Gas Exchange in Humans

Question 1

What happens as you breathe?

Answer 1

As you breathe in, air enters the trachea. The trachea splits into two bronchi - one bronchus leading to each lung. Each bronchus then branches off into smaller tubes called bronchioles. The bronchioles end in small air sacs called alveoli. The rib cage, intercostal muscles and diaphragm all work together to move air in and out

Question 2

What is ventilation?

Answer 2

Ventilation consists of inspiration and expiration

Question 3

What is ventilation controlled by?

Answer 3

The diaphragm, internal and external intercostal muscles and the ribcage

Question 4

What happens during inspiration?

Answer 4

The external intercostal and diaphragm muscles contract. This causes the ribcage to move upwards and outwards and the diaphragm to flatten, increasing the volume of the thoracic cavity. As the volume of the thoracic cavity increases, the lung pressure decreases. Air will always flow from an area of higher pressure to an area of lower pressure, so air flows down the trachea and into the lungs

Question 5

Does inspiration require energy?

Answer 5

Inspiration is an active process - it requires energy

Question 6

What happens during expiration?

Answer 6

The external intercostal and diaphragm muscles relax. The ribcage moves downwards and inwards and the diaphragm becomes curved again. The volume of the thoracic cavity decreases, causing the air pressure to increase (above atmospheric pressure). Air is forced down the pressure gradient and out of the lungs

Question 7

Does expiration require energy?

Answer 7

Normal expiration is a passive process - it doesn’t require energy

Question 8

What happens during forced expiration?

Answer 8

During forced expiration, the external intercostal muscles relax and the internal intercostal muscles contract, pulling the ribcage further down and in. During this time, the movement of the two sets of intercostal muscles is said to be antagonistic (opposing)

Question 9

What are the alveoli made from?

Answer 9

Each alveolus is made from a single layer of thin, flat cells called alveolar epithelium

Question 10

How is gas exchanged at the alveoli?

Answer 10

There’s a huge number of alveoli in the lungs, which means there’s a large surface area for exchanging oxygen and carbon dioxide. The alveoli are surrounded by a network of capillaries. Oxygen diffuses out fo the alveoli, across the alveolar epithelium and the capillary endothelium, and into the haemoglobin in the blood. Carbon dioxide diffuses into the alveoli from the blood, and is breathed out

Question 11

Give two features of alveoli that speed up the rate of diffusion

Answer 11

1. A thin exchange surface - the alveoli epithelium is only one cell thick. This means a short diffusion pathway
2. A large surface area - the large number of alveoli mean there’s a large surface area for gas exchange
3. There is also a steep concentration gradient of oxygen and carbon dioxide between the alveoli and capillaries, which increases the rate of diffusion - this is constantly maintained by a flow of blood and ventilation