AS Biology: Human Ventilation System: Gas Exchange

Gas Exchange

An efficient gas exchange system has:

• a larger surface area 
• a short diffusion distance
• a large diffusion gradient

Adaptations for efficient gas exchange

• Large surface area - individual alveoli are small (approx. 100-300 µm) yet surface area for gas exchange is about 70m^2
• Permeable barrier to exchange - the plasma membranes that surround the thin cytoplasm of the cells form the barrier to exchange - allows exchange of O2 and CO2.
• Short diffusion distance - the alveolus walls and capillary walls are one cell thick and are made upp of squamous epithelium. Capillaries are close contact with the alveolus wall and are so narrow that the RBC are squeezed against the walls.
• Diffusion gradient - this is maintained by the ventilation of the lungs and movement of the blood through the capillaries in the lungs.

Surfactant

• A thin layer of moisture lines the alveoli, it evaporates when we breathe out.
• The lungs must produce this substance called surfactant to reduce cohesive forces between water molecules - without it alveolus would collapse due to cohesive forces between water and air sacs.
• alveoli must be kept open for their extensive surface area used for gaseous exchange
• surfactant reduces the surface tension by occupying the space between the watery film and alveolar membrane.

Features

• A large surface area provided by many alveoli - increases the rate of diffusion of gases
• A good blood supply due to dense work of capillaries - capillaries ensure the gradient for the diffusion of gases is maintained.
• Thin surface layer (one cell thick) - shorter diffusion distance  greater rate of diffusion
• Partially permeable to respiratory gases - allows few movement of gases across alveoli walls
• Ventilation mechanism - ensure fresh oxygenated air is drawn into lungs to maintain the diffusion gradient

Blood reaching the alveoli has a low concentration of oxygen and high concentration of carbon dioxide than the alveolar air; and so there is a concentration gradient which helps the diffusion of O2 and CO2 in opposite direction. As blood flows past an alveolus, oxygen diffuses into it and carbon dioxide out of it - by that time blood leaves the alveolus it has the same concentration of oxygen and carbon dioxide as the alveolar air. Each pulmonary capillary is very narrow so that RBC are slowed as thy pass the capillaries allowing more time for diffusion.

Ventilation in lungs

• Ventilation is when air is constantly moving in and out of the lungs. 
• Intrapulmonary pressure - pressure within the lungs.
• The lungs are not muscular so pressure changed are achieved indirectly

• When you breath in and out you ribcage moves up and out, your abdomen moves in and out.
• Nerve impulses from the brain causes the diaphragm and external intercostal muscles to contract; he diaphragm flattens/ribcage moves up and out.
• Increasing the volume of the thorax and decrease pressure inside the lungs makes air move down pressure gradient from outside into the lungs. 
• Air pressure in lungs is reduces.
• Air enter the lungs - whenever air pressure inside the alveoli is reduced below atmospheric pressure. 

Breathing in

• Diaphragm contract and moves downwards at the same time the intercostal muscles contract and move the ribcage up and out. The volume inside the thorax increases,
• The increase in volume causes the pressure to drop. The pressure in the chest is lower than the atmospheric pressure outside. Air is forced down the through trachea into the lungs.

Breathing out

• Diaphragm relaxes and moves upwards - at the same time intercostal muscles relax and ribcage fall down and inwards. The volume in the thorax decreases - the decrease in volume causes the pressure to rise.
• The pressure in the chest is higher than the atmospheric pressure outside.
• Air is forced up out of the trachea and out of the mouth.

Pulmonary ventilation

Pulmonary ventilation (dm^3 min^-1) = tidal volume (dm^3) x ventilation rate (min^-1)

The tidal volume is the volume of air breathed in at each breath during normal, relaxed, rhythmical breathing and it's about 0.5dm^3.

Ventilation rate is the number of breaths taken in one minute. Normally about 12-20 breaths in a healthy adult.

Lung capacity

• the change in lung volume can be analysed by using a spirometer.
• this uses an oxygen filled chamber floating over a water bath. The lid of the chamber is hinged t one side.
• when a person breathes in and out of the apparatus the lid moves and these movements of the lid correspond exactly to changed in the volume of air held in the lungs.
• CO2 released is removed by passing expired air through soda lime before returning to its main chamber.
• A recording pen draws a trace on a rotating drum in response to movements of floating chamber.

Tidal volume is the volume of air moved in and out of the lungs with each breath when you are at rest.

Vital capacity - largest volume of air that can be moved into and out of the lungs in any one breath.

Residual volume - volume of air that always remains in the lungs even after the biggest possible exhalation.

Breathing during exercise muscles cells use up more oxygen an produce increased amounts of carbon dioxide. The lungs and heart have to work harder to supple the extra oxygen and removed the carbon dioxide. the breathing rate and depth of increases. heart rate also increases in order to transport oxygenated blood to the muscles.

So during exercises

• muscle cells respiration increases (more O2 is used and CO2 produced)
• increasing level of CO2 made is detected by the brain and a signal is sent to the lungs to increase breathing rate.
• breathing rate and volume of air in each breath increases meaning more gaseous exchange is taking place.
• the brain also signals the heart to beat faster to pump blood to the lungs for efficient gaseous exchange.
• more oxygenated blood gets to the muscle and CO2 is removed.