flattened shape of structures such as leaves. One of the ways in which gas exchange is carried out efficiently is by the countercurrent flow principle. We will be very happy to hear from you. The concentration of dissolved oxygen in water is higher than than the blood of the fish. The tips meet to form a sieve like arrangement for flow of water. Oxygen and carbon dioxide are exchanged in a process of simple diffusion; (passive movement from high to low concentration) The air in the alveoli contains a high concentration of oxygen. Abstract. Unlimited number of exercises. Always. The skin of anguillid eels may absorb oxygen directly. Explain the Mechanism of Stomatal Opening. The operculum has been lifted to show the arch. [7] Fish gill slits may be the evolutionary ancestors of the tonsils, thymus gland, and Eustachian tubes, as well as many other structures derived from the embryonic branchial pouches. Which part is directly involved in gas exchange in plant? "[8], Higher vertebrates do not develop gills, the gill arches form during fetal development, and lay the basis of essential structures such as jaws, the thyroid gland, the larynx, the columella (corresponding to the stapes in mammals) and in mammals the malleus and incus. The small round alveoli allow for an amazingly large surface area for this gas exchange to take place. The lamellae contain blood capillaries, which have blood flowing in the opposite direction to the water. Dordrecht: Springer. Both the gill filaments and lamellae provide a large surface area for gaseous exchange, increasing the efficiency of diffusion .The lamellae have many blood capillaries and a thin surface layer of cells (or epithelium), this means there is a short diffusion distance .The blood flows through the lamellae in one direction and the water flows over The first, titled Arturo Xuncax, is set in an Indian village in Guatemala. The diagram shows how the gill filaments and lamellae (also called gill plates) create the large surface area. Gills are branching organs located on the side of fish heads that have many, many small blood vessels called capillaries. These adaptations are gills. Thickness of A fish uses its gills to absorb oxygen from water. You also have the option to opt-out of these cookies. Because of poor elastic recoil, such patients have high lung compliance. All rights reserved. Protects your airways from harmful substances and irritants. Fish possess several gills located between their mouth cavity (buccal cavity). Toadfish live on the seabed in deep water. After completing his doctoral studies, he decided to start "ScienceOxygen" as a way to share his passion for science with others and to provide an accessible and engaging resource for those interested in learning about the latest scientific discoveries. However, if a fish swims forward with its mouth open, water will flow across the gills without active pumping by the muscles surrounding the buccal and opercular cavities. Ventilation is the fast movement of a fluid such that water through a medium like gills. These cookies ensure basic functionalities and security features of the website, anonymously. When the mouth closes, the gill cover opens and this forces the water into the gill chamber. Therefore, freshwater fishes must utilize their gill ionocytes to attain ions from their environment to maintain optimal blood osmolarity. Fish gills have many lamellae so larger surface area. These adaptations aregills. How do gills help gas exchange? In six adult human lungs, the mean alveolar number was 480 million (range: 274-790 million; coefficient of variation: 37%). The structures that aquatic animals use for gaseous exchange, absorbing oxygen (for respiration) from the water, excreting carbon dioxide (from respiration) into the water Gaseous exchange structures need to: have a large surface area maintain a steep concentration gradient have a copious blood supply Fish are aquatic animals that breathe using gills, which are specialized organs that are adapted for extracting oxygen from water. [1][2] Each filament contains a capillary network that provides a large surface area for exchanging oxygen and carbon dioxide. The table shows some features of the gills of these fish. [6] When a fish breathes, it draws in a mouthful of water at regular intervals. 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. Ram ventilation is efficient because the fish does not need to use the muscles around its buccal and opercular cavities to move water through the gills. What Wonders Can You Achieve With ChatGPT? What happens when alveoli lose their elasticity? As water moves over the surface of the gills, oxygen is absorbed - like lungs in land creatures. The breathing cycle is important for maintaining the pressure differential across the gills. Yes! 3 Tips for Beginner Players. What is rotational grazing, and how does it mimic natural processes? However, bony fish have a single gill opening on each side. Fish extract dissolved oxygen molecules from the surrounding water. This is a counter current flow. Gills are simply layers of tissue adapted specifically to gas exchange. Image showing the structure of a leaf from a dicotyledonous plant. This cookie is set by GDPR Cookie Consent plugin. The bony fish have three pairs of arches, cartilaginous fish have five to seven pairs, while the primitive jawless fish have seven. Alveolar number was closely related to total lung volume, with larger lungs having considerably more alveoli. The folds are kept supported and moist by the water that is continually pumped through the mouth and over the gills. the short distance required for diffusion - the outer layer of the gill filaments and the capillary walls are just one cell thick. Image showing the structure of the tracheal system of an insect. (2002): Gaining ground: the origin and evolution of tetrapods. These cookies will be stored in your browser only with your consent. Each filament is covered in lamellae. A fish had gills, made up of gill filaments containing lamellae, which provide a very large surface area for oxygen to diffuse out of the water as it moves over them. less energy needed / continuous flow of water or O2; The graph shows t he relationship between gill surface area and body mass for three species of fish. This cookie is set by GDPR Cookie Consent plugin. Part of two adjoining gill arches with their filaments. Part I-systematics, middle ear evolution, and jaw suspension. [7] The spiracle is thought to be homologous to the ear opening in higher vertebrates. How do fish gills achieve these requirements? Explain why this arrangement is important for efficient oxygen uptake. The base of the arch may also support gill rakers, small projecting elements that help to filter food from the water. . the short distance required for diffusion - the outer layer of the gill filaments and the capillary walls are just one cell thick. 8 study hacks, 3 revision templates, 6 revision techniques, 10 exam and self-care tips. The upper row is the oxygen concentration in the blood, while the lower is the one in the water. Fish exchange gases by pulling oxygen-rich water through their mouths and pumping it over their gills. Table of Contents show Learn how leaves and fish gills are adapted for efficient gas exchange with evulpo. Rich blood supply of lamellae. It ensures the maximum exchange possible occurs. [8] Oxygen has a diffusion rate in air 10,000 times greater than in water. Unlimited power Get the whole package! What happens to oxygen during gas exchange? A woman believes she is married to a rock singer. There are blood vessels running through a structure called the gill arch which deliver and remove blood. Born and raised in the city of London, Alexander Johnson studied biology and chemistry in college and went on to earn a PhD in biochemistry. Wittenberg is a nationally ranked liberal arts institution with a particular strength in the sciences. The fish opens its mouth to let water in, then closes its mouth and forces the water through the gills and out through the operculum (gill cover). [9][10], In bony fish, the gills lie in a branchial chamber covered by a bony operculum (branchia is an Ancient Greek word for gills). The gills (found under the gill covers) allow the fish to breath. In adult lampreys, a separate respiratory tube develops beneath the pharynx proper, separating food and water from respiration by closing a valve at its anterior end. [7], Gills usually consist of thin filaments of tissue, branches, or slender tufted processes that have a highly folded surface to increase surface area. This mucus also helps to trap and dissolve oxygen from the air. However, the fish needs to be swimming, which is energetically costly, and its body position with the mouth open may increase drag on the fish and increase the cost of locomotion. "Modifications of the Digestive Tract for Holding Air in Loricariid and Scoloplacid Catfishes", "Vestiges of the natural history of development: Historical holdovers reveal the dynamic interaction between ontogeny and phylogeny", "Cutaneous gas exchange in vertebrates: design, patterns, control and implications", "Spatial and temporal variations of the ectoparasites of seven reef fish species from Lizard Island and Heron Island, Australia", Tradeoffs for locomotion in air and water, https://en.wikipedia.org/w/index.php?title=Fish_gill&oldid=1138191810, Creative Commons Attribution-ShareAlike License 3.0, This page was last edited on 8 February 2023, at 13:53. Gas exchange in fish occurs in their gills which is supported by a bony arch. This device is used by fishes and crabs with the help of their locomotary organs for circulating the water. Repeat part (a) for a sodium ion (Na+)\left(\mathrm{Na}^{+}\right)(Na+) and a chloride ion (Cl).\left(\mathrm{Cl}^{-}\right).(Cl). Just a click away. Gills of fishes consist of several gill arches on either side. The high surface area is crucial to the gas exchange of aquatic organisms as water contains only a small fraction of the dissolved oxygen that air does. Some species retain gill rakers. The cookie is used to store the user consent for the cookies in the category "Other. The gill filaments have many protrusions called gill lamellae. These filaments have many functions including the transfer of ions and water, as well as the exchange of oxygen, carbon dioxide, acids and ammonia. Removes waste gases, including carbon dioxide, from the body when you exhale. Squamous epithelium of alveolar wall, endothelium of blood capillaries in alveoli and basement substance are the three layers forming diffusion surface or membrane. Use evidence from the table to explain how mackerel are able to swim faster than toadfish. Explain how a fish is adapted for gas exchange? lamellae / m Number of lamellae How are earthworms adapted for gas exchange? How do gills promote rapid gas exchange by ventilation mechanism (breathing)? There is therefore a very large concentration gradient and oxygen diffuses out of the water and into the blood. Printable summaries. He also shares personal stories and insights from his own journey as a scientist and researcher. Fish gills are the preferred habitat of many ectoparasites (parasites attached to the gill but living out of it); the most commons are monogeneans and certain groups of parasitic copepods, which can be extremely numerous. This is called a Organisms Respond to Changes in their Environments (A Level only), 6.1.9 Investigating Touch and Temperature Receptors, 6.1.12 Investigating Variables that Affect Heart Rate, 6.2.6 Maths Skill: Calculating Maximum Impulse Frequency, 6.2.8 Transmission Across a Cholinergic Synapse, 6.3.3 Examining Skeletal Muscle Under a Microscope, 6.4.6 Control of Blood Glucose Concentration, 6.4.8 Calculating the Concentration of Glucose in Urine, 7. This allows for more efficient gas exchange than if the water had to go in and out the same way. How do fins and gills help a fish? The gills are carried right behind the head, bordering the posterior margins of a series of openings from the esophagus to the exterior. Alveoli in the lungs, fish gills and the villi in the small intestine are adapted so that they have short diffusion distance, big concentration difference and large surface area. We also use third-party cookies that help us analyze and understand how you use this website. The adolescent protagonists of the sequence, Enrique and Rosa, are Arturos son and , The payout that goes with the Nobel Prize is worth $1.2 million, and its often split two or three ways. Fish do not have lungs like terrestrial animals do. As the blood flows in the opposite direction to the water, it always flows next to water that has given up less of its oxygen. This means that fresh water, high in oxygen is always [passing the gills and there is a steep diffusion gradient for oxygen between water and the blood, Michelle Provost-Craig, Susan J. Fish breathing Adaptations for Gas Exchange Mouth & Opercula Alternate opening of the mouth and two flaps of skin that cover the gills called the opercula (singular: operculum) helps to force water across the gill surface = ventilate the gas exchange surface. The main function of gas exchange is to bring oxygen into the tissues and expel carbon dioxide. The operculum can be important in adjusting the pressure of water inside of the pharynx to allow proper ventilation of the gills, so that bony fish do not have to rely on ram ventilation (and hence near constant motion) to breathe. Air-breathing fish have skin, lungs, or air bladders that enable gas exchange with surface air in addition to gills. The vertebrate ancestor no doubt had more arches, as some of their chordate relatives have more than 50 pairs of gills. They control water loss and gas exchange by opening and closing. A chamber at the sides of their mouth called the opercular cavity. This movement is aided by ciliary action as in gills of mussels and clams. This massively increases the fish's ability to absorb oxygen from the water as a diffusion gradient is always maintained. The exchange of oxygen and dioxide occurs between the alveoli and the capillaries in the lungs. This is, however, often greatly reduced, consisting of a small mass of cells without any remaining gill-like structure.[7]. The concentration of oxygen in water is lower than air and it diffuses more slowly. You need to ask yourself questions and then do problems to answer those questions. [11] Conversely, freshwater has less osmolytes than the fish's internal fluids. Both the gill filaments and lamellae provide a large surface area for gaseous exchange, increasing the efficiency of diffusion.The lamellae have many blood capillaries and a thin surface layer of cells (or epithelium), this means there is a short diffusion distance.The blood flows through the lamellae in one direction and the water flows over in the opposite direction. (2), What causes the pressure difference to fall below zero? The volume of the opercular cavity can also be changed by the movements of opercular flaps that swing out to enlarge the cavity and swing in to reduce it. Other uncategorized cookies are those that are being analyzed and have not been classified into a category as yet. This means the water flows through the gills in one direction, allowing for more efficient gas exchange than if the water had to go in and out the same way. Key points Gill filaments have lamellae which increase the surface area available for diffusion, while keeping the diffusion pathway short. A natural history of skin and gill parasites of fishes. All basal vertebrates breathe with gills. This jet propulsion also provides the locomotion. Breathing air is primarily of use to fish that inhabit shallow, seasonally variable waters where the water's oxygen concentration may seasonally decline. By using the pumping action of mouth and opercular cover, the aquatic animals move water over the gills. Therefore, even when the blood is highly saturated, having flowed past most of the length of the lamellae, there is still a concentration gradient and it can continue to absorb oxygen from the water. Removal of carbon dioxide also occurs, as the blood containing high concentrations of the waste gas goes to the gills, and the carbon dioxide diffuses out into the water down a diffusion gradient (external water has lower concentrations of carbon dioxide than levels in the blood, so this sets up a diffusion gradient.). In the ventilation cycle of a fish, water enters the mouth cavity and then passes through the gills into the opercular cavity.