3.3.2 - Gas Exchange in Fish & Insects

Tuesday 20th February ‘24

How is gas exchange affected by the size of an organism?

• Single celled organisms are very small, and so have a large SA:V. 

• Oxygen can be absorbed across the cell membrane and carbon dioxide can diffuse out.

• If a cell has a cell wall, this does not affect diffusion.

How do insects exchange gas?

What are the physical structures that allow this to happen?

• Trachea - microscopic air-filled pipes .

  • Divide into tracheoles, continue to divide until they penetrate into individual body cells.

  • Stiffened with bands of chitin to prevent collapse.

• Gas directly echanged between cells and atmosphere.

• Air enters trachea through pores on surface of exoskeleton called spiracles (openings along thorax and abdomen). CO2 & O2 diffuse in/out spiracles down conc gradient.

  • Exoskeleton - rigid outer skeleton that covers the insects’ body surface, covered with waterproof cuticle (layers of chitin form waterproof barrier on surface).

How are the structures adapted for gas exchange?

• Tracheoles have thin walls - shortens diffusion distance of gases to cells.

• Tracheoles are highly branched - increased SA for gas exchange.

• Fluid in ends of trachea where joins tissue - gas exchange made faster from air to liquid through tracheoles; tracheal fluid can be withdrawn into body fluid to increase SA of tracheole exposed to air.

• Muscles can pump body and force air in/out - maintains conc gradient for gasses.

• Spiracles can be closed - prevents water loss.

How is a fresh supply of air/water circulated to maintain the concentration gradient?

• Contracting muscles between each body segment means the insect can compress tracheae and so pump gases in/out of body.

• Abdominal pumping - raises pressure in body and forces air out of spiracles down pressure gradient.

  • Can be utilised to increase the removal of carbon dioxide when energy demands increase (respiration levels are highest).

How do fish exchange gas?

What are the physical structures that allow this to happen?

• Each gill made of lots of gill filaments (thin plates), attached to bony gill arch.

  • Gill filaments convered in lamella (many tiny folds with lots of blood capillaries and thin layer of cells) which further increases SA.

  • Gas exchange happens at lamellae (through countercurrent flow).

How are the structures adapted for gas exchange?

• Counter-current system ensures steep conc gradient maintained over whole length of gill - oxygen can diffuse from water into blood.

• Thin walls of lamellae - shortens diffusion distance.

• Large number of filaments and lamellae - incrreases SA.

• Large number of capillaries around lamellae - circulation constantly removes oxygenated blood, maintains steep conc gradient.

• Ventillation by operculum - ensires fresh water flow over gills to replace lost oxygen, maintains steep conc gradient.

How is a fresh supply of air/water circulated to maintain the concentration gradient?

• Countercurrent

  • Water & blood flow over and through lamellae in opposite directions to each other.

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

  • Blood absorbs more and more oxygen as it moves along.

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

  • Maintains favourable conc gradient.

  • Opposite to this is concurrent flow.