Why do fish have gills? Fish have gills because water contains dissolved oxygen but far less oxygen per unit volume than air, so an aquatic animal needs a large, thin, highly supplied exchange surface that can extract oxygen efficiently while releasing carbon dioxide. Gills solve that problem by exposing blood-filled filaments and lamellae to a continuous flow of water, allowing gases to diffuse across very short distances.
People searching for why fish have gills, how fish breathe underwater, how gills extract oxygen, what countercurrent exchange is, why fish suffocate out of water, why fish move their mouths and gill covers and why warm water can stress fish are asking about diffusion, anatomy, blood flow, water chemistry and respiration at the same time. A gill is not simply an underwater lung; it is a specialised gas-exchange organ built for a very different physical environment.
For students, fish gills are a strong systems example. Water movement, gill surface area, blood flow, haemoglobin, oxygen concentration, temperature and metabolism all interact. The gill works only because structure, fluid movement and circulation are matched: water brings oxygen to the surface, blood carries it away, and countercurrent flow helps maintain the diffusion gradient along much of the exchange pathway.
The Short Answer: Gills Extract Dissolved Oxygen
Fish gills expose a large surface area of thin tissue to water. Dissolved oxygen moves from water, where its concentration or partial pressure is higher, into blood where it is lower. Carbon dioxide diffuses in the opposite direction.
The exchange is passive at the gas-transfer surface, but the fish actively moves water and blood to keep fresh gradients available.
Why Water Breathing Is Difficult
Water contains much less oxygen than air and is far denser and more viscous. An aquatic animal must move a large mass of water across its respiratory surface to obtain enough oxygen.
Gills minimise this cost by creating a broad, delicate interface where oxygen can be removed efficiently from each volume of water.
Why Gills Have So Much Surface Area
Each gill contains arches supporting many filaments, and each filament carries numerous thin secondary lamellae. The repeated branching creates enormous surface area within a compact space.
Large area matters because diffusion rate increases when more membrane is available for gas exchange.
Why Gill Lamellae Are Thin
Oxygen must cross water, epithelial tissue and capillary walls before reaching blood. The shorter this diffusion distance, the faster gas transfer can occur.
Thin lamellae therefore bring water and blood extremely close while keeping the fluids physically separate.
Why Gills Need Capillaries
Capillaries carry deoxygenated blood close to the exchange surface and transport newly oxygenated blood away. Continuous circulation prevents oxygen from accumulating locally and weakening the gradient.
Blood flow is therefore as important as water flow; a stationary gill full of stagnant blood would soon become inefficient.
Why Countercurrent Exchange Matters
In many fish, blood flows through gill lamellae in the opposite direction from the water. This countercurrent arrangement preserves an oxygen gradient across nearly the entire length of the exchange surface.
Water with progressively less oxygen always encounters blood with even less oxygen, allowing diffusion to continue rather than reaching equilibrium early.
Why Parallel Flow Would Be Less Efficient
If water and blood flowed in the same direction, their oxygen levels would move toward equilibrium relatively quickly. Once the two streams became similar, little additional oxygen could diffuse.
Countercurrent flow avoids that early plateau and lets fish extract a larger fraction of the oxygen from passing water.
Why Diffusion Needs a Gradient
Gas molecules move randomly, but net movement occurs from regions of higher partial pressure to lower partial pressure. The gill does not actively pump individual oxygen molecules into blood.
It maintains conditions that make passive diffusion strongly favourable.
Why Oxygen Partial Pressure Matters
The driving force for oxygen exchange is related to the partial pressure of oxygen rather than simply the number of water molecules present.
Blood leaving the gill can become highly oxygenated because haemoglobin binds oxygen and keeps dissolved oxygen levels low enough to sustain diffusion.
Why Haemoglobin Helps
Haemoglobin inside red blood cells binds oxygen reversibly. This removes oxygen from simple solution in plasma and allows much more oxygen to be carried without eliminating the diffusion gradient immediately.
The gill and blood pigment therefore operate as one transport system.
Why Carbon Dioxide Leaves Through Gills
Fish cells produce carbon dioxide during respiration. Blood carries much of it toward the gills, where it diffuses into water.
Because carbon dioxide is relatively soluble, excretion across gills can be very effective.
Why Gills Also Handle Acid-Base Balance
Carbon dioxide chemistry affects blood pH, and gills also exchange ions involved in acid-base regulation. Specialised ion-transport cells help fish control hydrogen ions, bicarbonate and other solutes.
The gill is therefore both a respiratory and chemical-regulation organ.
Why Gills Help Control Salt and Water
Fish constantly exchange water and ions with their environment. Freshwater fish tend to gain water and lose salts, while marine fish tend to lose water and gain salts.
Gill ionocytes actively transport ions to help maintain internal balance.
Why Freshwater Fish Need Ion Uptake
Their body fluids contain more dissolved salts than surrounding freshwater. Ions tend to diffuse outward, so specialised gill cells actively take up sodium, chloride and other ions from dilute water.
Kidneys and gills work together to maintain homeostasis.
Why Marine Fish Need Salt Excretion
Seawater is more concentrated than most fish body fluids. Marine bony fish drink seawater and absorb water in the gut, then use gill cells to excrete excess salts.
Gills therefore solve a salt problem at the same time they solve a breathing problem.
Why Fish Move Their Mouths
Many bony fish draw water into the mouth and pump it across the gills. The mouth opening and floor movements create pressure differences that keep water moving.
The movement is part of ventilation, not chewing or thirst.
Why Opercula Move
The operculum is the bony gill cover in many fish. Coordinated movements of the mouth and operculum create a pressure pump that pulls water across the gill surfaces.
This buccal-opercular pump allows fish to ventilate while hovering or resting.
Why Some Fish Must Swim to Breathe Efficiently
Fast-swimming species such as some sharks and tunas can use ram ventilation, swimming with the mouth open so forward motion forces water across the gills.
Some can supplement this with pumping, while others depend heavily on continuous movement.
Why Sharks Have Gill Slits
Sharks lack the single large bony operculum typical of many bony fish. Water exits through separate gill slits along the sides of the head.
Their gill anatomy reflects a different evolutionary lineage while solving the same basic exchange problem.
Why Spiracles Matter in Some Sharks
Bottom-dwelling sharks and rays may draw water through openings called spiracles behind the eyes. This lets them ventilate without sucking sand or debris into the mouth.
The route is especially useful when the animal rests on the seafloor.
Why Fish Can Suffocate in Oxygen-Poor Water
Gills can only extract oxygen that is actually present. If dissolved oxygen falls below the fish’s needs, even healthy gills cannot maintain adequate blood oxygen.
The result is hypoxia, which can lead to rapid breathing, surface behaviour, loss of coordination and death.
Why Warm Water Holds Less Oxygen
The solubility of gases generally decreases as water warms. At the same time, a fish’s metabolic demand often rises with temperature because most fish are ectotherms.
Warm water can therefore create a double challenge: less oxygen is available while the animal may need more.
Why Fish Gasp at the Surface
Surface water can contain more oxygen because it is directly exposed to air and may be mixed by waves. A hypoxic fish may therefore gather near the surface or inflowing water.
Surface gasping is a warning sign of oxygen stress in many aquarium and pond situations.
Why Aeration Helps
Bubbles themselves contribute some gas exchange, but their most important effect often comes from moving water and increasing contact between water and air.
Mixing breaks up oxygen-poor layers and replenishes dissolved oxygen throughout the system.
Why Flowing Water Often Supports More Oxygen
Streams and riffles mix continuously with air, while moving water transports fresh oxygen past gill surfaces.
Still warm ponds can develop stronger oxygen gradients, especially at night or when organic matter is decomposing.
Why Algae Can Raise Oxygen by Day
Photosynthetic algae and aquatic plants release oxygen when light is available. This can increase dissolved oxygen during daylight.
At night, photosynthesis stops while respiration continues, so oxygen may fall sharply before dawn.
Why Nighttime Oxygen Can Crash
Fish, plants, microbes and algae all consume oxygen through respiration. Without daytime photosynthetic replacement, a pond can reach its lowest oxygen level just before sunrise.
Dense algal blooms or high organic loads can make the decline severe.
Why Decomposition Uses Oxygen
Bacteria breaking down dead algae, food or waste perform aerobic respiration. Heavy decomposition can consume dissolved oxygen faster than it is replenished.
A pond can therefore become hypoxic after an algal bloom collapses even though the dead material itself no longer photosynthesises.
Why Gills Are Vulnerable to Pollution
The enormous surface area and thin barrier that make gills efficient also expose them directly to dissolved chemicals. Irritants, metals, ammonia and suspended particles can damage tissue or disrupt ion transport.
Respiratory efficiency and homeostasis can therefore fail together.
Why Ammonia Damages Fish
Ammonia is toxic at sufficient concentrations and can injure gill tissue, disrupt ion balance and impair normal excretion. Its toxicity rises with pH and temperature as more un-ionised ammonia is present.
Aquarium filtration therefore manages nitrogen chemistry as well as water clarity.
Why Nitrite Is Dangerous
Nitrite can enter fish through gills and interfere with haemoglobin, forming methaemoglobin that carries oxygen poorly.
A fish may suffer oxygen deprivation even when water contains plenty of dissolved oxygen because the blood cannot transport it effectively.
Why Chlorine Can Harm Gills
Chlorine and chloramine used to disinfect tap water can damage delicate gill surfaces at concentrations safe for human drinking systems but not for fish.
Aquarium water is therefore commonly conditioned before fish are exposed.
Why Silt Can Be a Problem
Fine suspended particles can coat or irritate gill surfaces, increase mucus production and reduce effective gas exchange.
Sediment pollution can therefore stress fish even when it is not directly poisonous.
Why Fish Produce Gill Mucus
A thin mucus layer protects epithelial surfaces from abrasion, microbes and chemicals.
Too much mucus, however, can increase diffusion distance and interfere with exchange, so irritation can make a protective response become costly.
Why Gill Parasites Are Dangerous
Parasites attached to gills can damage tissue, stimulate mucus, block water flow and consume blood.
Because the gill has little spare diffusion distance, even small structural changes can have major respiratory consequences.
Why Gill Colour Can Reflect Blood Flow
Healthy gills often appear red or pink because capillaries lie close to the surface and are filled with blood.
Pale, brown or damaged gills can reflect different problems, but colour alone is not enough to diagnose a disease.
Why Gills Collapse Out of Water
Gill filaments and lamellae are supported by surrounding water. In air, surface tension and gravity cause the delicate structures to stick together and collapse.
Effective surface area falls dramatically, making ordinary fish gills poor at extracting oxygen from air despite air containing more oxygen than water.
Why Air Has More Oxygen Yet Fish Suffocate
The problem is structural rather than simply oxygen concentration. A gill adapted to water loses its spread-out architecture in air and can dry rapidly.
Without a moist separated surface, diffusion becomes ineffective.
Why Some Fish Can Breathe Air
Several fish lineages evolved accessory air-breathing organs, modified gills, vascularised mouths, intestines or labyrinth organs. These structures remain supported in air and can exchange gases directly.
Air breathing is therefore possible when anatomy changes to solve the collapse and drying problems.
Why Lungfish Are Special
Lungfish possess true lungs homologous in broad evolutionary terms to the lungs of tetrapods. They can gulp air and survive in oxygen-poor waters, with some species enduring seasonal droughts.
Their biology illustrates how aquatic vertebrates can combine gill and lung respiration.
Why Bettas and Gouramis Can Gulp Air
Labyrinth fish have a specialised suprabranchial organ containing vascular surfaces that absorb oxygen from swallowed air.
This lets them survive in warm stagnant habitats where dissolved oxygen may be low.
Why Mudskippers Can Leave Water
Mudskippers keep gill surfaces moist in enlarged chambers and also exchange gases through skin and mouth linings.
They remain dependent on moisture, showing that successful air exposure requires protecting respiratory surfaces from collapse and drying.
Why Eels Can Survive Briefly Out of Water
Some eels can exchange meaningful amounts of oxygen through moist skin and mouth tissues, especially in humid conditions.
This does not make them fully terrestrial; dehydration and prolonged air exposure remain dangerous.
Why Fish Gills Are Different From Lungs
Gills are outward-projecting exchange surfaces bathed by water, while lungs are inward-folded chambers protected inside the body.
The two designs reflect the physical properties of their respiratory media: water supports delicate external lamellae, while air would dry and collapse them.
Why Lungs Would Be Hard to Ventilate With Water
Water is dense and viscous, so pumping large volumes in and out of deep internal sacs would require substantial energy.
Unidirectional flow across gills is generally more efficient for extracting sparse dissolved oxygen from water.
Why Gill Water Flow Is Often One-Way
Fresh water enters through the mouth and leaves past the gills rather than moving back and forth through the same path.
This reduces mixing of oxygen-rich incoming water with depleted outgoing water and supports countercurrent exchange.
Why Human Lungs Use Tidal Flow Instead
Air is light and oxygen-rich, so breathing it in and out of branching lungs is energetically practical.
The physical medium therefore shapes the architecture of the respiratory system.
Why Fish Need Ventilation and Perfusion
Ventilation means moving water across gills; perfusion means moving blood through gill capillaries. Efficient respiration requires both.
Increasing only one side provides limited benefit if the other side cannot match the exchange demand.
Why Exercise Increases Gill Ventilation
Swimming muscles consume more oxygen and produce more carbon dioxide. Fish respond by increasing mouth and opercular movements, heart output and blood flow through gills.
The respiratory system scales its activity to metabolic demand just as human breathing and heart rate rise during exercise.
Why Fast Fish Have Large Respiratory Capacity
Tuna and other active swimmers have high metabolic rates and specialised gill structures with extensive surface area and efficient water flow.
Their respiratory anatomy matches the energetic cost of continuous high-speed swimming.
Why Slow Fish Can Manage With Less
Sedentary or ambush species have lower average oxygen demand and can function with lower ventilation rates.
Respiratory capacity therefore evolves in relation to lifestyle rather than being identical across all fish.
Why Fish Can Regulate Gill Surface Area
Some species alter perfusion, lamellar exposure or gill morphology depending on oxygen conditions, temperature or activity.
This plasticity can reduce ion loss when demand is low and increase gas exchange when demand rises.
Why More Gill Area Has Costs
A larger exchange surface improves oxygen uptake but also increases opportunities for unwanted ion and water movement.
Fish therefore balance respiratory capacity against osmoregulatory cost, another example of biological trade-offs.
Why Freshwater Fish Face Water Gain
Their body fluids are saltier than the surrounding water, so water tends to enter osmotically across gills and other surfaces.
They excrete large volumes of dilute urine while actively taking up ions to maintain balance.
Why Marine Bony Fish Face Water Loss
Seawater is more concentrated than their body fluids, so they lose water osmotically. They drink seawater and excrete excess salts through gill ionocytes and kidneys.
The gill is therefore a central organ in both respiration and marine water balance.
Why Sharks Handle Salt Differently
Sharks retain high levels of urea and other osmolytes so their body fluids are close to or slightly more concentrated than seawater.
They still use gills for gas exchange and ion regulation, but their overall osmoregulatory strategy differs from that of marine bony fish.
Why Gills Help Excrete Nitrogen
Many fish excrete much of their nitrogen waste as ammonia directly across the gills. Ammonia diffuses readily into surrounding water when gradients are favourable.
This reduces the need to convert all nitrogen into urea or uric acid before excretion.
Why Aquatic Life Makes Ammonia Practical
Ammonia is highly toxic but can be diluted immediately by the surrounding water.
Terrestrial animals cannot rely on that constant dilution, which is why many convert nitrogen waste into less toxic compounds.
Why Gill Function Depends on pH
Water pH influences ion transport, ammonia chemistry, haemoglobin oxygen binding and the effects of pollutants.
Extreme pH can therefore stress respiration even when dissolved oxygen seems adequate.
Why High Carbon Dioxide Can Stress Fish
Accumulated carbon dioxide reduces the gradient for CO2 excretion and can lower blood pH. In crowded aquaculture systems, carbon dioxide removal can be as important as oxygen addition.
Respiration depends on removing waste gas as well as acquiring oxygen.
Why Ice Cover Can Cause Winterkill
Ice and snow reduce contact with air and block light needed for photosynthesis. Fish and microbes continue consuming oxygen beneath the ice.
If the water body is shallow or nutrient-rich, dissolved oxygen can fall far enough to kill fish before spring.
Why Summerkill Can Happen
Hot weather lowers oxygen solubility while raising metabolic demand and microbial decomposition. Algal blooms can amplify day-night oxygen swings.
A sudden collapse can leave fish gasping or dying even when the pond looked productive days earlier.
Why Gills Are a Good Diffusion Lesson
They maximise surface area, minimise diffusion distance and maintain steep concentration gradients. These are the same design principles seen in human lungs, insect tracheoles and plant gas-exchange surfaces.
Comparative biology reveals common physical constraints beneath very different anatomies.
Why Gills Are a Good Countercurrent Lesson
Opposing flows allow exchange to continue across a long surface instead of reaching equilibrium early.
The same principle appears in heat exchangers, bird legs and other biological systems where efficient transfer matters.
Why Gills Are a Good Homeostasis Lesson
One organ participates in oxygen uptake, carbon-dioxide removal, salt regulation, acid-base balance and nitrogen excretion.
Damage to gills therefore produces multiple physiological problems at once rather than only “trouble breathing.”
Why Gills Are a Good Evolution Lesson
Aquatic vertebrates faced the challenge of extracting sparse oxygen from a dense medium. Large thin external exchange surfaces with controlled water flow solved that problem effectively.
Different fish lineages modified the basic design while retaining the same core physical requirements.
Why Gills Are a Good Systems-Thinking Lesson
Gill performance depends on water oxygen, temperature, flow, blood circulation, haemoglobin, ion transport and metabolic demand.
A fish can become oxygen-stressed because any one of these links fails, even when the gill tissue itself is structurally normal.
Common Myths About Fish Gills
Fish do not split water molecules to obtain oxygen, gills are not simply underwater lungs, and a fish out of water does not suffocate because air lacks oxygen.
The strongest model is diffusion of dissolved oxygen across thin lamellae supported by water flow and countercurrent blood flow.
Frequently Asked Questions
Why do fish have gills? To extract dissolved oxygen efficiently from water. Why do they move their mouths? To ventilate gills. Why do fish gasp at the surface? Surface water may contain more oxygen. Why do fish die out of water? Gill lamellae collapse and dry.
Can some fish breathe air? Yes, if they have accessory organs, lungs or other moist vascular surfaces adapted for it.
Where to Go Next
Fish gills connect naturally to Why Do We Need Oxygen? and the wider respiration, aquatic ecology and animal-physiology ecosystem.
Fish have gills because breathing water requires enormous exchange area, very short diffusion distances and continuous flow. Gill filaments, lamellae, countercurrent blood and water movement turn a low-oxygen medium into a usable respiratory supply.
Why Gill Ventilation Has an Energy Cost
Water is much denser and more viscous than air, so moving it across a respiratory surface requires substantial muscular work. A fish therefore benefits from gill structures that extract as much oxygen as possible from each litre of water rather than simply pumping unlimited volumes.
Countercurrent exchange and large lamellar area reduce the amount of water movement needed for a given oxygen demand.
Why Gill Resistance Matters
Narrow spaces between lamellae increase surface area but also resist water flow. If the gaps were too tight, ventilation would become energetically expensive; if too wide, exchange area would fall.
Gill design therefore balances diffusion efficiency against the mechanical cost of pushing water through the structure.
Why Water Must Contact the Lamellae Evenly
If some regions receive abundant water while others receive little, part of the exchange surface becomes underused. Gill arches and filament orientation help distribute flow across the respiratory area.
Efficient breathing depends not only on total water volume but on how evenly that water is delivered to functioning tissue.
Why Blood Flow Must Match Water Flow
Too much blood flow with too little ventilation leaves blood poorly oxygenated, while too much ventilation with too little blood flow wastes pumping effort. Fish adjust both sides according to activity and oxygen availability.
This matching of ventilation and perfusion is a general respiratory principle shared with lungs in air-breathing animals.
Why Exercise Changes Gill Blood Flow
When swimming muscles work harder, cardiac output rises and more blood is routed through active gill lamellae. At the same time, ventilation rate increases.
The fish therefore recruits extra respiratory capacity rather than operating every exchange surface at maximum intensity while resting.
Why Some Lamellae Can Be Poorly Perfused at Rest
Reducing blood flow through portions of the gill can limit unnecessary ion and water exchange when oxygen demand is low. During exercise or hypoxia, additional capillaries can carry more blood.
Respiratory capacity is therefore adjustable rather than fixed at one permanent level.
Why Gill Surface Area Can Be a Trade-Off
More exposed surface improves oxygen uptake, but every square millimetre of thin epithelium also permits unwanted movement of salts and water and increases exposure to toxins.
Fish evolve a compromise between respiratory performance and the costs of maintaining chemical homeostasis.
Why Hypoxia Can Change Gill Structure
Some species remodel their gills when oxygen remains low, exposing more lamellar area or altering epithelial coverage. These changes can improve oxygen uptake over days or weeks.
Physiological plasticity therefore extends beyond faster breathing; the organ itself can adjust to long-term environmental conditions.
Why High Oxygen Does Not Mean Infinite Performance
Even oxygen-rich water cannot remove limits imposed by cardiac output, muscle metabolism, temperature or acid-base balance. Once haemoglobin is nearly saturated, additional dissolved oxygen produces diminishing benefit.
Respiratory performance is limited by the whole oxygen-delivery system rather than the gill alone.
Why Fish Can Be Stressed by Sudden Temperature Change
Temperature changes metabolism, oxygen solubility, membrane properties and haemoglobin function at the same time. A rapid shift can therefore disturb several physiological systems before the fish acclimates.
This is one reason aquarium temperature changes are managed gradually rather than treating water temperature as a simple comfort setting.
Why Salinity Changes Can Stress Gills
Moving between fresh and salt water reverses major osmotic gradients. Species such as salmon can remodel gill ion-transport machinery as they migrate between environments.
The same organ must therefore switch from absorbing salts efficiently to excreting them efficiently while continuing to breathe.
Why Salmon Gills Change During Migration
Before entering seawater, hormonal signals increase the abundance and activity of salt-secreting ionocytes. Returning adults later reverse parts of this programme for freshwater.
Migration is possible because gill physiology is developmentally flexible rather than permanently set for one salinity.
Why Estuary Fish Need Flexible Gills
Estuaries can change salinity with tides, rainfall and river flow. Fish living there experience shifting osmotic conditions within hours or days.
Euryhaline species survive by regulating ion transport quickly enough to keep internal chemistry stable while the external environment changes.
Why Gill Disease Can Cause Several Symptoms at Once
Damaged gills can reduce oxygen uptake, disturb salt balance, impair ammonia excretion and alter acid-base control simultaneously. A fish may therefore breathe rapidly, lose equilibrium, become lethargic and show abnormal ion regulation from one organ problem.
This multifunctionality is why gill health is central to both wild-fish and aquarium medicine.
Why Gill Tissue Must Stay Wet
Gas exchange requires gases to dissolve in a thin water film before crossing cell membranes. Drying thickens barriers, damages cells and collapses the lamellae.
Even fish capable of short excursions on land protect their respiratory surfaces by retaining moisture in chambers or using alternative organs.
Why Gill Design Is Perfect for Water but Poor for Air
Water buoyancy supports thousands of delicate lamellae, letting them spread widely without heavy structural reinforcement. In air, that same delicacy becomes a weakness because gravity and surface tension collapse the filaments together.
Respiratory organs are therefore shaped by the physical medium in which they evolved.
Why Gills and Lungs Represent Different Engineering Solutions
Gills expose exchange surfaces outward into flowing water, while lungs protect exchange surfaces inside the body and move lightweight air in and out. Both maximise area and minimise diffusion distance, but the architecture is reversed.
Different environments produce different structures built around the same diffusion laws.
Why Fish Respiration Connects to Ecosystem Health
Dissolved oxygen reflects temperature, water mixing, plant photosynthesis, microbial decomposition and pollution. Fish breathing therefore depends on the condition of the whole aquatic ecosystem.
A mass fish kill can be evidence of environmental failure even when no poison directly attacked the fish.
Why Monitoring Dissolved Oxygen Matters
Aquaculture farms, aquariums and environmental agencies measure dissolved oxygen because fish can become stressed before visible signs appear. Continuous sensors reveal nighttime declines, stratification and sudden failures in aeration.
Measurement turns invisible respiratory risk into data that can guide intervention.
What Students Should Be Able to Explain
A strong answer should connect dissolved oxygen, gill filaments, lamellae, thin diffusion distance, water flow, capillary blood and countercurrent exchange. It should then explain why warm water, pollution and low oxygen can stress fish.
If the learner can also explain why gills collapse in air and why lungs use a different architecture, the topic has become transferable comparative physiology.
The Final Mechanism to Remember
The durable chain is simple: water carrying dissolved oxygen flows across thin lamellae, blood flows the opposite way, oxygen diffuses into blood, haemoglobin carries it away and carbon dioxide diffuses outward.
Countercurrent flow keeps the gradient useful across the surface, making a difficult low-oxygen medium biologically workable.
The Big Picture
Fish have gills because extracting oxygen from water requires a respiratory organ designed around water’s density, low oxygen content and ability to support delicate external surfaces.
Gills succeed by combining huge area, microscopic thinness, continuous ventilation, dense blood flow and countercurrent exchange in one compact organ.
The gill succeeds because every structural feature preserves diffusion gradients while minimising the energetic cost of moving dense water across a delicate respiratory surface.