2(g) Gas exchange

Breathing

The movement of gas in and out of the lungs involving intercostal muscles. Also known as ventilation.

Structure of the thorax

A - intercostal muscles

B - pleural membranes

C - bronchioles

D - voice-box

E - alveoli

F - rib-cage

G - pleural fluid

H - Diaphragm

I - trachea

J - bronchus

Trachea

Tube with incomplete rings of cartilage carries air to the lungs. Lined with cells making mucus and cells with cilia to move the mucus away.

Bronchi

Two tubes which carry air from the trachea to the lungs

Bronchioles

Alveoli

Diaphragm

Rib-cage

Intercostal muscles

Pleural membrane

Inhalation

The intercostal muscles contract, which moves the rib-cage up and out. The diaphragm contracts and flattens(moves down), so the volume of the thorax increases which reduces the pressure(lower than atm. pressure), causing the air to move into the lungs.

Exhalation

The intercostal muscles relax, which causes the rib-cage to move in and down. The diaphragm relaxes and moves up, so the volume of thorax decreases which increases the pressure. Therefore air moves out of the lungs.

How are alveoli adapted for gas exchange

1. There are a lot of alveoli in the lungs which creates a large surface area for diffusion.

2. It has a rich blood supply to maintain a steep diffusion gradient between the alveoli and the blood for diffusion of Co2 out of the blood into the alveoli and diffusion of oxygen out of the alveoli into the blood.

3. Short diffusion distance between the air and the blood because the alveoli and the capillary surrounding them only have walls that are one cell thick.

4. The walls are permeable and are moist to help dissolve gases and increases diffusion rate. Moist surfaces allow gases to dissolve and diffuse across the wall more effectively.