Learning outcomes:
describe the structure of the thorax, including the ribs, intercostal muscles, diaphragm, trachea, bronchi, bronchioles, alveoli and pleural membranes
understand the role of the intercostal muscles and the diaphragm in ventilation - b
explain how alveoli are adapted for gas exchange by diffusion between air in the lungs and blood in capillaries
understand the biological consequences of smoking in relation to the lungs and the circulatory system, including coronary heart disease
describe experiments to investigate the effect of exercise on breathing in humans.
Gas Exchange in Humans:
resp sys.GIF

How breathing works

Breathing in (inhaling)

1. Intercostal muscles contract, pulling the ribcage up and out
2. Diaphragm contracts moving down
3. The volume of the Thoracic Cavity increases
4. The pressure in the Thoracic Cavity decreases
5. Air is drawn into the lungs to equalize the pressure

Inhaling is an active process, i.e. it requires energy for muscle contraction

Breathing out (exhaling)

1. Intercostal muscles relax, the ribcage moves inwards and down
2. Diaphragm relaxes moving up
3. The volume of the Thoracic Cavity decreases
4. The pressure in the Thoracic Cavity increases
5. Air leaves the lungs to equalize the pressure

The entire process is passive, i.e. no energy is required as there is no muscle contraction.

Alveoli and their adaptations for gas exchange:


- Alveolus is one cell thick
- Capillary wall is one cell thick
- Many alveoli produce a huge surface area
- Alveoli wall is moist
- Breathing maintains a high concentration gradient for O2 and CO2
- Blood movement maintains a high concentration gradient for O2 and CO2


Cigarette smoke contains tar, nicotine, carcinogens, CO and poisons

Chemical and Effect

Tar - Blocks up alveoli, making gas exchange more difficult. Also clogs up cilia (little hairs lining the lungs, whose job is to “wave” and remove mucus and trapped bacteria out of the lungs).

Nicotine - Speeds heart rate and damages arteries, causing furring of artery walls (atherosclerosis). This leads to heart disease and vascular diseases. It is also addictive.

Carcinogens - Damages the DNA of alveoli cells. This can lead to them reproducing faster than normal, which will cause a tumour to form. The tumour is the start of cancer.

Carbon Monoxide - Attaches permanently to haemoglobin, reducing the ability of the blood to carry O2.

Poisons - The list is endless. There are over 5000 poisonous chemicals in cigarette smoke (e.g. benzene, arsenic, lead, cyanide etc).

You need to know an experiment that will show the effect of exercise on humans. The easiest experiment is to take your own heart rate, breathing rate and skin temperature at rest. Do some exercise, then take the same measurements again. You’ll find they’ve all increased. The reason for this is that your rate of respiration has increased (to supply the muscles with extra energy for contraction). In order to get respiration to happen faster, you need more O2, so the breathing and heart rate increase. Unfortunately, you also release more waste heat energy, so your body heats up and you might have to start sweating to cool it down again.