Human gas exchange system

• Define antagonistic muscles

  Where one muscle contracts and the other relaxes
• INSPIRATION

  The external intercostal muscles contract this causes the ribs to pull upwards and outwards. The internal intercostal muscles relax. The diaphragm contracts and pulls down from it's dome position. This increases the volume of the lungs. The air pressure in the lungs decrease below atm. The air then moves out of the lungs from a lower pressure to atm pressure
• Expiration

  The external intercostal muscles relax. The internal intercostal muscles contract this pulls the ribs downwards and inwards. The diaphragm returns to it's dome position this decreases the volume of the lungs. The air pressure in the lungs increases above atm pressure. This causes the air to move out the lungs from a higher pressure to atm pressure
• Alveoli

  They are lined with epithelial cells. They are also surrounded by a network of capillaries. These capillaries are one cell thick and have very thin walls. This provides a short diffusion pathway. The walls being thin cause red blood cells to flatten against the thin walls when they pass through, so it slows the passing of red blood cells to allow for more time for gas exchange.