Exchange Surfaces

• The need for specialised exchange surfaces (small organisms)

  - Low metabolic activity so oxygen demands and carbon dioxide production is also low

  - Large SA:V ratio meaning the distances the substances have to travel is small

  Simple diffusion is enough to meet their metabolic needs
• The need for specialised exchange surfaces (large organisms)

  - Higher metabolic activity

  - Small SA:V meaning the diffusion distance is large

  - Therefore, they require adaptations to increase efficiency of exchange across their surface
• What are the 4 features of an efficient gas exchange system?

  Increased surface area

  Thin barrier

  Good blood supply

  Ventilation to maintain a concentration gradient
• Increased surface area

  Provides the area needed for gas exchange

  
• Thin barrier

  The distance the that substances have to diffuse is short, making the process fast and efficient (short diffusion distance)

  
• Good blood supply

  Maintains the concentration gradient. The steeper the concentration gradient, the faster diffusion takes place

  
• Ventilation

  Maintains a concentration gradient, so that diffusion can take place more quickly

  
• What does the Mammalian Gas Exchange System consist of?

  Trachea

  Bronchi

  Bronchioles

  Alveoli
• Trachea

  Supported by incomplete, C-shaped rings of strong, flexible cartilage which stops the trachea from collapsing.

  The trachea is lined with ciliated epithelium (wafts mucus) and goblet cells.

  The walls of the trachea consist of smooth muscle which contracts if there are any harmful substances in the air. This causes the lumen to constrict the airways and reduce airflow. When smooth muscle relaxes, the lumen dilates

  Goblet cells secrete mucus to trap dust and microorganisms that have escaped the nose airway.

  
• Bronchi

  Bronchi have a similar structure to the trachea, except bronchi are narrower as they have smaller cartilage rings.

  Bronchioles split into many smaller tubes to create a network of bronchioles

  
• Bronchioles

  Bronchioles are much narrower than bronchi. Smaller bronchioles have no cartilage (larger ones may have some). The wall is mainly compromised of smooth muscle and elastic tissues

  
• Where do oxygen and carbon dioxide diffuse?

  Oxygen diffuses from the alveoli into the blood in the capillaries

  Carbon dioxide diffuses from the blood in the capillaries to the alveoli
• Alveoli (site of gas exchange)

  Large surface area --> There are millions of alveoli, providing a large surface area

  Thin barrier --> The alveoli walls are very thin, consisting of single-celled squamous epithelial cells

  Good blood supply --> Surrounded by a network of capillaries which means there is a constant blood flow through the capillaries that brings carbon dioxide and carries away oxygen (maintains a steep gradient)
• What is ventilation?

  Mechanism of breathing involving the antagonistic interaction between the internal and external intercostal muscle bringing about pressure changes in the thorax

  
• Inspiration (inhaling)

  Air moves to the lungs

  Volume of the chest cavity increases (thorax)

  External intercostal muscles rise

  Diaphragm contracts and becomes flatter (displacing digestive organs underneath)

  
• Expiration (exhaling)

  Air moves out of the lungs

  Volume of chest cavity decreases

  External intercostal muscles relax and internal intercostal muscles contract to push air more forcefully (e.g., sneezing) Diaphragm relaxes and is pushed up by displaced organs

  
• What does a spirometer do?

  Measures the volume of air inhaled and exhaled

  
• Vital Capacity

  Total volume of air an individual can inhale and exhale during a deep breath

  
• Tidal Volume

  The air inhaled (peaks) and exhaled (troughs) at rest

  
• Residual Volume

  The volume of air that always remains in the lungs so it does not collapse

  
• Breathing Rate

  Number of breaths taken per minute

  Count number of peaks in a minute
• Oxygen Uptake

  Volume of oxygen absorbed by the lungs in one minute

  Increase in ventilation rate, increases oxygen uptake (e.g. during exercise)
• Ventilation Rate

  tidal volume x breathing rate
• What is the gas exchange system in insects?

  The tracheal system which is made up of spiracles and trachea
• What are spiracles?

  Spiracles are small openings along the thorax and abdomen

  They can be open and closed by sphincters

  Helps prevent water loss

  
• When an insect is metabolically inactive...?

  Spiracles will be closed as oxygen demands are low
• When an insect is metabolically active...?

  Spiracles will be open as oxygen demands are high
• Trachea in insects

  The trachea in insects carries air into the body

  The trachea is lined with spirals of chitin which keep them open and prevent them collapsing

  
• What does the trachea in insects branch into?

  Trachea branches out into tracheoles
• Tracheoles

  Each tracheole is a greatly, elongated cell with no chitin lining so they are permeable to gases.

  The vast number of tracheoles provides a large surface area for gas exchange

  At the end of the tracheoles are tracheal fluid (residue water)

  
• Tracheal Fluid

  When an insect is metabolically active (flying), the muscle cells start to respire anaerobically to produce lactate. This lowers the water potential of the cells and water moves from the tracheoles (tracheal fluid) into the cells by osmosis

  This means there is a decreased volume of liquid in the tracheoles and more air can move in

  
• What do bony fish have to absorb oxygen?

  Gills
• Gill structure

  The gills are covered by a bony plate called the operculum

  Each gill consists of 2 pairs of gill filaments (primary lamellae) attached to the bony arch

  The filaments are very thin and fold into many gill plates (secondary lamellae). This provides a very large surface area
• Gas exchange in bony fish

  Large surface area --> Many gill filaments (primary lamellae) which fold into many secondary lamellae

  Thin barrier --> The gill filaments and secondary lamellae are very thin and have a network of capillaries

  Maintaining concentration gradient --> Counter-current flow
• Counter-current flow

  Blood flows along the gill arch and out along the filaments. Blood flows through the capillaries in the opposite direction to the flow of water over the lamellae, this absorbs the maximum amount of oxygen from water. This maintains a steep concentration gradient.

  
• Ventilation in bony fish

  Bony fish can keep water flowing over the gills by using a buccal-opercular pump.

  The floor of the mouth moves downwards, drawing water into the buccal cavity. This increases the volume of the buccal cavity and therefore decreases pressure

  When the fish closes its mouth, the operculum opens. This increases pressure in the operculum cavity and forces the water over the gills.

  This ventilation ensures there is a constant flow of water over the gills for gas exchange
• Spirometer


  A spirometer is a chamber filled with oxygen that floats on a tank of water. A pen tracks the movement of the chamber to produce a trace.

  • Breathing in takes oxygen from the chamber - the top of the chamber sinks
  • Breathing out pushes air into the chamber - the top of the chamber rises
• Extra - spirometer

  • As the air breathed out is a mixture of oxygen and carbon dioxide, carbon dioxide is absorbed by soda lime (so the overall volume of gas in the chamber decreases)
  • Disinfect and sterilise when a new person uses it
  • Make sure the people using the spirometer are healthy (e.g. not asthmatic)