exchange surfaces

• surface area : volume ratio

  • small organisms have a greater surface area in comparison to the volume
  • means a big surface area
  • a shorter distance from the outside of the organisms to the middle of it
• trachea

  • have c-shaped cartilage for support
  • line with ciliated epithelium with goblet cells
  • smooth muscle in walls, will contract if there are harmful substance detected in the air
  • the lumen of trachea with constrict reducing airflow into the lungs
  • when the smooth muscles relaxes the lumen dilates, stretch and recoil is possible due to elastic fibres in the wall
• bronchi and bronchioles

  • the trachea splits into two, bronchi which connect to the left and right lung
  • the bronchi split further into many smaller tubes to create a networks of bronchioles
  • both have cartilage in the walls for structural support to keep tube open
• alveoli

  • located at the end of the bronchioles
  • site of gas exchange
  • oxygen diffuses from the alveoli into the blood in the capillaries, then carbon dioxide is diffuse out of the blood into the alveoli
• large surface are (alveoli)

  1. a large number of alveoli creates a large surface area for diffusion
• short diffusion distance (alveoli)

  1. the alveoli walls are very thin, made up of squamous epithelial cells
• concentration gradient (alveoli)


  each alveolus is surrounded by a network of capillaries to remove exchange gas and maintain a high concentration gradient
• ventilation

  • maintains the conc gradient for gas exchange

  mechanism of breathing which involves:

  1. diaphragm
  2. external and internal intercostal muscles
  3. pressure changes in thoracic cavity
• inspiration

  1. volume of thorax increases
  2. air pressure inside the thorax is reduced
  3. causes air to flow into the lungs
  4. thoracic cavity increases :
  5. diaphragm contracts, down and flatter
  6. external intercostal muscle contract
  7. internal intercostal muscle relaxes
• expiration

  1. decrease in the volume in the thorax
  2. increase air pressure of the thorax
  3. causes air to be expelled from the lungs ,
  4. thoracic cavity decreases:
  5. diaphragm relaxes
  6. internal intercostal muscle contracts
  7. external intercostal muscles relax
• vital capacity 


  maximum volume of air an individual can inhale and exhale during a deep breath
• tidal volume 


  the air inhaled (peaks) and exhaled (through) when at rest
• residual volume 


  the volume of air that always remain in the lungs so they don't collapse
• breathing rate 


  the number of breathes taken per minute

  1. counting how many breaths are taken (how many full peaks + through)
• oxygen uptake 


  will increase when the ventilation rate increases , increase e.g. exercise
• ventilation rate 


  volume of air inhaled per minute , calculated via formula

  1. ventilation rate = tidal volume x breathing rate
• inspiration of fish

  1. swim with their mouths open to allow water to flow over the gills
  2. lower their buccal cavity and open their mouths
  3. increase the volume of the buccal cavity
  4. decrease pressure
  5. results water flowing into the buccal cavity
• operculum valve (inspiration of fish)

  1. the operculum will shut
  2. operculum cavity(location of gills) will expand
  3. increase in volume
  4. decrease in pressure
  5. raising floor of the the buccal cavity, forcing water from buccal cavity over the gills within the operculum cavity
• expiration of fish

  1. fish closes its mouth therefor operculum opens
  2. increase in pressure in the operculum cavity
  3. forces water over gills and out of the side of the fishs head
  4. ventilation ensures there is constant flow of water over the gills for gas exchange
• fish gills

  • have 4 layers of gill on each side of their head
  • gills are made of gill filaments and gill lamellae
• gill adaptations for gas exchange

  1. large surface area - there are many gill filaments and lamellae which are stacked at right angles to each other
  2. short diffusion distance - the gill lamellae and filaments are both thin and contain a capillary of network
  3. maintain concentration gradient - countercurrent mechanism
• countercurrent flow mechanism

  • for fish to maintain concentration gradient for diffusion
  • as oxygen in atmosphere is greater than concentration in the water
  • water flows over the gill lamellae in the opposite direction of the flow of blood in the capillaries, making sure the diffusion gradient is maintained across the entire lamellae
• tracheal system (gas exchange in insects )

  made up of :

  • spiracles
  • trachea
• insects contract and relax their abdominal muscles to moves gases on mass into and out of the spiracles to the trachea
• adaptations to efficient gas exchange in insects

  1. large surface area - many branching tracheoles
  2. short diffusion distance - many branching tracheoles reach muscle and thin-walled tracheoles
  3. maintaining a concentration gradient - when the cells respire, they use up oxygen and produce carbon dioxide, abdominal muscle contract to pump air
• • when the insect is in flight
  • muscle cells start to respire anaerobically to produce lactate
  • lowers the water potential of the cells and therefore water moves from tracheoles (tracheal fluid) into the cells by osmosis
  • decrease in volume of liquid in the tracheoles causes more air from atmosphere to move in
• open circulatory systems

  1. invertebrates such as insect have this
  2. the transport medium is usually pumped directly to the open body cavity and there are very few transport vessels
  3. transport medium is pumped at low pressure and will transport food and nitrogenous waste, not gases
  4. once exchange has taken place at the cells and tissues, the transport medium returns to the heart through an open-ended vessel
• closed circulatory system

  • all vertebrates like fish and mammals and some invertebrates like worms have this
  • transport medium (blood) remains inside o vessels
  • gases and small molecules can the leave the blood by diffusion or due to high hydrostatic pressure
  • closed circulatory systems transport oxygen and carbon dioxide, and the oxygen is usually transported by a pigmented protein like haemoglobin
• Single closed circulatory systems

  • The blood only passes through the heart once per cycle
  • Fish have this system
  • Blood passes through two sets of capillaries immediately after being pumped out of the heart the blood flows through capillaries in the gills to become oxygenated
  • Blood flows through capillaries deliver in blood to the body before returning back to the heart
  • This system is not efficient gas exchange for mammals but works for fish due to countercurrent mechanism
• Double closed circulatory system

  • The blood plug is through the heart twice per cycle
  • Birds and mammals have a double closed circulatory system
  • Second of blood vessel carries blood from the heart to the lungs for gas exchange
  • The second second of blood vessels carries blood from the heart to the rest of the body to deliver oxygen and nutrients and to collect waste
• Arteries( blood vessel)

  • The smooth muscle layer is thicker than veins so that construction and dilation can occur to control the volume of blood
  • The elastic is thicker than veins to help maintain blood pressure. The wall can stretch and recoil in response to the heartbeat.
  • There is a collagen outer layer to provide structural support
  • The wall is thicker than veins to help maintain blood pressure
• arterioles (blood vessel)

  • The smooth muscle layer is thicker than in arteries to help restrict blood flow into the capillaries
  • Elastic layer is thinner than in arteries as the pressure is lower
  • The collagen layer is thinner
  • The wall thickness is thinner as pressure is slightly lower
  • No valves
• Capillaries

  • No collagen or elastic layer
  • No smooth muscular wall
  • 1 cell thick consisting of only lining layer, this provides a short diffusion distance for exchanging materials between blood and cells
  • No valves
• Venules

  • Valves are present
  • No collagen or elastic layer
  • A thin layer of smooth muscle
  • Very thin wall, several venules join to form a vein
• Veins

  • Valves are present
  • Contains lots of collagen
  • Elastic layer is thinner as the pressure is much lower
  • Smooth muscle layer is thin so it cannot control the blood flow
  • Thin walis as the pressure is much lower so there is low risk of vessel bursting. The thinness means the vessels are easily flattened which helps the flow of blood up to the heart.
• Further capillaries info

  • they form capillary beds at exchange surfaces
  • have a narrow diameter to slow blood flow
  • Red blood cells can only just fit through and washed against the walls. This maximises diffusion.
  • Have squamous epithelium or cells in their endothelium
• Tissue fluid

  • Tissue fluid is formed by liquids in small molecules being forced out of the capillaries small gaps
  • Hydrostatic pressure is the pressure exerted by liquid.
  • Oncotic pressure is the tendency of water to move into the blood via osmosis.
• Tissue fluid formation

  • As blood enters, the capillary from the arterial is the smaller diameter results in high hydrostatic pressure
  • this pressure forces water, oxygen, amino acids, ions, fatty acids, glucose, to be exerted out of the capillaries at the arterial end
  • The solution that has been forced out is called the tissue fluid and it bathes the cells in substances they need
  • The hydrostatic pressure is greater than the oncotic pressure at the arterial end of the capillaries
• Tissue fluid reabsorption

  1. Large molecules like plasma proteins remain in the capillaries as they can't move out due to the size

  2. This lowers the water potential in the capillaries

  3. Lower water potential in the capillaries results in a higher oncotic pressure

  4. Towards the venue end of the capillary, there is a lower hydrostatic pressure as there is less liquid in the capillaries

  5. This leads to water moving from a higher water potential in the tissue fluid to low water potential in the capillaries via osmosis

  6. Once equilibrium has been reached, normal water is removed or reabsorbed into the blood in the capillaries
• Lymphatic system

  • Meaning liquid in the tissue fluid is absorbed into the lymphatic system and eventually drains back into the bloodstream near the heart
  • once the liquid is in the lymphatic system, it’s called lymph
  • lymph has a similar composition to plasma, except it does not contain large plasma proteins, and it has less oxygen and nutrients as this would’ve been absorbed by the cells