6.6 Gas Exchange in Humans

Question 1

What are the two gases humans need to exchange?

Answer 1

• oxygen into the blood for respiration
• get rid of carbon dioxide made by respiring cells

Question 2

Describe how gas exchange occurs in the lungs

Answer 2

1. Breathe in = air enters the trachea
2. Trachea splits into two bronchi ~ one bronchus into each lung
3. Each bronchus then branches off into smaller tubes called bronchioles
4. Bronchioles end in alveoli = where gas exchange takes place

Question 3

What does ventilation consist of and what is it controlled by?

Answer 3

• inspiration = breathing in
• expiration = breathing out
• controlled by diaphragm and intercostal muscles

Question 4

Describe inspiration

Answer 4

• external intercostal and diaphragm muscles contract
• causes the rib cage to move upwards and outwards
• diaphragm flattens, increasing volume in the thoracic cavity
• increase volume in thoracic cavity = lung pressure decreases
• air will always flow down a pressure gradient so air flows down the trachea and into the lungs
• inspiration is an active process = requires energy

Question 5

Describe expiration

Answer 5

• external intercostal and diaphragm muscles collapse
• ribcage moves downwards and inwards and diaphragm becomes curved again
• volume of thoracic cavity decreases = air pressure increases to above atmospheric pressure
• air is forced down pressure gradient and out of the lungs
• normal expiration is passive = doesn’t require energy

Question 6

Describe what forced expiration is and what happens during it

Answer 6

• external intercostal muscles relax and internal intercostal muscles contract
• ribcage is pulled further down and in
• intercostal muscles work antagonistically

Question 7

Describe the structure of the alveoli

Answer 7

• each alveolus is made from a single layer of thin, flat cells = alveolar epithelium
• huge number in the lungs = increase surface area for gas exchange
• surrounded by a network of capillary tissues = short diffusion pathway

Question 8

How are gases exchanged in the alveoli?

Answer 8

• O2 diffuses out of the alveoli, across epithelium cells and into haemoglobin
• CO2 diffuses into the alveoli from the blood, and is breathed out

Question 9

What adaptations do alveoli have for gas exchange?

Answer 9

• thin exchange surface as alveolar epithelium is one cell thick = short diffusion pathway
• large number of alveoli = large surface area

Question 10

What is the concentration gradient like and how is it maintained?

Answer 10

• steep concentration gradient of oxygen and carbon dioxide between alveoli and capillaries
• increases rate of diffusion
• maintained by flow of blood and ventilation