Gas exchange in humans

Describe the gross structure of the human gas exchange system.

Air moves into the mouth down a pressure gradient and travels down the trachea until it reaches the bronchi. Then it'll travel down a bronchus and into the bronchioles where at the end it reaches the site of gas exchange, the alveoli.

Name the adaptations of the alveolus.

1. Very large surface area of alveoli and the surrounding network of blood capillaries (provides rich blood supply).

2. The walls of the alveoli (one layer of epithelial cells) and capillaries (one layer of endothelial cells) are thin therefore providing a short diffusion pathway/distance.

3. The capillaries are narrow so that RBCs are squashed against the capillary wall to slow down flow and allow time for diffusion to occur. Also it shortens the distance between alveolar air and RBCs.

4. The lungs are constantly ventilated- maintaining a concentration gradient.

5. Blood is constantly moving (due to pumping by the heart) to maintain the concentration gradient.

6. Alveoli have a moist surface for gases to dissolve before they diffuse.

How is the trachea + bronchi prevented from collapse?

C shaped cartilage keeping airways open.

How is the alveoli prevented from collapse?

Pulmonary surfactant is a phospholipid that coats the surfaces of the lungs and has a low surface tension preventing the watery lining of the alveoli from creating a surface tension and collapsing.

Explain the process of inspiration.

1. External intercostal muscles contract, internal intercostal muscles relax and ribs rise.

2. Diaphragm contracts to become flatter and pushes digestive organs down.

3. Volume of chest cavity increases.

4. Pressure in chest cavity drops below atmospheric pressure.

5. Air containing oxygen is sucked in through the mouth.

6. Air moves into lungs.

Explain the process of expiration.

1. External intercostal muscles relax, internal intercostal muscles contract and ribs fall.

2. Diaphragm relaxes and is pushed up by displaced organs underneath.

3. Volume of chest cavity decreases.

4. Pressure in lungs increases and rises above atmospheric pressure.

5. Air moves out of lungs.

6. Air is forced out through the mouth, removing waste carbon dioxide.

What's the point of lung tissue being elastic?

It recoils and regains the original shape of the lungs when not being actively expanded which plays a major part in pushing air out of the lungs.

What's the point of the pleural membrane?

Secrets pleural fluid contained in the pleural cavity which acts as a lubricant which allows friction free movement against the inner wall of the thorax during ventilation.