2.44 – Describe the structure of the thorax, including the ribs, intercostal muscles,
diaphragm, trachea, bronchi, bronchioles, alveoli and pleural membranes.
Part of the Thorax Description
Ribs Acts as a protective cage for the lungs. Can be lifted by intercostal muscles to
support ventilation.
Intercostal
Muscles
Each rib is joined to two sets of muscles – intercostal muscles – that contract and
relax to move the rib cage to support ventilation.
Diaphragm A curved smooth sheet of muscle that contracts or relaxes to either increase or
decrease the volume of space in our lungs for ventilation.
Trachea Also known as the windpipe; air enters our nose or mouth and passes down the
trachea to get to the lungs.
Bronchi The trachea splits into two tubes called bronchi (singular: bronchus).
Bronchioles Smaller branched tubes from the bronchi.
Alveoli A microscopic air sac, located where each individual bronchiole ends.
Pleural
Membranes
Separates the inside of the thorax from the lungs and makes an air-tight envelope
around them.
Pleural Cavity Area between the two layers of pleural membrane.
Pleural Fluid The fluid that fills the pleural cavity; acts as a lubricant to prevent the lungs from
sticking to the inside of the chest when we breathe
2.45 – Understand the role of the intercostal muscles and the diaphragm in ventilation.
– Ventila
Mechanism
Internal
Intercostal
Muscles
External
Intercostal
Muscles
Diaphragm
Inhale Relax Contract Contract; flattens to allow more air to enter as ribs are lifted
and volume of space is increased.
Exhale Contract Relax Relax; air is forced our as ribs drop back down, the diaphragm
returns to a dome shape and the volume of space decreases.
2.46 – Explain how alveoli are adapted for gas exchange by diffusion between air in the
lungs and blood in capillaries.
– Thin capillary wall for short diffusion distance.
– Rich blood supply maintains concentration gradient from network of capillaries.
– Moist surface, encouraging diffusion.
– Large surface area.
2.47 – Understand the biological consequences of smoking in relation to the lungs and
the circulatory system, including coronary heart disease.
LUNGS
– Emphysema caused by damage to alveoli.
– Bronchitis caused by a bacterial infection of the bronchi.
– Cilia is damaged, resulting in a build up of mucus.
– Tar contains carcinogens which cause cancerous mutations.
HEART
– Carbon monoxide binds to haemoglobin, reducing the capacity to carry oxygen.
– Build up of plaque or cholesterol which blocks coronary arteries, increasing the blood
pressure.
– Blockage of coronary arteries increases anaerobic respiration while decrease aerobic
respiration.
– Build up of lactic acid causes fatigue and eventually heart disease.
2.48 – Describe experiments to investigate the effect of exercise on breathing in humans.
– The person rests for 5 minutes, making sure he (or she) is completely relaxed.
– He then counts the number of breaths in 1 minute.
• This could be repeated if necessary to get a steady ‘resting rate’.
– He then does some vigorous exercise (i.e. running on a treadmill for 3 minutes).
– Immediately after finishing, he records the breathing rate. He does this each minute
until his rate returns to normal.