# Gas Exchange in Fish & Insects ## The Tracheal System in Insects - All insects possess a rigid exoskeleton with a waxy coating that is impermeable to gases - Spiracles are openings in the exoskeleton of an insect which allow air to flow into the internal system of tubes known as the tracheal system - Tracheae (singular: trachea) are tubes within the insect respiratory system which lead to narrower tubes known as tracheoles - Rigid rings of chitin keep the tracheae open

Spiracles are external openings in the insect exoskeleton that permit gas exchange between the atmosphere and the tracheal system

### Structure and Function - Many tracheoles carry oxygen into the muscle fibres of the insect, where gas exchange takes place - The ends of the tracheoles are filled with tracheal fluid - Gases can dissolve in this fluid before diffusing to the cells for gas exchange - The large number of tracheoles that are in contact with the muscle cells provides a large surface area for gas exchange

Think of the tracheal system like an internal plumbing network that brings oxygen directly to where it's needed—unlike mammals that rely on blood transport

### Adaptations for Gas Exchange - The extensive branching of tracheoles maximises surface area in contact with tissues - Direct delivery of oxygen to respiring cells means diffusion distances are short - The tracheal fluid at the ends of tracheoles dissolves gases, facilitating diffusion across cell membranes ## Ventilation Mechanism in Insects ### At Rest - When insects are at rest their energy requirements are low and diffusion alone is fast enough to supply oxygen to the cells - In this state insects may close their spiracles to reduce water loss by evaporation - This is similar to how cacti close their stomata during the day to conserve water in hot climates

Closed spiracles prevent water loss but also limit gas exchange—insects must balance these competing needs

### During Activity Active insects need a rapid supply of oxygen; this can be achieved as follows: - **Contracting and relaxing the muscles** of the thorax and abdomen alters the volume, and therefore pressure, inside the tracheae - This draws air in and out - Similar to how squeezing and releasing a syringe moves liquid in and out - **During flight** the tracheal fluid at the narrow ends of the tracheoles is drawn into the respiring muscle - Removing fluid from the tracheoles reduces the diffusion distance between the air and the muscle cells - This speeds up diffusion - It's like removing a barrier to allow faster movement of gases ## Gills of Fish ### Why Fish Need Specialised Gas Exchange - Oxygen dissolves less readily in water - A given volume of air contains 30 times more oxygen than the same volume of water - Fish are adapted to directly extract oxygen from water

Fish must work much harder than land animals to obtain sufficient oxygen due to the low oxygen content of water

### Structure of Fish Gills in Bony Fish - Series of gills on each side of the head - Each gill arch is attached to two stacks of filaments - On the surface of each filament, there are rows of lamellae - The lamellae surface consists of a single layer of flattened cells that cover a vast network of capillaries

Lamellae (gill plates) are thin structures that project from gill filaments, providing a large surface area for gas exchange

### Counter-Current System The mechanism that makes fish gills so efficient: - The capillary system within the lamellae ensures that the blood flow is in the opposite direction to the flow of water - This is a **counter-current system** - The counter-current system ensures the concentration gradient is maintained along the whole length of the capillary - The water with the lowest oxygen concentration is found adjacent to the most deoxygenated blood - Water continues to supply the blood with oxygen along the whole gill arch - Ends with water with the lowest oxygen concentration adjacent to the most deoxygenated blood - Continued diffusion occurs throughout | Position along gill | Water $O_2$ concentration | Blood $O_2$ concentration | Diffusion occurs? | |---------------------|---------------------------|---------------------------|-------------------| | Front edge (entry) | High (saturated) | Low (deoxygenated) | Yes ✓ | | Middle | Medium | Medium-low | Yes ✓ | | Rear edge (exit) | Low | Very low | Yes ✓ | The chart below shows how oxygen concentration changes along the gill length. Notice how the concentration gradient is maintained at every point, allowing continuous diffusion from water to blood:

The counter-current system is like two conveyor belts moving in opposite directions—this maintains a gradient throughout, unlike parallel flow where equilibrium would be reached halfway

## Ventilation Mechanism in Fish The ventilation mechanism in fish constantly pushes water over the surface of the gills and ensures they are constantly supplied with water rich in oxygen (maintaining the concentration gradient). ### Inspiration (Water Intake) 1. When the fish **open their mouth** they lower the floor of the buccal cavity 2. This causes the volume inside the buccal cavity to increase 3. Which causes a **decrease in pressure** within the cavity 4. The pressure is higher outside the mouth of the fish and so **water flows into the buccal cavity**

The buccal cavity is the mouth cavity where pressure changes drive water flow across the gills

### Expiration (Water Exit) 1. The fish then **raises the floor of the buccal cavity** to close its mouth 2. Increasing the pressure within the buccal cavity 3. Water flows from the buccal cavity (high pressure) into the gill cavity (low pressure) 4. As water enters, pressure begins to build up in the gill cavity 5. This causes the **operculum** (a flap of tissue covering the gills) to be forced open 6. Water exits the fish - The operculum is pulled shut when the floor of the buccal cavity is lowered at the start of the next cycle - The pressure differences created by the opening of the mouth causes water to be constantly pushed across the surface of the gills

The operculum is a protective bony flap covering the gills that also functions as a valve in the ventilation mechanism

### Key Principle - Water will always flow from an area of **high pressure** to an area of **low pressure** - This pressure gradient is maintained by the coordinated movement of the mouth and operculum - Think of it like a pump that alternates between creating suction (mouth opens) and pushing (mouth closes) ### Summary of Pressure Changes | Stage | Mouth | Buccal cavity floor | Buccal pressure | Operculum | Water movement | |-------|-------|---------------------|-----------------|-----------|----------------| | Inspiration | Opens | Lowers | Decreases | Closed | Into mouth | | Expiration | Closes | Raises | Increases | Opens | Over gills, out |

The fish ventilation mechanism ensures continuous water flow over the gills, unlike mammalian breathing which has distinct in and out phases

## Comparing Gas Exchange Systems | Feature | Insects | Fish | |---------|---------|------| | Medium | Air | Water | | Structure | Tracheal system with spiracles | Gills with lamellae | | Ventilation at rest | Diffusion only | Active pumping | | Ventilation during activity | Muscle movements change body volume | Mouth and operculum movements | | Oxygen delivery | Direct to cells via tracheoles | Via blood in capillaries | | Efficiency mechanism | Large surface area, short diffusion distance | Counter-current system | ### Adaptations to Different Environments **Insects (terrestrial):** - Must balance oxygen uptake with water conservation - Can close spiracles when oxygen demand is low - Air is oxygen-rich so less ventilation effort needed **Fish (aquatic):** - Water contains much less oxygen than air - Must maintain continuous water flow across gills - Counter-current system maximises oxygen extraction - Cannot "hold their breath" like terrestrial animals