Gas exchange and breathingIB MYP Biology: Revision notes
Section 1
The gas exchange system
Air enters through the nose or mouth and passes down the trachea (windpipe), which is kept open by rings of cartilage. The trachea divides into two bronchi, one to each lung. Each bronchus divides into many smaller tubes called bronchioles, which end in clusters of tiny air sacs called alveoli. The ribs, intercostal muscles (between the ribs) and the diaphragm (a sheet of muscle under the lungs) are used in breathing.
The trachea and bronchi are lined with cilia and mucus that trap and sweep away dust and microbes.
Section 2
Inhaling and exhaling
Inhalation: the diaphragm contracts and flattens; the intercostal muscles contract and the ribs move up and out. The volume of the chest increases, so the pressure drops below atmospheric pressure and air flows in.
Exhalation: the diaphragm relaxes and moves up; the intercostal muscles relax and the ribs move down and in. The volume of the chest decreases, the pressure rises and air is pushed out.
The lungs do not suck air in. Air moves in because the pressure in the chest falls below the pressure outside.
Section 3
Alveoli: gas exchange
Oxygen diffuses from the air in the alveoli into the blood, and carbon dioxide diffuses from the blood into the alveoli to be breathed out. The alveoli are adapted for fast gas exchange:
- a very large surface area (millions of alveoli)
- walls one cell thick, a short diffusion distance
- a rich blood supply of capillaries, so a steep concentration gradient
- ventilation brings fresh air and removes carbon dioxide
- a moist lining so gases dissolve
Section 4
Inhaled and exhaled air
Approximate composition:
- Inhaled air: about 21% oxygen, 0.04% carbon dioxide, 78% nitrogen, variable water vapour.
- Exhaled air: about 16% oxygen, 4% carbon dioxide, 78% nitrogen, more water vapour.
Exhaled air has less oxygen and more carbon dioxide because cells use oxygen in respiration and make carbon dioxide. Limewater turns cloudy with carbon dioxide, so it is cloudy faster with exhaled air. Exhaled air is also warmer and more moist.
Nitrogen is the same in inhaled and exhaled air because the body does not use it.
Section 5
Effects of smoking
Tobacco smoke contains:
- Tar, which paralyses and destroys cilia, so mucus builds up and causes a cough and bronchitis, and contains chemicals that cause lung cancer.
- Carbon monoxide, which binds to haemoglobin so red blood cells carry less oxygen.
- Nicotine, which is addictive and raises the heart rate.
Smoking also damages the walls of the alveoli, reducing the surface area.
Section 6
Gas exchange in leaves
Leaves exchange gases through tiny pores called stomata (singular: stoma), mostly on the underside. Carbon dioxide diffuses in for photosynthesis; oxygen diffuses out. Water vapour also leaves.
Each stoma is controlled by two guard cells. When they take in water they become turgid and curve apart, opening the stoma. When they lose water they become flaccid and close it, saving water. Inside the leaf, air spaces between cells and a large surface area help gas exchange.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Gas exchange and breathing
- A swimmer in Sydney takes a deep breath before a race. Her chest expands and air flows down her airways into her lungs.Explain why air flows into the lungs when the diaphragm contracts.2 marks
- A student in Dublin compares the carbon dioxide in air from the atmosphere with the carbon dioxide in air she breathes out. In tube A she breathes out through a straw into limewater for 1 minute. In tube B she uses a pump to bubble the same volume of air from the room through the same volume of limewater. Limewater turns cloudy when carbon dioxide is present. The limewater in tube A turns cloudy after 20 seconds. The limewater in tube B is still clear after 4 minutes.Limewater only shows whether carbon dioxide is present. Suggest one improvement that would allow the student to compare the amounts of carbon dioxide more accurately.2 marks
- A biology student in Toronto is comparing how a human lung and a leaf are adapted for gas exchange. Both have structures that allow oxygen and carbon dioxide to diffuse in and out of the organism.Describe how the guard cells control the movement of gases in and out of a leaf.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).