Gas Exchange in Fish and Insects Review

Fish Gas Exchange

Adaptations for Efficient Gas Exchange in Fish

• Structure:

  • Thin epithelium/walls of lamellae: Shortens the diffusion distance for gases from water to blood.

  • Large number of filaments and lamellae: Increases the surface area for gas exchange.

  • Countercurrent flow system: Maintains a concentration gradient as water always flows next to blood with a lower concentration of oxygen.

  • Large number of capillaries around lamellae: Circulation constantly removes oxygenated blood to maintain a steep concentration gradient.

  • Ventilation by operculum: Ensures constant fresh water flow over gills to replace lost oxygen and maintain a steep concentration gradient.

Countercurrent Flow vs. Concurrent Flow

• Countercurrent Flow:

  • Water and blood flow over and through the lamellae in opposite directions.

  • Blood always flows next to water that has a higher oxygen concentration, so diffusion happens along the full length of the lamellae.

  • The blood absorbs more and more oxygen as it moves along.

  • Even when the blood is highly saturated, there is still a concentration gradient, so more oxygen can flow into the blood.

• Concurrent Flow:

  • Water and blood flow over and through the lamellae in the same direction.

  • At first, there is a very large concentration gradient as water has a much higher oxygen concentration, so diffusion occurs.

  • As they flow along the lamellae, the concentration gradient decreases until equilibrium is reached, and no more oxygen diffuses into the blood.

  • Less oxygen would be absorbed into the blood overall because diffusion only happens in the first part of the lamellae.

Saturation with Oxygen

• Countercurrent flow maintains a higher oxygen saturation in the blood along the entire length of the gill plate compared to concurrent flow.

• In concurrent flow, the saturation reaches equilibrium, limiting further oxygen absorption.

Ventilation

• The internal gills are protected by an operculum and therefore need to be actively ventilated.

• The fish takes water in through its buccal cavity, which then flows through the pharynx and over the gill plates, leaving via the opercular openings on each side of the fish's head.

• The rows of gill filaments have many folds called lamellae. The folds are kept supported and moist by the water that is continually pumped through the mouth and over the gills.

• This also ensures fresh water with oxygen is always passing over the gills to maintain the concentration gradient as oxygen diffuses into the blood.

Ventilation Steps

1. Mouth opens, operculum shuts.

2. Water enters cavity due to decreased pressure/increased volume.

3. Mouth closes, operculum opens.

4. Results in increased pressure/decreased volume.

5. Increased pressure forces water out over gills.

• Pressure dynamics:

  • When the mouth is open, the pressure in the buccal cavity is low.

  • When the mouth is closed, the pressure in the buccal cavity is higher than in the opercular cavity.

Gill Structure

• Each gill is made of lots of thin plates called gill filaments which are attached to a bony gill arch; these create a large surface area for water to flow over.

• The gill filaments are covered in lots of tiny folds called lamella which further increase the surface area of the gills.

• The lamellae have lots of blood capillaries and a thin layer of cells.

• Gas exchange happens at the lamellae. Water flows over them in an opposite direction to the blood (countercurrent flow).

Insects Gas Exchange

Insect Structure and Adaptations for Gas Exchange

• Tracheae: A large internal network of tubes in insects with supported rings (chitin) to prevent them collapsing.

• Tracheoles: These tubes extend from the tracheae and extend throughout all the body tissues of the insect to allow atmospheric air to be brought directly to respiring tissues. Tracheoles have thin walls and are highly branched.

• Spiracles: Tiny pores that allow gases to enter and leave the tracheae (and water vapor to leave as well). They are opened and closed by a valve.

Gas Exchange Process

• Insects that have evolved to live on land have microscopic air-filled pipes called tracheae.

• The tracheae divide into smaller tubes called tracheoles which continue to divide until they penetrate into individual body cells.

• This means that gases are directly exchanged between cells and the atmosphere - there is no need to transport them.

• Air enters the trachea through pores on the surface of the exoskeleton called spiracles. $$CO2$and$$$ and $$O2$$ will diffuse in/out of the spiracles down their concentration gradient.

• The ends of tracheoles are filled with fluid, primarily consisting of water.

Adaptations for Increased Diffusion

• Shortens diffusion distance of gases to cells.

• Increases surface area for gas exchange.

Role of Tracheal Fluid

• Gas exchange from air to liquid occurs in the tracheole which allows gases to diffuse to tissues faster.

• Tracheal fluid can be withdrawn into the body fluid to increase the surface area of the tracheole exposed to air.

  • This occurs during intense activity because muscle cells respire anaerobically, producing lactate, which lowers water potential, causing water to move into cells by osmosis.

  • This reduces water in tracheoles, drawing in more air and increasing diffusion speed but also more water loss.

Ventilation in Insects

• Through contracting muscles between each body segment, the insect can compress the tracheae and so pump gases in and out of its body - this is a type of ventilation.

• Pumping raises pressure in the body and forces air out of the spiracles down the pressure gradient; maintains concentration gradient for gases.

Spiracle Control

• Spiracles can be closed/not open all the time; this prevents water loss and keeps the organism waterproof.