Unit+5

=Gaseous exchange and Ventilation=

=Meeting the muscles oxygen demand=

__Key Terms__ Tidal Volume - The volume of air inhaled and exhaled in a normal breath Vital Capacity - Is the maximum amount of air that can be inhaled and exhaled in a single breath Ventilation Rate - The rate at which a person breathes per minute

A calculation which uses some of these terms, is the one for working out ventilation rate. This calculation is:
 * Ventilation rate = tidal volume x number of breaths per minute**

An area in the brain called the **Medulla** controls breathing, it is split into two sections the **Inspiratory centre** and the **Expiratory centre.**

**__Inhalation__**
To inhale messages are sent from the inspiratory centre down the **intercostal nerve** and the **phrenic nerve** causing the diaphragm and the intercostal muscles to contract. Air then moves into the lungs. As the lungs inflate the stretch receptors in the bronchial tree are activated.

__**Exhalation**__
The stretch receptors then send a message up the **vagus nerve** to the expiratory centre which stops the inhalation messages. When expiration has occured the messages stop and inhalation occurs once more. During exercise the cells demand for oxygen increases, which means that this cycle happens more frequently.

=__Gas Exchange at the Alveoli__=

This Diagram shows the basic structure of the human gas exchange system. The Alveoli's are air sacs, which have capillaries lying close to their surface. Gas Exchange happens in the Alveoli by diffusion. In the Alveoli the oxygen concentration is high, where as it is low in the capilaries. Therefore the oxygen moves from a high concentration to a low concentration. Where as in the cappilaries the carbon dioxide concentration is high, and in the alveoli's it is low. So as oxygen diffuses out of the alveoli, whilst carbon dioxide diffuses into the alveoli. Fick's law is that: Rate of Difffusion=__Surface Area X Concentration Gradient__ length of diffusion pathway A rapid rate of diffusion can be achieved in the human gas exchange system.
 * The Alveoli**

This is because there is a large number of alveoli in the human lungs, therefore there is a large surface area. Each one of theese alveoli has lots of capillaries surounding it. This creates a large surface area.

The Alveoli's have very thin walls, and as their are capillaries lying close to the surface of the alveoli their is a short diffusion pathway.

The concentration gradient between the capillaries and the alveoli is very steep. It is important that their is a constant supply of blood to every alveoli in order for the body to constantkly have nough oxygen in order for respiration to be carried out.